ANDHRA PRADESH POWER GENERATION CORPORATION LIMITED RAMAGUNDAM THERMAL STATION, RAMAGUNDAM.

From ToThe Superintending Engineer, The Head of Department,Operation & Maintenance, Electrical Engineer Department,RTS-B, Ramagundam. Sindhura College of Engg &

Tech,NTPC.

Sir,

Sub:-Industrial Oriented Mini Project Work At RTS-B to The Students of 3rd year, B.Tech (Electrical). Ref:-SE/O&M/RTS/DE/AT&P/AM(HR)/SUB ENG2/F.PROJECT WORK/D.No. 292/14, Date: 15.05.2014

With reference to the above it is to inform that the following students of “Sindhura College of Engineering & Technology”, NTPC, Ramagundam, have carried out a Industrial Oriented mini Projects training on “TRANSFORMER PROTECTION & MAINTENANCE” at RTS-B Station, Ramagundam during the period “15.05.2014 to 28.05.2014” under the guidance of Sri. K. Sridhar (Assistant Engineer /EM).

1. ZEBA ANJUM2. G.RAJKUMAR3. O.BHARGAV4. MD.KHAJA MUNWAR SHARIEF5. K.SAINATH REDDY6. A.PRADEEP

Their conduct during this period is found satisfactory.

SUPERINTENDING ENGINEER/O&M,

RTS-B, RAMAGUNDAM. ANDHRA PRADESH POWER GENERATION CORPORATION

LIMITEDRAMAGUNDAM THERMAL STATION, RAMAGUNDAM.

CERTIFICATE

This is to certify that the following students of “

Sindhura College of Engineering & Technology ”, NTPC, Ramagundam, have undergone project work on “ TRANSFORMER PROTECTION & MAINTENANCE ” at RTS-B, Ramagundam during the period “15.05.2014 to 28.05.2014” under the guidance of Sri. K. Sridhar (Assistant Engineer/EM).

1. ZEBA ANJUM2. G.RAJKUMAR3. O.BHARGAV4. MD.KHAJA MUNWAR SHARIEF5. K.SAINATH REDDY6. A.PRADEEP

Their conduct during this period is found satisfactory.

Project Guide:(K. Sridhar)Assistant Engineer,Electrical Maintenance, RTS-B, Ramagundam.

ACKNOWLEDGEMENT

We are very thankful to Mr. B. Surya Narayana, Superintending Engineer R.T.S ‘B’ STN, RAMAGUNDAM for giving this opportunity of doing the project work.

We are very grateful to Mr. K. Sridhar, Assistant Engineer EM Division R.T.S ‘B’ STN under whose guidance we are able to complete the project.

We are also grateful to Sub Section Engineers of EM and MRT section especially Mr. D. Shankaraiah, Assistant Divisional Engineer R.T.S ‘B’ STN who gave their valuable suggestions and cooperation.

We are grateful to our principal Mr. P. Sushanth Babu, Sindhura college of engineering and Technology, who give their kind cooperation and allowed us to do the project outside the college campus.

ABSTRACT

A power transformer is most costly and essential equipment of an electrical transformer. It is well known fact that power transformers are the heart of the power systems which enable us to establish very large power systems networks. Failure of any transformer will create critical situation hence extra attention is required in commissioning, maintenance & protection of these transformers. The periodical maintenance practice should improve the life period of transformer.

For getting high performance and long functional life of

the transformer, it is desired to perform various maintenance activities. Not only that, a power transformer also requires various maintenance actions including measurement and testing of different parameters of the transformer.

This synopsis deals with power transformers, (especially 75 MVA, 13.8/132 KV transformer and 10 MVA,132/3.3 kV Station Transformer, available at RTS) maintenance, overhaul & protection.

The Electrical Engineer of any power plant, Sub station or switching station regularly monitors the transformer in his day to day walk down check list and act accordingly to rectify problems that are encountered.

The power transformer is subjected to many internal or external faults during its life, depending up on the intensity of the fault; it deteriorates the winding, insulation and core of the transformer. Hence different types of protection like Differential, Over Current, and earth fault protection are provided for the transformer to reduce the damage, caused by the faults.

Transformer oil testing, DGA (Dissolved Gas Analysis) are some other methods to identify gradually developed faults and take necessary measures to improve the oil properties by filtration or complete oil replacement.

PROFILE OF RTS.B, STATION

Ramagundam thermal power station (R.T.P.S) of Andhra Pradesh state electrical board (APSEB) is situated at Ramagundam in the district of Karimnagar. The plant is situated at 60 Km from Karimnagar and 4 Km from Ramagundam railway station. The power station is about 0.5 Km from the state high way connecting Hyderabad – Mancheriyal. The power station has only one unit of 62.5 MW. The unit was commissioned in 1972. The plant was financed under AID scheme and unit comprises of boiler of CE, USA & turbine generator of GE USA. The coal is received at the power station by road and rail from Godhavarikhani of Singareni collieries. Adequate facilities are provided for unloading coal from the rail wagons. However at present the entire coal requirement is received at plant by road by means of trucks.

