Asset Management and Innovative Solutions using High Voltage GIS :Experience of The Tata Power Co. Ltd.

ANJALI RAJE MURUGAN P RAINA DAsst. Manager Deputy General Manager General Manageraraje@tatapower.com pmurugan@tatapower.com draina@tatapower.com

THE TATA POWER COMPANY LTD.

1. INTRODUCTION

Metropolitan areas in rapidly developing regionsare faced with special problems in augmentationof their power supply system due to uncontrolledgrowth, uncertainty of development plans,extreme congestion and very high load densities.

The continuous growth of large metropolitan citiesand industries, which results in concentration ofthe demand for electricity, necessitates installationof distribution equipment upto highest voltage.Use of large size transformers to achieve economyof scale and space results in high fault levelswithin these areas.

The population in Mumbai has been growing at avery rapid rate and today , it is around 15 to 16million resulting in total congestion of the existingland and leaving no Right Of Way for anyadditional HV overhead line input into the city. Infact, finding a suitable route for undergroundcables is also becoming almost impossible due tonarrow carriageways, large number ofunderground utilities and non co-ordinateddevelopment of underground services.

In addition, land in South Mumbai is at very highpremium, which makes it practically impossible toplan conventional indoor or outdoor EHVReceiving Stations economically with highreliability.

To meet the above constraints, installation of GasInsulated Switchgear (GIS) was the onlyalternative to conventional outdoor/ indoorswitchgear to ensure economical supply of powerwith a high degree of reliability.

GIS has excellent adaptability in meeting mostdifficult terrain or space constraints. Low spacerequirements on account of high dielectric strengthof gas leads to flexibility of application. Entireswitchgear being in a sealed environment does notget impacted by polluting conditions like dust,impurities and deteriorating lubricants used indisconnectors. There has been besides above,other factors such as safety, longevity andadaptability which make GIS an attractiveproposition as compared to Air InsulatedSubstation (AIS). It is upto 220 kV level,transportation conditions permit factory assembledbay which is most attractive and favourable

feature for indoor & outdoor application alike dueto very short erection and commissioning time.Reduced requirement of maintenance make themsuitable option for most reliable operation. In fact,quality consciousness of equipment alone makeGIS as favoured equipment choice for remoteoperation. All these factors in totality make GISmost attractive equipment options to adopt.Though initial cost of equipment is higher thanthat of AIS, the choice of GIS with respect to AISis a techno economic decision to be made takinginto account technical economical &environmental factors.

Considering all the above factors Tata Power hadtherefore decided to adapt GIS at 245 kV and 145kV and commissioned in 1988 India’s first suchsubstation at Carnac Receiving Station (R/s)situated in the heart of Mumbai city. Since thenTata Power has experience of operating more than600 bay years of 245 kV GIS and 284 bay years of145 kV GIS respectively.

2. EXPERIENCE OF TATA POWER

The performance of GIS in Tata Power has beenquite good as compared to conventional AIS.However, some problems faced with GIS arebriefly described below:

• 245 kV SF6 gas to oil transformer bushings at oneof Tata Power’s substation failed which resulted ina flashover in the power transformer exit bus ductcompartment. Impending failure of a similarbushing was detected in incipient stage bymonitoring of dissolved gases in bushing oil. Thisbushing was dismantled and stripped whichindicated Very Fast Transient Over Voltage(VFTO) as a possible cause of failure of thebushing. The computer simulation of GIS modelwas carried out by using the Electro MagneticTransiemnt Programme (EMTP) for analysis oftransients generated by closing and opening of 245kV breaker of 315 MVA auto transformers. EMTPstudies indicated that the maximum rise in voltageduring switching operation of breaker was 2 pu in530 nano secs. The computer model of 245 kVGIS and corresponding theoretical voltage waveform obtained using EMTP simulation is indicatedin Figure 1 and 2 respectively. Tests wereconducted at site to correlate and validate the

Fifteenth National Power Systems Conference (NPSC), IIT Bombay, December 2008

427

theoretical test results. During actual tests at site,the peak value of the VFTO was of the order of1.4 pu in 364 nano secs., when the breaker wasclosed. VFTO waveform record from oscillogramis indicated in Figure 3. The variation intheoretical and actual measurements could havebeen due to the following:

a) Non inclusion of arc impedance in thecomputer model since no reliable means areavailable to assume value of the arcimpedance.

b) In the computer model all the three poles ofthe circuit breaker are assumed to closesimultaneously whereas in practice due tomechanical linkages the poles may not closesimultaneously

c) Effect of resistive and inductive values ofearth and earthing connections.

