Vocational Training Report

Report on Summer Training done atNTPC Dadri(NCPS)

Submitted By:

CERTIFICATE

This is to certify that type name has completed his summer

training at the department of Electrical maintenance

department N.T.P.C Dadri. The project entitled “Electrical

Maintenance “under the guidance and supervision of Mr.

name of the Executive Officer Electrical Main Plant gas,

NTPC Dadri.

Mr. __________________Electrical Main Plant Gas NTPC Dadri

Overview of NTPC

NTPC was set up in the central sector in the 1975 in response to widening demand

& supply gap with the main objective of planning, promoting & organizing an

integrated development to thermal power in India. Ever since its inception, NTPC

has never looked back and the corporation is treading steps of success one after the

other. The only PSU to have achieved excellent rating in respect of MOU targets

signed with Govt. of India each year. NTPC is poised to become a 40,000 MW gint

corporation by the end of XI plan i.e. 2012 AD. Lighting up one fourth of the nation,

NTPC has an installed capacity of 19,291 MW from its commitment to provide quality

power; all the operating stations of NTPC located in the National Capital Region &

western have acquired ISO 9002 certification. The service groups like Engineering,

Contracts, materials and operation Services have also bagged the ISO 9001

certification. NTPC Dadri, Ramagundam, Vindhyachal and Korba station have also

bagged ISO 14001 certification.

Today NTPC contributes more than 3 / 5th of the total power generation in India.

Station At Glance

NTPC dadri is model project of NTPC . also it tit the best project of NTPC also

known as ncps ( National capital power station ). Situated 60 kms away from Delhi

in the Distrct of gautam budh Nagar, Uttar Pradesh. The station has an installed

capacity of 1669 MW of power – 840 MW from Coal based units and 829 MW Gas

Based Station . the station is excelling in performance ever since it’s commercial

operation . consistently in receipts of meritorious projectivity awards, the coal based

units of the station stood first in the country in terms of PLF for the financial year

1999 – 2000 by generating an all time national high PLF of 96.12 % with the most

modern O & M Practices. NTPC – Dadri is committed to generated clean and green

Power. The Station also houses the first HVDC station of the country (GEP project)

in association with centre for power efficiency and Environment protection

(CENEEP) – NTPC & USAUID. The station has bagged ISO 14001 & ISO 9002

certification during the financial year 1999 – 2000, certified by Agency of

International repute M/s DNV Netherlands M/s DNV Germany respectively.

Index:

Sl. No. Description Page No.

1. Introduction 1

2. Objective 2

3. Power Transformers 3

4. Generator Transformer 4

5. Specification of Generator Transformer 5

6. Unit Auxiliary Transformer (UAT) 6

7. Station transformer 7

8. Interconnecting Transformer 8

9. Specifications of I.C.T 9

10. Parts of Transformer 10

11. Conservator 11

12. Breather 12

13. Bushing 13

14. Tap Charger 14

15. Cooling of transformer 15

16. Types Of Cooling 16

17. Fans and Pumps for Cooling 17

18. Transformer Protections 18

19. Temperature indicators 19

20. Oil Temperature indicator 20 - 21

21. Buchholz relay 22

22. Lightning Arrester 23

23. Bushing testing 24 - 25

24. Oil sampling , testing 26 - 28

25. Conclusion 29

26. Preferences 30

27. Switchgear 31 - 34

28. Generator (steam turbine) 35 – 40

29. 220 KV switchyard equipment descriptions 41

1.0 INTRODUCTION:

The testing of the generator Transformer (GT) of Unit # 2 at NCPS, Dadri was done during the annual overhauling of the Unit. The transformer maintenance group is responsible for the maintenance of the transformer and the testing group is responsible for all the testing of the transformer during overhaul. The maintenance group has taken the PTW (Permit to Work) before starting of work on transformer. After taking all the clearances, they have started isolation and earthing of the transformer.The servicing of the all fans was carried out at the site. The oil filtration of the transformer oil was done through oil filtration unit the rest values of the oil like BDV (Break Down Voltage), moisture ppm. Tan - values was under acceptable limits. The oil filtration is done through vaccum pulling and using heaters and filters. After completion of oil filtration the settling time of at least 48 hours must be given before commencement of the testing to settle down the oil. After the settling time of 24 hours transformer test started on the transformer. The capacitance & Tan delta test on bushing of HV side and transformer winding magnetized current test, winding resistance test, turns ratio test and insulation resistance test were performed on the transformer.After these testing the protection testing were performed in the protection testing the proper functioning of all protection devices was tested through simulated conditions. The protection devices consist of Pressure relief valve (PRV), Bucchulz relay, winding temperature indicator (WTI), winding temperature indicator (WTI) of LV and oil temperature indicator (OTI). The alarm and trip settings of the Buchholz relay. OTI and WTI were got checked. In this project report the transformers protective devices and its working testing types are given first in this all the type of testing are given.Then the details of the instruments used during the actual testing are given. The accuracy ranges and types of instruments used affect the value of the reading during testing. The knowledge of how to use a measuring instrument is very important for finding out the accurate results.The safety of the personal performing the tests is of utmost important. The safety measures are given next. Then the same of the limitations and the problem faced during the test at site are discussed. The test results are given in li standard test format of the testing methods used on the site and for getting the best interpretation from the test results.