The raw water to the plant is drawn from the river Godavari situated about 8 Km from the plant. The water from the river is pumped to a reservoir on the top of the hill near the plant. Water is supplied by gravity to DM plant through clarifier and directly to CW to cooling tower basins as make up.

The ash from the boiler is disposed to the ash disposal area situated about 1 Km from the plant towards east of the plant. This area is getting filled up & extension of the area has to be developed.

The present coal quality as reported is about 3700 K cal/Kg and ash content 42%. Due to ageing deterioration of equipment, controls and non-availability of spares the performance has deteriorated. The boiler was designed for a

coal quality of 4050 K cal/Kg with ash content of 38.7%. At present the unit is operating at a PLF of 65% and heat rate has been reported to be 2660- 2730 K cal/ KWh, while design heat rate is 2616 K cal/KWH. The unit has been adequate residual life and also for improving the performance by implanting renovation and modernization works. There fore APGENCO (erstwhile APSEB) has decided to carry out renovation & modernization (R & M) works on the unit to restore the unit to operate at its rated capacity and at designed efficiency. The Unit was taken for R&M on 20.09.2006 and continuing till today. The major R&M works includes Boiler, Turbine & Generator, and Generator transformer.

CONTENTS

CHAPTER-1 INTRODUCTION1.1 Introduction

CHAPTER-2 CONSTRUCTION FEATURES OF TRANSFORMERS

2.1 Core2.2 HV & LV winding2.3 Conservator2.4 Bucholtz relay 2.5 Vent pipe & Diaphragm 2.6 Breather2.7 Bushings2.8 Tap Changer2.9 Cooling

CHAPTER-3 RECOMMENDED MAINTENANCE SCHEDULE FOR POWER

TRANSFORMERS4.1 Limits of insulation resistance of windings of power

transformers

CHAPTER-4 POWER TRANSFORMER MAINTENANCE SCHEDULE

CHAPTER-5 LOSSES IN TRANSFORMER5.1 Core Losses5.2 Copper Losses

5.3 Stray Losses5.4 Dielectric Losses

CHAPTER-6 TRANSFORMER PROTECTION6.1 Types of faults.

6.1.1. Through faults 6.1.2. Internal faults

6.2 Different types of relays6.2.1 Buchholz relays.6.2.2 over fluxing relays6.2.3 REF relays6.2.4 O/C & E/F relays.6.2.5 Differential relays.

CHAPTER-7 TESTINGS7.1 Ratio test7.2 Open Circuit and Short Circuit 7.3 Transformer Oil testing7.4 Dissolved Gas Analysis7.5 Magnetizing current test7.6 Magnetic balance test

POWER TRANSFORMERS:

PROTECTION & MAINTENANCE

CHAPTER-1

INTRODUCTION: Power transformers are the basic building blocks of the power system, the capital investment involved in power system for the generation transmission and distribution of the electrical power is so great that proper precautions must be taken to ensure that the equipment not only operates as nearly as possible to peak efficiencies, but also that it is protected from accidents.

Power Transformer

The power transformer could be used in a power station or power system could be a bank of three single phase transformers connected in either star/delta or star/star etc., or could be a single three phase transformer with single core. Normally for large capacity transformers, a three phase is used because of it lighter weight, cheaper in cost, occupies less space and more efficient. The only disadvantage is that any thing that effects the winding of one phase will effect the other also, whereas in single phase transformers this is not so, as one transformer can be replaced and the operation can be continued. The power generated at power stations is stepped up and transmitted on extra high tension lines of 132 KV or 220 KV. The voltage is again step down to 33 KV or 11 KV at various distribution transformer were voltage is stepped down to 440/400 V before supply is made available at consumer installation. It is roughly estimated that the power generated is transformed 3 or 4 times before it reaches a consumers system is 10 - 12 times its generating capacities have to be provided at various stages. In the distribution net work a transformer is most common of all electrical equipment.

CHAPTER-2

CONSTRUCTION FEATURES OF TRANSFORMERS:

2.1 CORE:Core is used to support the windings in the

transformer. It also provides a low reluctance path to the flow of

magnetic flux. It is made up of laminated soft iron core in order to reduce eddy current loss and hysteresis loss. The composition of a transformer core depends on factors as voltage, current and frequency. Diameter of the transformer core is directly proportional to copper loss and is inversely proportional to the iron loss. If the diameter of the core is decreased, the weight of the steel in the core is reduced which leads to less core loss of the transformer and the copper loss increase. The vice versa happen when the diameter is increased.

Laminated Steel Transformer Core

2.2 HV & LV WINDINGS:

The LV & HV windings are generally circular and concentrically arranged. When a transformer is opened the HV coils are seen first. When the HV coils are lifted LV coils are seen.