After detailed analysis and investigations oncareful dismantling of bushing, it was observedthat edges of inner 12 layers of aluminium foiltowards the oil end had marks of combustion andpuncture of the paper. The size of combustionmarks increased towards the main copperconductor. Based on these observations, thefollowing corrective measures were taken:

Fig 1 : Computer Model of 245 kV GIS

Fig 2 : Theorotical Voltage Form by EMTP Simulation

↑ kV

Time→

Fig 3 : VFTO Wave Form Record Test at Site

i) Existing OIP bushings were replaced by newbushings in which the zero foil connection to mainconductor was shifted from oil end to SF6 gas endto clamp VFTOs as the gas end itself.

ii) The insulation strength of the first layer ofinsulating papers was improved alongwith itsthickness.

iii) The number of connections to connect zero foilwith the main conductor was increased from one

Fifteenth National Power Systems Conference (NPSC), IIT Bombay, December 2008

428

to two and thickness of connecting wire was alsoincreased.

iv) The new bushing was satisfactorily tested atbushing manufacturer’s works to simulate VFTO.The torque withstand capability of the bushing capwas improved from 120 Nm to 190 Nm, asrequired by GIS manufacturer, to prevent rotationof bushing top cap which was the interface pointwith the GIS. Earlier lesser top cap torquecapability had led to snapping grading leadconnections.

v) Thus the results of these studies helped themanufacturer to modify the design & adopt correctinterface between GIS, bushing & transformer.

• The earthing system of the GIS requires greatercare as compared to that for a conventionaloutdoor substation. During initial charging of 145kV GIS when 110 kV XLPE cable was earthedthrough high speed earth switch, a flash over wasobserved from 145 kV GIS cable compartment tothe metallic gland on which the copper screen ofXLPE cable was terminated. The flash over wascaused due to non provision of shorting stripswhich were meant to directly connect GISenclosure to copper screen of XLPE cables. Thishad resulted in discontinuity in the GIS earthingsystem. The case of the flash over was attributedTransient Ground Potential Rise (TGPR). Whenthe cable was switched off, it retained a trappedDC charge between the conductor and the screenof the cable. When the earth switch was closed,this caused a discharge at the shortest path acrossthe sealing end enclosure to the cable screen, dueto high inductance of the GIS earthing system inthe absence of the shorting strips. After provisionof shorting strips, there was no incidence of flashover during abovementioned operation. Figure 4indicates connection for GIS enclosure to copperscreen of XLPE cable.

Fig 4 : GIS Enclosure Connection to Copper Screen ofXLPE

• 245 kV GIS high speed earth switch bushings usedto crack during the operation of high speed earthswitch. It was observed that due to solidconnection and inadequate dampner, the shockwas getting transferred to the porcelain bushing ofthe high speed earth switch. Subsequently thesolid connection was replaced by the manufacturerand there were no subsequent failures of the highspeed earth switch bushings.

• In 145 kV GIS circuit breaker compartment onesafety disc ruptured. The sequence of eventslogged by SCADA indicated initially Lowpressure alarm and LOLO pressure trip within 715mseconds. The bay was carrying current of 270amps prior to tripping. No other relay operatedand the breaker tripped on LO-LO SF6 pressure.The internal inspection of the breakercompartment did not reveal flash over marks orany other abnormality. The ruptured disc wasreplaced by a new safety disc. After reinstallingthe top cover of the circuit breaker and evacuation,SF6 gas was filled and breaker taken into loadservice after carrying out timing, contactresistance and insulation resistance checks.Broken pieces of the safety disc were sent to themanufacturer. After studying the pieces of thefaulty disc no defect was found. Carbon matteralso did not show any micro cracks. Themanufacturer opined that the defective disc wasprobably knocked during the transport or handlingof the circuit breaker, which induced stressedinside the carbon matter until the disc burstsuddenly. After the replacement of the safety discthe GIS has been operating satisfactorily.

