CONDITION MONITORING OF POWER TRANSFORMERS

CONDITION MONITORING OF POWER TRANSFORMERS

CONDITION MONITORING OF POWER TRANSFORMER

CONTENTS Page NoCHAPTER-I

3 1.1INTRODUCTION TO APGENCO

3

1.2 INTRODUCTION TO KTPS

5 1.3 GENERAL LAYOUT

6 1.4 BASIC OPERATION

8 1.5 BASIC EQUIPMENTS

9CHAPTER-II

TRANSFORMERS 2.1 INTRODUCTION TO TRANSFORMER

12 2.2 WORKING PRINCIPLE OF TRANSFORMER

13 2.3 DIFFERENT PARTS OF TRANSFORMER

13 2.4 TYPES OF TRANSFORMERS

20 2.5 FAULTS AND FAILURES OF TRANSFORMER

23 2.6 TRANSFORMER LOSSES

25CHAPTER-III CONDITION MONITORING OF POWER TRANSFORMER 3.1 INTRODUCTION

27 3.2 TRANSFORMER ASSESMENT

28 3.3 CRITICAL COMPONENTS

29 3.4 TYPES MAJOR FAILURES

30CHAPTER-IV TRANSFORMER TESTS

4.1 INTRODUCTION

32 4.2 WINDING RESISTANCE MEASUREMENTS

32 4.3 CAPACITANCE AND TAN FOR WINDING

33 4.4 INSULATION RESISTANCE(IR) AND POLARIZATION INDEX

34CHAPTER-V DISSOLVED GAS ANALYSIS 5.1 INTRODUCTION

36CHAPTER VI TROUBLE SHOOTING CHART FOR ALL TRANSFORMERS

37CHAPTER VII CASE STUDIES

43CONCLUSION

46BIBOLOGRAPHY

47 CHAPTER.IBRIEF HISTORY OF KOTHAGUDEM THERMAL POWER STATION:1.1 INTRODUCTION TO APGENCO:Andhra Pradesh Power Generation Corporation Limited (APGENCO) is the electricity generation company of the Government of Andhra Pradesh found in the year 1998 as a part of the network of APSEB (Andhra Pradesh State Electricity Board) in India. It has an installed capacity of 7048.4 MW which makes it the third largest power generation company in India.

Andhra Pradesh Power Generation Corporation Limited is one of the pivotal organizations of Andhra Pradesh, engaged in the business of Power generation. Apart from operation & Maintenance of the power plants it has undertaken the execution of the ongoing & new power projects scheduled under capacity addition programme and is taking up renovation & modernization works of the old power stations. APGENCO came into existence on 28.12.1998 and commenced operations from 01.02.1999. This was a sequel to Governments reforms in Power Sector to unbundle the activities relating to Generation, Transmission and Distribution of Power. All the Generating Stations owned by erstwhile APSEB were transferred to the control of APGENCO. The installed capacity of APGENCO as on September 30, 2010 is 8135.9 MW comprising 4382.50 MW Thermal, 3751.40 MW Hydro and 2 MW Wind power stations, and contributes about half the total Energy Requirement of Andhra Pradesh. APGENCO is third largest power generating utility in the Country next to NTPC and Maharashtra. It's installed Hydro capacity of 3703.4 MW is the second highest among the Country. Types of power plants APGENCO operates:

Source Installed Capacity (MW)

Thermal 4382.5 MW

Hydel 3751.40 MW

Wind 2.00 MWTable1.1: Statistical Information Regarding Different Thermal Power Plants Operating Under APGENCO:

Power

Station

OperatorLocationDistrictUnit

Wise

capacityInstalled

capacityPlant Coordinates

Ramagundam

B thermal

Power

Station

APGENCORamagundamkarimnagar162.562.5184331N793047E

Kothagudem

Thermal

Power

Station

APGENCOPaloncha Khammam 460,

4120720173718N8041,15E

Kothagudem

Thermal

Power

Station V&IV

Stage

APGENCOPaloncha Khammam 2250

1500500

500173724N8042060E

Dr Narla

Tatarao TPS

APGENCOIbrahinpatnam Krishna 6210,

15001760163558N803212E

Rayalaseema

Thermal

Power

Station

APGENCOCuddapahYSR4210840144214N782729E

Kakatiya

Thermal Power

APGENCOChelpurWarangal1500500182302N794942E

1.2 ABOUT KTPS:

Electricity is being produced at thermal, nuclear, hydel generating stations. Kothagudem thermal power station is one of the major power generating stations of the Andhra Pradesh. The main raw material is coal is supplied by singereni collieries, Kothagudem and a water source is from kinnerasani project, which is about 12kms. From paloncha.

For the power generation with 2x110 MW and 3x210 MW of K.T.P.S. authorities are required to be operative to active full operation. The auxiliaries are basically operation either on L.T. System i.e. 415 V 3 power supply is made available to the system after providing the station transformer of 3x50 MVA capacity with voltage 220 KV/ 7.2/7.2 KV & different service transformers of capacity 1.0 MVA, 1.5 MVA, 2.0 MVA, which are located near the load centre as the transformer having the voltage of 6.6 KV /415 V. The 6.6 KV power is distributed through 6.6 KV interconnected Bus System for all the five units with a control through DC of 220 V.

The 415 V power supply is done through a L.T. SWGR (Switchgear) which are located nearby the distribution transformer as well as the load centers. The all in -comers, which are breaker controlled , are having the control the L.T. SWGR are having the control system on 110/ 220 V AC. The 6.6 KV power supply which are either MOCB (Minimum Oil Circuit Breaker) of JYOTI MAKE or Air Circuit Breakers.

The 6.6 KV power supply to various draining equipments i.e. more is made through breakers which are either MOCB of jyothi make air circuit breaker which are either of voltage makers as well as SF 6 of NGEF make. The LT supply is also controlled through air break circuit breaker which are either L&T make or English Electric Company of India. The various H.T. motors are switched on / started through on direct ON line (DOL) in order to inverse the availability of equipment at full efficiency without time gap.

Further , the 6.6 KV system which is normally in delta configuration and terms as an unearthed system so also to keep the running motor complete in operating condition in case of any one .phase of motor winding is earthed due to any one reason. Earthling is detected by an protection system with alarm facility to take remedial measures immediately and at the same time to maintain the generation level in the same condition,prior to occurring the earth fault the single phase earth fault is detected in due course till the motor is not earthed to other or another phase. Soot Blowers are there in the boiler area on the furnace side or Zone which helps in blowing the soot / ash deposition regularly of the furnace wall / economizer tubes to keep heat transfer at the required parameter. In April 1973, Central Electricity Authority prepared a Project Report for power station comprising of the two units of each of capacity 110 MW for RSEB subsequently in September, 1975 this was revised by the Consultant Thermal Design Organization , Central Electricity Authority for invention of 2x110 MW units being manufactured by BHEL, Hyderabad in 1st Stage.

