Bhel Journal Apr 07

BHEL JOURNAL

Volume 28 No. 1 April 2007

Editorial Advisory Committee

C.P. SinghK. RavikumarK.V. MuthukrishnanS.K. Goyal

Editor : R.K. Bhattacharya

Associate Editor : Alok Mathur

BHEL JOURNAL is published quarterly.All correspondence and enquiries are to beaddressed to:

Mr. R.K. BhattacharyaEditor, BHEL JournalBharat Heavy Electricals LimitedBHEL House, Siri Fort,New Delhi-110 049

The statements and views expressed in thisJournal are entirely those of the authors, andnot necessarily that of the Organisation.

Contents may be referred to or reproducedpartially with due acknowledgements.

Copyright reserved.

CONTENTS Page

LAMINAR HEAT TRANSFER AND PRESSUREDROP IN THE AXIAL CIRCULAR DUCTS OFTHE ROTOR OF AN ELECTRICAL MACHINE 1

ALUMINIUM TRIHYDRATE (ATH) —A VERSATILE MATERIAL 12

APPLICATIONS OF HIGH POWER LASERSIN MATERIAL PROCESSING 20

CONTROL & DIAGNOSTIC FEATURES OF500 MW TURBO GENERATORS 31

IMPROVED THYRISTOR TURN-OFFCHARACTERISTICS THROUGHSELECTIVE EMITTER DOPING 43

INNOVATIONS — FROM BHEL 55

RECENT MAJOR ACHIEVEMENTS OF BHEL(during September'06-March'07) 61

1

2

4

3 Cover Photographs

1. Singrauli STPS, equipped with BHEL's first indigenouslybuilt 500 MW set

2. 290 MW Almatti Dam HEP

3. 4x15 MW Kurichu HEP, Bhutan

4. 2x77 MW Captive Power Plant at Hindustan Zinc Ltd.

1BHEL JOURNAL, April 2007

LAMINAR HEAT TRANSFER AND PRESSURE DROPIN THE AXIAL CIRCULAR DUCTS OF THE

ROTOR OF AN ELECTRICAL MACHINE

N. Gunabushanam

SYNOPSIS

Experimental results have been obtained on pressuredrop and heat transfer for laminar flow of air in anaxially cooled rough circular duct of a rotor of anelectrical machine, with circumferential variation ofheat flux. The entry conditions, and the range ofparameters covered, simulate the real conditions moreclosely than in the work reported in literature. The localand mean heat transfer characteristics are evaluated

and their variation with rotational Grashof numberand aspect ratio are reported. The influence of rotationon hydraulic friction factor is studied.

Key Words:

Laminar Flow; Rotating Circular Ducts; HeatTransfer; Pressure Drop.

NON DIMENSIONAL GROUPS

Ac

- Acceleration ratio = HΩ2/g

Grg

- Grashof number = gβd3Δt/γ2

Grr

- Rotational Grashof number = AcGr

g

J - Rotational Reynolds number = 2Ω a2/γ

Nub

- Bulk Nusselt number = q/πK(Tw-T

b)

Num

- Mean Nusselt number = q/πK(Tw-T

m)

Pr - Prandtl number = γ/α

Rag

- Gravitational Rayleigh number = Grg.Pr

Rar

- Rotational Rayleigh number = HΩ2βτa4/αγ = Grr. Pr.

Re - Pipe flow Reynolds number = Wmd/γ

ε - Eccentricity Parameter = a/H

aR

- Aspect Ratio = L/d


1. INTRODUCTION

Cooling passages are often used in the rotors of largehigh performance electrical machines to allow forincreased electrical and magnetic loadings. In someof these designs, a major portion of a typical rotorcooling circuit involves flow channels which areparallel to, but displaced from the rotor axis. In sucha geometry, the presence of centripetal and Coriolisacceleration forces may cause secondary flow tooccur in a plane perpendicular to the axis of therotating channel, leading to considerable differencesin fluid friction and heat transfer between rotating

ducts and stationary ducts. The density gradients inthe fluid resulting from the temperature differencegive rise to buoyancy forces, which further influencethe flow field. For larger angular velocities, thesebuoyancy effects must be taken into account in thecalculation of temperatures and design of fancapacities for optimum machines.

Most investigations reported in the literature areconcerned with laminar flow in parallel rotatingpassages. A few concern the theoretical analysis fordeveloped flow, with very weak secondary flows andextremely low Reynolds numbers. Morris [1], Mori

NOMENCLATURE

a - Radius of circular duct

Cf

- Friction factor in rotating tube

Cfo

- Friction factor in stationary tube

d - Hydraulic diameter of the duct

g - Acceleration due to gravity

H - Radius of rotation of pipe axis

L - Length of test section

K - Thermal conductivity

p - Pseudo pressure

q - Heat transfer rate per unit length

T - Temperature

Tb

- Bulk Temperature

Tm

- Mean temperatures

Tw

- Wall temperature

ΔT - (Tw - T

b) Temperature difference

Wm

- Mean axial velocity

α - Thermal diffusivity

β - Coefficient of volumetric expansion

γ - Kinematic viscosity

ρ - Density

τ - Axial temperature gradient

Ω - Angular velocity


and Nakayama [2] have studied the case of fullydeveloped laminar and turbulent flows. The authorsassumed a gross secondary flow consistent with highrotational speeds and derived heat transfer andpressure drop for a variety of Prandtl numbers. Theyalso showed that the influence of Coriolis forces wassignificant. Nakayama [3] considered the fullydeveloped turbulent flow using the boundary layerconcept. Humphreys, Morris and Barrow [4]presented the results of an experimental investigationusing air as the working fluid in the entry region byforcing air through a radial rotating pipe and a bend,thereby introducing considerable inlet swirl. Hencethe results may not be directly useful to axiallycooled electrical motors. Le Feuvre [5] has alsopresented results of an experimental programmewhere this type of cooling system is used in the rotorof an electrical motor, but for a limited range ofparameters. However, he did not investigate thepressure drop aspect of the rotating ducts. Generally,improved heat transfer was noted with increase inrotational speed and, although reliable quantitativepredictions were not possible, qualitative agreementwith the analytical work was observed.

In the present work, experimental measurements ofpressure drop and heat transfer in the axial circularducts formed by the assembly of rotor laminationsare made under conditions of rotation. The entry ofthe flow to the duct is axial and swirl-free. Onlyweak secondary forces, i.e., very slow speeds, areconsidered and it is shown that the effect of rotationis to increase the heat transfer and friction, the effectbeing larger in laminar flow than in turbulent flow.The experiment simulates the condition of anaxially-cooled rough circular duct of a rotor of anelectrical machine with circumferential variation ofheat flux.

2. EXPERIMENTAL SET-UP

The experiments were performed using a rotatingduct facility shown in Fig. 1. This consists basicallyof a built-up rotor of circular ducts formed by anassembly of electrical steel sheets. There are six axialducts on the rotor. Thermocouples are embedded in

the walls of the ducts and in the air passage alongthe rotor length to measure the temperatures and arebrought out through slip rings. Two sets of ductswere chosen for the purpose of checking theconsistency. Pressure taps are made in the axial ductsand they are brought out through a 36 port rotatingscanivalve. The pressure tap can be changed bymeans of a 90 psi compressed air supply to thescanivalve, through a solenoid operated controlvalve. Complete details of the test set-up are shownin Fig. 2. The heating elements are embedded in theconductor slots of the rotor and brought outthrough power slip rings. The rotor surface isinsulated uniformly with fibre-glass tape and asbestospowder. Heat flux meters are mounted on theinsulated rotor surface, along with the thermocouples,to measure surface losses. The ends of the rotor areprovided with 5.0 mm thick glass textolite insulatorsand a 1.0 mm thick layer of asbestos powder tominimize end losses. Embedded thermocouplesmeasure the end losses across the textolite thermalinsulators, whose thermal conductivity is known. Ad.c. motor drives the rotor whose speed is measureddigitally by a magnetic pick-up. The air is directlydrawn from the atmosphere into the rotor axialducts through an inlet nozzle. The flow rate ismeasured by pitot probes and hot wire anemometersat around 50 locations, at a stabilised distance fromthe outlet of the test section.

3. EXPERIMENTAL PROCEDURE

The power input to the heating elements wasadjusted so that it caused a measurable temperaturedifference between the air and the wall of the ductfor maximum air flow. The power input wasmaintained constant and the steady state temperatureswere recorded on a data logger. The pressure droptap readings were measured through the scanivalve.

The experiments were repeated for different heatinputs and rotational speeds of 0 to 1200 r.p.m.Data was obtained for rotor power inputs in therange of 100 to 300 W. For each power input andspeed, eight axial flow Reynolds numbers, rangingfrom 800 to 14 000, were calculated.


FIG

. 1

: E

XP

ER

IME

NT

AL

SET

-UP


FIG

. 2

: E

XP

ER

IME

NT

AL

TE

ST S

EC

TIO

N


4. METHOD OF DATA ANALYSIS

The wall heat flux was calculated from the mainpower input by subtracting the brush contact losses(calibrated earlier), rotor surface losses and endlosses. The wall heat flux so obtained was comparedwith the thermal balance made by measuring themass flow and the temperature rise of air. The twoagreed within ±3.5%. The pseudo Nusselt numberswere calculated using the local temperature differencebetween the wall and the local centre line temperatureof the air. The local heat flux was evaluated bycalculating the heat carried by the air up to the pointof consideration. For evaluating mean Nusseltnumber, the difference between the mean walltemperature profile and the mean of the inlet andoutlet temperatures of air in the duct, was used. Thehydraulic friction factor, C

f was calculated from the

pressure gradient, measured through the scanivalve,using the Fanning friction equation.

5. DISCUSSION OF EXPERIMENTALRESULTS

The test data obtained are represented graphically inFig. 3 to 11, for the minimum and maximumReynolds numbers. Fig. 3 shows the axial temperaturedistribution obtained for mean gravitational Rayleighnumber of 5320. The curves for zero speed andsome typical speeds of the rotor ducts are also given.The influence of rotation on the mean and maximumwall temperature, for various mean Reynolds numbers,is explained in Fig. 4. It is observed from the figurethat the effect of reduction in wall temperature isgreater at lower speeds than at higher speeds. Fig. 5shows the variation of the mean motivatingtemperature differential (used in the evaluation ofmean Grashof number) vs. speed for typical meanaxial flow Reynolds numbers. An indication of thevariation in heat transfer owing to duct rotation isshown in Fig. 6. Fig. 7 explains the variation of localNusselt number with aspect ratio for various valuesof acceleration ratio (A

c) and pipe flow Reynolds

number (Re). The variation of hydraulic frictionfactor with nominal Reynolds number, for variousacceleration ratios and mean rotational Rayleigh

numbers, is given in Fig. 8. It can be seen from thefigure that there is a steep decrease in the frictionfactor in the range of 0 to 400 r.p.m. and furtherincrease in speed marginally decreases the frictionfactor for a given nominal Reynolds number.

The calibration of the test equipment can bechecked by comparing the local Nusselt numbers forthe stationary case. Even the small difference for zerospeed can be attributed to the roughness involved inthe formation of the duct by the assembly of thelaminations.

6. UNCERTAINTIES IN THEMEASUREMENTS

The basic measurements involved in the experimentalinvestigation are the voltage and current for powerinput calculations, wall tap pressures and wall andair temperatures for pressure drop and heat transfercalculations, respectively. The brush contact lossesfor power input and the rotor surface and end losseshave been measured by highly sensitive embeddedheat flux sensors, drawn through the slip ringmechanism. These losses have been calibrated forvarious speeds. The net heat flow into the rotorducts is compared with the heat flow obtainedthrough the thermal balance and it compares well.The percentage of unaccounted losses is showngraphically in Fig. 9. It can be seen that, forReynolds number from 800 to 9500, and for thespeeds 0 to 1200 r.p.m., the unaccounted losses varyfrom 2.0 to 8.0%.

It can be seen from Fig. 10 that the temperaturedifferential between the wall and the air decreaseswith increase in Reynolds number for a given speed.An increase in the speed brings down the motivatingtemperature differential. At higher Reynolds number,the speed has a minimal effect on the temperaturedifferential. Similar trends can be noted in Fig. 11.

The thermocouples are of sheath type, pre-calibratedusing a thermostat bath. The measuring spot is 0.15to 0.20 mm from the duct inner surfaces, which isanother source of error.


FIG. 3 : AXIAL TEMPERATURE PROFILES FOR ROTATING CIRCULAR DUCT

FIG. 4 : INFLUENCE OF ROTATION ON MAXIMUM AND MEAN WALL TEMPERATURE

FIG. 5 : INFLUENCE OF ROTATION ON MEAN MOTIVATING TEMPERATURE DIFFERENCE BETWEEN WALL DUCT AND COOLANT


FIG. 6 : VARIATION OF MEAN NUSSELT NUMBER WITH ROTATIONAL GRASHOF NUMBER

FIG. 7 : VARIATION OF LOCAL NUSSELT NUMBER WITH ASPECT RATIO

FIG. 8 : INFLUENCE OF ROTATION ON HYDRAULIC FRICTION FACTOR


FIG. 9 : PERCENTAGE OF UNACCOUNTED LOSSES

FIG. 10 : REYNOLDS NUMBER VS MOTIVATING TEMPERATURE DIFFERENCE

FIG. 11 : VARIATION OF WALL TEMPERATURE WITH REYNOLDS NUMBER FOR VARIOUS SPEEDS

600 rpm

400

1000

1200 rpm


7. CONCLUSIONS

The following broad conclusions can be drawn fromthe present investigation:

1. Rotation of the rotor duct enhances heattransfer and increases the hydraulic frictionfactor relative to the stationary case. There isa steep change from zero speed to a nominalspeed.

2. The effect of Coriolis forces, as identified bythe parameter J, is to decrease the heattransfer and also the hydraulic friction factor.

3. The axial entry condition of the flow in therotor ducts is very important as this cancompletely change the picture of heat transferand pressure drop.

Acknowledgement

The author wishes to thank the management ofBharat Heavy Electricals Limited for providingassistance in building the rig, to carry out theinvestigation and to publish the paper.

References

1. MORRIS, W.D., -- Laminar convection in aheated vertical tube rotating about a parallelaxis. Journal - FLUID MECHANICS Vol. 21,Part 3, 453-464, 1965.

2. MORI, Y and NAKAYAMA, W., -- Forcedconvective heat transfer in a straight piperotating around a parallel axis - Laminarregion. International Journal of Heat and MassTransfer Vol. 10, 1179-1194, 1967.

3. NAKAYAMA, W., -- Forced convective heattransfer in a straight pipe rotating around aparallel axis - Turbulent region. InternationalJournal of Heat and Mass Transfer Vo.11,1185-1201, 1960.

4. HUMPHREYS, J.F., MORRIS, W.D. andBARROW, H., -- Convective heat transfer inthe entry region of a tube which revolves aboutan axis parallel to itself. International Journalof Heat and Mass Transfer. Vol.10, 333-347,1967.