2.0 OBJECTIVE

To study the power transformer Equipments & Protections.

To study the site testing of generator transformer during O/H of unit at NTPC

Dadri and to study of the testing method and maintenance practices adopted at

NTPC.

To study the safety measures and precaution taken during any testing of the

transformer.

To achieve maximum generation with minimum cost and downtime.

To study benefits of advanced maintenance practices.

METHODOLOGY: - Analysis of different type of testing technology, advance maintenance practices & healthiness checking of protective equipments for power transformer. Testing at site for generation of data, interpretation of data for condition monitoring of EHV Transformer.

3.0 POWER TRANSFORMERS

Transformer is a device or a machine with two or more stationary electrical circuits that transfer electrical energy from one electrical circuit to another electrical through the medium of magnetic field without change in frequency.

The electrical circuits which receive energy from the supply mains is primary winding and the other circuit which delivers electrical energy to the load is called the secondary winding. transformer is a electromagnetic energy conversion device. The windings are not electrically connected but they are magnetically coupled to each other by a common time varying magnetic field. the voltage level at the primary and secondary windings are usually different and any increase of, decrease of the secondary voltage is accompanied by corresponding decrease of increase in current. Transformers are among the most efficient machines, efficiency of the order of 99% is achievable in high capacity range.Power transformers are backbone of the large grid. The power is generated at low voltage level and has to be carried to far away load centers. Typically the power is generated at the pit heads i.e. power source like coal, water. It is uneconomical carry the bulk power at low voltage levels. Depending upon the requirement the voltage level is stepped up to the transmission level i.e. 220 or 400 KV. At higher voltages the transmission losses are less. Similarly at the remote end the voltage is stepped down the distribution level. To accomplish the task power transformers are installed and act as bi- directional element in the system.Transformer is a device or machine with two or more stationary electrical circuits that transformer electrical energy from one electrical circuit to another electrical circuit through the medium of magnetic field without change frequency. Transformers are among the most efficient machines, efficiency of the order of 99 % is achievable in high range. Different types of transformers are used at NTPC Dadri as per requirement some of them are given below;

3.1 Generator Transformer (G.T.)3.2 Unit Auxiliary Transformer (U.A.T.)3.3 Station Transformer (S.T.)3.4 Interconnecting Transformer (I.C.T)

3.1 Generator Transformer:

The generator is connected to this transformer by means of isolated bush ducts. This transformer is used to step up the generating voltage 16.5 KV to 220 KV. It is an important unit because the failure of any one causes the unit to trip. Only one generator transformer is installed per unit. This transformer is generally provided OFAF cooling system consists of six numbers of oil pumps & 12 fans. It is provided with off circuit taps on the high voltage side.

Specification of Generator Transformer:

Make BHELNo. One per unit Rating 270 MVAPhase 3No. Load Voltage HV 235 KVNo. load Voltage LV 16.5 KV Rated current HV 664.12 ARated current LV 9458.15 AType of Cooling OFAF

TABLE -1 Specification of Generator Transformer.

Tap Position

Lead Connection

HV line terminals IU, IV, IW LV line terminal 2U, 2V, 2W

Line Voltage(KV)

Line current ( Amps)

Voltage(KV)

current ( Amps)

1 (Max.)2344

5-65-74-74-83-8

240.875235.000229.125223.250217.375

647.93664.12981.15699.08717.97

16.5 9485.75

TABLE -2 Specification of Tap Change of Generator Transformer.

3.2 Unit auxiliary transformer (UAT) : The UAT draws its input from the main bus duct connecting generator to the Generator transformer. There are two unit auxiliary transformers for each unit. It steps down the voltage from 16.5 KV to 6.6 KV which is required for major plant auxiliaries to drive conveyors, BFP, FD fans, PA Fan, ID Fans, bowl Mills, CWP, CEP, ESP and crushers. These transformers have a capacity of 16 MVA each.

Specification:

Make NGEF Ltd. Bangalore No. 2 per unit Rating 16 MVAVoltage ratio 16.5 / 6.9 KVType of cooling ONAN / ONAF

Table-3 Specifications of Unit Auxiliary Transformer (UAT)

3.3 Station Transformer :

The station transformer is required to feed to the auxiliaries during start ups. This transformer is normally rated for the initial auxiliary load requirements of unit. There are two station transformer provided to step down the voltage form 220 KV to 6.6 KV. One station transformer for two units. These transformers have capacity of 27.5 MVA. They serve as the standby source of supply to auxiliaries.