The planning commission cleared the project report in Sept., 1976 for installation of two units each of 110 MW in first estimated cost of Rs. 143 ChoresTable 1.2: total installed capacity:

NAMENOOF UNITSIndividual capacityTotal capacity

Station A460 MW240 MW

Station B2120 MW240 MW

Station C2120 MW240 MW

Stage V2250 MW500 MW

Stage VI1500 MW500 MW

TOTAL1720 MW

1.3 General Layout & Basic Idea: A control system of station basically works on Rankin Cycle. Steam is produced in Boiler is exported in prime mover and is condensed in condenser to be fed into the boiler again. In practice of good number of modifications are affected so as to have heat economy and to increase the thermal efficiency of plant.

The Kothagudem Thermal Power Station is divided into four main circuits:

Fuel and Ash Circuit.

Air and Gas Circuit.

Feed water and Steam Circuit.

Cooling Water Circuit.

Objectives

One of the important objectives of K.T.P.S. is to generate thermal power efficiently and economically. It is also fulfilling the role of social responsibility objective by encouraging local small-scale industries, providing employment to the people of the backward and tribal areas. It has crores of rupees controlling pollution by installing Electrostatic Precipitators (ESP).LOCATION:

The Kota Thermal Power Station is ideally on the left bank of Chambal River at Up Stream of Kota Barrage. The large expanse of water reached by the barrage provides an efficient direct circulation of cooling system for the power station. The 220 KV GSS is within km \ from the power station.LAND: Land measuring approx. 250 hectares was required for the project in 1976, For disposal of ash tank very near to power station is acquired which the ash in slurry form is disposed off through ash and slurry disposal plants.

1.4 BASIC IDEA OF OPERATION: The basic principle involved is Faradays law of electro-magnetic induction i.e. whenever a conductor cuts a magnetic flux emf is induced across its ends. The very first thing we need to provide is a conductor cutting magnetic flux. So this can be done in two basic ways i.e. either the conductor can be moved in the magnetic field or the field can be varied according to the required emf that is to be generated. The process we follow here is we rotate the rotor of a generator in the magnetic field and emf is generator at the stator and this generated emf is further utilized according to the purpose. To meet the purpose of rotating the rotor of a generator, the rotating shaft is in turn connected to a turbine which is made to rotate at a rated speed by an external energy source. So we need an energy source to rotate the turbine. To rotate the turbine energy must be transferred from a medium to the turbine so that energy from the external source is converted to rotational energy of turbine. In general to rotate an object which is mounted we need to apply some torque. To produce torque we need to apply force in the tangential direction. For the purpose of application of force we chose steam as a medium of transfer. For hydel plants water is directly allowed from a very great height to collide with the turbine blades with a great force.

In the same way we need to send the steam with a greater force in turn with a greater pressure to make the turbine rotate. The basic physics involved in this is the internal energy and enthalpy of the steam gets converted to mechanical energy that rotates the turbine.

Our target is to produce steam at a very high pressure. Pressure of the steam can be increased by various auxiliaries through different mechanisms. So basically we need to produce steam. For the production of steam water is to be heated to high temperatures with the help of available fuel. Combustion of fuel is done and evolved heat is utilized for production of steam.

Total idea is to be implemented in a highly efficient way to balance the finance and economy. Environmental protection should also be the point of concern because burning of fuel may evolve gases which are responsible for harmful effects that distract our ambience.

1.5 BASIC REQUIREMENTS: Fuel Water Heating system

Steam circuit Regenerating system Steam turbine Generator Transformer

COAL: Coal India limited owns and operates all the major coal fields in India through its coal producing subsidiary companies viz. Eastern Coal Fields Limited, Western Coal Fields Limited/Coal India limited is supply coal from its coal mines of coal producing subsidiaries BCCL, SECL & ECL to Kota Thermal Power Station through railway wagons. The average distances of SECL, ECL & BCCL are 800, 950 and 1350 Kms. respectively.WATER: The source of water for power station is reservoir formed by Kota Barrage on the Chambal River. In case of large capacity plants huge quantities of coal and water is required. The cost of transporting coal and water is particularly high. Therefore, as far as possible, the plant must be located near the pit rather than at load centre for load above 200 MW and 375 MW. The transportation of electrical energy is more economical as compared to the transportation of coal. The design of steam power station requires wide experience as the subsequent operation and maintenance are greatly affected by its design. The most efficient design consist of properly sized component designed to operate safely and conveniently along with its auxiliaries and installation.HEATING SYSTEM: Aheating systemis a mechanism for maintaining temperatures at an acceptable level; by using thermal energy within a power plant.REGENERATIVE SYSTEM: It is a designed loop for effective utilization of energy to increase the efficiency of the process. In general furnace is meant to produce the steam from water. But total energy evolved from combustion of coal is excessive for this. So this heat energy from the furnace is repeatedly utilized wherever necessary through

SUPER HEATER COILS REHEATER COILS ECONOMISER STEAM TURBINE: To generate EMF the rotor of the generator need to be rotated which in turn is operated by a shaft which is rotated with the help of three turbines.

HIGH PREESSURE TURBINE (HP TURBINE)

INTERMEDIATE /MEDIUM PRESSURE TURBINE (IP TURBINE)

LOW PRESSURE TURBINE

Each turbine has its own operating temperature and pressureGENERATOR: It converts mechanical energy to electrical energy. As we have discussed the 3 turbines rotate a single shaft at a rated speed of 3000 RPM. This shaft is in turn connected to a TURBO GENERATOR which can generate an EMF of 11 KV. TRANSFORMER: Transformers are static devices used for transferring power from one circuit to another without change in frequency.

Different types of transformers are used for stepping up and stepping down the generated voltage either for supplying to the grid or for self utilization. 1. GENERATOR TRANSFORMER 2. UNIT AUXILIARY TRANSFORMER 3. STATION TRANSFORMER

CHAPTER.II

Transformers

2.1 Introduction:

A transformer is a device with two or more stationary electrical circuits that are conductively disjointed magnetically coupled by a common time-varying magnetic field. Transformers are static devices used for transferring power from one circuit to another without change in frequency. Transformers are basically passive devices for transforming voltage and current. It can raise or lower the voltage corresponding decrease or increase in current. One of the windings, generally termed as secondary windings, transforms energy through the principle of mutual inductance and delivers power to the load. The voltage levels at the primary and the secondary windings are usually different and any increase or decrease of the secondary voltage is accompanied by corresponding increase or decrease in current.

Transformers are among the most efficient machines 95% efficiency being common in lower capacity ranges, while an efficiency of 95% is achievable in high capacity ranges. Theoretically, there is no upper limit to the power handling capacity; transport constraints, handling facilities, etc. being the limiting factors. The lower limit is governed by the allowable no-load losses.