5. Le FEUVRE, R.F., -- Heat Transfer in rotorcooling ducts. Proceedings of Institution ofMechanical Engineers, London. Vol.182, Pt3H,232-240, 1967-68.

6. GUNABUSHANAM, N and REDDY, R.S., -- Heat transfer in the axial ducts of a rotor ofan electrical machine. BHEL R&D Report.April 1990.

7. GUNABUSHANAM, N. -- Heat transfer inthe air gap of an electrical machine. BHELR&D Report. June 2002.


Mr. N. Gunabushanam completed his B.Sc.(Math.) in 1967, B.E. (Mech. Engg.) in 1970from Madras University, and M.Tech. (ThermalEngg.) from REC Bhopal. After working for twoyears in the Department of Mechanical Engineeringat IIT Madras he joined BHEL Corporate R&D,Hyderabad, in 1974 as a Design Engineer andheld successive positions until his retirement as

General Manager in 2005. He was trained inStuttgart University, KWU and Dorniers, Germany.He is currently working as General Manager-Design in a heat exchanger manufacturingcompany in Hyderabad. In addition, he is achartered consultant for many energy engineeringindustries.

His fields of interest include thermal engineering,heat exchangers, ventilation and cooling of electricalmachines, braking and starting resistors forlocomotives, etc. He has published and presentedseveral papers in national and international journalsand conferences. He can be contacted by email [email protected].


ALUMINIUM TRIHYDRATE (ATH) —A VERSATILE MATERIAL

Shilpa Hiremath and Sukumar Roy

SYNOPSIS

Aluminium trihydrate, popularly known as ATH, is aversatile material that has been catering to a wide rangeof applications over the past few decades, starting frommetallurgical to ceramic processing industries, as FRLS(fire retardant low smoke) fillers in electrical/ electroniccable industries, as catalyst supports and absorbents inchemical industries, as functional additives in polymers,paints, composites, automotive and in pharmaceuticalindustries. The commercial growth of this material hasbeen significant over the years. In the meantime,substantial innovations have taken place worldwide ondeveloping new or modified techniques to synthesizeboth conventional coarse-grained ATH and fine-grainedATH powders, which have made it possible to synthesizethe material in a variety of forms, suitable for differentpurposes. In this paper, an attempt has been made tobring out the current international status of thismaterial on important synthesis techniques andapplication areas. The paper also highlights how thematerial was made technically suitable from time totime to fit into various application needs.

Key Words:

Hydrated Aluminas; Transition Aluminas; Synthesis& Application of ATH; Composite Fillers; FRLSFillers.

1. INTRODUCTION

1.1 Background

Aluminous materials have been symbolic of progressthroughout human history. Before 5000 BC,aluminous clays were being used in Mesopotamia for

the manufacture of fine pottery. After 3000 BC,Babylonians and Egyptians began employingaluminous materials in chemicals such as dyes andmedicines [1]. Romans utilized aluminous materialsin the manufacture of perfumes. Emerald, sapphireand ruby, which are crystalline forms of aluminacoloured by the presence of metal ion impurities,were used in jewellery as early as 800 BC.

Bauxite, the most common ore of aluminium, wasdiscovered by Beethier in 1821, near the village ofLes Baux, France. However, it was only at the endof the 19th century that bauxite was recognized ascontaining useful forms of aluminium hydrates, i.e.,Al (OH)

3 and AlOOH, and aluminium silicates. In

1889, Karl Bayer invented the so-called Bayer'sprocess which is, even today, the most economicprocess of alumina or hydrated alumina production.

Alumina (Al2O

3) and its hydrates have always

generated a great deal of interest amongcrystallographers, as it undergoes several intermediatemodifications before it transforms into itsthermodynamically stable alpha phase or corundum.

The intermediate phases, commonly called 'transitionphase' or 'transition aluminas', exist in a variety ofstructures and have widely varying properties.Interestingly, all these phases are stable andreproducible at room temperature. For example, onheating (calcination), gibbsite ultimately stabilizes ascorundum (α–Al

2O

3) in the following sequence:

Gibbsite [γ-Al(OH)3] ➞ Boehmite (γ-AlOOH) ➞

γ-Alumina (γ-Al2O

3) ➞ δ-Alumina (δ-Al

2O

3) ➞

θ-Alumina (θ-Al2O

3) ➞ α-Alumina (α-Al

2O

3)

These phase transitions are of fundamentalimportance in designing the manufacturing process,


depending on which intermediate phase of aluminais used in the application. Partially calcined aluminais often used as the starting material for processingto get specific properties. The phase transitiontemperatures of aluminium hydrates are depicted inTable 1 [2].

1.2 Properties of ATH

Aluminium usually forms two types of hydroxides –tri-hydroxide and mono-hydroxide – some havingwell-characterized crystalline structures, whilst othersare amorphous. The most common tri-hydroxidesare gibbsite, bayerite and nordstrandite, and themore common mono-hydroxides are boehmite anddiaspore. Commercially, the most important form isgibbsite, although bayerite and boehmite are alsomanufactured for various purposes.

Hydrated aluminas are often loosely referred to asATH (aluminium trihydrate), although technically,

only the trihydrate forms can be called ATH. It isalso known as hydrated alumina, alumina hydrate,alumina trihydrate, alhydrogel, superfos, amphogel,aluminium (III) hydroxide, amorphous alumina,trihydrated alumina, trihydroxy aluminium.

The classification of aluminium hydroxides, basedon the degree of hydration and crystal structure, ispresented below:

● Bayerite [alpha-aluminium trihydroxide,alpha-Al(OH)

3 or alpha-Al

2O

3.3H

2O]

● Gibbsite [gamma-Al(OH)3 or gamma-

Al2O

3.3H

2O]

● Nordstrandite [beta-aluminium trihydroxide,beta-Al(OH)

3 or beta-Al

2O

3.3H

2O]

● Boehmite [gamma-AlO(OH) or gamma-Al

2O

3.H

2O]

● Diaspore [alpha-AlO(OH) or alpha-Al

2O

3.H

2O]

TABLE 1 : SEQUENCE OF THERMAL TRANSFORMATIONS OF ALUMINIUM HYDRATES [2]


ATH is a non-abrasive material with Mohs' hardnessindex of 2.5 - 3.5 and specific gravity of 2.42.Detailed physical and chemical properties of ATHcan be found elsewhere.

2.0 APPLICATIONS OF ATH

Aluminium trihydrate is a versatile material with awide range of industrial applications. Starting withmetallurgical processes, it spread to allied industriesmanufacturing aluminium chemicals or salts, eitherin pure form or derivatives like refractories, cements,abrasives, etc. Other products include alum(aluminium sulphate), poly-aluminium chloride(PAC), sodium aluminate, zeolites, aluminiumfluoride for manufacturing glass and glazes, catalysts,fertilizers, and fibre cement board products. ATHand its derivatives then found application in flameretardant and smoke suppressant fillers in plastics,cables, rubber products and carpet backing. The useof aluminium hydroxide as adsorbent, emulsifier, ionexchanger, mordant, antacid, and filtering medium,etc., was also then realized. Later, it found applicationsin the manufacture of technical ceramic products,printing inks, detergents, waterproofing fabrics,dentifrices and anti-perspirants. Recently, ATH hasbeen used in waste water treatment. Another largeapplication of ATH is in cast polymers, which arechemically bonded, ATH-filled polymer compositematerials. It is believed that the presence of ATH inthe material enhances the flexibility in moulding andhardening parameters of the composite, that in turnmeets diverse design requirements.

2.1 Applications of Fine-Grained ATH

In the past, ATH has been available as a coarse-grained material, composed of grains in the range of5 - 50 micron. However, with the development ofnano-technology, ultra fine or nano-sized ATH isalso finding new applications. Ultra fine-grained(grains below one micron) or nano-grained (grainsin the range of 1-100 nm) ATH has advantages overcoarse grained ATH in many applications:

● Use of nano-grained ATH can significantly

reduce the requirement of filler load in flameretardant cables for the same level of flameretardancy, thus making them lighter andimproving their thermal stability, mechanicalstrength and electrical properties.

● Nano grained ATH is also known to improvearc-track resistance in plastics for electricalapplications.

● As filler in fine printing papers, it increasesopacity and brightness; and in paper coatings,it imparts brightness, gloss and high inkreceptivity.

● It could be a better reinforcing pigment inadhesives, where it improves cold flowproperties and cohesion, besides stabilizingpH.

● Finer ATH also improves surface properties(surface roughness), which are exploited inpolishing applications, cleansing agents,mould wash and separating agents.

● Finer ATH with high purity finds newapplications in pharmaceuticals, high puritychemicals and paints.

2.2 ATH and Flame Retardancy

ATH is the largest volume flame retardant used inthe world. The flame retarding property of ATH inpolymers is based on its thermal decomposition inthe temperature range of 200 - 400 °C. Duringthermal decomposition, an endothermic (energyconsuming) process, ATH releases its chemicallybonded water (34.6 wt.%), while the correspondingaluminium oxide remains as char residue in the bodymass. Thus, heat energy is removed from theburning zone and the released water, in the form ofsteam, cools the surface. The oxygen content in theair surrounding the burning surface is diluted by thegases released from the polymer matrix. Thealuminium oxide formed provides a protective layer,which is chemically inert and insensitive to burning,on the surface, thereby preventing oxygen and heatfrom reaching in. Finally, due to the high specific


surface area of the oxide layer, absorption of smokeand other toxic or decomposed carbonaceous gaseousproducts takes place, making ATH a very effectivesmoke suppressant as well.

The flame retardant property of ATH has beenexploited in the paint industry also as a pigmentfiller. Fine-grained ATH is believed to have a morepronounced effect on flame retardancy and smokeabsorption properties, compared to coarse grainedATH.

2.3 Commercial Growth of ATH

According to Martinswerk GmbH, the world'sleading consumer of commodity ATH, the globalmarket for ATH as flame-retardant is growing atapproximately 4 - 6 % annually.

The total consumption of ATH in polymerapplications has reached a volume of 470,000 tonnesworld wide during the year 2000 [3]. Consideringthe total consumption of flame-retardants in WesternEurope, ATH has a market share of 42% by volume.

The U.S. market for ATH as a filler in plastics was50 million pounds in 1993 (US$ 3.7 million)increasing to 63 million pounds in 1998 (US$ 18.6million) and 92 million pounds in 2003 (US$ 25.4million), an annual growth of 6.2% in volume and6.4% in value [4].

The commercial consumption of ATH has increasedin India also over the years. The consumer industriesbroadly include metallurgical, ceramics, polymers,insulation electrical/electronic cable, chemical, etc.However, the annual consumption of the material inthe country is not known exactly.

3.0 SYNTHESIS OF ATH

3.1 Bayer's Process

The most widely used method for large-scale synthesisof ATH is the Bayer's Process. Bauxite ore is treatedwith sodium hydroxide, which dissolves thealuminium component in the ore and separates it

from the impurities. ATH is allowed to precipitateout, filtered and heat treated at appropriatetemperature/s. Industrially, a Bayer Process is a plantfor heating and cooling a large recirculating streamof caustic soda solution in which bauxite is addedat high temperature. Red mud (mainly ironimpurities) is separated at an intermediatetemperature, and alumina is precipitated at a lowtemperature in the cycle. The Bayer process producesATH of high purity in the form of gibbsite.

Bauxite contains 40-70% of natural aluminiumhydroxide in the crystalline form of gibbsite (tri-hydrate form, γ-Al(OH)

3 or Al

2O

3.3H

2O) and

boehmite (mono-hydrate form, γ-AlOOH orAl

2O

3.H

2O). Another mono-hydroxide, diaspore,

the alpha form of aluminium oxide hydroxide,α-AlOOH requires much higher temperatures ofextraction and concentrated caustic solution.

ATH manufactured by the Bayer process primarilymeets the demand for metal grade alumina, whichis the largest market of ATH. However, Bayer's ATHdoes not qualify for non-metallurgical applications,which is the second largest market.

3.2 Precipitation Techniques

Csige et al. [5] and Sleppy et al. [6] developed amilling process to generate finer (<2 micron) particlesby grinding coarse ATH powder, but as the methodis highly energy intensive it did not draw muchattention. Baksa et al. [7] produced several finehydrates in an air jet mill by precipitation in thepresence of a modifier, namely aluminium sulphate.They also studied the production of 0.5 micronATH particles by decomposing gallium-aluminiumalloy with caustic soda and/or water [8]. Martinswerk,Germany [9] has also produced special grade aluminasince 1970 and supplies one micron precipitatedATH. Interestingly, Shibue et al. [10] found that thesize of primary particles is not affected significantlyby mechanical stirring and the size of the particledecreases with increase in tank size. Also, particle sizedistribution is sharper in batch than in continuousprecipitation.


T. Tran et al. [11] studied the effect of 3,4Dihydroxy Benzoic Acid (3,4 DHBA), an organicacid, on the precipitation of ATH particles andestablished a second order rate equation in order tounderstand the rate of nucleation and crystal growth.In another study, the same group, M.J. Kim et al.[12] reported that precise control of supersaturationcondition and temperature as a function of time wasimportant in producing ATH particles with desirableshape and size.

J.K. Pradhan et al. [13] studied various factorsaffecting the quality of ATH precipitates, such asprecipitation temperature, amount of seed, seedsurface area, precipitation time, soda content ofpregnant liquor and presence of modifier additives.It was observed that at higher temperatures, purerATH with larger particle size, without soda wasobtained. At lower temperatures, though the particlesize of ATH was smaller, the soda content in theproduct was more. The single most importantparameter that influenced the precipitation processwith respect to yield, particle size and purity was thepresence of a modifier, namely aluminium sulphate.An improvement in yield and finer size with organicadditives was also observed.

3.3 Rotating Packed Bed Reactors (RPB)

J. Chen et al. [14] developed a route for synthesizingso-called nano-fibrillar ATH by carbonation in arotating packed bed at room temperature. Theformed gel was subsequently thermally treated for 30minutes at a temperature of 85 °C to yield pseudo-boehmite fibres of 1-10 nm diameter and 50-300nm length. Factors affecting the carbonation processwere gravity level of the reactants in the RPB andconcentration as well as the ratio of the reactants.Z. Peng-yuan et al. [15] worked on the preparationof ultrafine ATH via a new method combined withhigh gravity reaction and thermal hydrolysis. Theexperimental processes from a small-scale experimentto pilot-plant test and the scale-up principle werediscussed. In another report, Z. Peng-yuan et al. [16]prepared nano-ATH by the RPB method, in whichindustrial ATH was used as raw material for the

preparation of sodium aluminate, which wasprecipitated by carbonation and ATH of below 50nm was prepared.

3.4 Sonochemical Synthesis

Sonochemistry is the application of ultrasound tochemical reactions and crystallization or precipitation.It has been reported that high-power ultrasound canenhance or alter chemical reactions [17-21]. Liu Ji-bo et al. [22] studied the effect of ultrasoundfrequency on the precipitation process ofsupersaturated sodium aluminate solution underconditions similar to those in industry. The resultsindicated that ultrasonic treatment at 16 kHzenhanced the decomposition rate of sodium aluminatesolutions and also affected particle morphology andparticle size distribution of ATH precipitates.