The LT coil is normally of copper strip insulated by manila paper. In between LT coils & HT coils places concentrically: A separator is used made of leatheroid paper on a bakelite cylinder. The HV coils are normally of double paper covered or double cotton covered or enameled copper wire of suitable guage wound in the layers. In between layers press pan paper & manila paper is used for insulation for 2 - 16 nos. of coils in each HV winding are used in which two or tapped coils the connection leads between coils and from the coils to tapping switch are insulated by sleeves.

High Voltage Windings Low Voltage Windings

2.3 CONSERVATOR:

This is a reservoir for oil. Whenever the oil in the transformer contracts during low temperature the oil is drawn from this and when the temperature is high the oil expands and the excess volume of oil goes into this and is store.

Conservator Tank

2.4 BUCHOLTZ RELAY:

It consists of a case in which two spherical floats are provided. Each assembly of floats is arranged in such a way that when the transformer oil is completely filled and ready for service, the contact of both the switches are open when minor fault cause, e.g., some insulation break down between the turns or core is over heated or transformer has been over loaded and raising the temperature of oil, small bubble of gas due to vaporization oil will

pass through the relay and gradually go on accumulation above the assembly of float to alarm. Circuit when gas pressure becomes sufficient, float alarm is forced to move down wards and thus close the circuit of alarm. This alarm circuit will also operate if the oil in the transformer is insufficient for cooling (i.e. oil might have leaked out).

When serious internal short circuit between the phases, earth faults due to break down of insulation, puncture of bushings, etc., generation of gas will be rapid owing to high current. Due to this, oil will rush suddenly through the pipe line causing the trip circuit to short circuit of the two contact points of trip circuit and hence a relay operates and it isolates the transformer.

*Bucholtz relay and Conservator of 10 MVA, 132/3.3 kV Transformer

2.5 VENT PIPE & DIAPHGRAM:

The vent pipe is pressure relief device for the main tank provided for oil to gush out when ever fault develops in transformer. This is only safety device to avoid major damage inside or to prevent the tank from bursting. The vent pipe is closed at the end by a diaphragm. In fact for some makes of transformer to diaphragms are provided on at the bottom and other at the mouth of the vent pipe. The diaphragm gets broken when pressure is developed in the tank & oil gushes out. It is to be ensured that this diaphragm is intact and air-tight as otherwise moisture may enter through this and cause damage to the oil in the transformer.

2.6 BREATHER:The insulating oil of transformer is provided for

cooling and insulating purpose. Expansion and contraction of oil during the temperature variations cause pressure change inside the conservator. This change in pressure is balanced by the flow of atmospheric air into and out of the conservator. Transformer breather is a cylindrical container which is filled with silica gel. Insulating oil reacts with moisture can affect the paper insulation or may even may lead to internal faults. So it is necessary that the

air entering the tank is moisture free. It consists of silica gel contained in a chamber. For this purpose breather is used. When the atmospheric air passes through the silica gel breather the moisture contents are absorbed by the silica crystals. Silica gel breather is acts like an air filter for the transformer and controls the moisture level inside a transformer. It is connected to the end of breather pipe.

Silica Gel Breather

2.7 BUSHINGS:Up to 33 KV voltages ordinary porcelain bushings

are used. Above this voltage condenser and oil filled terminal bushings or a combination of both are employed.

Bushings

2.8 TAP CHANGER:The output voltage may vary according to the input

voltage and the load. During loaded conditions the voltage on the output terminal fall and during off load conditions the output voltage increases. In order to balance the voltage variations tap changers are used. Tap changers can be either on load tap changer or off load tap changer. In on load tap changers the tapping can be changed without isolating the transformer from the supply and in off load tap changers it is done after disconnecting the transformer. Automatic tap changers are also available.

Tapings in transformer

2.9 COOLING:

The cooling of a transformer is carried out by following methods.

ON: Majority of transformers are oil immersed with natural cooling that is the heat developed in the cores and coils is passed on to the oil and hence to the tank valves for which it is dissipated. Thus has an advantage that moisture can not easily affect insulation.

OB: In this method the cooling of an ON type transformer is improved by air blast over the outside tank.

OFB: For last transformer artificial cooling may be used. This method comprises forced circulation of oil to a radiator were oil is cooled and again let in to the transformer.

OW: An oil immersed transformer of this type is cooled by the circulation of water in cooling tubes.

CHAPTER-3

RECOMMENDED MAINTENANCE SCHEDULE FOR TRANSFORMER

Items to be inspected

Inspection notes frequency Action required

1. Ambient temperature

Daily

2. Winding temperature &oil temperature

Check the temperature

Daily Shutdown transformer & investigate if found abnormal.

3. Load & voltage Check against rated figures

Daily

4. Oil level Weekly If low top up with dry oil, examine transformer for leakage

5. Oil level in bushing

Weekly If low top up with dry oil, examine transformer for leakage

6. Relief diaphragm

Monthly Replace if cracked or broken.