Specification:

Make NGEF Ltd. Bangalore No. 2 per unit Rating 55 MVAVoltage ratio 220/ 6.6 KVType of cooling at different rating

ONAN ONAF OFAF60% 80% 100%

3.4 Interconnecting Transformer:

At NTPC Dadri interconnection of 220 KV switch yard to 400 KV switchyard carried out by bank of

single phase 400/220 KV Interconnecting transformer. Autotransformers are used when

transformation ratio is between 1 and 2 and above 315 MVA, due to size and weight constraints

all the transformers single phases. Three such single phase transformers three phases to make

one bank of transformer.

The three bank of transformers are installed to evacuate power from the 220 KV switchyard

generated by 4 X 210 MW thermal Units.

All these transformers are star-star connected transformers with neutral solidly grounded. A third

winding called tertiary winding at much lower voltage i.e. 33 KV, is also provide and is connected

in delta to facilitate the flow of third harmonic current to reduce distortion of in the output voltage.

To reduce overall size of the transformer, the transformer is provided with oil force and air forced

type cooling at its 100% rating. However, to save the energy, the cooling system is controlled by

the temperature of the winding.

The transformers are also equipped with On Load Tap Changer to meet the change in voltage

variation. Typically the Tap changer provides variation between 10 % of the nominal voltage. i.e.

400 KV with a variation of 0.5% at each Tap.

Specification of ICT:

Table - Specification of ICT

Make Compton GreavesNo. 3Rating 167*3=500MVATap 17No Load Voltage KV (HV side) 4003No Load Voltage KV (IV side) 2203No Load Voltage KV (LV side) 33 KVLine Current Amp. ( HV side) 289.25, 433.88, 723.13Line Current Amp. ( IV side) 525.91, 788.87, 1314.78Line Current Amp. ( LV side) 1688.48 Connection symbol YNaOd11 for three bankType of cooling ONAN / ONAF / OFAFFrequency 50 Hz.

4.0 PARTS OF TRANSFORMER:

4.1 Main Tank

Transformer winding would on the core is placed in the tank which is filled with insulating or transformer oil. A pipe that connects main tank to conservator tank is provided for oil expansion. Oil circulates through radiating tubes or fins provided in the radiator banks to dissipate heat into the atmosphere. This tank is sealed to avoid ingress of’ moisture and air.

4.2 Magnetic core And Winding:

Core of transformer is made of stampings of Cold-Rolled Grain Oriented (CRGO) silicon steel. These stampings arc insulated from each other by applying thin layer of varnish to reduce eddy current loss.Transformers for use at power or audio frequencies have cores made of many thin laminations of silicon steel. By concentrating the magnetic flux, more of it is usefully linked by both primal) andsecondary windings. Which are made of copper? Since the steel core is conductive, it too, has currents induced in it by the changing magnetic flux. Each layer is insulated from the adjacent layerto reduce the energy lost to eddy current heating of the core. The thin laminations arc used to reduce the eddy currents, and the insulation is used to keep the laminations from acting as a solid piece of steel. The thinner the laminations, the lower the eddy currents, and the lower the losses

4.3 Conservator:

The transformer is filled with oil. As the oil heated its density reduces and thus volume increases and when oil is cooled, it contracts thus creating vacuum. To take care of the oil expansion andcontraction conservator is provided and it is open to air through silica gel breather. Even though silica gel absorbs the moisture entering into the conservator. however it not completely moisture free. The oil absorbs oxygen and moisture from the air and starts degrading. To prevent the oil degradation large transformers are provided with air bag in the conservator. Air hag prevents the contact of air with the oil and provides the suitable expansion space for oil.

4.4 Breather:

It is outside the main tank and is connected with the conservator tank. During expansion of oil. the oil level inside the conservator increases and air cell contracts and thus air inside is pushed out. But hen oil cools down. air cell expands and air is sucked inside. This atmospheric air contains moisture and if it comes in contact with oil, the property of oil degrades. This is avoided by placing drying agent in the breather i.e. Silica Gel, which sucks the moisture from air. In normal conditions colour of silica gel is blue: when colour changes to Pale Pink it should be replaced

4.5 Bushing:

Porcelain bushings are provided on both sides of the tank from which end terminals of HV and LV windings are connected to external circuit. These bushings insulate the winding terminals from the body. Bushings are also filled with the transformer oil, which serves two functions i.e. cooling and insulating.