3.5 Biomimetic Mineralization Synthesis(BMS)

Mineral synthesis by biological means has generatedmuch attention in recent years as it is an environmentfriendly approach. In contrast to conventional mineralprocessing techniques, BMS generates materials withhighly controlled size, homogeneity, texture,composition and structure and, most importantly, inan environmentally benign manner. Bio-mineralization processes occur naturally in a diversityof species and tissues, including bone mineral,marine shells, tooth enamel, and marine algae shells.Several excellent reviews of biomineralization havebeen published [23-27]. One of the most significantfindings of these studies is that in virtually all casesof bio-mineralization, macromolecular structurescomposed of lipids, proteins, and/or polysaccharidesare intimately associated with mineral phases andserve vital roles in their crystallization.

3.6 Hydrothermal Synthesis

Hydrothermal synthesis is a method to producemetal oxide crystals from metal salt aqueous solutionsby heating the aqueous solution [28-29].


Hydrothermal synthesis of AlO(OH) was conductedusing a flow type apparatus over the range oftemperature from 523 to 673 °K at 30 MPa. Nano-sized crystals were formed at supercritical condition[30].

4.0 CONCLUSION

The present survey shows that aluminium trihydrate(ATH) has always been an important material fornumerous applications across many interdisciplinaryareas. Enormous research work has been carried outthroughout the world to develop the same ATHmaterial with new sets of physical and chemicalproperties by adopting various innovative synthesistechniques. As the demand for the finer or nanometersized ATH powders increases, more innovations arelikely to take place in the future to address theaspects of large-scale synthesis. Sonochemical andhydrothermal processing appear to have the potentialfor preparing finer or nano-sized ATH powders ona commercial scale.

Because of their remarkable properties, various formsof ATH, specifically those with finer or nano-sizedgrains, are expected to find new applications inpower generation and distribution related products,particularly in the area of nano-polymer composites.

References

1. Van Horn, K.R., Bridenbaugh, P.R. and Staley,J.T., Aluminium Processing Encyclopedia,Britannica, 2002.

2. Wefers, K., Misra, C., Oxides and Hydroxidesof Aluminum; ALCOA Laboratories,Pennsylvania, USA, 1987, 20.

3. Roskill, The Economics of Bauxite and Alumina,5th ed., 2002.

4. Business Communications Co., Inc, 25 VanZant Street, Norwalk, CT 06855, Study RP-231, Advanced Inorganic Fillers for Plastics,Nov. 1998.

5. Csige, J., Matyari, J., Banerjee, M.T., Kaptay,G., Light Metals. The Minerals, Metals andMaterials Society, TMS, Warrendale, PA. 1990.

6. Sleppy, W.C., Pearson, A., Mishra, C., Maczura,G., Light Metals. The Minerals, Metals andMaterials Society, TMS, Warrendale, PA, 1991,117.

7. Baksa, G., Szalay, G., Siklori, P., 2nd Int.Alumina Quality Workshop, Perth, WesternAustralia, 1990, 39.

8. Baksa, G., Foeler, J., Horvarth, J., Somosi, I.,Szabo, B., Szalay, G., Hung. Teljes HU 49,28th Nov. 1989, 831.

9. Greenway, P., Brandt, W., Light Metals. TheMinerals, Metals and Materials Society, TMS,Warrendale, PA, 1992, 237.

10. Shibue, Y., Sakamoto, A., Kawai, Y., Morihira,M., Light Metals. The Minerals, Metals andMaterials Society, 1989.

11. T. Tran, M.J. Kim and P.L.M. Wong, MineralsEngineering, 1996, 9 [5] 557-572.

12. M.J. Kim, P.L.M. Wong, T. Tran, Journal ofCrystal Growth, 1997, 178 360-366.

13. J.K. Pradhan, P.K. Gochhayat, I.N.Bhattacharya, S.C. Das, Hydrometallurgy 2001,60, 143-153.

14. Jian-Feng Chen, Lei Shao, Fen Guo, Xing-Ming Wang, Chemical Engineering Science,2003, 58, 569 - 575.

15. Zhang Peng-yuan, Zheng Li-li, Gong Yang-ming, CHEN Jian-feng, Research Report fromthe Research Center of the Ministry of Educationfor High Gravity Engineering and Technology,Beijing University of Chemical Technology,Beijing 100029, China.

16. Zhang Peng-yuan, Gong Yan-ming, CHENJian-feng, Research Report from the ResearchCenter of the Ministry of Education for HighGravity Engineering and Technology, Beijing


University of Chemical Technology, Beijing100029, China.

17. Yin Z.L., Wu Z.P., Chen Q.Y., et al., Journalof Chinese J. Nonferr. Met., 2002, 12(3), 596-601 (in Chinese).

18. Zhao J.H., Chen Q.Y., Journal. Acta Metall.Sinica, 2002, 38(2), 171-173 (in Chinese).

19. Enomoto N., Katsumoto N., Nakagawa Z.,Journal of Ceram. Soc. Japan, 1994, 102(12),1106-1110.

20. Imamura T., Deformation of Ultrasonic Pulsewith Diffraction [J], Ultrasonics, 1999, 37,71-78.

21. Mason T.J., Practical Sonochemistry [M], NewYork, Ellis Horwood Press, 1993,17-51.

22. Liu Ji-bo, Chen Jin-qing, Yin Zhou-lan, ZhangPing-min, Chen Qi-yuan, The Chinese Journalof Process Engineering, 2004.

23. Mann S., Nature 1988, 332(6160), 119-24.

24. Mann S., Nature 1993, 365(6446), 499-505.

25. Mann S., J. Mater. Chem., 1995, 5(7), 935-46.

26. Vogel J.J., Boyan-Salyers B. D., Clin. Orthop.Relat. 1976, R118, 230-41.

27. Lowenstam, H.A., Weiner, S., OnBiomineralization, Oxford University Press NewYork 1989.

28. Dawson W.J., Am. Ceram. Soc. Bull. 1988,67, 1673.

29. Matijevic E. and W.P. Hsu, J. Col. InterfaceSci. 1987, 118(2), 506.

30. Tadafumi Adschiri, Yukiya Hakuta, KiwamuSue and Kunio Arai, Journal of NanoparticleResearch, 2001, 3, 227-235.


Dr. Sukumar Roy is currently working as Managerat the BHEL Ceramic Technological Institute,Bangalore. His R&D activities include synthesis,processing and application development of nano-ceramic materials. He is in the process ofdeveloping large-scale synthesis of various nano-materials for in-house applications through design

and development of low-cost indigenous precursorsand processes. He has earlier executed projects forthe development of micro-filtration ceramicmembranes, processes and precursors for variousnano-material synthesis, polymer-based compositeinsulators using nano-additives, nano-modifiedPVC-based FRLS materials, etc.

BHEL has sponsored him to work in the area ofnano-technology at the Max Planck Institute forMetals Research, Stuttgart, Germany (under DAADfellowship and Max Planck Institute fellowshipprogramme) and also at the Alfred University, USA(under UNIDO fellowship). He has authored/co-authored several papers and patents.

Ms. Shilpa Hiremath is a post graduate studentat the National Institute of Technology, Surathklal,Karnataka, having obtained her bachelor's degreein Chemical Engg. from KLE Engineering College,Belgaum. As part of her postgraduate curriculum,

she carried out project work at the BHELCeramic Technological Institute, Bangalore, for aduration of 10 months starting June 2005. Thetheme of her work is 'Synthesis, characterizationand application studies of fine and nano-grainedaluminium trihydrate (ATH)'. She has also workedas a lecturer in KLE Engg. College, Belgaum, fora year. During her undergraduate course, she hasworked on projects such as colour removal of dyeeffluents by using rice husk carbon as an adsorbent,heat transfer characteristics of two immiscibleliquids in falling film heat exchangers, etc., at theIICT, Hyderabad.


APPLICATIONS OF HIGH POWER LASERS INMATERIAL PROCESSING

K. Venugopal and Manish Agrawal

SYNOPSIS

The demand for laser systems for surface treatmentapplications is growing rapidly due to the uniqueadvantages offered by lasers. High productivity, reducedmanufacturing cost, repeatability and high quality arethe major advantages of using lasers in the manufacturingsector. The most widespread use of lasers at present isfor cutting, cladding, heat treatment and welding.Laser surface treatment techniques like hardening andcladding can be used to improve the life of severalcomponents used in thermal, hydro and gas turbineindustries, besides reducing the cost of manufacturing.The paper describes some important applications oflasers in material processing.

Key Words:

Lasers; Material Processing; Laser Cladding; LaserTransformation Hardening; TIG Welding.

1.0 INTRODUCTION

There is a growing demand for use of high powerlasers in industrial applications, especially for theimprovement of wear, corrosion and fatigue propertiesof components made out of different materials. Laserbased technologies are increasingly replacingconventional manufacturing technologies due toimprovements in efficiency, quality and productivity.Compared to ordinary light, the laser beam iscoherent, i.e., it has low divergence. It is amenableto focusing into a very narrow light beam, resultingin very high energy density, which can be utilizedfor several surface modification applications.

Some important applications of laser radiation arecutting, welding, drilling, cladding, surface hardening,

glazing and alloying [1-7]. In addition, lasers are alsoused for marking or engraving on components onproduction lines. Laser cladding and surface hardeningof materials to improve corrosion and wear resistanceare used on a production scale as laser technologyoffers advantages over conventional techniques likeTungsten Inert Gas (TIG) welding, inductionhardening and plasma spraying.

The major advantages of laser surface treatment are:

● High productivity

● Elimination or reduction of post finishingoperations

● Possibility of automation

● Reduced process cost

● Improved quality of finished products

● Elimination of heat affected zone relatedproblems

● Non-contact processing

● Greater flexibility

Realizing the potential of the technology in improvingthe performance of power plant components, severalorganizations have intensified their research effortsin laser surface treatment. Critical components suchas blades, buckets, valves, etc., used in power plantequipment for thermal, gas and hydropowergeneration undergo severe wear and corrosion damageduring use. Laser surface treatment techniques likehardening and cladding can be adopted to improvethe life of such components. Further, componentsthat have already suffered damage can be restored totheir original shape and dimensions with improvedlife.


2.0 MAJOR APPLICATION AREAS

Some typical application areas where laser equipmentcan be effectively used are:

● Laser surface transformation hardening (heattreating)

● Laser cladding

● Laser alloying, laser melting and laser glazing

These are described in the following paragraphs.

2.1 Laser Transformation Hardening

The laser beam serves as a tool for localized and non-distortion surface hardening of hardenable steels.Laser hardening improves the strength, hardness,fatigue life and wear resistance at the localized areas,as compared to untreated areas. The surface scannedby the laser beam gets heated rapidly to theaustenising temperature and, as the beam moves overthe surface, the localized heated area cools veryrapidly to form martensite, thereby resulting in highhardness. Laser hardening relies on the core metalbeing self-quenching. The depth of hardening varies

from 0.5 mm to 2.5 mm. Steels with carbon contentgreater than 0.15% and with alloying additions, canbe effectively laser hardened. The principle of laserhardening is shown in Fig. 1.

Comparison with flame hardening and inductionhardening techniques (Table 1) clearly shows thatlaser hardening is the most advantageous process.With laser hardening, the major advantages are highprecision, very low distortion of the component andno need of quenchant.

Laser hardening can also be carried out on carburised,nitro-carburised and boronised surfaces and has beenshown to produce surface properties superior tothose of the individual treatments.

2.1.1 Application of Laser TransformationHardening

2.1.1.1 INLET EDGE HARDENING OF STEAMTURBINE BLADES

The leading edges of long last stage blades of steamturbines are subject to water droplet induced erosion,which limits the useful life of the blades. The pits

FIG. 1 : LASER TRANSFORMATION HARDENING OF AN ALLOY STEEL


and craters formed due to erosion can act as stressconcentration points, initiating cracks and subsequentblade failures. Traditionally, the problem of waterdroplet erosion has been mitigated by adoptingprocesses such as flame hardening, inductionhardening and stelliting during blade manufacture.As mentioned earlier, these processes have their owndisadvantages like distortion and short life. Somecompanies have adopted automated laser surfacehardening, which is reported to have lead to excellentperformance of the blades in service, with remarkablereduction in erosion. The major advantages of lasersurface hardening of turbine blades, as reported inliterature, are:

● Distortion of the finished component isnegligible.

● The presence of surface compressive residualstresses after hardening significantly reducesthe susceptibility of the material to stresscorrosion cracking.

● The fatigue limit of the laser hardened zoneis enhanced compared to that of the basematerial. Further, the fatigue resistance of alaser hardened turbine blade, loaded in itsfirst harmonic, is remarkably higher thanthat of a conventionally protected one.

● The higher hardness resulting from fastercooling results in better erosion resistance ofthe material, thereby enhancing the life.

Published wear resistance data [8] on laser hardenedblade samples, along with other samples hardened byconventional techniques, are shown in Fig. 2. Fromthe results, it is evident that there is negligible massloss with laser treatment as compared to othermethods. This has lead to the establishment of laserhardening technology for steam turbine blades.

Several laser hardened blades have been in use in anumber of steam turbines supplied by variousEuropean turbine manufacturers and the performanceas reported has been satisfactory.

2.1.1.2 BHEL'S EXPERIENCE

BHEL has successfully established, after extensiveexperimentation, the technology for hardening theleading edges of 12% Cr steam turbine blades, usingcarbon-di-oxide laser. Establishment of the processinvolved the following activities:

● Study of laser absorption techniques forhardening

TABLE 1 : COMPARISON OF LASER HARDENING WITH OTHER COMPETING METHODS OF HARDENING

Parameter Laser Inductive Carburising Flame ElectronHardening Beam

Max treatment depth, mm 1.5 5 3 10 1

Distortion Very low Medium Medium High Very low

Flexibility High Low Medium High Medium

Precision High Medium Medium Low High

Operator skill Medium Medium Medium High Medium

Environmental impact Low Low High Medium Low

Quenchant required No Sometimes No Yes No

Material flexibility High Medium Low Medium High


● Establishment of laser process parameters

● Design and fabrication of suitable highprecision fixtures to hold the blade

● Detailed programming of laser workstationto track the zone to be hardened

● Laser hardening of actual blade profiles

● Establishment of quality checks

The laser hardening facility and the results obtainedare shown in Fig. 3. More than 700 LP stage bladesfor steam turbines of different ratings have beenhardened by BHEL and put in operation.

2.2 Laser Cladding

Cladding refers to the process of developing a hardlayer of an alloy of a different composition on thesubstrate. The hard layer protects the substrate

underneath from wear and corrosion. It can also beused to restore worn out parts of components bybuilding up the eroded areas with new materials.Laser cladding has been successfully applied to anumber of components on a production basis, asshown in Table 2. The major advantages of lasercladding over conventional techniques like TIGwelding and plasma spraying are:

● The metallurgical bonding between thecladding and substrate is very strong,preventing removal of the cladding underloads.