The physical basis of a transformer is mutual induction between two circuits linked by a common magnetic field. The primary circuit carrying a current has associated with it, as a manifestation of electrical phenomenon of current flow, a magnetic field in its immediate vicinity. When the circuit is alternating, the magnetic field at any point in the surrounding medium will vary in both magnitude and direction I accordance with the change of current with time. The secondary being in the vicinity the primary circuit will link with some of the primary magnetic flux produced. With an alternating primary current, and therefore flux, the changing linkages will produce in secondary winding an emf. The more closely the primary and the secondary circuits are mutually linked, the more direct becomes the change of energy between them. Major electrical parameters of a transformer are iron and copper losses, hysteresis losses, efficiency, regulation. Essentially the chief elements of construction of a transformer comprise materials for magnetic circuit, terminals, tapping switches, oil as well as cooling devices.

2.2 Working principle of a transformer:

The physical basis of a transformer is mutual inductance between two circuits linked by a common magnetic flux through a path of low reluctance. The two coils named primary and secondary posses high mutual inductance. If one coil connected to a source of alternating voltage, alternating flux is set up in laminated core, most of which is linked up with the other coil in which it produces mutually induced emf according to the faradays law of electromagnetic induction, ie E=M*(di/dt)

Where E=induced emf, M=mutual inductance.EMF induced in a transformer is given by the equationE=4.44 m f N volts, m=BmA

If second circuit is closed, a current flow in it and so electrical energy is transferred entirely magnetically from the first coil to the second coil.

2.3 Different Parts of transformer:Core:

The core forms the magnetic circuits of a transformers. There are two 1) core type and 2) shell type. In core type transformer, the windings surround a considerable part of core where as in shell type transformers; the core surrounds a considerable part of windings. In both core and shell type transformers, the individual laminations are cut in the form of long strips of Ls, Es, Is and the laminations are butted against each other.

The material used are COLD ROLLED GRAIN ORIENTED ELECTRICAL STEEL SHEETS (CRGO). CRGO made from ferrous base present maximum magnetisability i.e. permitting high induction. Iron crystallizes into body centre cubic lattice with the cube edges of lattice pointing in the direction of easiest magnetizability and lowest core loss. Grain oriented electrical sheets consists of silicon-iron alloy, with the crystallites being predominately oriented by the means of a specific manufacturing process, in such a way as to have four cube edges pointing the rolling direction and diagonal plain being parallel to the sheets surface. In this way the rolling direction becomes the direction of maximum magnetic properties direction and approaching the ideal properties of the individual crystallite. The CRGO has the following properties such as maximum magnetic properties, minimum specific core loss, low apparent power input, low magnetostriction, high grade surface insulation, good mechanical processing properties.

Insulating oil:

Insulating oil forms a very significant parts of a transformer insulation system and has the important function of acting as an electrical insulation as well as coolant to the dissipate heat losses. This basic raw material for the production of transformer oil is a low viscosity lube termed as TRANSFORMER OIL BASE STOCK (TOBS) which is normally obtained by fractional distillation and subsequent treatment of crude petroleum. TOBS is further refined by acid treatment process to yield transformer oil.

Chemical Properties:

Transformer oil consist of four major generic classes of organic compounds, namely paraffins, naphthenes, aromatics and olefins. All these are hydrocarbons and hence insulating oil is called a pure hydrocarbon mineral oil. For good fresh insulating oil, it is desirable to have more of saturated paraffin, less of aromatic or naphthenes and none of olefins. However, for better stability of properties, it is necessary to have optimum aromatic or naphthenes hydrocarbons. Such as optimum balance is carefully struck by a carefully controlled refining process. Depending upon the predominance, oil is usually termed as of paraffinic base or naphthenic base.

Electrical properties: Electric strength (Breakdown voltage): BDV is the voltage at which breakdown occurs between two electrodes when oil is subjected to an electric field under prescribed conditions. Electric strength is the basic parameter for insulation system design of a transformer. It serves to indicate the presence of contaminating agents like moisture, fibrous material, carbon particles, perceptible sludge and sediment.

Resistivity (specific resistance): This is the most sensitive property of oil requiring utmost care for its proper determination. Resistivity in ohm-cm is numerically equivalent to the resistance between opposite phases of a centimeter cube of liquid. Insulation resistance of windings of a transformer is also dependent upon the resistivity of oil. A low value indicates the presence of moisture and conductive contaminants.

Dielectric dissipation factor (DDF): DDF is numerically equal to sign of the loss angle (approx. equal to tangent of loss angle for dielectrics) and is a good tool to indicate the quality of insulation. A high value of DDF is an indication of the presence of contaminations or deterioration products such as water, oxidation produced.

Table 2.1: Characteristics of oil:

Characteristic Requirement as per standards

Density at 27c, max0.89g/cu.cm

Kinematics viscosity at 27c, max27cSt

Interfacial tension at 27c, mini0.04N/m

Flash point, mini140c

Pour point, max-9c

Neutralization value, max0.03mg KOH/g

Corrosive sulphurNon-corrosive

Electric strength as received30KV(rms)

Electric strength after filtration50KV(rms)

Dielectric dissipation factor(tan delta) at 90c, max0.005

Specific resistance at 90c30*10^12ohm cm

Specific resistance at 27c500*10^ohm cm

Oxidation stability neutralization value after oxidation, max0.4mg KOH/g

Total sludge after oxidation0.1%by weight

Presence of oxidation inhibitor The oil shall not contain antioxidant additives

Water content as received, max50 ppm by weight

Winding:

The transformer consists of two coils called WINDINGS which are wrapped around a core. The transformer operates when a source of ac voltage is connected to one of the windings and a load device is connected to the other. The winding that is connected to the source is called the PRIMARY WINDING. The winding that is connected to the load is called the SECONDARY WINDING.

High Voltage Coil:

It has more number of turns.

Thickness of the wire is less.

Tapings are taken from this coil because of low currents.

Low Voltage Coil: It has less number of turns.

Thickness of the wire is high.

Tapings are not taken from this coil because of high currents.

Tap-Changing: There is considerable voltage drop between generating sources and consumers in modern electricity supply. So transformers provided with number of taps at the ends of the low-current winding i.e. H.V side so that the voltage ratio can be adjusted to suit loading conditions. There are two types of tap-changers, they are-

Off-load Tap- changer

On-load Tap-changer

Off-load Tap-changer: The transformer is normally fitted with a off-load tap changing to obtain required tap voltage. It can be hand operated by a switch handle mounted in tank. Locking device is fitted to handle to padlock it on any tap position and also to prevent any unauthorized operation of switch. The switch mechanism is such that it can be locked only when it is bridging two contacts on any particular tapping position and cannot be locked in any intermediate position.