7. Dehydrating breather

Check for air passage color of the agent

Monthly If found pink change by spare charge or old charge may also reactivated.

8. Bushing Examine for cracks & dirt

Quarterly Clean or replace

9. Oil Check for the di electric strength & water content

Half yearly Take suitable action to restore quality of oil

10. Cooler fans, bearings motors & control mechanism

Lubricate bearings, check gear box, examine contacts, controls & interlocks

Half yearly Replace burnt or warm contact or other parts

11. Oil in coolers Test for pressure Half yearly 12. Oil in transformer

Check for sludge Yearly Filter or replace

13. Oil filled bushing

Test oil Yearly Filter or replace

14. Gasket Yearly Tighten bolts evenly

15. Cable box Check for ceiling arrangements, examine compound cracks

Yearly Replace

16. Relays, alarms & circuits

Examine relays & alarm contacts

Yearly Clean components, replace contacts & fuses if necessary

* 17. Earth resistance

Yearly Take suitable action if resistance is high

18. O.L.T.C over hauling

Check O.L.T.C. R.T.C.C. of proper functioning

Quarterly Clean & grease all moving contacts check oil in diverter arrangements

19. Bucholtz relay contacts

Check contacts & floats

Monthly Rectify or replace defective

20. IR test of windings

Measure by MEGGER

Yearly Take suitable action if found low

21. Overall inspection including lifting of core

Once in 15 years Wash by hosting down with clean dry oil

22. Sludge Oil for all values Once in 10 years Replace if tests values are not attained.

Permissible values of Earth resistance at Power stations 0.5 ohm Major substation 1.0 ohm Small Sub stations 2.0 ohms

LIMITS OF INSULTATION RESISTANCE OF WINDINGS OF POWER TRANSFORMERS

Rated voltage of the winding

Minimum safe insulation resistance in mega ohms at winding temperature of given above

30°C 40°C 50°C 60°C66 KV & above

600 300 150 75

33 KV 500 250 155 65

6.6 KV & 11KV

400 200 100 50

Below 6.6 KV

200 100 50 25

CHAPTER-4 POWER TRANSFORMER MAINTENANCE

SHEDULE

The following maintenance schedule is followed in power stations and switching stations in the A.P power system.

PARTICULARS PERIOD

REQUIRED SATISIFACTORY RESULTS

1 Checking of oil level in conservator & bushing, examining for leaks.

Daily shift

No leaks

2 Checking for unusual noise. Daily shift

1. No unusual noise.2. No sparks

3 Noting the loading in amp. Daily shift

----

4 checking for leakage of water into coolers (forced cooling systems)

Daily shift

----

5 checking relief diaphragm for cracks Daily shift

No cracks

6 Cleaning of bushings Monthly or during shutdown

7 Ensuring that oil comes out when air release valve is opened

Monthly or during shutdown

Without air bubbles

8 Checking the color of silica gel (replacement or recondition if necessary).

Monthly or during shutdown

Blue color

9 Inspection & cleaning of breather Monthly or during shutdown

Vent hole should not be blocked. Small quantity of oil should be in the bottom

10 Measuring insulation resistance of windings with 1000 V Megger

Monthly or during shutdown

See chart

11 Checking up of temperature bucholtz Monthly Alarm should come when points of

alarms for correct operations or during shutdown

thermostat touches set point

12 Noting the oil level tanks in the inspection glass of bucholtz relay

Monthly or during shutdown

CC level shall be fault

13 Testing of oil from tank and conservator for di-electric and testing strength (above 10000kVA).

Quarterly

30 KV -60sec.40 KV- instant40 KV -60sec.50 KV-instant

14 Checking Bucholtz relay for any gas collection and testing the gas collected

Quarterly

If gas collected switch off & intimate TRE30 KV -instant

15 Testing of oil for dielectric strength of tap changer

Quarterly

30 KV -instant

16 Megger testing of motor of forced cooling systems

Quarterly

17 Check transformer ground connection for lightness

Quarterly

18 Cleaning of water jacket (forced cooling). Quarterly

19 Testing of oil in the conservator for dielectric strength for transformer below 1000 KVA (or before after wet season).

Yearly 30 KV -instantly

20 Checking up of gap setting of bushing of transformers.

Yearly

21 Pressure testing of oil coolers (forced cooling system)

Yearly

22 Testing motors, pumps & calibrating pressure gauges etc (forced cooling).

Yearly

23 Calibration of temperature indicator by MRT.

Yearly +/- 2.5%

24 Testing of oil in the conservator and tank acidity (neutralization valve)

Yearly 0.3mg KOH/gm of air.

25 Testing the di-electric strength of oil in oil bushing when ever the di-electric strength is unsatisfactory filtering of transformer of oil should be done.

Yearly 40 KV - 60sec40 KV -60 sec50 KV- instant

26 Checking operation of bucholtz relay by air injection

Yearly alarm shall come

27 Tap changer maintenance. a. Over hauling.

b. Checking up of contacts

c. Testing of oil for acidity

d. Filtering or renewal of oil (yearly or after 1000 operations or when test results are poor)

Yearly

28 Major over haul (complete) of the transformers with capacity and below should be done whenever test results are unsatisfactory.