● Fast cooling (104 °C/sec) yields refinedmicrostructure, resulting in improved hardnessand better wear resistance.

● Very low thermal distortion of thecomponent, which reduces scrap rate andrework.

FIG. 2 : COMPARISON OF WEAR RESISTANCE OF SEVERAL STEAM TURBINE BLADE PROTECTION METHODS USINGWATER DROPLET EROSION TEST


(c)

FIG. 3 : (a) LASER HARDENING OF LP STAGE BLADE (b) BLADE CROSS-SECTION SHOWING THE DEPTH OF LASER HARDENED ZONE(c) HARDNESS ACROSS THE LASER HARDENED ZONE

(a)

(b)


● High processing speed and automation leadto improved productivity and reduced labourcosts.

● Controlled surface finishes and near netshape geometry of the product reduce postcladding machining operations and materialconsumption.

● Very low dilution and elimination of heataffected zone cracking.

Thus laser cladding offers significant economicbenefits as compared to conventional techniques,leading to reduced manufacturing costs and superiorproducts.

2.2.1 Applications of Laser Cladding

2.2.1.1 GAS TURBINE BLADE SHROUD INTERLOCKS

High-pressure gas turbine blades are of shroudedconstruction, manufactured from a cast nickel basedsuper alloy. The mating surfaces of the 'Z' shaped

TABLE 2 : REPRESENTATIVE LASER CLADDING EFFORTS ON COMMERCIAL SCALE

Company Component Material & DepositionMethod

Rolls Royce Turbine Blade Triboloy/Nimonic alloysShroud Interlock Powder Feed

Pratt & Whitney Turbine Blade PWA 694/Nimonic alloysShroud Interlock Pre-Placed Chip

Westinghouse Turbine Blades Stellites, ColmonoysPre Placed Beds, Gravity feed

Rockwell Aerospace T-800, StellitesPowder feed

Fiat Valve Stem CrC2, Cr, Ni Mo/Cast Fe

Valve Seat Pre-placed Powder

General Motors Automotive Cast Iron Systems

shroud interlocks (Fig. 4) undergo severe fretting wearduring operation due to blade vibration. Excessivewear of the interlock would lead to blade rejection onoverhaul or, in extreme cases, to blade fracture. Inorder to reduce fretting wear, the interlocks are cladwith a cobalt based hard facing alloy.

The conventional method of cladding, TIG welding,produces heat affected zones stretching into the

FIG. 4 : GAS TURBINE BUCKET SHROUD INTERLOCK


airfoil section. Lack of precise process control canresult in excess deposit of costly cobalt based alloy,requiring expensive grinding operations, and highreject rates. Use of laser cladding eliminates most ofthese problems. The cladding time is reduced from14 minutes to 75 seconds for each blade. Also,elimination of intermediate grinding operation,reduced post-machining operation, and reducedconsumption of hard facing material, result in 85%cost reduction, despite the higher capital costs of thelaser equipment.

Typical results of the experimental work carried outby BHEL using TIG welding and laser cladding ona GT Superalloy are shown in Fig. 5. It can be seenfrom the graph that the sample with laser claddinggave a much higher hardness as compared to the onewith TIG welding. Further, due to very low dilutionof base metal, the laser clad sample gave a higherhardness of 750 HV at a clad height of 1.5 mm, as

against a hardness of 600 HV at a height of 4.5 mmfor TIG welding.

Laser cladding technology for turbine blade interlockswas introduced by Rolls-Royce and Pratt & Whitneyof United Technologies in the year 1982. More than100,000 jet engine blades are laser hard-facedannually now.

2.2.1.2 LP STAGE STEAM TURBINE BLADES

The leading edges of low pressure steam turbineblades undergo water droplet erosion damage, asdescribed earlier. In order to protect the leadingedges, an erosion resistant cobalt base strip of stellite6 is silver brazed along the blade edge, which is alabour intensive process. Also, brazing can introducedefects, which can subsequently act as stressconcentration points for fatigue crack initiation.

FIG. 5 : MICROHARDNESS COMPARISON OF TIG AND LASER CLAD SAMPLES


Laser cladding with stellite powder offers a viablealternative to silver brazing. Westinghouse ElectricCorporation has discovered that laser clad bladesexhibit good bonding and high cycle fatigueendurance that is superior to blades with brazed-onprotection. Further, the clad layer has high integrityto withstand extreme wear conditions.

2.2.1.3 GATE VALVES

The sealing surfaces of gate valves for oil fields,chemical plants and nuclear or geothermal energyinstallations are being laser clad (Fig. 6) to resistcorrosion and wear, thereby offering improved life.

3.0 OTHER AREAS OF DEVELOPMENT

A number of research studies, as described below,are being carried out to enhance the life ofcomponents.

3.1 Laser Alloying

Laser alloying is a process in which the metal surfaceis intentionally melted by a laser beam and alloyingmaterials are added to the melt pool to change thesurface chemistry to a controlled depth. Differentalloying materials can be added to tailor the surfacechemistry to achieve higher wear and corrosionresistance.

3.2 Laser Melting

Laser melting is a rapid heating and cooling processwhich produces extremely fine microstructures, givingrise to improved wear resistant properties. In the caseof some metals, such as cast iron, rapid cooling fromthe melt solidifies leduburite structure that exhibitshardness characteristics greater than those of heat-treated cast iron. Laser melting has been used withsuccess on automobile parts such as camshaft lobes,where a superior hard, shallow surface providesimproved hardness characteristics.

3.3 Laser Glazing

Laser glazing is used to seal the pores in a thermalspray coating. Studies have revealed that there isconsiderable densification of coatings after laserglazing treatment.

3.4 Laser Powder Fusion Technology forGas Turbine Repair

The advancement in laser material processing systemshas led to a unique method of depositing complexmetal alloys onto gas turbine component surfacesrequiring material build-up using co-axial cladding.In co-axial cladding, a mixture of the powder to bedeposited and inert argon gas is fed through a water-cooled conical co-axial nozzle surrounding the laser

FIG. 6 : LASER CLADDING OF GATE VALVE


beam. The powder-gas mixture passes throughspecially shaped channels and a ring shaped slit,which focuses it onto the laser spot on the workpiece. The laser melts the powder and the base metaland the movement of the laser beam results in a cladlayer of the deposited powder.

Critical components of compressors and gas turbineslike blades, vanes, shrouds, discs and stator assemblies,which are used in aggressive combustion gases athigh temperatures, undergo localized damage due toerosion, corrosion and foreign body impact.Replacement of these components, especially blades,involves high costs since blades are made ofdirectionally solidified castings or single crystals,which are very expensive. As keeping large stocks ofblades to replace defective blades during overhaul isuneconomical, repairing worn blades is the onlyalternative. Conventional repair techniques, such asTIG, plasma and electron beam welding can causedistortion of the component and microstructuraldegradation of the base alloy due to excess heatinput.

Laser powder fusion technology has been acceptedby leading engine manufacturers such as General

Electric, Pratt & Whitney, Rolls-Royce and AlliedSignal for repair procedures, since the process resultsin very low heat input, metallurgical bonding ofdeposited alloy and nil distortion of the component.The reconditioned blades can be considered asalmost original, thereby saving cost and increasinglife by almost 100%.

4.0 LASER EQUIPMENT

Regarding selection of the type of laser, the choicesboil down to three basic equipment: carbon di-oxide(CO

2) lasers, neodymium: yttrium-aluminium-garnet

(Nd: YAG) lasers, and high-power direct diode(HPDD) lasers. The preferred choice earlier wereCO

2 lasers but, due to the commercial availability

of both high-power Nd: YAG and HPDD lasers,there is a wider choice available now. Thecharacteristics of various laser systems are given inTable 3.

From the table it can be seen that the diode laseris very compact and has better absorption in steelsamples. There is no need for applying absorption

TABLE 3 : COMPARISON BETWEEN VARIOUS LASER SYSTEMS

Parameter Direct CO2 Flowing Nd:YAG Nd:YAG

Diode ISL Flash pumped Diode pumped

Net system efficiency 30% 6% 1% 6%

Hourly operating cost ($) $1.50 $10.00 $30.00 $6.00at 100% power

Wave length, micron 0.8 10.6 1.06 1.06

Absorption, %-steel 40% 12% 35% 35%

Footprint for laser supply 8 sq. ft. 50 sq. ft. 100 sq. ft. 60 sq. ft.and chiller, sq. ft.

Replacement, hours Laser Optics-2,000 hrs, Lamps-1000 Pumping Arrays-Arrays Blower/Turbine hrs 10,000 hrs.10,000 hrs 20-30,000 hrs


coatings on the samples, as required in the caseof CO

2 laser. They are gaining importance for

hardening and cladding applications and can beeasily handled by a robot, resulting in greaterflexibility in working.

5.0 CONCLUSION

Considering the need for improved availability andreliability of power plants, laser surface treatmentssuch as hardening and cladding are gainingimportance to improve the life of power plantcomponents. There is a tremendous growthopportunity which should be exploited fully.

Acknowledgement

The authors thank the management of BHEL for itssupport and permission to present this paper.

References

1. David E. Fly, J.T. Black, Ben Singleton, Jl. ofLaser Applications, 1996, 8, p. 89-93

2. N.B. Dahotre, Lasers in Surface Engineering,ASM International, Oct. 1998

3. Bachmann, F., Applied Surface Science, 2003,p. 208-209, 125-136

4. P. Hoffmann, R. Dierken, Proc. of Second Intl.Conference on Lasers in Manufacturing, 2003,Munich, p. 1-5

5. L. Giordano, E. Ramous, Jl. of HighTemperature Technology, Nov. 1984, p. 213

6. B.L. Mordike, ECLAT 96, p. 253.

7. G.L. Goswami, Santosh Kumar, R. Galun &B.L. Mordike, Lasers in Engg., Vol.13, p. 1.

8. W. Storch, G. Blum, B. Borsig, B. Brenner,Turbinenschaufel, 1988, p. 16-17


Mr. K. Venugopal graduated in MetallurgicalEngineering from Guindy Engineering College,Chennai, in 1975 and subsequently obtained hisM.S. degree from IIT Madras. Mr. Venugopal iscurrently working as Addl. General Manager in

the Surface Coatings & Treatment Laboratory ofBHEL Corporate R&D. He was responsible forestablishing the high velocity oxygen fuel andtwin wire arc thermal spray techniques forproviding wear resistant coatings for CFBC, AFBCand thermal power plant boiler tubes and forother applications. He was also associated withestablishing the laser hardening technology forinlet edge hardening of LP stage steam turbineblades using CO

2 laser. He is currently working

in the area of nano-coatings and is involved inestablishing the High Power Diode Laser facilityfor surface hardening and cladding applications.

Mr. Manish Agrawal graduated in MechanicalEngineering from Dr. B.R. Ambedkar University,

Agra, in 2001. Mr. Agrawal joined BHELCorporate R&D, Hyderabad, as Engineer Traineein 2002. At present, he is working as Engineer inthe Surface Engineering Lab. He has been workingin the area of laser material processing anddevelopment of wear resistant coatings using twinwire arc spray and HVOF techniques for hydroand thermal power plant components. He is alsoworking on the development of nano-crystallinecoatings.


CONTROL & DIAGNOSTIC FEATURES OF500 MW TURBO GENERATORS

M.R. Bhardwaj

SYNOPSIS

With the increase in the power rating of generating sets,the issue of availability has become very important. Theoutage of even one large capacity machine from the gridcauses a significant shortfall in power generation. Highavailability and reliability are therefore fundamentalrequirements of power generating units. To achieve this,the health of machines needs to be monitoredcontinuously. Over and above this, we also requiretechniques for forecasting imminent problems. Diagnostictools are therefore gaining increasing importance by theday. This paper gives details of the control anddiagnostic tools used in 500 MW generatorsmanufactured by BHEL.

Key Words:

Diagnostic Tools; Distributed Control System;Temperature Control; Conductivity Monitoring;Thyristorized Switching; Piezo-electric Accelerometers;Grounding Brush Monitor; Partial Discharge;Quadrature Axis Voltage.

1.0 INTRODUCTION

To ensure uninterrupted operation, generators andtheir auxiliary systems are supplied with field sensingdevices, signal conditioners, control elements andsecondary instrumentation for control, monitoringand protection. All the field signals are taken to thecentral Distributed Control System (DCS) formonitoring, control, protection and recording. Faultsand abnormalities are annunciated in the DCS inthe control room. In the event of critical parametersexceeding set limits, the control system provides fordisconnection of the generator through protections.

2.0 CONTROL SYSTEMS

The main systems for control, monitoring andprotection of a 500 MW set are described in thefollowing paragraphs.

2.1 Hydrogen System

Hydrogen (H2) gas is the main cooling medium for

the internals of the stator and rotor. As the mixtureof hydrogen and air becomes explosive when thepurity of hydrogen is in the range of 4 to 76% byvolume, a special procedure is followed to fill orpurge the gas from the generator. Air is first replacedby carbon dioxide (CO

2) gas. Thereafter, carbon

dioxide is replaced by hydrogen. The reverse procedureis followed during purging. The followingmeasurement and monitoring functions are performedfor reliable operation of the gas system.

2.1.1 HYDROGEN PURITY MONITORING

Hydrogen purity measurement is done in threeranges, i.e., 90 to 100% hydrogen in air; 0 to 100%hydrogen in carbon dioxide; and 0 to 100% carbondioxide in air. Both thermal conductivity and gasdensity measurement based systems are used formeasuring the purity. The thermal conductivitybased system is low pressure type, with gas passingthrough the analyzer being purged to the atmosphere.The gas density based system is high pressure type,with gas being returned to the generator, avoidingcontinuous bleeding. Alarms are generated foroperator intervention if purity falls below 95%. Themachine is not recommended for operation ifhydrogen purity falls below 90%.


2.1.2 HYDROGEN GAS PRESSURE MONITORING

The hydrogen gas pressure is maintained at 3.5 kg/cm2 (gauge). Alarms are generated if pressure deviatesby ±0.2 kg/cm2.

2.1.3 BEARING OIL VAPOUR EXHAUSTERS

Two bearing oil vapour exhaust fans are provided toexhaust oil vapours and hydrogen gas which getaccumulated from the seal oil supply and bearinghousings of the generator. The fans are in oneworking and one standby configuration, with anautomatic switching over interlock.

2.1.4 GAS MOISTURE CONTENT MONITORING

Any contamination of the gas with moisture or oildrastically reduces the thermal conductivity of thegas, resulting in poor heat transfer. To help maintainthe gas inside the machine dry, a fraction of the gasis continuously passed through the gas drier (blowerheater or refrigeration type). To monitor the moisturecontent in the gas, a dew point temperaturemonitoring system, shown in Fig. 1, is supplied. Inthis scheme, only one thin film aluminium oxide orceramic sensor is used to measure the dew pointtemperature at both inlet and outlet of the drier. Thefunctionality is realized with the help of four

FIG. 1 : SCHEMATIC DIAGRAM OF DEW POINT TEMPERATURE MEASUREMENT


solenoid valves provided. At any given time, onlyone circuit is in use. The dew point temperatureshould be below 10 °C.