It is important that the transformer should be isolated from the live line before moving the switch. Operating the switch when the transformer is energized, will damage the switch contacts due to serve arching between the contacts, and may damage the winding also.

On-load Tap-changer: On-load tap changers are employed to change turn ratio of transformer to regulate system voltage while the transformer is delivering normal load. With the introduction of on-load tap changer , the operating efficiency of electrical system has considerably improved. Now-a-days, almost all large transformers are fitted with on-load tap changer. All forms of on-load tap changing circuit posses impedance, which is introduced to prevent short circuiting of tapping section during, tap changer operation. The impedance can be either a resistor or a center tapped reactor. The on-load tap changer can be classified as two types i.e. resistor type and reactor type.

Conservator: Conservator is a sort of a drum, mounted on the top of the transformer. A level indicator is fixed to it.

Conservator is connected through a pipe to the transformer tank containing oil. This oil expands and contracts depending upon the heat produced and so the oil level in the conservator rises and falls. Pipe connected to the conservator is left open to the atmosphere through a breather so that the extra air may go out or come in.

Breather:

The breather is a box containing calcium chloride or silica gell to absorb moisture of air entering the conservator as it is a will known fact that the insulating property of the transformer oil is lost if a small amount of moisture enters in it, so dry air is allowed to pass in through this breather.

Breather should be inspected frequently especially in a situation where temperature and humidity changes are considerable and when transformer is subjected to fluctuating loads. So long as silica gel is in active stage, it color is dark blue but as it becomes saturated moisture, its color changes to pale blue/pink when it should be reactivated.

valves: Transformer is equipped with drain cum filter valve at the bottom of the tank and filter valve at the top of the tank. Valves are fitted with plug/blanking plates to stop the dirt or moisture entering inside the valve and avoid the contamination of the transformer oil.

Types of valves:

Plug type

- Up to 500 KVA units Wheel valve with female screw threads - 501 to 2000 KVA units Wheel valve with flanges

- Above 2001 KVA units

Buchholz relay: This relay gas actuated relay which is meant for the protection of oil immersed transformer from insulation failure, core heating or any type of internal fault which may cause the heating of coil beyond the specified temperature. Due to any internal fault, oil is heated up and oil vapour so formed causes either the alarm circuit(for less fault) or the trip circuit (for severe faults). The relay is situated in the pipe connected between the transformer and the conservator. Explosion Vent: It is also a safety device of a transformer which protects the transformer tank from the gases induced by any type of short circuit in the transformer.

Temperature gauge: It is also a protection device fitted to a transformer to indicate the temperature of transformer and operates the alarm, trip and cooler control contacts. The instrument is provided with a maximum temperature indicator .

Radiators: Radiators are commonly used for cooling. ONAN, ONAF and OFAF are the types of cooling. Radiators consist of elements joined to top and bottom headers.

Elements are made by welding two previously rolled and pressed thin steel sheets to from a number of channels or flutes through which oil flows. These radiators can be either mounted directly on the transformer tank or in the form of a bank or connected to tank through pipes. The surface area available for dissipation heat is multiplied manifolds by using various elements in parallel. As oil passes downwards, either due to natural circulation or force of a pump in the cooling circuit, heat is carried away by the surrounding atmosphere air

There are two types of radiators named tank mounted radiators and bank mounted radiators.

The total number of radiators required for the cooling of transformer can be arranged in many ways. Usually one bank of radiators for 100% capacity is sufficient. However, if desired 2-50% banks can be formed by dividing number of radiators into two equal parts. But each part is completed with separate auxiliary cooling equipment like fans and a pump. So if we divide the bank into number of parts then the auxiliary parts increase and ultimate in increase of cost.

Bushings:

Porcelain insulators and connectors should be cleaned at convenient intervals and minutely examined for the cracks or other defects. Small or narrow cracks are difficult to detect. However, they are likely to develop rapidly. All such bushings should be replaced. Similarly oil communicating type bushings top. The cause of any serious loss of oil should be investigated. In case of any sign of oil leakage in the bushings, the matter should be referred to the corresponding company.

2.4 TYPES OF TRANSFORMER:

Depending upon the type of construction used, the transformers are classified into two categories, they are- Core type transformer

Shell type transformer

Core type transformer:

It has one magnetic path only.

Average length of core is more.

Here area of cross-section is less, so more turns are required.

On every leg both primary and secondary windings are there so leakage flux is less.

It is used for high voltage and less output.

Most portion of winding is visible so it is easy to insulate and repair.

Shell type transformer:

In this type core encloses the winding, so the cooling is good for core.

It has two magnetic paths.

Average length of core is less.

Area cross-section is more. So less turns are required.

It is used for low voltage and high output.

To reduce losses here the winding is done in PANCAKES-primary, secondary windings are on the central limb.Depending upon the cooling medium. The transformers are classified into two types. They are- Dry type transformers

Oil filled transformers

Dry type transformer: These are further classified as air cooled transformers and air blast transformers. The air cool transformers are of very small output (say 5or 10 KVA) and cooled by circulation of air. Air blast transformers are cooled by a forced circulation of air through core and windings. Such transformers are limited to voltages not exceeding 25 KV. These are used in substations located in thickly populated areas where oil is considered a fire hazard.Oil filled transformer: in general transformers are of oil-immersed type. The oil used for this purpose is mineral one which provides better insulation in addition to cooling. These transformers are further classified as oil immersed natural cooled, oil immersed forced air cooled, oil immersed water cooled, oil immersed forced oil cooled transformers.

Depending upon the power rating, the transformers are classified into two categories, they are-

Distribution Transformers

Power Transformers

Distribution Transformers: Transformers of rating up to 200KVA , used to step down the distribution voltage to a standard service voltage are known as distribution transformers. They are kept in operation all the 24 hours a day whether they are carrying any load or not. They are of the self cooling type and are almost invariably oil-immersed. Energy is lost in iron losses throughout the day while the copper losses account for ioss in energy when the transformer is loaded. Therefore distribution transformers should have their iron losses small as compared to full load(about 50%). Owing to low iron loss, the distribution transformers have good all-day efficiency. These transformers should have a good voltage regulation.

Power transformers: The transformers of rating above 200KVA used in generating station and substations at each end of a power transmission line for stepping up or stepping down the voltage are known as power transformers. They are put in operation during load hours and disconnected during light load hours. Therefore, power transformers should be designed to have maximum efficiency at or near full load. Power transformers are designed to have considerably greater leakage reactance than is permissible in distribution transformers because in the case of power transformers, voltage regulation is less important than the current limiting effect of higher leakage reactance. They may be self oil cooled, forced air cooled or forced water cooled.

2.5 Faults and failures: Although failure in transformers is rare, faults do occur and the reasons may be broadly classified as:

Failure in magnetic circuit

Failure in electric circuit

Failure in dielectric circuit

Failure in structural and mechanical fittings.