• Gas leak from gaskets particularly from theoutdoor flanges of GIS is frequently experienced.The ultra violet radiations causes fasterdeterioration of gaskets. The gradual corrosion ofthe aluminium metal enclosure was also observeddue to moisture ingress in outdoor GIS gaskets.

Tata Power replaced leaking gasket alongwith theassociated gaskets. The hydraulic mechanism oilleakage was also attended.

• The enclosures of SF6 gas density monitors,especially those which are mounted outside GISbuilding, were getting corroded and subsequentlyresulted in mal operation of the same. Duringinvestigation, it was observed that environmentalcondition of Mumbai i.e. extremely humid climateand heavy rains during monsoon season,enclosures of outdoor SF6 monitors are gettingcorroded. Suitable protective measures have beenadopted to address this problem.

Fifteenth National Power Systems Conference (NPSC), IIT Bombay, December 2008

429

3. MONITORING AND MAINTENANCEPHILOSOPHY

Achieving and maintaining high level ofavailability of equipment requires integratedapproach to quality control by both users andmanufacturers. Tata power has standerdisedquality assurance programme to be carried out atthe time of purchase of GIS, as well as it followsstandardised (internally) operation andmaintenance philosophy for GIS.

As a part of quality assurance program duringinitial purchase stage and commissioning stage ofGIS, cleanliness during assembly and erection isvery vital since any contamination causesdeterioration in the properties of SF6 gas. Accessto current carrying parts during services is notpossible as all the live parts of GIS are enclosed inmetal enclosure. Opening of the enclosure forinspection at site can lead to introduction ofimpurities which can deteriorate dielectricproperties. This necessitates stress on diagnosticmeasurement in case of GIS instead of usualmaintenance practices for outdoor switchyardequipment. The values measured during initialcommissioning are very useful as a reference infuture. Following tests are performed aftercompletion of erection of GIS at site :

1. Checking the moisture content of SF6 gas ofvarious gas compartments.

2. Measurement of contact resistance, operatingtime of circuit breakers, isolators etc. by usinginsulated bushings of the earthing switches.

3. Caliberation of gas density monitors.4. Partial discharge test.

5. Power frequency test / Oscillating switchingimpulse test.

Dielectric tests are carried out on completeinstallation (except surge arrestors, Cables,Voltage Transformers and Power Transformers) toconfirm dielectric integrity of GIS and detectpresence of foreign particles and verifycorrectness of connections. The dielectric tests arecarried out through special test set with bushingsor backcharged VTs which are speciallystrengthened. Once GIS is commissioned itrequires no or very little maintenance. Themaintenance and inspection philosophy for GISfollowed by Tata Power consists of following.

1. Visual Inspection – To be carried out onperiodic internals i.e. every month.

2. Detailed Inspection – To be carried out every6 years or 1000 times of no-load switchingoperation.

3. Disassembling Inspection – Every 18 yearsor the initial stage and afterward carrying outreferring to the initial stage inspection resultsas per manufacturer’s standards and as perTata Power, follow trends of variousinspection & testing results.

4. Special Inspection – (i) In case of anyabnormality. (ii) For GCBs : After 10 times ofbreaking at rated breaking current. (iii) After1000 times of breaking at rated normalcurrent. (iv) After 2000 times of no-load orsmall current switching

The brief procedures for visual, detailed anddisassembling inspection (special inspection) areindicated below :.

Inspection Description Brief DetailsExterior in general Abnormal noise or odour, Rust damage of any part,

tightness of bolts/nuts.Operating Mechanism Box Checking of position indicator, operation counter, oil

pressure gauge.

Visual Inspection

Leakage Oil & Gas leakage from operating mechanism systemand gas respectively.

Switching Operation Checking of position indicator, operation counter,auxiliary switches, low pressure operation and breakeroperation.

Exterior In General Check the tightening of bolts and nuts, Rusting orpeeling of pains,

Operating Mechanism Checking operation of the oil pressure switch anddensity switch, greasing, checking oil level of pumpunit, exchange of operating oil.

Detailed Inspection

Measuring Test Measurement of Insulation resistance, measuring initialoil charging time, checking accumulator gas (N2)pressure.