2.1.5 COLD GAS TEMPERATURE CONTROL

Due to load variations during operation and theresulting thermal expansion and contraction, thegenerator is subjected to stresses. To reduce thesestresses, the hydrogen cooling circuit is providedwith an automatic temperature control system,shown in Fig. 2, to maintain the active generatorcomponents at the proper temperature level. Amotorized temperature control valve, withthyristorized switching, maintains the cold gastemperature at set value. To preclude the possibilityof any condensation on stator bars, the cold gas

temperature set point is kept 5 °C below the primarywater temperature set point.

Tripping based on '2 out of 3' logic is provided inthe event of emergency high value of the cold gastemperature. The main exciter is also protected foremergency high air temperature, which is caused byinsufficient cooling of the air circulating in theexciter.

2.2 Primary Water Cooling System

In large rating machines, hydrogen cooling is notsufficient to remove the entire heat generated. Foradditional cooling, a Primary Water (PW) coolingsystem, with demineralised water flowing throughthe hollow stator conductors and terminal bushings,

FIG. 2 : COLD GAS TEMPERATURE CONTROL LOGIC FUNCTION BLOCK


is provided. The following main measurement andmonitoring functions are performed for reliableoperation of the PW system.

2.2.1 PW FLOW MONITORING

The flow through the main winding, as well asthrough the three bushings, is monitored and alarmsare generated at set values. If the flow falls below theemergency value, a tripping signal is generated witha time delay of 60 seconds to shut down the unit.

2.2.2 PW CONDUCTIVITY MONITORING

This measurement is very important for healthyrunning of the machine. The primary water shouldalways be slightly alkaline, at pH of 8 to 9, tominimize the possibility of corrosion and formationof oxides inside the hollow conductors. Also, apolishing unit is used to remove harmful ions(copper, iron, chlorine, carbon dioxide) from thewater system. However, as sodium ions also getremoved in the process, this elimination is requiredto be compensated. An alkaliser dosing system isused to add caustic soda (NaOH) in the water circuitintermittently to refurbish with sodium ions and tokeep the solution slightly alkaline. PW conductivityis maintained between 1.0 and 2.5 micro Siemens/cm. Alarms are generated if the dosing controllerfails to maintain the conductivity within the specifiedrange.

2.2.3 PW TANK LEVEL MONITORING

The hot water returning from stator conductors istaken to the PW tank mounted on top of thegenerator. The tank level is monitored for anyleakages. The gas pressure in the tank is alsomonitored to detect leakage of hydrogen through theconductors into the water circuit.

2.2.4 LIQUID LEVEL IN GENERATOR TERMINALBOX

The liquid level in the terminal box is monitored.The presence of liquid in the terminal box generatesan alarm for operator intervention. If the level

crosses a limit, the generator is tripped based on '2out of 3' protection, with the help of level switchesmounted at the top position in the terminal box.

2.2.5 PW TEMPERATURE CONTROL

To maintain the windings and terminal bushings atthe proper temperature, a motorized control valvewith thyristorized switching is provided in the PWcircuit, as shown in Fig. 3. As mentioned in 2.1.5,the PW temperature set point is kept 5 °C above thecold gas temperature set point.

2.3 Seal Oil System

The seal oil system is required to retain hydrogeninside the generator without affecting its purity. Thefollowing main measurement and monitoringfunctions are performed for reliable operation of theseal oil system.

2.3.1 SEAL OIL PRESSURE MONITORING

The oil pressure at seals has to be maintained higherthan the gas pressure. The system also takes care ofemergencies like total AC failure or failure of thecontrol system. The emergency DC seal oil pump ismade available automatically to operate in sucheventualities.

2.3.2 SEAL OIL/HYDROGEN DIFFERENTIALPRESSURE MONITORING

The differential pressure (DP) is monitored at bothturbine and exciter end shaft seals. Alarms aregenerated if the DP goes higher or lower than theset value band (<3.3 or >3.7 kg/cm2). The differentialpressure signal is generated by comparing the seal oilpressure at seals and the hydrogen pressure in thecasing. A height correction factor (offset) is appliedas the seal oil transmitters are located near the sealoil unit while oil seals are at the operating floor.

2.3.3 SEAL OIL LEVEL MONITORING

The seal oil level is monitored at different locationsinside and outside the generator to ensure


uninterrupted operation of the machine and alarmsand protective actions are initiated in the event ofa malfunction.

2.3.4 SEAL OIL INLET TEMPERATURE

High seal oil inlet temperature endangers performanceof the shaft seals. The situation can arise due to thefailure of seal oil coolers, causing a reduction in oilviscosity. In such an event, the gas can penetrate theseal oil film at the shaft seal contact face and enterthe bearing housing. Therefore, protection trippingbased on '2 out of 3' logic is provided.

2.4 Bearing/Shaft Vibration Monitoring

The bearing vibrations are measured at generatorturbine end and exciter end (TE/EE) bearings and

the exciter bearing in the X-Y plane. The relativeshaft vibrations are also measured in the X-Y planeat generator (TE/EE) and exciter ends. The absoluteshaft vibrations are derived mathematically in thecontrol system. The alarms for high and emergencyhigh values are generated as per ISOrecommendations. Expert system software, whichgives detailed analysis and advance information ofthe faults developing in the bearings, can also besupplied as an option.

2.5 Generator Winding/Core TemperatureMonitoring

Twelve simplex RTDs are used to monitor statorwinding temperature. These are wired to paperlessrecorders/DCS for monitoring and alarming.Additional twelve RTDs (physically located near the

FIG. 3 : PW TEMPERATURE CONTROL LOGIC FUNCTION BLOCK


above RTDs) are provided as spare. Similarly, twelvesimplex Cu-CuNi thermocouples are provided in theend zones for monitoring the core temperature.These are also wired to paperless recorders/DCS formonitoring and alarming. Additional twelvethermocouples (physically located near the abovethermocouples) are provided as spare. No tripping isprovided for generator winding/core temperaturehigh values. The tripping is taken care of by cold gastemperature monitoring.

3.0 DIAGNOSTIC TOOLS

Following are the diagnostic systems provided ingenerators.

3.1 Generator End Winding VibrationMonitoring System

The continuous vibration monitoring system enablesmonitoring of mechanical integrity of the stator endwinding (overhang area) over a period of time. Itthus helps in routine monitoring, trending as wellas analysis of the performance of the generator. Theadvantages of the system are as follows:

● Helps in minimizing the frequency of forcedoutages by identifying potential problems.

● Increased vibration beyond set value causesan alarm, drawing operator intervention forintensive monitoring.

● The availability of the generator can beenhanced through predictive analysis and byway of warning for taking remedial measuresin case there is any significant increase invibration level.

● In case of stable vibration level, the timebetween overhauls could be extendedconsidering the constraints in withdrawingthe set from grid at time.

The monitoring system comprises the following:

a) 12 low magnetic sensitivity piezo-electricaccelerometers.

b) 12 miniature charge amplifiers directlymounted on BNC connectors.

c) Vibration monitoring panel containing 12vibration channels. Each channel monitors2f component, i.e., 100 Hz vibration levelfor 50 Hz generators. The 2f vibrations areproduced by the stator current as well as theelectro-magnetic flux produced in the statorcore. The system monitors 100 Hz vibrationsin the range 0-50 mm/sec.

d) Very low noise cable: This is used forconnecting the signal from the accelerometersfixed on the stator winding to the chargeamplifiers.

The vibration signal leads are brought out frominside the TG through the two lead-in-plates withflanges provided in the stator casing. Theaccelerometers are fixed on the nose joints of thestator end winding and are distributed asfollows:

a) Six on the turbine end; three in radialdirection and three in tangential direction atneutral bars.

b) Six on the exciter end; three in radialdirection and three in tangential direction atneutral bars.

The accelerometer sensors are mounted in generatoroverhang at works. These are mounted behind thewater boxes to avoid physical dislocation due tomovement of maintenance personnel duringinspection. The signal cables are brought out throughspecial gas tight flanges.

The vibration monitoring modules can displayvibration in terms of mm/sec in bar graph or digitaldisplay, simultaneously or cyclically, for all channels.A 4-20 mA signal, corresponding to 0-50 mm/sec,is available for each channel, which can be interfacedto the DCS for vibration trending at selectedsampling interval. A typical scheme is shown inFig. 4.


3.2 Individual Bar PW Flow Monitoring

Primary water flow monitoring of individual bars isprovided to alert the operating staff, in case there isa choking trend in any of the hollow conductors ofa stator bar of turbogenerator. It also helps inassessing the temperature rise of individual bars, inaddition to the normal winding temperature detectorsprovided. The PW flow monitoring system is suppliedas an integral part of the GAMP (Generator AuxiliaryMonitoring Panel).

The rise in PW temperature is an indirect replica ofthe flow in each bar. One simplex platinum RTD(PRT) is provided at each nipple of the outlet PWheader. All the PRTs are laid and covered individuallywith ceramic fibre cast cover to insulate themthermally from the environment. The leads of PRTsare taken out from generator through 'lead in plate'and connected to a terminal box outside thegenerator.

Individual PRTs can be monitored either continuouslyor intermittently, depending upon customer choice.

Each PRT is connected to the DCS for bar graphdisplay and alarming.

The choking trend can be inferred by comparing thetemperatures measured by PRTs, after appropriatemodifications, with the reference finger-print values.The reference finger-print values are decided basedon test results at works.

The measured values are modified as follows:

(a) Due to stator current 'I'

The measured temperature is directly proportionalto the square of stator current. If 'I' is the measuredcurrent, temperature measured is modified as:

(b) Due to cold distillate (PW) temperature,Tpw

FIG. 4 : CONTINUOUS VIBRATION MONITORING SYSTEM SCHEME


(c) Due to differential pressure across thewinding, ∂∂∂∂∂P

Combining the three factors together, the overallcorrected value can be expressed as:

The correction logic is realized in the DCS. Theinitial condition data is entered as signature dataduring commissioning. During operation, the systemcompares the generator individual bar flows (modifiedvalues as above) with the reference flows. The twovalues are displayed in terms of bar graphs for all the96 slots. If limits are crossed, alarms are annunciatedin the DCS.

3.3 Grounding Brush Monitor

The Grounding Brush Monitor (GBM) is an on-linecondition monitoring instrument for predictivemaintenance of grounding brushes, to protect thethrust and journal bearings of the generator andexciter from damage due to the flow of current. Thefunctions of the instrument include the following:

1) Shaft voltage monitoring

2) Shaft current monitoring

3) Generator rear bearing and exciter bearingpedestal insulation healthiness checking bymeasurement of pedestal voltages

Two current brushes, displaced 90 degrees spatiallyto ensure firm contact for proper earthing, are used.Annunciations are provided when actual values ofparameters exceed set values.

The scheme of shaft grounding is shown in Fig. 5.The shaft is grounded at the turbine end of thegenerator shaft, through the GBM instrument.

The working principle of this diagnostic equipmentis as follows: Static electricity is developed in the lowpressure stages of steam turbines when wet steamdroplets impinge on the turbine blades at highvelocity; the static charges accumulate and build upvoltage (which can go up to a high level of 50 V) tillit can discharge through the thinnest part of the oilfilm in a bearing. In the process, the bearing can getdamaged by electrical pitting. To avoid such damage,carbon brushes are provided for grounding. However,due to high temperature, presence of oil film formedby oil vapours in the surroundings and high slidingvelocity between the brush and the shaft, a highimpedance path forms between the grounding brushand the shaft, inhibiting flow of charge to ground.Due to accumulation of charge, the shaft voltagebuilds up till it discharges through the bearings. Tomeasure this voltage build up, an additional carbonbrush is put in a high impedance path. Two levels areset in the GBM for warning and alarm.

FIG. 5 : GROUNDING SCHEME FOR SHAFT


Magnetic asymmetry also gives rise to shaft leakagevoltage. This shaft voltage, in the nature of alternatingvoltage, tends to cause flow of current in a closedloop through generator outboard bearings, groundand any of the other bearings on the turbine sideof the generator. Hence, insulation is provided in thegenerator outboard bearing and the exciter bearingto prevent heavy flow of current through thebearing, which can damage the bearing babbitt liner.

3.4 Partial Discharge Monitoring System

Partial discharge (PD) refers to the small electricaldischarges that typically occur within or betweeninsulation materials, usually across voids. Partialdischarge is also referred to as corona or surfacetracking. The visible evidence of corona presentsitself as a white, powdery residue. It is important todetect increased frequency and amplitude of suchdischarges in generators before they develop intoserious faults. The PD monitoring system is providedto monitor the healthiness of the winding insulationand detect any deterioration or loosening.

Partial discharges tend to generate radio frequency(RF) signals. These RF signals are isolated from the

electromagnetic noise, due to surroundings or griddisturbances, with the help of special noisecancellation techniques. The system pre-empts fullblown discharges by detecting their onset beforetime. At the time of first commissioning, the initialsignature of the machine is recorded. Thereafter,whenever there is significant increase in PD activity,an alarm is generated in the control room. Athorough analysis can then be done with the data,collected at predetermined intervals and stored inthe memory of the system.

Three coupling capacitors are used for detection ofthe RF signals from partial discharge - one installedin each phase. Additional information is derivedwith the help of RTDs embedded in the windingslots. A typical scheme is shown in Fig. 7. Theexternal electromagnetic noise is eliminated with thehelp of special algorithms in the software to recognizeand discard such noise. In one of the variations,three coupling capacitors are installed each atgenerator terminals in the terminal box and three inPT cubicles. The additional capacitors help in noiseelimination through the time of arrival technique.The distance between the coupling capacitors installedin terminal box and in the bus duct near the PTcubicles causes a delay in the arrival of noise signals

FIG. 6 : GROUNDING BRUSH MONITOR (DISPLAY UNIT)


from the grid at terminal box with respect to thoseat the PT cubicles. This delay is utilized to eliminatethe noise. The coupling capacitor connectionarrangement in one phase in generator terminal boxis shown in Fig. 8.

BHEL has installed and commissioned periodic aswell as continuous online PD monitoring systems inmany 500 MW units. Continuous online systemsare preferred as these act as continuous watch dogsand give alarms whenever PD activity increases

FIG. 7 : TYPICAL PD SCHEME WITH COUPLING CAPACITORS & RTDs

FIG. 8 : COUPLING CAPACITOR CONNECTION ARRANGMENT


beyond the set value. Software is supplied for in-depth analysis. The data can also be sent to thesupplier for in-depth interpretation. This system canalso be offered as retrofit as it is non-invasive.

3.5 Generator Rotor Winding TemperatureMonitoring

In Static Excitation System (SEE), the field voltageand field current signals are directly available for V/I calculation to calculate rotor winding temperature.However, this facility is not available in generatorswith brushless excitation where AC from the mainexciter is rectified with the help of rotating diodesand fed into generator field winding directly. Thefield voltage and current signals are not available formeasurement. In such cases, an indirect approximatemethod is used to get an indication of the fieldwinding temperature. The quadrature axis voltage vs.current characteristic curve, taken during shop testingof the exciter, is stored in the GAMP logic, andforms the basis for calculation and display ofapproximate rotor winding temperature duringoperation.