4.6 Tap changers:

Windings on both the primary and secondary of power transformers may have external connections (called taps) to intermediate points on the winding to allow adjustment of the voltageratio. Taps may be connected to an automatic/manual, on-load tap changer .or off-load tap change for voltage regulation of distribution circuits

4.0 COOLING OF TRANSFORMER:

During operation of transformer, the temperature of oil and the winding in the transformer increases w.r.t. load current. For proper operation, temperature of transformer oil and winding must be kept within limit respectively by cooling of transformer5.1 Transformer oil for cooling:

The oil helps cool the transformer. Because it also provides part of the electrical insulation between internal live parts, it must remain stable at high temperatures over an extended period. To improve cooling of large power transformers, the oil-filled tank may have radiators through which the oil circulates by natural convection. Very large or high-power transformers (with capacities of’ millions of watts) may have cooling fans. oil pumps and even oil-to-water heat exchangers. Large and high-voltage transformers undergo prolonged drying processes, using electrical sell-heating. (lie application of a vacuum, or both to ensure that the transformer is completely free of water vapor before the cooling oil is introduced. This helps prevent corona formation and subsequent electrical breakdown under load.

It is EHV grade Naphthenic based transformer oil. The various desirable properties of’ the transformer oil is as follows:

Higher Breakdown Voltage

More Resistivity

Lesser Dissipation Factor

Higher Flash Point

5.2 Types of Cooling:

During operation of transformer, the temperature of oil and the winding in the transformer increases w.r.t. load current. For proper operation. temperature of transformer oil and winding must he kept within limit i.e. below So C and 90 C respectively. In transformer there are three types of cooling systems:

1 .O.N.A.N

2. O.N.A.F

3. O.N.A.F

Oil Natural Air Natural (ONAN)

In this type of cooling system. oil and air flows naturally.

Oil Natural Air Forced (ONAF)

When load on the transformer is between 125 MVA this type of cooling is provided. As natural air is not so efficient to cool down the oil and bring it to safe limits. So air is forced on the radiating fins with help of fans installed below the radiator fins.

Oil Forced Air forced (OFAF)

With further increase in load, more heat is generated which necessitates forced cooling of oil. This is achieved by energizing the pumps placed at the bottom near the main tank. These pumps force the oil to flow frequently through the radiating fins for cooling of the transformer.

5.2 Fans and pumps For Cooling:

The fans are intended for blowing air vertically upwards on the bank of radiators to increase the rate of cooling in radiators to increase the rate of cooling in radiators.

The circulating oil pump set for transformers consists of a centrifugal pump with in-line suction and

delivery openings at 900 to each other. The pump has top delivery arid end suction. This pump is

driven by a three phase, 50 cycles, two pole, and squirrel cage induction motor suitable for

operation at 4 15 ±10%. The set can he mounted on the transformer. The oil is sucked from the

tank of the transformer and after flowing through the transformer oil cooler returns to the tank.

Thus, this is a closed cycle operation and the pump is required to just develop the pressure

necessary for overcoming the frictional head during the flow through the cooler.

6.0 TRANSFORMER PROTECTIONS:

For protection of transformer different types of protective equipments arc used some of them are given bellow;

1. Temperature Indicators

OTI High Alarm& Trip

WTI High Alarm& Trip

2. Oil Level Indicator

Low Oil Level Alarm

3. Buchholz Relay

Alarm & Trip

4. Pressure Relief Valve Alarm& Trip

5. Magnetic Oil Flow Indicator Oil Flow Low Alarm

6. Lightning Arrester

7. Mulsifire Protection

8. Differential Protection

9. Directional Over Current Protection

10.Restricted Earth Fault Protection

11.Over Fluxing Protection

6.1 Temperature Indicators

Temperatures indicators are precision instruments, specially designed for protection lion of transformers perform the following functions. Indicate maximum oil temperature and maximum or hottest spot temperature of winding. Operate an alarm or a trip circuit at a predetermined temperature, Switch on the cooling equipment when the winding attains a preset high temperature and switch it off when the temperature drops by an established differential. Two separate instruments arc used for indicating oil and winding temperatures.

6.11 Oil Temperature IndicatorThe equipment operating on the principle of liquid expansion provides local indication of the top oil

temperature. The temperature is given by a thermometer bulb with is connected by capillary tubing

to the local indicator. The pocket of the indicator is to he filled with oil. The indicator is fitted with

maximum pointes and two mercury switches. Out of which one is used for alarm and other is

for tripping switches are suitable for 5A, 250V AC or D.C. oil alarm temperature is 80°C and

tripping temperature is 90°C.

6.12 Winding Temperature Indicator:

This equipment gives continuous indication of the temperature of the windings and initiates alarm

when the temperature exceeds a certain limit and trip the transformer from the circuit.

The winding temperature transmitter is places in an oil filled pocket on the transformer tank cover.

Inside the transmitter is fitted with a heating resistance, which is fed by current proportional to the

loading current of transformer from a current transformer associated to the loaded winding of the

transformer. The temperature increase of the leading resistance is thereby proportional to the

ten1perature of the winding over the top oil temperature.

The winding temperature transmitter is located in the zone of hottest oil. Therefore, it senses both

the temperature. increase of the heating resistance & lop oil temperature. In this ay transmitter

measures the temperature in the hottest part of the winding i.e. hot spot temperature.