Failure in magnetic circuit:

Common causes for this failure are-

Breakdown of insulation of core bolt.

High flux density in the core resulting in large magnetizing current increase during switching.

Failure in electric circuit:

Common causes of this failure are-

Presence of sharp edges on conductors.

Drying of a transformer is not carried out properly

If moisture penetrates into the winding insulation

Incorrect pressure on windings resulting in dislodging of turns

Switching, lightening surges producing high voltage at the end turns

Loose connections, bolted joints

Sustained overloads resulting in over heatingFailure in dielectric circuit:

Common causes of this failure are-

Moisture entering oil due to breathing

Narrow oil ducts in winding causing insufficient cooling

Sustained overloading resulting in deterioration due to excessive oil temperature

Failure in structural and mechanical fittings:

Common causes of this failure are-

Inadequate clamping of leads from windings to terminal boards resulting in short circuit.

Poor welding, leaky fittings cause leakage of oil resulting in overheating. Improper ventilation causes overheating of oil.

Dos for power transformers:1. Connect gas cylinder with automatic regulator if transformer is to be stored for long duration, in order to maintain positive pressure.2. Fill the oil in the transformer at the earliest opportunity at site and follow storage instructions. It must be commissioned as soon as possible.

3. Open the equalizing valve between tank and OLTC diverter compartment, whenever provided, at the time of filling the oil in the tank and close the same during operation.

4. Clean the oil conservator thoroughly before erecting.5. Check the pointers of all gauges for their free movement before erection.

6. Inspect the painting and if necessary do retouching.

7. If inspection covers are opened or any gasket joint is tightened, tighten the bolts evenly with the proper sequence to avoid uneven pressure.

8. Clean the buchholz relay and check the operation of alarm and trip contacts.

9. Check the oil level in oil cup and ensure that the air passages are free in the breather. If oil is less, make up the oil level.

10. Check the oil in the transformer and OLTC for the dielectric and moisture content, and take suitable action for restoring the quality of oil.

11. Attend to leakages on the bushing immediately.12. Check the diaphragm of the relief vent. If cracked or broken, replace it.

13. Remove the air from vent plug of the diverter switch before energizing the transformer.

14. Check the gear box oil level in the tap changer. If less, top up with specified oil.

15. Check the OTI and WTI pockets and replenish the oil, if required.

16. Examine the diverter and selector contacts of tap changer and if found burnt or worn out, replace the same.

17. Check and thoroughly investigate the transformer whenever any alarm or protection is operated.

18. Examine the bushing for dirt deposits and coats and clean them periodically.

19. Check all bearings and operating mechanism of the tap changer and lubricate as per schedule.

20. Keep the wall connected between the conservator of tap changer and its diverter compartment open, during transformer operation.

Donts for transformer:

1. Do not allow WTI, OTI temperature to exceed 75c during dry out of transformer, and filter machine temperature beyond 85c.2. Do not re-energize the transformer, unless the buchholz gas is analyzed.

3. Do not re-energize the transformer without pre-commissioning checks.

4. Do not energize the transformer, unless the off-circuit tap switch handle is in locked position.

5. Do not leave off-circuit tap switch handle unlocked.

6. Do not leave tertiary terminals unprotected outside the tank.

7. Do not leave any connection loose.

8. Do not meddle with the protection circuit.

9. Do not leave maximum temperature indicating pointer behind the other pointer in OTI and WTI.

10. Do not change the setting of WTI and OTI alarm and trip frequently.

11. Do not allow oil level in bushings to fall, they must immediately top up.

12. Do not allow conservator oil level to fall below one-fourth level.

13. Do not leave secondary terminals on an unloaded CT open.

2.6 Transformer losses: In transformer as there is no rotating part, there is no friction and wind age losses. Hence, the only losses occurring are- Core losses: Core losses are caused by alternating flux in the core. These are also called as iron losses. The iron losses are constant on every load. These losses can be found out by open circuit test on the transformer. It consist of

Hysteresis losses Eddy current losses

Hysteresis losses: Each time the magnetic field is reversed, a small amount of energy is lost due the hysteresiswithin the core. For a given core material, the loss is proportional to the frequency, and is a function of the peak flux density to which it is subjected.

Eddy current losses: due to variation in magnetic flux eddy currents are induced on the surface of iron core which in turn produce heating and therefore reduce the amount of power to the secondary coil. In order to avoid eddy currents, the core is laminated, made of thin sheets of soft iron. Each sheet is separated from the next by a layer of insulating varnish.Copper losses: These losses are due to resistance of winding and are proportional to (current)^2 or (KVA)^2. These can be obtained experimentally by means of short circuit test.

Copper Loss =I2RWhere I is the current flowing in the conductor and R the resistance of the conductor. With I inampereand R in ohms, the calculated power loss is given inwatts.With high-frequency currents, winding loss is affected byproximity effectand skin effect, and cannot be calculated as simply. For low-frequency applications, the power lost can be minimized by employing conductors with a large cross-sectional area, made from low-resistivitymetals.

CHAPTER.III

Condition monitoring3.1 Introduction:

Reliable and quality power is need of the hour for the economic development of a country. For providing reliable electrical energy, it is very necessary to have highly reliable associated electrical equipment. The transformer, being a key element in the transmission and distribution of electrical energy, improving its reliability is of utmost importance. System abnormalities, loading, switching and ambient condition normally contributes towards accelerated aging and sudden failure, hence, it is, all the more essential, to employ continuous monitoring techniques and on-site diagnostics followed by quality maintenance for having trouble-free and reliable operation with minimum outage.

The article, being submitted, shall present a survey of new monitoring and diagnostic technologies in power transformer for the purpose of condition assessment. Also, life assessment and extension program for transformer in service will be highlighted. Case studies citing site experiences of problem faced on transformer in service and various diagnostic tools employed for finding solutions will be cited.Insulation monitoring: Insulation is the major component, which plays an important role in the life expectancy of the transformer. Transformer life known to us is based on the designed parameter with respect to normal operation and climate conditions. To determine the performance and aging of the asset, insulation behavior is a main indicator [1]. Most of the transformers in a system, around the world are exceeding their designed life. In the absence of insulation assessment, good number of transformer failed due to insulation problems, before reaching to their designed technical life.

It is important to investigate the cause(s) of the insulation degradation with respect to age. Average age of the transformers that failed due to insulation deterioration during the last ten years was 17.8 years [2]. A good number of aged transformers are still performing well, it is vital to monitor the insulation behavior rather than replacing with new one. Transformer insulation behavior is different with respect to operation mode, climate (ambient condition) and frequency of subjected faults. Load growth has influence on the insulation degradation. The insulation degradation trend needs regular assessment.