3.6 Generator Condition Monitoring

The system is used to provide very early warning ofgenerator overheating or the development of hotspots, which can lead to serious damage if notdetected in a timely manner. Tagging compounds areapplied in different zones in the machine. A highconcentration of semi-micron particles is producedwhenever any materials (tagging compounds) within

the generator are heated sufficiently to producethermal decomposition. These pyrolytic particles arequickly detected in the ion chamber of themicroprocessor based Generator Condition Monitor(GCM). In the event of an alarm condition, a self-diagnostic feature starts the verification sequence tocheck genuineness of the alarm. Thereafter, a sampleof the contaminated gas is collected, and sent to thelaboratory for analysis of the nature and source ofthe problem. A base line sample may be maintainedand additional samples taken every six months todetermine any deviation.

4.0 CONCLUSION

With the increase in ratings of generating sets, therequirement of maximum availability and minimumoutage of turbogenerators is becoming absolutelyessential. Modern diagnostic tools can help a lot toforewarn about impending catastrophes. The newcontrol features also help in running theturbogenerators within operating limits, withoutrequiring much human intervention. This paper hasattempted to give an insight into the control anddiagnostics philosophy of 500 MW turbo generatorsmanufactured by BHEL.

Acknowledgement

The author places on record his gratitude to ShriDev Raj, AGM (EME & CIE), Shri S.C. Tyagi,AGM (CIE) and Shri A.K.L. Rao, AGM (GRI &ISE) — all from HEEP, Haridwar, for their supportand guidance in bringing out a comprehensive paper.


Mr. M.R. Bhardwaj graduated in Electronics &Communications Engg. from University ofRoorkee (now IIT Roorkee) in 1978 and obtaineda PG Diploma in Management from AIMA in1988.

Mr. Bhardwaj joined BHEL Power Sector TechnicalServices in 1978 as Engineer Trainee and workedin the area of installation and commissioning ofgenerator C&I, excitation and generator protectionsystems for thermal power projects. After a four

year stint, he moved to HEEP, Haridwar, wherehe joined Turbo Generator Engg. (TGE). In TGE,he worked for engineering of C&I and excitationsystems for thermal, gas and nuclear sets. He wasresponsible for the introduction of microprocessorbased C&I for THRI and THDF type generators.He also developed the microprocessor based on-line diode fuse monitoring system for brushlessexciters for 500 MW turbogenerators. He hasvisited Germany for in-depth training in C&I forair cooled generators and 500 MW sets.

Mr Bhardwaj joined the newly created C & IEngg. (CIE) department in July 2003. At present,he is working as Deputy General Manager in CIEand is looking after project and product engineeringfor C&I for thermal, gas and nuclear sets. He hasfiled a patent for the improved moisture measuringsystem.


IMPROVED THYRISTOR TURN-OFF CHARACTERISTICSTHROUGH SELECTIVE EMITTER DOPING

N. Harihara Krishnan, S. Govindaraj, Maheshwar Dehury,Vikram Kumar Yadav, P.K. Kumaravyasa and S.K. Ramesh

SYNOPSIS

This paper reports a process optimization study toimprove the turn-off characteristics of an 1800 Vthyristor. Thyristor chips, fabricated by conventionaldoping and alloyed molybdenum discs, displayedinconsistency in the turn-off time (t

q), when measured

values either exceeded the USL (prescribed level of 500μs) or the thyristors lost blocking capability permanently.Cause-effect analysis and trial runs using two golddoping temperatures allowed us to exclude electron-holerecombination in n-base and p-base regions as a processfailure mode. It was surmised that the presence of anultra-thin buried n+ layer above the compensated p+

region, and the ensuing reverse-biased junction, delayedcarrier depletion. This hypothesis was verified byselectively doping phosphorus (n+) on the cathode faceof the thyristor and also by entirely compensating theburied n+ layer by a prolonged alloying process. Turn-off time was found to conform to the normal values insubsequent production cycles that employed selective n+

layer doping.

Key Words:

Thyristor; Turn-off Time; Reverse RecoveryCharacteristics; Emitter Doping; PhosphorusDiffusion; Gold Diffusion; Electron-HoleRecombination Rate; Carrier Life Time; Aluminium-Silicon Eutectic; Alloying; Photolithography; ProcessOptimization.

1.0 INTRODUCTION

The manufacturing process of semiconductor devicesentails a series of steps, each of which has to be

optimized to deliver devices with the desiredcharacteristics and a reasonably good yield. However,specific aspects of device characteristics are oftenattributable to parameters related to multiple processsteps, which complicates the optimization of processparameters. In order to resolve process relatedproblems, a good understanding of devicecharacteristics as well as manufacturing processes isessential. Presented in this paper is a processoptimization study of a silicon thyristor device,which resulted in improved turn-off characteristics.

2.0 THYRISTOR DEVICES

A thyristor is a four-layer (p-n-p-n), three terminal(anode, cathode and gate) semiconductor switch,which is open (OFF) in forward as well as reverseblocking modes and closed (ON) in forwardconduction mode. It can be triggered ON in itsforward mode by applying a signal to the gateterminal. By adjusting the time and position (firingangle) of the gate signal, the conduction current canbe controlled and hence, the device is popularlyknown as silicon controlled rectifier.

It finds application in power electronic circuits forAC-DC conversion (rectifer), AC-AC conversion(power converter, cyclo-converter, matrix converter),DC-AC conversion (inverter) and DC-DC conversion(chopper).

2.1 Thyristor Manufacturing Process

A p-n junction is formed on NTD FZ-Silicon wafer(Orientation <111>, Resistivity 55 ohm-cm, thickness470 µm, diameter 33 mm) by conventional high


temperature (~1230 °C) diffusion, using high purityAluminium (Al) and Boron (B) sources. Phosphorus(n+) is diffused on both sides of the silicon wafer toform an n+-p-n-p-n+ structure. Gold impurity is usedto control the turn-off time within its nominal value(500 µs) by doping at a temperature of 805 °C.

A molybdenum disc is alloyed with the anode sideof the diffused wafer using an Aluminium-Silicon(Al-Si) eutectic foil as binder. This converts the n+

layer (on anode) into p+ to form a p+-p-n-p-n+

thyristor structure. A cross-section of the fabricatedthyristor chip is shown in Fig.1. The anode, cathodeand gate terminals are formed using physical vapourdeposition (PVD) coating of aluminium and silver(coating thickness: 2 µm on anode, 25 µm oncathode and gate). The circumferential edge of thechip is bevelled at an angle of 20° using siliconcarbide abrasive and subsequently polished using a1:1 mixture of hydrofluoric acid and nitric acid. Thebevel is then protected from exposure to theatmosphere by applying a coating of silicone rubber,

which ensures stable blocking characteristics by wayof making the bevel surface electrically passive.

2.2 Thyristor Turn-off Characteristics

The thyristor 'switch' is turned OFF (open) fromconduction to blocking mode by reverse-biasing, orby withdrawing anode current below the holdingcurrent. The response is not instantaneous, and afinite time elapses before the switch is sufficientlyOFF, during which it can be spuriously triggeredON, in the presence of a forward-bias voltagegradient (dV/dt). This response time is termed asturn-off time (t

q), as illustrated in Fig. 2.

The carrier concentration within the thyristor waferis sketched in Fig. 3 at different instants during theturn-off process [1]. At t=t

0, the thyristor is in

conduction mode and the current has just started towithdraw. The rate at which the current reduces (di/dt) is determined by the inductance in the loadcircuit, but the thyristor is tested by manufacturers

FIG. 1 : STRUCTURE OF THYRISTOR CHIP. J1, J2, J3 ARE p-n JUNCTIONS OF THYRISTOR


FIG. 2 : TURN-OFF TIME CHARACTERISTICS OF THYRISTOR

FIG. 3 : CARRIER DISTRIBUTION WITHIN BULK OF THYRISTOR WAFER DURING TURN-OFF PROCESS


at a value of di/dt that guarantees its safe operation.The current is zero at t=t

1, but withdrawal continues

at the same rate due to the inductive effect till t=t2,

when the carriers are cleared from the p-base region.Further carrier depletion from the n-base preparesthe junction J

1 to support reverse voltage. Now

onwards, the reverse current reduces exponentiallyand is influenced by the recombination of carriers(holes and electrons).

Two categories of failures can take place during theturn-off time measurement, which is one of theroutine tests conducted on thyristor chips.

a) Measured value of tq > USL (500 μs);

USL: Upper Specification LimitThis category is termed as Not-Turning-Offand is partially recoverable.

b) Thyristor loses its blocking capability.This category is termed as Short and is non-recoverable.

Typical statistics of failures in turn-off time testindicate that during the test period, 61% of thyristorspassed the turn-off test, while 32% did not turn-off

and 7% were shorts. With rework on thyristor chips,the turn-off time test yielded 70% acceptance.However, as the rework process involved chemicalcleaning, re-alloying and re-polishing, which werefound to endanger the blocking capability ofthyristors, it was not pursued further.

3.0 EXPERIMENTAL RESULTS

The effects of selected key process parameters, whichwere considered to have a significant influence onturn-off characteristics, were estimated and a cause-effect diagram was constructed (Fig. 4). Duringbrainstorming sessions, three parameters from thisdiagram (gold diffusion temperature, phosphorusdiffusion (n+) time and diffusion of n+ layer onanode side) were identified as key variables for thepurpose of experimentation. Two levels were assignedto each of these variables. However, out of the eightpossible combinations, four were judiciously selectedand a set of three experiments was conceptualized asshown in Table 1. These experiments were conductedto measure the impact of changes in these variableson turn-off time and yield.

FIG. 4 : CAUSE-EFFECT DIAGRAM


The turn-off time was measured as per internationalstandard IEC 747-6 using an LEM-make tester (Model4030). The thyristor is triggered to ON state usinga gate circuit. It is turned OFF by means of a reversevoltage and by withdrawing the current at a ratecontrolled by MOSFETs. The zero-crossing end ofON state current pulse (1 ms, 1 Hz) is detected anda forward ramp-up voltage is applied at a delayedinstant away from zero-crossing. The delay time isadjusted by a potentiometer and the value at whichthe thyristor toggles between its ON and OFF statesis sensed and displayed as turn-off time.

Table-2 summarizes the process variables adopted inthis study and the corresponding experimental results.

4.0 ANALYSIS OF EXPERIMENTALRESULTS

4.1 Gold Diffusion Temperature

Recombination rate is one of the causal factors thataffect turn-off time. The higher the recombination

rate, the shorter is the turn-off time. In the thyristormanufacturing line, this rate is controlled by golddiffusion. As it increases with density of goldimpurity in silicon, the gold diffusion temperaturepresents an easy diagnostic tool to estimate thyristorturn-off characteristics. It is observed fromExperiment-1 that, in the range 805 °C to 820 °C,the turn-off time is marginally lower and somestatistical improvement in manufacturing yield isapparent. However, other experiments, as analyzedbelow, reveal that the problem is not associated withthe recombination rate and hence, the gold diffusionprocess.

4.2 Phosphorus (n+ Emitter) Diffusion Time

The n+ emitter layer on cathode side influences gatetriggering characteristics, and is adjusted by diffusiondepth. The total amount of n+ impurity, which isdriven in from both anode and cathode faces of thewafer, is controlled by diffusion time. Experiment-2 shows that the manufacturing yield of thyristors

TABLE 1 : EXPERIMENTAL VARIABLES, LEVELS, COMBINATIONS AND LAYOUT


with 6-hour diffusion time is higher (72%) thanwith 7-hour period, when doping was effected fromboth sides of the wafer. This result is furtherdiscussed under 4.3 below.

4.3 Diffusion from One Side/Both Sides

It is evident from Experiment-3 that when n+

diffusion occurs from the cathode face alone, thereis a spectacular improvement in the turn-offcharacteristics, compared to the case in whichdiffusion takes place from both sides. It is interestingto observe that this improvement is seen in spite ofthe longer (7-hour) diffusion, which showed loweryield than the shorter (6-hour) diffusion inExperiment-2. These observations suggest that the n+

layer on the anode face is greatly responsible for theincrease in thyristor turn-off time and also that adeeper n+ layer on the anode side affects turn-offtime adversely.

4.31 Analysis of Buried n+ Layer

The eutectic temperature of Al-Si foil is 577 °C [2],[3]. During the alloying cycle, the wafer surface iseroded through dissolution of silicon into Al-Si melt,as well as by formation of molybdenum silicide(MoSi

2) layer. This process entails compensation of

the n+ region on the anode face to form a p+ dopedlayer. If the n+ region is not fully converted, an ultra-thin buried n+ layer remains, which is difficult tomeasure. As a result, two additional p-n junctions,J

4 and J

5, are created, as shown in Fig. 5.

During turn-off, junction J2 is forward biased, while

J1 and J

3 are reverse-biased. Carriers are depleted

from the p-emitter region during the time period, t0

< t < t3. Since turn-off time is measured with

reference to t1, two components T

1 = t

1 < t < t

3 and

T2 = t

3 < t < t

q are defined for the sake of analysis.

At t = t3, the reverse current passes through its peak,

known as reverse recovery current (IRR) as reverse

TABLE 2 : RESULTS OF EXPERIMENTS


voltage just begins to build up at junction J3. During

the period T2 = t

3 < t < t

q, the recombination process

lowers carrier concentration in n-base and p-baseregions. This results in exponential decay of currentwith time constant related to recombination rate.

In the case of buried n+-layer, additional junctions J4

and J5 exist and the reverse-biased junction J

5 hampers

the current flow. Carriers are withdrawn from p-emitter rather slowly, which prolongs time period T

1.

As a result, reverse current IRR increases to a very highlevel. The turn-off process is further delayed (T

2) as

current decays from a relatively higher value comparedto the normal case. The sum of T

1 and T

2 results in

high turn-off time (tq = T

1 + T

2).

It was possible to verify this hypothesis by capturingthe current waveform for several thyristors (reverserecovery current, IRR) at standard test condition (T

J

= 95 °C, ITM

= 1000 A, di/dt = -20 A/μs). Measuredtraces of reverse recovery current of normal and hight

q thyristor are given in Fig 6. The values of I

RR were

169 A and 886 A and the corresponding turn-offtimes were 350 µs and >1000 µs (USL=500 µs),respectively.

4.4 Extended Alloying Cycle

In order to confirm the hypothesis that anuncompensated n+-buried layer is indeed present, thealloying cycle (time) was deliberately extended toconsume the n+ layer entirely. This is brought outas Experiment-4 in Table-2. The duration of alloyingwas extended from 25 minutes to 40 minutes andthe turn-off characteristics of the resulting thyristorswere evaluated. All thyristors qualified the test,validating the earlier findings.