When temperature approaches at 55°C and give the commend to start of fans. Capillary tubing to

the local indicator connects thermometer bulb. The indicator is fitted with a maximum pointer and

tour mercury switches. Out of the four one contact is for alarm, other for tripping, third one is for

controlling fan and Fourth is spare itches are suitable for 5A. 250 V AC’ or DC. Winding alarm

temperature is 90°C and tripping temperature is 105°C.

6.2 Oil Level Indicator:

Normally all transformers are provided with an expansion vessel called conservator, to take care of

expansion in the oil volume due to rise in temperature. When the load on the transformer increases

or due to increase in ambient temperature. 1’he oil level in the conservator consequently goes up.

Conversely, it Falls when the temperature or load reduces.

It is essential that the oil level in the conservator is maintained above pre—determined minimum

level. The audible alarm operates in the event of oil level drop to near empty position in the

conservator. A bloat is used as a sensor which moves with rise and fall of the oil level. The pointer

is also magnetically operated and picks tip the correct oil level.

6.3 Buchholz relayThe relay serves as main protection for any minor or major faults that may develop inside a transformer. Such faults always result in generation of gases which causes the operation of mercury switches giving signal for audible alarm or isolates the transformer from the network.It comprises a cast housing which contains two pivoted buckets, each bucket being counter-balanced by a weight. Each assembly carries a mercury switch, the leads from which are taken to a mounded terminal block.

The relay is mounted in the pipe at an inclination of 3-7 degree. In healthy condition of the

transformer, the relay is full of oil and both mercury switches are open. In the event of a minor fault

like damage to core bolt insulation, local overheating etc., the arcing causes slow generation of

gas in the oil. this passes up in the pipe and gets trapped in the relay housing. As gas

accumulates, the oil level in the relay falls, leaving the top bucket full of oil.

When a sufficient volume of gas is collected in the relay, the top bucket, because of its extra

weight due to oil contained tilts. overcoming the balance weight which closes the mercury switch

and initiates an audible alarm.

With a major fault like short-circuit between turns, coils or between phases; the generation of

gases is rapid and the gas and the displaced oil surges through the relay and impinges on the

baffle plates, causing the lower assembly to tilt and close the mercury switch and provide signal for

tripping the circuit breaker, which disconnects the transformer from the network.

6.6 Lightning Arrester:

Generally LA’s are installed in the switchyard at the entry point of the transmission line and on

both sides of large power transformers to protect the equipments from Lightening and switching

surges. The lightening arrestors provide protection against the high voltage lightening waves

on the transmission line and from the switching surges. All the windings of the transformers are

protected with the LA’s against switching surges and lightening waves. The LA’s are installed to

protect the windings of the transformers. The present practice is to install Metal oxide i.e. ZnO type

LA’s because of the superior characteristic.

Test Report Of Bushing Tan Delta And Capacitance

Phase Voltage Before overhauling After overhauling Capacitance pf

Tan delta% Capacitance pf

Tan delta%

R 10 V Avg 389.6 .425 384.7 00. 36 1

Y 10 V Avg 379.3 .410 372.7 00.197

B 10 V Avg 379.5 .402 379.4 00.471

9.8 Transformer Earth Pit Resistance Measurement:

Purpose: - To check the healthiness of Earthing of transformer and neutral grounding

Neutral Grounding:

The process of connecting neutral point of transformer to earth either directly or through some circuit element is called neutral grounding.

Neutral grounding provides protection personal and equipment. It is because during earth fault the current path is completed through the earthed neutral and the protective devices operate to isolatefaulty conductor from the rest of the system.

Observation: Ambient Temp.32

S. No.

EARTH RESISTANCE IN OHM AVERAGE EARTH PIT

RESISTANCEREF VALUE OBSERVED VALUE

1 .01 .010.26 2 .01 .01

3 .08 1.014 .01 .01

Table – 12 Earth Pit resistance test reportNote: - Measured Value Should Not Be More Than 0.5 Ohm.

9.9 Oil Sampling, Testing

(Before shut down/Prior to charging/24 Hrs after charging)

The oil provides an electrically insulating and cooling medium. The service reliability of an

immersed transformer is therefore mainly depending on the oil quality. In service oil is subjected or

normal deterioration due to the condition of use mainly due to water, and solid particles/sediment.

The oil should be maintained as per the guidelines of IS I 86 periodic test of oil is necessary.

Oil in transformer can he sampled through sampling valve for Break down Voltage

(B.D.V) .Dissolved Gas Analysis (DGA) water content etc.