Yearly

CHAPTER-5

LOSSES IN TRANSFORMER

Losses can be considered as the difference between the Input power and the output power. All electrical machines has certain losses. There is no equipment which has zero loss or whose output power is equal to the input. Losses occur in all electrical equipment and these losses are dissipated in the form of heat.

Transformer is the most efficient electrical machine. Since the transformer has no moving parts, its efficiency is much higher than that of rotating machines. The various losses in a transformer are enumerated as follows:

1. Core loss

2. Copper loss

3. Load (stray) loss

4. Dielectric loss

5.1 Core loss:

When the core of the transformer undergoes cyclic magnetization, power losses occur in it. There losses are together called as core loss. There are two kinds of core losses namely hysteresis loss and eddy current loss. Core loss is important in determining heating, temperature rise, rating and efficiency of transformers. The core losses comprises of two components:

Hysteresis loss

Eddy current loss

Hysteresis loss

This phenomenon of lagging of magnetic induction behind the magnetizing field is called hysteresis.

In the process of magnetization of a ferromagnetic substance through a cycle, there is expenditure of energy. The energy spent in magnetizing a specimen is not recoverable and there occurs a loss of energy in the form of heat. This is so because, during a cycle of magnetization, the molecular magnets in the specimen are oriented and reoriented a number of times. This molecular motion results in the production of heat. It has been found that loss of heat energy per unit volume of the specimen in each cycle of magnetisation is equal to the area of the hysteresis loop.The shape and size of the hysteresis loop is characteristic of each material because of the differences in their retentivity, coercivity, permeability, susceptibility and energy losses etc.

Hysteresis loop

Click thumbnail to view full-size

The net unrecoverable energy lost in the process is area of abco which is lost irretrievably in the form of heat is called the hysteresis loss. the total hysteresis loss in one cycle is easily seen to be the area of one complete loop abcdefa.

If wh indicates the hysteresis loss/ unit volume, then hysteresis loss in volume V of material when operated at f Hz is given by the following equation.

Ph=whVf W

Steinmetz gave an empirical formula to simplify the computation of the hysteresis loss based on his experimental studies. The formula given by him is as follows:

Ph=khfBnm W

where kh is a characteristic constant of the core material, Bm is the maximum flux density and n is caller steinmetz constant

Permissible core losses in transformerkVA Core loss (W)16 15525 19540 26050 29563 35075 38588 400100 500

kVA Core loss (W)125 570160 670200 800250 950315 1150400 1380500 1660860 1980900 24001000 2800

All the above losses are subjected to positive or negative variation of 10%

Eddy current Loss

When the magnetic core flux varies in a magnetic core with respect to time, voltage is induced in all possible paths enclosing the flux. This will result in the production of circulating currents in the transformer core. These currents are known as eddy currents. These eddy currents leads to power loss called Eddy current loss. This loss depends upon two major factors. The factors affecting the eddy currents are:

Resistivity of the core andLength of the path of the circulating currents for a given cross section.

The eddy currents can be expressed as,

Pe =kef2B2 W/m3ke = ke'd2/p

Where, d is the thickness of the lamination.p is the resistivity of material of the core

Pe = ke'd2f2B2/p W/m3

Hence from the above equations it is evident that Eddy current loss is directly proportional to the square of the thickness of the lamination and that of the frequency of supply voltage.

Total core lossTotal core loss = Hysteresis loss + Eddy current loss.

Reduction of Eddy Current LossReduction of eddy current loss can be achieved by using

core with high resistivity and increasing the path of circulating currents.

By increasing the length of the path, the resistance offered by the material to the induced voltages will increase, resulting in the reduction of Eddy current loss.

High resistance can be achieved by using silicon steel cores. The resistance of the steel can be increased by adding silicon to it. The cores can be laminated along the flow of flux. Each lamination is insulated from the adjoining one. This increases the path length of the circulating currents with consequent reduction in Eddy current loss.

5.2 Copper Loss:

It is a well known fact that whenever there is a resistance to the flow of current in a conductor, power loss occurs in the conductor due to its resistance. Copper loss occurs in the winding of the transformer due to the resistance of the coil. When the winding carries current, power loss occurs due to its internal resistance. This loss is known as copper loss. The copper loss can be expressed as below

Pcu = I2R

Where, I is the current through the winding and R is the resistance of the winding.

Copper loss is proportional to the square of current flowing through the winding.

Permissible copper losses at 75 degree Centigrade

kVA Copper losses (W)

16 50025 70040 97550 118063 140075 160088 1650100 2000125 2350160 2840200 3400250 4000315 4770400 5700500 6920860 82601000 11880All the above losses are subjected to positive or negative variation of 10%

5.3 Stray Loss:

Stray loss results from leakage fields including Eddy currents in the tank wall and conductors. The winding of the transformers should be designed such that the stray loss is small. This can be achieved by the splitting of conductors in to small strips to reduce Eddy currents in the conductors. The radial width of the strips should be small and they should be transposed.