An accurate analysis of the insulation can suggest operating condition, de-rating of the transformer will increase the life expectancy [3, 4]. The unit can be proposed for relocation, subjected to less stress. Cost effective maintenance strategies can be developed. Insulation aging in transformer is a complex and irreversible phenomena. To ensure higher reliability and safety, insulation condition monitoring and trend analysis are of major concern. Insulation trend analysis will conclude type of failure as well as severity of the fault. This will make easy to understand type of maintenance required, loading constraints and future management required. The analysis will predict the life expectancy of the asset. It is significant to recommend insulation assessment for the aged and suspicious behavior transformers. The overall integrity of the asset can be assessed, with minimum risk of sudden failure. The environmental risk can be reduced. Effect of aging rate on the life expectancy can be established. Condition monitoring provides information on the developing insulation problems and incipient faults [5, 6]. Thus early warning of any abnormality can avert the catastrophic failure.Purpose of condition monitoring: To avoid forced outages

To minimize failures and optimize the maintenance cost

Monitoring scheme must be:

Simple

Low cost

Without disruption of power

Data must be focused on results to:

Prevent problems

Define the severity of a problem

Provide information to take action

Provide on-line and off-line diagnostics

Enable trending of data

Avoid intrusive maintenance3.2 TRANSFORMER ASSESSMENT: Since 1885 transformers (0.15 MVA) are serving the power industry and are being produced with higher rating (> 2000MVA). Majority of transformer population is serving in many of the transmission and distribution utilities are 20 to 40 years old. As an example the installed power transformer (United States) capacity has reduced from 185 GVA (Giga Volt Amperes) to 50 GVA per year over the past twenty-five years [7].The average load growth rate observed is approximately 2% [7]. Transformer utilization has increased by 22% on average, causing oil hot spot temperature to increase by approximately a 48%, at normal peak load [7, 8]. Due to gradual increase in the temperature, peak load insulation life will be reduced by a factor of approximately 8 [7, 9]. Economic pressures and factors such as an increasing proportion of aged power transformers are combining to dictate more efficient plant maintenance management.

Life assessment is becoming increasingly important as the average age of the asset increases, due to economic pressures and a relatively low load growth, with fewer major re-development projects.

A scientific remnant life assessment would be an important tool towards higher reliability of the system and asset management. After determining the critical indicator responsible for aging as well as asset technical assessment, the rate of ageing can be reduced by implementing the correct operational and maintenance strategies. The early and failures due to aging can be effectively minimized. Better asset management system can be implemented (timely relocation / replacement can be planned).

The transformer's condition assessment can be broken down into the following areas of concern:

Operating performance to design criteria.

Aging of insulating materials due to stress imposed both thermal and electrical.

Chemical deterioration from moisture, oxygen and acidity and other contaminations.

Mechanical strength of the solid insulating and bracing materials.3.3 Critical Components: Core, windings, insulation oil, bushing and on-load tap-changer are the main active parts of the transformer insulation chain . The degradation of insulation systems is accompanied by phenomenon of changing physical parameters or the behavior of insulation systems. The degradation of insulation systems is a complex physical process. Many parameters act at the same time thus making the interpretation extremely difficult. The aging process in the oil/cellulose insulation system under thermal stress and their measurable effects are due to chemical reactions in the dielectric. The temperature of the oil/paper dielectric is the critical aging parameter to cause enough change in the mechanical and electrical properties of the material. Apart from high temperatures, other important parameters affecting the aging of the solid and liquid insulation include the presence of water and oxygen in the system . The monitoring and assessment of such components is vital to achieve better reliability of the system. By implementing correct operational and maintenance strategies the insulation aging/ degradation process can be controlled and the asset life can be extended effectively. Assets critical component monitoring (strict) is required for the technical assessment (normal to end of life) to ensure economical and safe operation. Also better asset management policies can be implemented .3.4 Types of Major Failures: Following are the major components, which have a direct bearing on reliability of the transformers

Winding and electrical circuit,

Core and clamping structure,

Bushings and external connections,

Tap chargers,

Coolers and cooling medium,

Control and supervisory equipment The types of failure which occur on transformer are many, one with serious concerns to condition monitoring techniques are listed below -

Core:Breakdown in core bolt insulation, core plate insulation or insulation between core and core clamps leads to circulating currents and usually sparking at the fault. Gases are evolved, which dissolve in oil. These can be monitored by Dissolved Gas Analysis (DGA). Windings and Inter-winding Insulation: Overheating due to poor joints is a common fault in any part of the electrical circuit. Breakdown of inter-strand insulation results in circulating current causing overheating of insulation and hot spots at point of fault. This can be a result of winding movement. A turn-to-turn fault produces a similar effect but with much more energy and can usually be detected and identified. Partial discharge faults can develop between various parts of the insulation structure as a result of contamination (including moisture) or due to poor impregnation or overstressing. Over heating of stress shields results in breakdown and circulating current. A fault between windings and a fault from line-to-ground usually results in serious damage.

Tanks, Flux shields and Fittings: The breakdown of insulation between portions of the tank shields or between the shields and tank can lead to circulating current, which is a function of load current. Circulating current in the tank due to proximity of heavy current conductors can produce hot spots in the tank and across gasket joints.

Bushings:

Ingress of moisture, loosed/bad joints may lead to failure of bushings.

Deterioration and failure factors: The factors responsible for failures and accelerated deterioration are categorized as:

Operating Environment (electrical)

Transient over-voltages, load current, short circuit (fault currents), lightening and switching surges.

Operating Environment (physical)

Temperature (operating full load with high ambient temperature-humidity index), wind, rain, seismic and pollution Operating Time Time in service and time under abnormal conditions or extreme condition (load variation, change in thermal stresses).

Number of Operations of Tap-changer Number of on-load tap-changer operation.

Vibration Effect Sound and material fatigue.

Contaminants Moisture (water content in oil), presence of oxygen and particles in oil.

CHAPTER.IVTRANSFORMER TESTS4.1 INTRODUCTION:

A Transformer is very vital equipment in a power system & its availability, reliability is very important. The transformer primarily comprises of core, winding, and insulation. The insulation comprises of solid, liquid and combination of oil and cellulose paper. The condition monitoring of oil is performed by prescribed tests as below-Things to be monitored:

Winding resistance measurements Capacitance and tan for winding

Insulation resistance(IR) and Polarization index(PI) measurement

Oil parameters Furan Analysis Degree of polymerization(DP) Partial Discharge(PD) Measurements Frequency Response Analysis(FRA) Recovery voltage measurement(RVM)

Surge comparison test

Dc step voltage measurement Capacitance and tan for bushings Dissolved Gas Analysis(DGA)

4.2 Winding Resistance Measurements:

This is nothing but the resistance measurement of the windings by applying a small

d.c. voltage to the winding and measuring the current through the same. The ratio gives the winding resistance, more commonly feasible with high voltage windings. For low voltage windings a resistance-bridge method can be used. From the d.c resistance one can get the a.c resistance by applying skin effect corrections. Winding resistance is measured by using MEGGER.