Gases To Be Analyzed And Criteria:

1. Gases to be analyzed normally 02, N2, H2, Co, Co2, CH4

2. Gases to estimate abnormality H2, CH4, C2H2, C2H4, C2H6

3. Gases to estimate deterioration Co, Co2, CH4

Gas Content In Oil By Faults In Transformer Oil:

S. No. Type of faults Decomposable gases in transformer oil

1. Overheat of oil CH4, C2H4, H2, (C2H6, C2H2, C3H6, C3H8)

2. Arcing in oil H2. C2H2, (CH4, C2H4)

3. Overheat of’ solid insulating materials CO. C02, (H2, C2H4)

4. Overheat of oil arid paper combination CH4, C2H4, CO, C02, H2.

5. Arcing of oil and paper combination H2, C2H2, CO, C02, (C2H2)

Standard Norms for oil As per IS: 1866

Oil Test Report After Overhauling:

BRFAK DOWN VOLTAGF. (B.D.V.)1……….85…..........KV 2. ……....90………….KV 3. ………….92………… KV4……… 87……….. KV 5………...88…………...KV 6………….... 87............. KV2) Resistivity At 90°c -- Ohm-Cm.3) Tan Delta At 90°c -- .0504) DISSOLVE GAS ANALYSIS (DGA) -- (In ppm)

Date H2 CH4 CO CO2 C2H4 C2H6 C2H2 C3S26.10.06 3 14 51 922 0 2 0 0

Table- 13 Oil Test Report.

Remarks: The test results are within the permissible limit.

Tests on oil Sampling

bottles

Oil lest kits

a) BDV

b) Moisture

c) Resistivity

d) 1FT

e) Tan delta

f) Acidity

Min. 60 KV

Min 10 ppm

Mm. 0.IX 1012 at

9QU

Mm. 15 mN/m

Max. 0.2 at 90 0C

Max. 0.3 mgKOH/g

Oil DGA Oil sampling bottle DGA

test instrument

Dissolved Gas

Analysis

As per ANSI / IEEE

(C57.104-1991)

Maximum gas limits

are as follows:

H2 100 ppm

CH4 120 ppm

C2H2 35 ppm

C2H4 50 ppm

C2H6 65 ppm

CO 350 ppm

CO2 2500 ppm

9.10 Oil temperature indicator (OTI) Winding Temperature Indicator (WTI) Calibration Purpose: — To check the OTI & WTI and comparison of OTI / WTI temperature readings with standard thermometer at different temperature.

Calibration Result: S. No. Standard

Thermometer 0COTI 0C WTI 0C WTI-II 0C

1. 30 31 30 312. 35 36 36 363. 40 41 42 414. 45 46 45 465. 50 51 52 516. 55 54 54 547. 60 62 60 608. 65 66 65 659. 70 71 70 70

10. 75 74 73 7411. 80 81 81 8112. 85 86 87 8613. 90 91 90 9114. 95 97 96 9615. 100 101 103 10116. 105 106 106 106

Table – 15 OTI, WTI Calibration Report.

13.0 CONCLUSION

Routine testing of transformers is very much essential to ensure good health of

transformer with more and more stress on quality and availability of power.

availability of transformer which is the vital link between generation and

transmission! distribution is must. To achieve the objective of availability and

efficiency regular testing with modern testing techniques have evolved as the most

effective means. Combination of off line testing and on line condition monitoring with

complimenting technique have been successfully used to identify and pin point

potential problems hug before they manifest as failure. All the testing elaborated in

this paper and inference derived are with reference to the tested transformer.

14.0 REFERENCES:

1. THE PERFORMANCE AND DESIGN 01: A/C Machines

BY- M.G.SAY,

CBS PUBLISHERS & DISTRIBUTORS, NEW DELHI

THIRD EDITION

2. BOOK- ELECTRICAL MACHINES

By- P.S. BHIMBRA,

KI IANNA PUBLISHERS, NE\V DELHI- 6

THIRD REVISED EDITION

3. BOOK- ELECTRICAL M/C DESIGN

B- A.K.SAWHNEY.

DHAN PAT RAI & SONS NEW DELHI- 6

FIFTH EDITION

4. POWER TRANSFORMER INSTRUCTION MANUAL. (VOLUME-2) BHEL Ltd.

5. SCOPE OF OVERHAULING FOR ELECTRICAL, EQUIPMENT CORPORATE

OPERATION SERVICES (NTPC)

SWITCHGEAR

SF6 245KV BREAKER

1. ELECTRICAL DATA

1. Make BHEL2. Type 3Avl SF-63. Rated voltage 245KV4. Rated normal current 2000A5. Rated short circuit 40 KA6. Rated duration of the short circuit 3 Sec

2. ERATING TIMES (MAX)

1. Minimum command duration 50 sec2. Closing time 110m Sec3. Operating time 50m Sec4. Arching time at 50Hz 21m Sec5. Break Time at 50 Hz 71 m Sec