FIG. 5 : STRUCTURE OF THYRISTOR CHIP, WHEN n+ LAYER ON ANODE SIDE IS NOT FULLY COMPENSATED BY ALLOYING.J4 AND J5 ARE ADDITIONAL p-n JUNCTIONS CREATED.


However, when the silicon wafer bonds with themolybdenum disc, it develops a warp due tomismatch in their thermal expansion coefficients.Also, it is difficult to estimate the exact alloying cyclefor a given batch of thyristors, since the depth of n+

layer (on anode) varies with different diffusion timesadopted. Extended duration of alloying is thereforenot recommended for fear of excessive warp, thermalstrain and poor reliability in blocking characteristics[4].

5.0 OPTIMIZED PROCESS

Based on the above discussions, it was preferred toadopt selective (cathode) n+ layer diffusion. Themanufacturing process was accordingly modified andimplemented for regular production.

In the normal process, the silicon dioxide layer thatis thermally grown all around the silicon wafer ischemically etched away from both anode and cathode

sides prior to phosphorus diffusion. Usingphotolithography steps, it is selectively retained onlyon a small circular gate region located at the centreof the cathode side. As silicon dioxide acts as a maskto prevent n+ layer formation during phosphorusdiffusion, the n+ layer is formed all around the waferexcept the gate region.

In the modified process, silicon dioxide layer isretained on anode side in addition to gate region.This is achieved by suitably modifying thephotolithography steps as shown in Fig. 7.

6.0 PRODUCTION DATA

Run-charts involving 363 thyristors, manufacturedwith the pre-optimized process, were plotted duringa period of seven months (May to November 2004)(Fig. 8). Wide inconsistency in manufacturing yieldcan be clearly seen, with turn-off time in the rangefrom 150 µs to >1000 µs.

FIG. 6 : MEASURED TRACES OF REVERSE RECOVERY CURRENT (IRR)


FIG. 7 : OPTIMIZED PROCESS

FIG. 8 : MANUFACTURING YIELD IN TURN-OFF TIME TEST


In the post-optimized regime, nearly 100% yield wasachieved, with 283 thyristors tested in the periodJanuary to March 2005, having an average turn-offtime of 199 µs (range = 110-380 µs).

7.0 SUMMARY

This paper summarizes the investigations and processoptimization concerning turn-off characteristics of athyristor. An ultra-thin buried n+ layer was found tobe responsible for the high values of turn-off timemeasured in these thyristors. This anomaly could beeliminated by adopting a selective diffusion processof n+ layer on the cathode face of the wafer.Production runs using the optimized process achievedsignificantly improved results and a consistentmanufacturing yield.

Acknowledgement

The authors would like to thank the Managementand Technical Staff of the Semiconductors andPhotovoltaics (SC&PV) department in BHEL forproviding the necessary facility and support duringthe course of this study. Motivation andencouragement given by Mr. K.P. Raghunath

(Additional General Manager), Mr. Ranjan Sahi(General Manager), Mr. V. Vishwanathan (ExecutiveDirector) and Mr. A. Bhattacharya (ExecutiveDirector) is gratefully acknowledged.

References

[1] B.E. Danielsson, 'Studies of Turn-off Effects inPower Semiconductor Devices', Solid StateElectronics, Vol. 28, No. 4, pp. 375-391,1985.

[2] D.E. Crees, G. Humpston, D.M. Jacobson andD. Newcombe, 'Silicon/Heat-sink Assembliesfor High Power Device Applications: PresentTechnology and Developments', GEC Journalof Research, Vol. 6, No. 2, pp. 71-79, 1988.

[3] F.M. Roberts and E.L.G. Wilkinson, 'TheEffects of Alloying Material on Regrowth-layerStructure in Silicon Power Devices', Journal ofMaterials Science, Vol. 6, pp. 189-199, 1971.

[4] G. Humpston, D.M. Jacobson, D.E. Crees,D.R. Newcombe and M. Zambelli, 'RecentDevelopments in Silicon/Heat-sink Assembliesfor High-Power Device Applications', GECReview, Vol. 7, No. 2, pp. 67-78, 1991.

Mr. N. Harihara Krishnan received B.E. (Honours)degree in Electronics and CommunicationsEngineering from PSG College of Engineering

and Technology, Coimbatore (Madras University)in 1984 and MBA degree in Finance Managementfrom Bangalore University in 1997.

He joined BHEL at its Electronics Division,Bangalore, in 1984. Since then, he has beeninvolved in design and development of powersemiconductor diodes and thyristors. At present,he is working as Deputy General Manager (Design)in the Semiconductors and PhotovoltaicsDepartment.


Mr. S. Govindaraj received Diploma inTelecommunication Engineering in 1979.

He joined BHEL in 1980 at its ElectronicsDivision, Bangalore, in the Semiconductors andPhotovoltaics Department. After a decade ofservice in production planning and control, he hasbeen closely associated with process control andmanufacturing activities of power semiconductordevices. He is at present working as SeniorEngineer (Production).

Mr. Maheshwar Dehury received B.Sc. degree in

Electrical Engineering from University College ofEngineering, Burla, Sambalpur, in 1990.

He joined BHEL at its Electronics Division,Bangalore, in 1994. Since then, he has beeninvolved in manufacturing of power semiconductordevices. Currently, he is working as DeputyManager (Production) in the Semiconductors andPhotovoltaics Department.

Mr. Vikram Kumar Yadav received B.Tech. degreein Electronics and Instrumentation Engineeringfrom Bundelkhand Government EngineeringCollege, Jhansi, in 1996 and M.Tech. degree in

Digital Communication Engineering from RECKurukshetra in 1998.

After working as lecturer in REC Kurukshetra fora period of two years, he joined BHEL at itsElectronics Division, Bangalore, in 2000. Sincethen, he has been involved in design anddevelopment of power semiconductor diodes andthyristors. At present he is working as SeniorEngineer (Design) in the Semiconductors andPhotovoltaics Department.


Mr. P.K. Kumaravyasa received B.E. degree inElectronics Engineering from Bangalore Universityin 1989.

He joined BHEL at its Electronics Division,Bangalore, in 1980. Since then, he has beeninvolved in design, construction and maintenanceof process and test equipment for manufacturingsemiconductor and photovoltaic products. Atpresent, he is working as Deputy Manager in theFactory Services Department.

Mr. S.K. Ramesh, after completion of IndustrialTraining Institute course, joined BHEL at its

Electronics Division, Bangalore, in 1979. He hassince been associated with testing of powersemiconductor devices. Having an in-depthpractical knowledge in electrical and electronicsengineering, he has developed specialized expertisein measurement of various electrical, mechanicaland thermal characteristics of diodes and thyristors.At present, he is working as Chargeman(Production) in the Semiconductors andPhotovoltaics Department.


INNOVATIONS — FROM BHEL

100 TO 140 MW SINGLE CYLINDER NON-REHEAT STEAM TURBINE

A single cylinder, non-reheat steam turbine for the100 to 140 MW range has been developed, basedon the well proven building block principles. It isthe largest single cylinder steam turbine engineeredso far in BHEL, with overall dimensions of 9.06m length, 8.04 m width and 6.72 m height. Thedesign can be customized to meet specific plantrequirements like power, inlet/exhaust steamconditions, extractions for feed heating system andco-generation.

The main features of the design are:

● Integral valve chest for compact design

● Nozzle control governing to improve partload performance

● Two shell construction, with guide bladecarriers, to facilitate parallel machining andhence manufacturing cycle time reduction

● Advanced 3D LP blading to improveefficiency

● Pelton wheel design for hydraulic barring

● Electro-hydraulic governing system

The design is suitable for plants where a large amountof waste heat is available and reheat is not feasible andin co-generation plants where low pressure steam isrequired for the process. An order has been receivedfrom TISCO for a 120 MW turbine.

Cross-section of a Single Cylinder Turbine


GAS LIFT COMPRESSOR

BHEL is executing its largest compressor exportorder of Gas Lift Compressor package to PDOOman, where it is under installation at the LekhwairGas Lift Plant. The package, with a capacity of 1.6mmscmd, will be the plant's fifth gas-lift compressortrain, operating in parallel with the four existingcompressor trains, which handle 1.0 mmscmd each.The centrifugal compressor train has been designedto boost the pressure from 200 kPa (abs) to 7800kPa (abs) for injection into the gas lift wells.

The compressor has two casings – LP & HP. The LPcasing is horizontally split while the HP casing isvertically split. Each casing accommodates two phasesarranged back to back. Thus the overall compressionis achieved in four phases, having three inter-stagecoolers and one final discharge cooler, along withseparators. The final phase discharge cooler outlettemperature is maintained at 45 °C. All four phaseshave independent anti-surge recycle protection.

It is the largest synchronous motor (14.8 MW)driven compressor and the largest compressor train(14 × 4 × 6 m) from BHEL. The long Vorecon(Voith coupling) supplied is the first of its kind inAsia and the eighth in the world.

To validate the design, the entire compressor packagehas been simulated under dynamic conditions. For

the first time, the entire motor driven compressortrain has been tested in the factory.

158 kW, 375 RPM BRUSHLESS EXCITER FORHYDROGENERATORS

Traditionally, hydrogenerators have been suppliedwith static excitation systems, which make use ofslip rings and brushes that require frequentmaintenance. Brushless excitation systems overcomethese limitations in terms of enhanced reliability andeasy maintenance. BHEL Corp. R&D and Bhopalhave recently developed and successfully tested a158 kW, 350 V, 375 rpm brushless exciter forhydrogenerators of ratings up to 12 MW and 375rpm. The brushless exciter developed is suitable forboth vertical and horizontal mounting.

The exciter was subjected to the open circuit, shortcircuit, heat run, and the voltage response tests andthe test results were satisfactory. The over-speed testhas also been carried out to check the mechanicalintegrity and the capability of the flat-pack typediodes, manufactured at BHEL Electronics Division,Bangalore, to function under high g-forces andunder run-away conditions experienced by a hydro-generator. The temperature rise was found to bewithin the Class-F limit of insulation.

BHEL is now in a position to design, manufactureand supply brushless exciters to meet the excitationrequirements of hydrogenerators. An export orderfor three sets has been received from the 3x5 MWDevighat Hydro Power Station, Nepal.

Compressor Train End view of Brushless Exciter (Drive End Side)


TWO STAGE TRIGGERED SPARK GAP FORPROTECTION OF 400 kV SERIESCAPACITORS

Series capacitor banks are employed in fixed as wellas thyristor controlled series compensation schemesfor enhancing the power transfer capability of EHVtransmission lines. The capacitor bank can experiencetemporary over-voltages during fault conditions.Triggered spark gaps are used to serve as primaryprotection devices, in conjunction with metal oxidevaristors, for the protection of series capacitorsagainst onerous over voltages.

With a view to meeting the demand for suchprotective devices, a two stage triggered spark gaphas been developed to meet the requirements of faultcurrent, capacitor discharge current and recoveryvoltage expected in present-day 400 kV transmissionsystems. The spark gap consists of main electrodesof copper and graphite, in addition to the triggeredgap, with precise spark-over voltage setting.

The triggering scheme, integrated with the mainspark gap assembly, has been tested successfully inthe laboratory for establishing operation in both thetriggered and self-firing modes. The equipment has

also successfully undergone type tests (short circuitcurrent, capacitor discharge current and recoveryvoltage) as per International Standard (IEC 60143-2) at CESI, Italy.

ONLINE TURBINE BLADE CONDITIONMONITORING

Steam turbine blades are subject to a severe vibrationenvironment and are directly exposed to a widerange of aerodynamic excitations. Although bladesare designed to withstand such conditions, failuresdo occur due to unfavourable operating conditionsthat cause blade resonance and a high level oftransient loading.

Online vibration monitoring of turbine blades canhelp predict the health of blades and warn aboutimpending failures to enable plant personnel toavoid any catastrophe. BHEL has recently developedan online blade condition monitoring system, whichprovides feedback on blade vibration, blade resonanceand change in the blade tip displacement, whichhave a direct relation to blade failures. The BVMSmonitors the vibration behaviour of blades duringturbine operation through online measurement ofimportant parameters, such as blade clearance, bladerub and blade frequencies. It generates informationabout bent or missing blades, turbine blade creepand crack detection and growth.

400 kV Triggered Spark Gap BVMS Connected to 8 MW Steam Turbine


The BVMS has been tested extensively in thelaboratory on a turbine blade test rig and the resultshave been verified through telemetry techniques. Ithas also been tested on an in-house developed 8MW steam turbine and plans are being finalized forinstallation in turbines of higher rating. BHEL aimsto offer BVMS as a Prognostic Health MonitoringSystem with its turbines in future.

HOTEL LOAD TRACTION ALTERNATORWITH COMPANION ALTERNATOR

To enhance the passenger carrying capacity oftrains, BHEL has designed and developed, withthe full support of Indian Railways, a 320 kWtraction alternator (TA10103AZ) with hotel loadcompanion alternator (CA10104AZ) for 3600h.p. broad gauge diesel electric locomotives.

In air-conditioned trains, an additional generatorcoach is generally provided to provide power forair-conditioning and other loads. With thecompanion alternator supplying power to ACcoaches and other necessary loads, there is noneed for the generator coaches, which can bereplaced by passenger coaches, enabling theRailways to provide better passenger services whileincreasing revenue. Maintenance costs are alsoreduced by the elimination of generator coachesand provision of sturdy companion alternator inthe locomotive. Mechanical drives in the

locomotive can be replaced by electrical auxiliaries(because of the extra power available from thecompanion alternator), thereby increasing theoverall efficiency and reliability.

The first machine has been manufactured andsuccessfully type tested in presence of RDSO anddespatched to DLW for further assembly andcommissioning. BHEL had already developed andsupplied a hotel load inverter to provide power atthe required voltage and frequency.

MICRO CONTROLLER BASED FLAMESCANNER (BHELSCAN)

BHEL has developed an advanced micro controllerbased flame scanning system. The new product,'BHELSCAN', with its superior features, willensure continuous and reliable flame monitoringin boilers, thus ensuring safe and trouble freeoperation.

The flame scanner consists of a scanner headassembly with a quartz lens for collecting the lightsignal from the flame, a fibre optic cable fortransmission of the light signal and head electronicsto convert the light signal to electrical signal. Theelectrical signal is transmitted to the control unitthrough special cables. Embedded software enablesthe controller to discriminate between flames of

Blade Vibration Monitoring System (BVMS)

Hotel Load Traction Alternator


different fuels by detecting the characteristic flickerfrequency range of the flame.

To establish satisfactory field operation, theprototype was tested at Raichur and RayalaseemaTPS. Subsequently, an improved version wasinstalled in Unit - 3 boiler at Raichur TPS and hasbeen in continuous operation since January 2005.

It was rigorously tested at Electronic Testing andDevelopment Centre, Bangalore. The CanadianStandards Association International, a world leaderin product testing and certification, has qualified,after reviewing the test results, the flame scannerfor award of the 'Certificate of Compliance' toInternational Standards requirements - CSA 61010-1. The certification will go a long way towardsenhancing customers' confidence in the product.