Megger used: To measure resistance of windings. To compare with factory results.

As part of a regular maintenance program. To help locate the presence of defects in transformers, such as loose connections. To check the make-before-break operation of on-load tap-changers. PURPOSE OF TESTING:Winding resistance measurements in transformers are of fundamental importance for the following purposes:

Calculations of the I2R component of conductor losses.

Calculation of winding temperature at the end of a temperature test cycle.

As a base for assessing possible damage in the field.

Increase in resistance indicates:

Loose joints- leads to local hot spots and eventual melting of joints. Worn out contacts- leads to contact erosion.4.3 Capacitance and tan for winding: This test measures and records the capacitance between the high and low voltage windings, between the high voltage winding and the tank(ground), and between the low voltage winding and the tank (ground).Changes in these values as the transformer ages and events occur, such as nearby lightning strikes or through faults, indicate winding deformation and structural problems such as displaced wedging and winding support.

Similarly in a Dielectric material when a cavity or deterioration starts, the life of thematerial starts deteriorating, as there is a resistance getting added and henceleakage current increases In tan Delta we find the difference in the angle andperiodically note down the pace at which deterioration takes place. Measure capacitance and tan of each pair of windings and windings with respect to earth. Compare with factory results It indicates healthiness of insulation system-paper, press-board and oil.

Increase in tan indicates :

Deterioration of insulation system. Contamination. Moisture absorption.4.4 Insulation resistance(IR) and Polarization index:Insulation failure can cause electrical shocks, creating a real hazard to personnel and machinery. A regular program of testing insulation resistance is strongly recommended to prevent this danger, as well as to allow timely maintenance and repair work to take place before catastrophic failure. All new equipment, motors, transformers, switch gears, and wiring should be tested before being put into service. This test record will be useful for future comparisons in regular maintenance testing. Some of the more common causes of insulation failure include excessive heat or cold, moisture, aging, corrosive atmospheres and vibration. Insulation values are in ohms, and insulation values change according to temperature. Take all of your readings at 20 C. A general rule is to take 1/2 the resistance reading for every 10 deg C increase, and double the resistance for every 10 deg C decrease. For instance, if you measure 10 mega ohms at 30 deg C, a 10 deg increase, your true reading is 5 mega ohms.

Measure the insulation resistance values of each pair and with respect to earth. Compare with factory results. To determine gradual decrease in insulation resistance. This provides a means for predicting future insulation failure. Lower values indicate poor insulation.

PI: It is ratio of insulation resistance (IR )for 10 minutes to insulation resistance for 1 minute. Measure PI values of each winding in pairs and with respect to earth. I t should be 1.5Table 4.1: range of polarization index PI(Ratio of 10 min to 1min)

condition

Less than 1 Dangerous

1.0-1.1 poor

1.1-1.25 Questionable

1.25-2.0 fair

CHAPTER.V

DISSOLVED GAS ANALYSIS5.1 INTRODUCTION: Transformers are vital components in both the transmission and distribution of electrical power. The early detection of incipient faults in transformers is extremely cost effective by reducing unplanned outages. The most sensitive and reliable technique used for evaluating the health of oil filled electrical equipment is dissolved gas analysis (DGA). Insulating oils under abnormal electrical or thermal stresses break down to liberate small quantities of gases. The qualitative composition of the breakdown gases is dependent upon the type of fault. By means of dissolved gas analysis (DGA), it is possible to distinguish faults such as partial discharge (corona), overheating (pyrolysis) and arcing in a great variety of oil-filled equipment. Information from the analysis of gasses dissolved in insulating oils is valuable in a preventative maintenance program. A number of

Samples must be taken over a period of time for developing trends. Data from DGA can provide

Advance warning of developing faults. A means for conveniently scheduling repairs. Monitor the rate of fault development

NOTE: A sudden large release of gas will not dissolve in the oil and this will cause the Buchholz relay to activate.

Transformer insulating oils consists of different hydrogen molecules splitting some of hydrocarbon bonds occur due to electrical and thermal faults, forming gases

Hydrogen (H2)

Methane(CH4) Ethane(C2H6)

Ethylene(C2H4)

Acetylene(C2H2)

Low energy faults(like partial discharges) sufficient to split weak H-C bonds result in hydrogen as main gas. Higher temperatures are needed for splitting of C-C bonds .Higher temperature results in

Ethane, methane and ethylene at 500C

Acetylene requires temperature 800-1200C

Carbon particles from at 500 to800C as results of arcing in oil or around very hot spots. CHAPTER.VI

TROUBLE SHOOTING CHART FOR ALL TRANSFORMERS Trouble

(1)Cause

(2)Remedy

(3)

Rise in Temperature

High TemperaturesOver voltage

Over current

High ambient temperatures

Insufficient Cooling

Lower liquid level

Sledged oil

Short-circuited core Change the circuit voltage or transformer connection to avoid over excitation.

If possible, reduce load. Heating can often be reduced by improving power factor load. Check parallel circuits for circulating currents which may be caused by improper rations or impedances. See Electrical Trouble, below.

Either improve ventilation or relocate transfer in lower ambient temperature.

If unit is artificially cooled, make sure cooling is adequate.

Fill to proper level.

Use filter press to wash off core and coils. Filter oil to remove sludge.

Test exciting current and no-load loss. If high, inspect core and repair.

See Electrical Trouble, below.

Electrical Troubles

Winding failureLightning, short-circuit.

Overload. Oil of low

Directive Foreign Material

Usually, when a transformer winding fails, the transformer is automatically disconnected from the power source by the opening of the supply breaker of

fuse.

Trouble

(1)Cause

(2)Remedy

(3)

Core failureCore insulation breakdown

(core, bolts, clamps or between laminations)

Smoke or cooling liquid may be expelled from the case, accompanied by When there is any such evidence of a winding failure, the transformer should not be re-energized at full rated voltage, because this might result in additional internal damage. Also it would introduce a fire hazard in transformers.

After disconnection from both source and load, the following observations and tests are recommended:

a) External mechanical or electrical damage to bushings, leads, patheads, disconnection switches, or other accessories.

b) Level of insulating liquid in all compartments.

c) Temperature of insulating liquid whenever it can be measured.

d) Evidence of leakage of insulating liquid or sealing compound.

High exciting current

Trouble

(1)Short-circulated core

Open core joints

Cause

(2)Test core loss. If high, it is probably due to a short-circuited core. Test core insulation. Repair if damaged. If laminations are welded together, refer matter to the company.

Core-loss test will show no appreciable increase. Pound joints together and retighten clamping structure.