5.4 Dielectric Loss:

This loss occurs in the transformer oil and other solid insulating materials in the transformer.

The major losses occurring in the transformer are Core loss and copper loss. Rests of the losses are very small compare to these two. All the losses occurring in transformer are dissipated in the form of heat in the winding, core, insulating oil and walls of the transformer. Efficiency of the transformer increases with decrease in the losses.

CHAPTER-6TRANSFORMER PROTECTION

6.1 TYPE OF FAULTS:The type of faults that the power transformers are

subjects to are classified as:1. Through faults.2. Internal faults

6.1.1 THROUGH FAULTS: These are due to over load condition and external short circuits.

The transformers much be disconnected when such faults only after allowing a predetermined time during which are other protective gear would have operated. A sustained over load conditions can be detected by thermal relay which gives an alarm so that the situation can be attended to or the supply disconnected, if necessary. For the external short circuit conditions, time graded O/C relay are generally employed. Fuses are provided for low capacity transformers (distribution transformers).

6.1.2 INTERNAL FAULTS The primary protection of a transformer is intended for

the conditions which arise as a result of faults inside the protected zone. Internal faults are very serious & there is always the risk of fire. These internal faults are classified into two groups.Electrical faults which cause immediate serious damage but are

generally detectable by unbalance of voltage or current such as phase to phase faults, short circuits between turns of high & low voltage winding etc.,.

Incipient faults: which are initially minor faults, causing slowly developing damage? They include: A poor electrical connection of conductors or a core faults

due to break down of the insulation of the lamination bolts or clamping rings.

Coolant failure which will cause a rise of temperature even below full load operation.

Possibility of low-oil content or clogged oil flow, which readily cause local hot-spots on windings.

Bad load-sharing between transformers in parallel, which can cause overheating due to circulating currents.

Generally for group (1) it is important that the faulted transformer should be isolated as quickly as possible after the fault has occurred to limit the damage to the equipment. The

faults of group (2) through not serious in their incipient stage may cause major faults in the course of time and should thus be cleared as soon as possible. It should be emphasized that the means adopted for protection against faults of group(1) or not capable of detecting faults of group(2), where as the means applicable to detect the fault of group(2) may detect some faults in group(1) but are not quick enough. These ideas are basic to transformer protection and the means for protection against group (1 & 2) should not be treated as alternative but as supplements to each other.

In A.P. system, the rating of power transformers at EHV substations in general are as follows:

1. 220/132 KV 100 MVA auto transformers.2. 220/33 KV 50 KVA & 31.5 MVA transformers.3. 132/66 KV 40 KVA & 27.5 MVA transformers.4. 132/33 KV 50 MVA, 31.5 MVA, 25, 16, 15 & 7.5 MVA

transformers.5. 132/11 KV 16, 15 & 7.5 MVA transformers.

Most of the power transformers are of star-star type with neutral solidity earthed. There are few transformers with delta-star windings (delta on HV side). Norms of transformer protection generally followed in A.P. system are indicated below:

VOLTAGE RATIO & CAPACITY.

HV SIDE. LV SIDE COMMON RELAYS

1. 132/33/11 KV up to 8 MVA.

3 O/L relays + 1 E/L relay

2 O/L relays + 1 E/L relay

Bucholtz, OLTCBucholtz, OT, WT

2. 132/33/11 KV above 8 MVA & below 31.5 MVA.

3 O/L relays + 1 dir. E/L relay

3 O/L relays + 1 E/L relay

Differential, Bucholtz, OLTCBucholtz, OT, WT

3. 132/33 KV, 31.5 MVA & above.

3 O/L relays + 1 dir. E/L relay

3 O/L relays + 1 E/L relay

Differential over flux, bucholtz, OLTC bucholtz, PRV, OT, WT.

4. 220/33 KV, 31.5 MVA & 220/132 KV, 100 MVA.

3 O/L relays + 1 dir. E/L relay

3 O/L relay + 1 dir. relay

Differential over flux, bucholtz, OLTC bucholtz, PRV, OT, WT.

5. 400\220 KV, 3 dir. O/L 3 dir. O/L Differential

315 MVA relays (with dir. Highest) + 1 dir. E/L relays restricted relay.

relays (with dir. Highest) + 1 dir. E/L relays restricted relay

over flux, bucholtz, OLTC bucholtz, PRV, OT, WT & over load (alarm) relay.

6.2 TRANSFORMER PROTECTION - DIFFERENT TYPES OF RELAYS

Bucholtz relays. Over fluxing relays. REF relays. O/L & E/L relays. Differential relays.