The following are the unique features ofBHELSCAN:

● Advanced digital micro controllertechnology

● Multi fuel flame detection capability (oil,coal)

● Digital display of flame intensity/ frequencyparameters

● Digital setting of process parameters

● Compact size - fewer electronic modules

● RS 232C output for DCS connectivity

● 4-20 mA output for remote monitoring

COMPOSITE INSULATORS WITH NANOADDITIVES

BHEL has successfully established the technologyfor manufacturing polymer based compositeinsulators. A state-of-the-art plant with aproduction capacity of 450 T per year, based onthe injection moulding method and speciallydesigned moulds, has been set up and its processparameters optimized. The facility was inauguratedby the Chairman & Managing Director, Mr.Ashok K. Puri on 4 January 2007.

A significant achievement has been the improvementin intrinsic properties of silicones, the raw material,by incorporating nano-materials in the siliconmatrix, thus making the composite material moresuitable for service in high pollution levels andharsh environments. The arc track resistance of thenano-modified silicones is better (by ~ 45%) thanstandard silicones. Their hydrophobicity is alsobetter. Improved arc track resistance gives BHEL'scomposite insulators a technical edge over the othercommercially available varieties.

Technical parameters for injection moulding of thenano material-modified silicone dough intonumerous shapes/dimensions so as to cater tovarious pollution levels into sheaths, sheds, etc.,depending on design of the composite insulatorand the application areas, have also been established.

Trial production of long rods and railway insulators

BHELSCAN - Microcontroller Based Flame ScannerCMD of BHEL, Mr. Ashok K. Puri Inaugurates the CompositeInsulator Manufacturing Facility


using silicones with nano-additives has beencompleted. The materials and the products werecharacterized and the insulators have been typetested successfully. The manufacturing process forthe composite insulators has been established on aproduction scale, with ratings in the range of 25 to400 kV for railway traction and transmission lines.

The first developmental order for supply of 400kV long rod insulators has been received fromPower Grid. The development has enabled BHELto take the leadership position in 400 kV long rodinsulators in the country.

CENTRE OF EXCELLENCE FOR SURFACEENGINEERING (COE-SE)

In line with its strategy to achieve technologicalleadership, BHEL has established the Centre ofExcellence for Surface Engineering (COE-SE) – thefourth in the series of COEs – at Corp. R&D,Hyderabad. The Centre was inaugurated by theChairman & Managing Director, Mr. Ashok K. Puri,on 29th March 2007. Set up with an investment ofRs. 8.4 crore, the COE-SE will further strengthenBHEL's leadership position as a pioneer in surface

Composite Insulators with Nano Additives

engineering R&D. BHEL's earlier developments inthis field have been successfully implemented atpower stations, industrial establishments, the spaceprogramme and other important applications.

The COE-SE will be a major facility for carryingout surface coatings and treatment and developstate-of-the-art technologies for products like hydroturbines, steam turbines, gas turbines and boilers.It will also explore new applications for BHELproducts and other strategic applications and facilitatetransfer of technology to BHEL's manufacturingunits. The facility will also be used in BHEL'sfrontier research projects for exploring the use ofnano materials for surface engineering applications.

The high bay new building houses major facilitiessuch as robotic thermal spray, robotic 4.6 kWdiode laser hardening and cladding system, imageanalyzer, macro-micro hardness tester andmechanical grit recovery system.

Robotic High Power Diode Laser Facility

Centre of Excellence for Surface Engineering


RECENT MAJOR ACHIEVEMENTS OF BHEL(during September'06-March'07)

ORDERS BAGGED

Overseas

● Won its second power project contract inBangladesh, wresting it from Chinese powerequipment major - Harbin PowerEngineering, in an intensely competitiveglobal tender. Valued at Rs.5,050 million,the Asian Development Bank (ADB) fundedcontract has been placed on BHEL by theElectricity Generation Company ofBangladesh (EGCB) for a turnkey 2x100-120 MW Gas Turbine Power Plant atSidhirganj. BHEL has earlier successfullyexecuted a turnkey 100 MW gas turbinepower plant for BPDB at Baghabari, whichhas been operating successfully. The companyis also executing an ADB funded turnkeycontract for a 220 kV substation at Baghabariand extension of a 220 kV substation atIshurdi with the Power Grid Company ofBangladesh.

● Achieved a major breakthrough in theEuropean market for compressors by securinga prestigious order worth about Rs.45 crore,for a compressor train package in France.Won in the face of intense competition fromseveral internationally renownedmanufacturers, the order has been receivedfrom Grande Paroisse, France, a Total Groupcompany — one of the leading globalcompanies in the oil and gas sector. Theorder, to be completed in a challengingschedule of 15 months, envisages design,manufacture, supply, erection andcommissioning supervision of a CO2compressor train package for a urea plant.The equipment will be manufactured andsupplied by BHEL's plant at Hyderabad.

● Achieved a major breakthrough in the IPPsegment by securing an order for a 500 MWgas turbine based power plant from MGI,Jordan, on EPC basis.

● Secured its first order for gas turbines (2x26MW) for co-generation application fromOman Refinery Company, Oman.

Domestic

● Won a contract from Damodar ValleyCorporation (DVC) for two units of 500MW each at Mejia Thermal Power Station(TPS) in West Bengal, on turnkey basis.Slated for synchronisation during the 11thPlan, the project will add nearly 24 millionunits every day to the grid, a major portionof which will be transmitted to Delhi for theCommonwealth Games in 2010. Mejia TPSis already equipped with four BHEL-builtunits of 210 MW each, and the company isexecuting a turnkey contract for another twounits of 250 MW each. Notably, BHEL setsaccount for 12,000 MW or nearly 69% ofthe cumulative generating capacity in theEastern region.

● Bagged orders for the Main Plant Package attwo power stations in Maharashtra againstInternational Competitive Bidding. Valuedat over Rs.39,000 million, the orders havebeen placed by Maharashtra State PowerGeneration Company Limited(MAHAGENCO), for one 500 MW unit atKhaperkheda TPS Expansion Project andtwo 500 MW units at Bhusawal TPSExpansion Project. Slated for synchronisationduring the 11th Plan, these units will add 36million units every day to the grid. BHEL'sscope of work involves design, engineering,


manufacture, supply, erection andcommissioning of steam turbines, generators,boilers, state-of-the-art C&I for the entiremain plant and electricals, besides auxiliaries.Khaperkheda TPS is already equipped withfour BHEL-built units of 210 MW each,while Bhusawal TPS is equipped with twoBHEL-built units of 210 MW each.

● Outbidding Indian and multinationalcompanies in an open tender, BHEL hassecured a turnkey contract worth nearlyRs.3,800 million from Punjab State ElectricityBoard (PSEB) for Renovation, Modernisationand Uprating of 2x110 MW Units (3&4) ofGuru Nanak Dev Thermal Plant (GNDTP)at Bhatinda in Punjab. BHEL will alsouprate the units from their existing capacityof 110 MW to 120 MW each. This will leadto increased availability and generation,besides life extension of the units by over 15years. The first set is targeted forcommissioning in a tight schedule of 20months and the second unit in ten monthsthereafter. BHEL is one of the few companiesin the world to have established state-of-the-art technology and expertise for R&M andUprating of both hydro and thermal powerstations.

● Against stiff competition from leading French,Austrian and German multinationals, BHELhas secured a major contract for a 520 MW(4x130 MW) Hydro Electric Project (HEP)in Himachal Pradesh. Valued at over Rs.4,000million, the order for Parbati HEP, Stage-III,has been placed on BHEL by NHPC.Located at Bihali in Kullu district, about 250km from Chandigarh, the undergroundpowerhouse will generate electricity utilisingthe waters of the river Sainj, a tributary ofBeas. The project is slated for commissioningin a tight schedule of 50 months. Majorequipment includes four high-head Francishydro turbines with matchinghydrogenerators; digital governors;microprocessor-based control, monitoring and

protection system, besides other associatedequipment and auxiliaries. BHEL is alsoexecuting its first ever mega hydro powerproject contract for the 800 MW ParbatiHEP, Stage-II, to be equipped with thecountry's largest capacity (4x200 MW) Peltonhydro turbines.

● Secured an order worth Rs.9,500 million, forsetting up a 99 (3x33) MW captive powerplant on EPC basis from Bharat OmanRefinery Limited (BORL) as part of itsrefinery project at Bina in Madhya Pradesh.The power plant will be equipped with eco-friendly Circulating Fluidised BedCombustion (CFBC) boilers of 275 tons perhour capacity, specifically designed for petcokefuel. Won against international competitivebidding, the order assumes special significanceas this is the first order for petcoke-firedCFBC boilers on BHEL, as also the largest-value single order secured by the IndustrySector business segment. The power plantwill meet the process steam and powerrequirement of the upcoming refinery project.While the first unit is slated forcommissioning in a schedule of 26 months,the project will be completed within 30months. The equipment has to meet verystringent technical specifications to ensureuninterrupted and high-quality power andsteam to the refinery.

● Against International Competitive Bidding,BHEL has bagged another major order fromIndian Oil Corporation (IOC) for setting upan energy efficient and environment friendlyco-generation power plant at its HaldiaRefinery Complex. IOC has placed an orderworth over Rs.1,650 million, on LumpsumTurnkey (LSTK) basis. The project is beingset up to meet the requirement ofuninterrupted power supply, in addition tothe steam needs of the refinery and isscheduled for commissioning in 22 months.BHEL's scope of work includes design,engineering, manufacture, supply, erection


and commissioning of a Frame-5 Gas TurbineGenerator and a Heat Recovery SteamGenerator (HRSG) of 130 tons per hourcapacity with associated auxiliaries and balanceof plant, in addition to complete civil worksand select spares.

● BHEL will supply the largest size (210 TPH)BFBC (Bubbling Fluidised Bed Combustion)Boiler along with a 50 MW steam turbinegenerator against an order from IND Energy,Raigarh. The boiler will operate on washeryrejects, a fuel having the least commercialvalue and will generate power at a very lowcost.

● Once again demonstrated its competitiveedge by bagging a World Bank fundedturnkey contract for a new 400 kV substationand extension of three existing substations inMaharashtra, against stiff competition fromEuropean MNCs and Indian companies.Valued at Rs.1,440 million, the order hasbeen placed by Power Grid Corporation ofIndia (PowerGrid) for a new 400 kVsubstation at Wardha and extensions of 400kV substations at Seoni, Akola andAurangabad, associated with the Sipat SuperThermal Power Project Stage-IIsupplementary transmission system. Aimedat improving power supply in the WesternRegion, the project will be commissioned byBHEL in a tight schedule of 19 months.

● BHEL will supply 500 MVA convertertransformers - the largest such equipment tobe manufactured in India and amongst thelargest worldwide for PGCIL's ±500 kV,2500 MW HVDC terminal stations at Balliaand Bhiwadi.

● BHEL's in-house developments have receivedcommercial recognition in the form of thefirst order for an indigenously-developeddevice for use in high-voltage (400 kV)transmission lines. The first-of-its-kind inthe world Controlled Shunt Reactor (CSR)

has been developed by BHEL in-house. Thedevice, which operates automatically,depending on system requirement in lessthan 10 milliseconds, improves power transfercapability of transmission lines and reducessystem losses, besides improving systemstability in high-voltage transmission lines.BHEL has received the first commercialorder for supply, erection, testing andcommissioning of an 80 MVAR, 400 kVControlled Shunt Reactor at Karad Substationon Karad - Lonikand line on turnkey basisfrom Maharashtra State ElectricityTransmission Company Limited. The contractwill be completed in a schedule of 18months. The reactor transformer will bemanufactured at BHEL's Bhopal plant whilethe company's Electronics Division,Bangalore, will supply the control equipment.

The first 50 MVAR, 400 kV CSR,commissioned at a 400 kV substation atItarsi on the Itarsi-Jabalpur transmission linein Madhya Pradesh has successfully completedtrial operation, providing impetus to theestablishment of the new technology. SinceCSRs can be taken out of the circuit inloaded line conditions, 25-30% more powercan be transmitted over the line throughimprovement in voltage profile, as comparedto a fixed shunt reactor configuration.

AWARDS

● Twenty nine of the thirty four power stationsin the country awarded with the Governmentof India's Meritorious Productivity Awardsfor excellent performance, are equipped withgenerating equipment manufactured byBHEL, once again establishing the reliabilityand quality of BHEL equipment.Significantly, all the five power stationsawarded the Gold Shield for 2005-06 and sixof the seven power stations awarded theGold Shield for 2004-05 are equipped withBHEL-built sets. These include: NTPC'sSimhadri and Ramagundam; Tata Power


Company's Trombay; KPCL's Almatti andVarahi; HPGCL's Tau Devi Lal; NPCIL'sKaiga; RIL's Dahanu; APGenco's UpperSileru and NHDC's Indira Sagar powerstations.

● BHEL has become the first Public SectorCompany in the country to win the 'PRIZE',under the CII Exim Award Scheme. Theprestigious recognition has been bestowed onBHEL's Heavy Electrical Equipment Plant atHaridwar, for business excellence, as per theglobally recognized model of EuropeanFoundation for Quality Management(EFQM). The other major units of BHEL atTrichy, Hyderabad and Bhopal have alsoreceived commendation certificates for'Significant Achievements in TQM' while itsElectronics Division at Bangalore has receivedthe commendation for 'Strong Commitmentto TQM'. A quality conscious organisationwith a strong customer focus, BHEL hasacquired ISO-9001 (2000 version), ISO-14001 and OHSAS-18001 certification, forall its operations.

● Three Vishwakarma Rashtriya Puraskars,under different categories, have been sharedby eight employees from BHEL's Haridwarunit, for their innovative suggestions leading

to cost reduction, higher productivity, safetyand quality of products, etc. In addition, two'National Safety Awards' have been won byBHEL's manufacturing plants at Trichy andBangalore, for outstanding achievements interms of longest accident free period andlowest accident frequency rate at their works.The awards were presented by the UnionMinister of State for Labour & Employment,Mr. Chandra Sekhar Sahu, at a function inNew Delhi, on September 17, 2006.

● Prime Minister Dr. Manmohan Singhpresented the 'MoU Award for Excellence inPerformance' to Mr. Ashok K. Puri, Chairmanand Managing Director, Bharat HeavyElectricals Limited (BHEL). Awarded toBHEL for exceeding the overall targets setout in the MoU with the Government ofIndia for the year 2004-05, the prestigiouscommendation was bestowed on the companyat the Conference of Chief Executives ofPublic Sector Enterprises, jointly organisedby the Department of Public Enterprises andSCOPE. BHEL was also awarded the MoUExcellence Certificate for exceeding the overalltargets set out in the MoU for the year 2005-06, by Mr. Jairam Ramesh, Minister of Statefor Commerce.

Ramagundam STPS, equipped with BHEL sets, winner of GOI Meritorious Productivity Gold Shield

Talcher STPS

Registration No. RN-27700/76

Bharat Heavy Electricals LimitedBHEL House, Siri Fort, New Delhi-110 049

Visit our Website at : http://www.bhel.com

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Bhel Journal Apr 07

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