Remedy

(3)

Incorrect voltage

Improper ratioChange terminal-board connection or ratio-adjuster position to give correct

voltage.

Audible internal arching

Supply voltage abnormal

Isolated metallic part

Loose connection

Low liquid level,

Exposing live parts

Change tap connections or readjust supply voltage.

The source should be immediately determined. Make certain that all normally

grounded parts are grounded, such as the clamps and core.

Same as above. Tighten all connections.

Maintain proper liquid level.

Bushing flash over Lightning

Dirty bushings

Provide adequate lightning protection.

Clean bushing porcelains, frequency depending on dirt accumulation.

Mechanical Troubles.

Leakage through screw

Joints

Trouble

(1)Foreign materials in threads. Oval nipples. Poor threads. Improper Filler. Improper assembly

Cause

(2)

Make tight screw joints or gasket joints.

Remedy

(3)

Leakage at gasketsPoor scarped joints

Insufficient or uneven

Compression improper

Preparation of gaskets and

Gasket surfaces

Make tight screw joints or gasket joints

Leakage in weldsShipping strains, imperfect

Weld

Repair leaks in weld

Pressure-relief diaphragm

Improper assembly.

Mechanical damage

Replace diaphragm. Inspect inside or pipe for evidence of rust or moisture.

Be sure to dry out transformer if there is a chance that drops of water may have settled directly on winding or other vulnerable locations, as oil test may not always reveal presence of free water.

Pressure-relief diaphragmInternal fault in conservator transformer obstructed oil flow or breathing.

In gas-seal transformer obstructed pressure relief value.

In sealed transformer liquid level too high.Check to see that valve between conservator and tank is open and that ventilator on conservator is not blocked.

Make certain that relief valve functions and that values in discharge

line are open.

Liquid level should be adjusted to that corresponding with liquid temperature to allow ample space for expansion of liquid.

Trouble

(1)Cause

(2)Remedy

(3)

Moisture condensation in open type transformer and air filled compartments

Improper or insufficient ventilators Make sure that all ventilator openings are free .

Moisture condensation in sealed transformers

Cracked diaphragm.

moisture in oil See remedies above for cracked and ruptured diaphragms.

Filter oil.

Audio noise Leaky gaskets and joints .

Accessories and external transformer parts are set giving off loud noise.

Make certain all joints are tight. Tighten loose parts. In some cases parts may be stressed into resonant state. Releasing pressure and shimming will remedy this condition.

Rusting and deterioration of paint finishAbraded surfaces and weathering.Bare metal of mechanical parts should be covered with grease.

Fractured metal are porcelain parts of bushings

Trouble

(1)Unusual strains placed on terminal connection.

Cause

(2)Cables and bus-bars attached to transformer terminals should be adequately supported. In the case of heavy leads, flexible connections should be provided to remove strain on the terminal and bushing porcelain.

Remedy

(3)

Oil troubles

Low dielectric strength Condensation in open type transformers from improper ventilation.

Broken relief diaphragm

Leaks around cover

Leaky cooling oil

Make sure that ventilating openings are unobstructed.

Replace diaphragms.

Replace gaskets if necessary.

Test cooling and repair.

Badly discolored oil.Contaminated by

Varnishes Carbonized

oil due to switching winding or core failure.

Retain oil if dielectric strength is satisfactory.

Oxidation (sludge of acidity)Exposure to air

High operating temperature wash down core and coils and tank. Filter and reclaim or replace oil.

wash down core and coils and tank. Either reduce load or improve cooling.

CHAPTER.VIICASE STUDIES

CASE STUDY 1:1. 125MVA, 11kv/ 220 kv , 50Hz, 3 generator transformer:

Sl . No.

: 6002681

Make

: Heavy Electricals (INDIA) Ltd, Bhopal

Year of manufacture

: 1973

Oil Temp.

: 28C

Winding Temp. : 28C

Ambient Temp.

: 28C

Date of test

: 11.10.2010 & 12.10.2010

ANALYSIS:

HV& LV Windings:

Results of IR,PI and Tan test obtained on the generator transformer are presented below;

Table 6.1: case study of generator transformer:

Insulation section

Insulation resistance

60 sec (G)

Polarization indexTan (%)

@10 KV

HV vs LV connected to grounded tank

1.722.240.249

HV vs LV ungrounded

1.762.830.243

LV vs HV connected to grounded tank

1.193.230.248

The IR and PI values obtained are in the normal acceptable range. The PI is regarded as index of dryness of the insulation system. For a good, healthy and dry paper-oil insulation system the PI shall be higher than 1.5. The Tan test values obtained on three insulation sections of the transformer are low and lie in the normal permissible range. These results indicated low dielectric losses in the transformer insulation system. Typical values of Tan for a new transformer are 0.5%. the maximum permissible value of Tan for an in-service and aged transformer is 2%.CASE STUDY FOR DGA:

2. 750KVA, 11KV/ 433V, 50HZ failed transformer:Commissioning & filtration date of analysis 3.11.1999

TGC

8.45

CH4

237

C2H6

54

C2H4

588

C2H2

2961

H2

1982

CO2

1548

Observation:

DGA indicates that the failure was due to severe arcing in the transformer.

CASE STUDY 3:

3. 20MVA POWER TRNASFORMER ROUTINE MAINTAINCE

TGC

6.90

CH4

53

C2H6

4

C2H4

28

C2H2

437

H2

140

CO2

1125

Observation: Rogerss ratio (0011) indicates arc with persistent sparking, IEC ratio (202) indicates continuous sparking in oil.CONCLUSION

In the conditional monitoring, we test the transformer for reliability of the equipment and also to avoid the forced outage. If faults are found to be occurring outages can be planned and the fault can be rectified before major occur. With strict monitoring, accurate diagnostics interpretations and realistic operational/ maintenance

Strategies implementation the following would be achieved effectively:

Asset economic loading conditions identification and assessment for maximum practicable operating efficiency.

Premature failures risk minimization.

Remnant life estimation and timely asset replacement/ retiring planning.

Asset life extension by implementing correct operational and cost effective maintenance strategies

Improvement in the system performance ensuring good reliability as well as plant availability.

Minimization of the long-term operational cost.

Cost saving by eliminating the unplanned maintenance.

Minimizing the outage period.

Relocation/ retirement planning.

In time procurement of spare parts to get

BIBOLOGRAPHY

Hand Book Of Transformers, BHEL, Tata McGraw Hill Basic Electrical Engineering by M.L.Anwani Electrical Technology by B.L.Theraja Electrical Machines by J.B.Guptha Electrical Machines by P.S.Bhimbra DEPARTMENT OF ELECTRICAL AND ELECTRONICS ENGINEERING ANNAMACHARYA INSTITUTE OF TECHNOLOGY & SCIENCES

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