6.2.1Bucholtz relays:When ever a fault in transformer develops slowly, heat

is produced locally, which begins to decompose solid of liquid insulated materials & thus to produce inflammable gas & oil flow. This phenomenon has be used in the gas protection relay or popularly known as bucholtz relay. This relay is applicable only to the so called conservator type transformer in which the transformer tank is completely filled with oil, & a pipe connects the transformer tank to an auxiliary tank or “Conservator” which acts as an expansion chamber. Figure shown as bucholtz relay connected into the pips leading to the conservator tank an arrange to detect gas produced in the transformer tank. As the gas accumulates for a minor fault the oil level falls &, with it a floar ‘F’ which operates a mercury switch sounding an alarm. When a more serious fault occurs within the transformer during which intense heating takes place, an intense liberation of gases results. These gases rush towards the conservator and create a rise in pressure in the transformer tank due to which the oil is forced through the connecting pipe to the conservator. The oil flow develops a force on the lower float shown as ‘V’ in the figure and over tips it causing it contacts to complete the trips circuit of the transformer breaker. Operation of the upper float indicates & incipient fault & that of the lower float a serious fault.

ANALYSIS OF GASES IN BUCHOLTZ RELAY:

The gas collected from bucholtz relay is passed through the bottle containing the 5% Silver Nitrate solution ( Ag No3) and allowed react with the with it , Depending up on the color obtained from this reaction, the type of fault in the transformer can

be easily analyzed.

Color of the gas Identification1. Colorless Air2. White precipitate

insulationgas of decomposed paper and cloth

3. Yellow gas of decomposed wood insulation

4. Gray gas of over heated oil due to burning of iron

5. Black gas of decomposed oil due to electric arch.

BUCHOLTZ RELAY OPERATION: CERTAIN PRECAUTION: The bucholtz relay may become operative not only during faults within the transformer. For instance when oil is added to a transformer, air may get in together with oil, accumulate under the relay cover & thus cause a false operation of the gas relay. For this reason when the “gas” alarm signal is energized the operators must take a sample of gas from the relay, for which purpose a special clock is provided. Gases due to faults always have color & an order & are inflammable.

The lower float may also falsely operate if the oil velocity in the connection pipe though not due to internal faults, is sufficient to tip over the float. This can occur in the event of an external short circuit when over currents flowing through the windings over heat the copper & the oil & cause the to expand. If mal-operation of bucholtz relay due to over loads or external short circuits is experienced it may be necessary that the lower float is adjusted for operation still higher velocities.

In installing these relays the following requirements should be fulfilled.

1. The conductor connection the contacts to the terminals on the cover must have paper insulation, as rubber insulation may be damaged by the oil.

2. The floats must be tested for air tightness by for example, submerging them in hot oil to create a surplus pressure in them.

3. The relay cover & the connection pipe should have a slope of 1.5 to 3% & not have protruding surface to insure unrestricted passage of the gasses into the conservator

A large number of faults gas protection operations may results from failure to fully observe the above precautions.

6.2.2 OVERFLUXING PROTECTION: PRINCIPLES & RELAYS IN A.P. SYSTEM:

The fundamental equation for generation of E.M.F in a transformer to give flux

=K (E/F)

The over fluxing condition in transformer can occur during system over voltage & or under frequency condition. This will cause an increase in the iron loss & disproportionately great increase in magnetizing current. In addition flux is diverted from the laminated core structure into steal structural parts. In particular under condition of over excitation of core, the core bolts which normally carry little flux may subjected to large component of flux diverted from highly saturated & constricted region of core along side. Under such condition, the bolts may be rapidly heated to temperature which destroys their own insulation & will damage the coil insulation if the condition continues.

The over fluxing condition does not call for high speed tripping. The tripping delayed for a minute or two by which time; the condition may come too normally.

Of late the margins between the operating flux density & design flux density are coming down due to economic consideration for the manufacturer of the transformer. More over with sustained low frequency operation, the transformer are naturally subjected to more than the rated values.

These condition prompted provision of over fluxing relays from 80’s in the system.

6.2.3 RESTRICTED EARTH FAULT PROTECTION:

An earth fault in the winding is the most common type of transformer fault and is best detected by using a “restricted” form of earth fault protection. In this way time and current settings can be made independent of other protection system, thus low settings and fast operating times can be achieved.

The restricted scheme is a balanced system of protection and can be applied to either star or delta windings. The scheme connections for either type of windings are shown in figure. (1.4.2.4)

For the star winding, 3-line current transformer are balanced against a CT in the neutral connection; while on the delta side, the 3-line CT’s are connected in parallel.

An external fault on the star side will result in the line current transformer of the affected phase and a balancing current in the CT’s, the resultant current in the relay is therefore zero. During an internal fault, the neutral CT only carries current & operation results.

The arrangements of residually connected CT’s on the delta side of a transformer is only sensitive to earth faults on the delta side because zero sequence is blocked by the delta winding. For example, on earth fault on the star side transferred to the transformer appears on the delta as a phase fault. There the arrangement is an inherently restricted earth fault scheme in this application.