3. DETAILS OF OIL /SF6/N2 LOCKOUT VALUES/ ALARMS:

1. General lock out SF6 6.5 + 0.05 bar at 20 0C

2. Loss of SF6 6.7- 0.1 bar at 20 0C

3. General lockout oil 253-3 bar ( open lockout)

4. Minimum pressure lockout oil 273-3 bar (close lockout)

5. Pump control on 313-3

6. General lockout N2 315-3

6.6KV SWITCHCEAR (VACUUM BRAKER)

1. ELECTRICAL DATA

Sr. No. Make SIEMENS

1. Type 8 BK 20 metal clad draw out

2. Rated Voltage 6.6 KV

3. Max System Voltage 7.2 KV

4. Short Circuit at stand current and at rated Voltage 31.5 KVA

5. Degree of protection IP 40

6. Rated current 2500.630 Amp

7. Minimum clearance in air between plates 135 mm

8. Rated Symmetrical interrupting current 31.5 KA

9. One second current carrying capacity 31.5 KA

10. Rated current of bus bar 4000 A

2. OPERATING TIME AND OTHER DIMENSION DETAILS

1. Dimension of breaker W*H*D 800*2050*1775

2. Type of vacuum bottle 3 AF

3. Closing time 75m Sec

4. Opening time 60m Sec

5. Closing time 80m Sec

6. Spring charging time 15 Sec

10.5 KV SF6 GENERATOR CIRCIJIT BREAKERS

One of the unique features of this plant is that a circuit breaker is provided between the

generator output terminals and the unit transformer input terminals. No such circuit breaker

is present in a coal— based plant. This circuit breaker eliminates the requirement of a

station transformer. When auxiliaries are required to be charged and the unit is not in

operation. The generation transformer terminals are kept open and unit transformer is back

charged for the grid. Once the unit transformer is charged the auxiliaries are charged using

the unit auxiliary transformer. By this means the high cost of station transformer is

eliminated.

1. ELECTRICAL DATA

1. Make ABB Germany

2. Type HKK-3

3. Rated current 10300 amps @ 40 0C

4. Rated voltage 24 KV @ 50 Hz

5. Operating voltage 10.5 KV @ 50 Hz

6. Type of power drive Compressed air

7. Rated short time current 1 Sec 100000 A

8. Peak making current 300000 A

9. Rated breaking current 100000A & 135000A

10. Rated making current 300000A

2. OPERATING TIMES AND OTHER DATA

1. Circuit breaker closing time 53m sec

2. Circuit breaker break time 47m Sec

3. Close open operating time 90m Sec

4. SF6 gas Pressure 0.62 Mpa

5. Actuating air pressure 0.96 Mpa

6. Control Voltage 220 V DC

7. Compressor start (Auto) 0.93 Mpa

8. Compressor stop 0.96 Mpa

9. Safety vale operation 1.09 Mpa

10. Breaker blocking ( closed operation) 0.90 Mpa

GENERATOR (STEAM TURBINE)

The generator is hydrogen cooled with direct hydrogen cooling for rotor winding and indirect

hydrogen cooling for stator winding. The heat losses in the generator interior are dissipated to the

secondary coolant through hydrogen. The hydrogen cooler is a shell and tube type heat

exchanger. The heat removed from hydrogen is dissipated through the cooling water. The cooling

water flows through the tubes, while the hydrogen is passed around the finned. The stator winding

is a fractional pitch two layer types consisting of two individual bars. The stator winding is

connected iii the generator interior. The stator winding connections are brought out to the bushings

located at the exciter end.

1. ELECTRICAL DATA

01. Make BHEL Hyderabad

02. Type THRI – 1000 – 3GP

03. Code VDE 530, ICE 34

04. Type of cooling Hydrogen

05. Ratings 198.1 MVA

a) Rated output 154.5 MW

b) Active power 18.5 KV or 10%

c) Terminal voltage 3000 RPM ( 50 Hz)

d) Rated Speed 10618 A

e) Power factor 0.8 (LAG)