Fifteenth National Power Systems Conference (NPSC), IIT Bombay, December 2008

430

Inspection Description Brief DetailsDisassemblingInspection

To be carried out undersupervision ofmanufacturer’srepresentative

Checking condition of main contacts, nozzle, cleaningof interrupting chamber and gas tank, replacement ofadsorbent, greasing, checking resistance acrossterminals of each pole.

Special Inspection To be carried out undersupervision ofmanufacturer’srepresentative

To be carried out as and when the conditions indicatedabove are observed. The inspection content is same incase of an detailed and disassembling inspections.During O&M, parts may be exchanged, greasing,cleaning of all ports done.

Besides the above, certain novel modifications in GISarchitecture, incorporated due to operating experiencewere carried out by Tata Power while procuring new

GIS substations. The advantages and disadvantages ofmodifications in GIS architecture are indicated below:

S.No. Modifications Advantages Disadvantages1. Reduction in number of gas

chambers in a bay by combingCT.

New GIS is much more compact. Volume of gas handled at atime is more.

2. Three position switch i.e.isolator, and earth switch

Space required for mountingisolator / earth switch reducedand hence further reduction inspace requirement

Mechanical interlock therebyavoiding mal operation isolator /earth switch.

Nil

3. Cable side earth switch andcable compartment in singlegas compartment

Reduction in size of GIS Quantity of gas at the time ofevacuation of gas from cablechamber is more.

4. INNOVATIVE SOLUTIONS

In today’s changing market, the substation isbecoming more and more a key element tomeet end users requirements successfully andeconomically. Many existing substations haveoutlived their operational life and a one-to-onereplacement of conventional AIS componentslike circuit breakers and disconnectors is noteconomically advisable. Completely newsubstations have to meet tough requirements ofin terms of occupied space, environment andavailability. Also, extension of existingsubstations require high availability of primarycircuit components, to cope with alreadyexisting control systems, lack of availability ofspace, limited down time.

Taking into consideration all the above factors,to expand existing substation, Tata Power hasintroduced new concept of “HybridSwitchgear” in HV/EHV switchyards. HybridSwitchgear is a combination of AIS and GISand it can be thought as “Performance andSave Space” and it can meet any substationlayout requirement while making efficient use

of available space. Hybrid GIS bay consists ofbreaker, isolator, earthing switch enclosed inan enclosure filled up with SF6 gas. Theinterface with the existing switchyard can bethrough air bushings if it is to be connected tobusbars or through cable if the bay is locatedaway from busbars. Sometimes, based on thecustomer’s requirement, the hybrid GIS baycan be equipped with conventional currenttransformers.

In one of Tata Power’s Generating Station atLodhivli, three 110 kV outdoor bays were tobe created for Generator Transformer and twooutgoing lines in a very limited space(Trapezoidal plot 10M x 23M x 20M x 25M)for evacuation of power. It was not possible toinstall conventional outdoor bay within spaceavailable. Also, it was not economicallyadvisable for installation of complete GIS.Hence a midway solution of installation ofHybrid GIS was considered andcommissioned. Figure 5 shows Hybrid GIS byinstalled at Tata Power’s Lodhivli Station.

Fifteenth National Power Systems Conference (NPSC), IIT Bombay, December 2008

431

Fig 5 : Hybrid 145 kV GIS

Fig 6 : 145 kV GIS Building at Carnac

References :

1. “Site Investigations and study of VFTO in245 kV GIS of Tata Electric Companiesby A M Sahni etal”, CIGRE 1996 session(15/21/33-01)

5. CONCLUSION

The paper highlights the experience of TataPower on High Voltage GIS indicating someminor problems, operation and maintenancephilosophy of GIS including quality controlaspects of GIS and innovative modifications inthe switchyard to meet the exceptionallygrowing load requirement from existingsubstations. GIS are now exhibiting anexcellent in service performance in terms ofavailability, compactness and reliability, whichwas made possible only through moreintegrated approach to quality control by bothusers and manufacturers during design,manufacture, testing, shipping, assembly,operation and maintenance.

GIS has resulted in widespread use over theyears and has helped in optimally using thespace available. (Refer Figures .6 & 7)

Fig 7 : 145 kV GIS in ground floor of GISbuilding at Carnac

Fifteenth National Power Systems Conference (NPSC), IIT Bombay, December 2008

432