f) Rated current 10618 A

06. Insulation class F

07. Phase connection YY ( 2 path per phase )

08. Hydrogen pressure 3.0 bar

09. Field current at full load/ at no load 2362/802 A

10. Rated field voltage 272 V

11. Continuous permitted unbalance current 8% of rated current

12. Generator efficiency at full load 98.75 %

13. Generator reactive capacity 146.2 MVAR ( over excitation )

91.6 MVA ( Under excitation )

14. Generator losses at full load 2094 KW

15. Stator winding resistance between phases 0.0005845 at 20 0C

16. Rotor winding resistance 0.08529 at 20 0C

17. Nos. Terminals 6 ( 3 Phase, 3 Neutral)

2. MATERIAL

01. Rotor shaft 26 Ni Cr Mov 145

02. Rotor copper Cu AgO IPF30

03. Rotor wedges Cu Ni 2 Si

04. Retaining rings X 8 Cr Mn N 1818 K

05. Slip rings ST 52-3

06. Labyrinth ring outer G- Al Si 10 Mg WA

07. Stator frame Rst 37-2

Electrical sheet steel V350-65A-ST

1.3/gen.502.10

Stator copper E-Cu 58 F20

Bearing Sleeves GS+ Lg Sn 80 Fs

3. SLIP RING / CARBON BRUSHES:

01. Nos. 30 at +ve side

a) Slip Ring 30 at –ve side

02. Rotor grounding 4 Nos.

03. Brush dimensions

a) Slipping 64 / 32 / 32

b) Grouping 28 / 8 / 32

4. CRITICAL SPEED ETC.

01. First critical speed 1425

02. Second critical speed 3906

03. Materials

Rotor shaft 28 26 Ni Cr Mov 115

Material of retaining forgings X8 Cr Mn N 1818 K

Material of rotor wedge Cu Ni Si

5. SLIP RING / CARBON BRUSHES LIMIT FOR WEAR

1. Slip ring – original size 450mm

- Minimum 430mm

2. Brush holder type 27-0703 (slip ring)

27- 0707 (grounding )

3. Brush Qty. 16 Nos. ( Slip ring )

04 Nos. ( grounding )

4. Slip ring brush – normal size 32 / 32 / 64mm

- Min. size 32 / 32 / 20mm

5. Rotor grounding brush

- Normal size 20 / 8 / 32 mm

- Minimum size 20 / 8 / 12 mm

6. EXCITATION SYSTEM

01. Make BHEL

02. Type Thyristor controlled / static

03. Range of voltage

a) In auto 90 to 110 % En.

b) In manual 75 % No load current to 2110% of

full load field current.

04. Frequency range of operation 47 to 52 Hz

05. Response time

- Rated current 2855 A

- Rated voltage 350 V

- Ceiling current rating 4728 A

- Ceiling voltage rating 566 V for Seconds

- Limiters in service Rotor current limiter 2362 Amp.

Stator Current limiter 10618 Amp.

Under excitation limiter ( 906

MVAR at rated voltage ) power

System stabilizer .

7. EXCITATION TRANSFORMER

01. Manufacturer BHEL

02. Type Dry cast resin

03. Capacity 2300 KVA

04. Voltage 10.5 KV/0.460 KV

05. Current HV / LV 126.5 A/28886.8A

06. Parentage impedance 7.2+10 %

07. Vector group DY 5

08. Insulation level 75 KVP , 28 KV RPM

09. Type of cooling Natural with addition (AF) forced

cooling also.

10. Class of insulation F

11. Degree of protection IP 20

7. GAS SYSTEM DATA :

01. volume of Hydrogen 55 m3

02. Space in generator cooling gas flow 24 m3/sec

03. Purity of Hydrogen required >98%

04. Leakage of H2 by volume per data at rated H2 Pressure 12m

05. Cold gas temperature ( Rated) 42 0C

06. Max. hot gas Temperature ( Before cooler) 75 0C

07. Max. hot gas temperature ( after cooler ) 55 0C

08. Hydrogen pressure 3 bar

09. Hydrogen flow at a 42 0C 24 m/ sec

10. Heat dissipating capacity 2350 KW

11. Cold gas temperature 42 0C

12. Gas pressure drop 20 mm of WC

13. Cooling water flow / cooler M3/hr

14. Maximum water infer temperature 3.6 0C

15. Pressure drop on water side 0.5 bar

8. SEAL OIL COOLER

Design data for the seal oil cooler

type: 4x 3C – 2720 – 4050 – 060 ad ( 2

sections of 100% each)

Material and pressure

Oil flow 3.33 dm3/s Tubes Adm. Brass

Heat dissipating capacity 140 KW Tubes Sheets Cu ZN40

Oil inlet temperature 65 0C Water sheets with protective

coating

Oil outlet temperature 40 0C

Oil pressure Drop ( approximate ) 0.75 bar Design pressure 10 bar

Cooling water flow 40 m3/hr Test pressure 15 bar

Maximum water inlet temperature 36 0C Oil side pressure

Water outlet temperature 40 0C Designed pressure 10 bar

Water pressure drop 0.7 bar Test pressure 15 bar

220 KV SWITCHYARD EQUIPMENT DESCRIPTIONS

01. Circuit Breaker type SF6 10 Nos.

02. Isolator without earth switch Staggered type

1600A

05 Nos.

03. Isolator with two earth switch 1250 A 06 Nos.

04. Isolator with two earth switch 2500A 02 Nos.

05. Isolator without earth switch Staggered type

1250A

12 Nos.

06. Isolator with one earth switch 1600 A 03 Nos.

07. Isolator with two earth switch 1600 A 03 Nos.

08. Isolator with one earth switch 1250 A 06 Nos.

09. Current transformer 27 Nos.

10. Capacitor voltage transformer 06 Nos.

11. Post insulators 74 Nos.

12. Current transformer with extended current 150% 03 Nos.

13. Lightening arrester 18 Nos.