Modern practice is to employee a voltage operated (high impedance principle) relay for this application. The relay is set to operate with a certain minimum voltage across its terminals. The value of this operating voltage is chosen to be slightly higher than the maximum voltage which can possible appear across the relay terminals during external faults conditions.

6.2.4 BACKUP O/L & E/L RELAYS :

The following O/L & E/L relays are provided on transformers in A.P. system.

Make of relay HV O/L & E/L (type) LV O/L & E/L (type)EE/GEC CDG (with highest) +

CDDCDG (with out highest) + CDG (CDD for 100MVA transformers)

ABB ICM 21P (with highest)

ICM21 NP + ICM21NP

ERALIND

TJM1 (highest) + TJM12

TJM10 + TJM10TMAS301a + TMAS

TMAS311a + TMAS101a + TMWD (dir. Element)

101a

6.2.5 DIFFERENTIAL RELAYS

A simple differential relay compares the currents at both ends of a protected element as indicated below.

As long as there is no fault within the protected

equipment the current circulates between the two CT’s & no current flows through the differential element. But for internal faults the sum of the CT’s secondary will flow through the differential relay making it to operate.

PERCENTAGE DIFFERENTIAL RELAYSTwo basic requirements that the differential relay

connections are to be satisfied. It must not operate for load or external faults.It must operate for internal faults.

As on-load tap change facilities are invariably provided in the grid transformers, any departure from the nominal tap position will results in spill currents in the relay circuits. Further, the CT’s are often of different types and have dissimilar magnetization characteristic, again resulting in spill current during heavy through fault condition.

To avoid unwanted relays operation under the above two conditions a “percentage bias” differential relays is used.

The operating characteristic of percentage bias differential relay is shown in following figure.

In general the transformer primary current does not equal their secondary current and the connections of the secondary winding do not correspond to those of the primary. In order that the current flowing through the relay should nearly equal zero during normal operating conditions and when external short circuit appear, it is necessary to do every thing to have secondary currents of the current transformers on the transformer primary & secondary sides of equal order and coincide in phase. This is achieved by accordingly selecting the current transformer ratios, having the method of connection CT’s made in conformity with the vector group of three phase power transformer and by the use of additional auxiliary CT’s in the scheme.

CURRENT TRANSFORMER RATIO & CONNECTIONS FOR DIFFERENTIAL RELAYS:

A simple role of thumb is that the current transformer on any star winding of a power transformer should be connected in delta and the CT’s on any delta winding should be connected in star. Very rarely this rule is broken. In case of winding connected in zigzag the CT’s will be connected in star. This arrangement of CT

connections will compensate for the phase shift due to power transformer vector group connection.

The significant point is that, when grounded current can star winding for an external fault, we must use the delta connection (or resort a “zero phase sequence current shunt” that will be discussed late). The delta CT connection circulates the zero sequence components of the currents inside the delta and there by keeps them out of the external connection to the relay. This is necessary because there are no zero phase sequence components of currents on the delta side of the power transformer for a ground fault on star side; therefore, there is no possibility of the zero phase sequence currents simply circulating between the sets of CT’s and, if the CT’s on star side were no delta connected, the zero phase sequence components would flow in the operating coils and cause the relative to operate undesirably for external ground faults.

Transformer full load current:

In =Transformer capacity in MVA/3*rate KV

If the CT’s are to be connected in star, the CT ratio will be IN / 1A.

If the CT’s are to be connected in delta.

The CT ratio will be: IN / 0.5775 A.If the 0.5775A rated secondary core is not available, an auxiliary

CT of 1 / 0.5775 A ratio can be used and its secondary connected in delta.

If the available CT’s on HV & LV side are not in inverse ratio of voltage, auxiliary CT’s of suitable ratio have to be selected to match the currents to the relay equal from both HV & LV side.

The Generator transformer at RTS, is protected for such internal fault by General Electric make Differential Relay, type BDD 15B.Settings adopted: 3.8 (132 kV side)

4.2 ( 13.8 kV)These relays are meant for the overall protection of the Generator, UAT and GT. This relay is provided with Percentage and Harmonic restraint and with a sensitive polarized main unit as operating element. Percentage restraint permits accurate discrimination between internal and external fault of high currents and harmonic restraint enables the relay to distinguish by difference in waveform, between the differential caused by transformer internal fault and that caused by magnetizing inrush currents.

10 MVA Power Transformer Relay settings

132 kV/ 3.3 kV, CTR = 400/5, PTR 1200:1

S.No. Type/Make Protection Settings adopted

1 GE Differential 3.2 (LV)5 (HV)

2 GEC Over current

(HV) PS 2.5TL 0.45

(LV) PS 6.0 TL 6.0

3 GEC Earth fault HV – PS 0.6 TL 0.10

MAGNETISING INRUSH CURRENT:

When a power transformer with its secondary circuit open, is switched on, it acts as simple inductance and a magnetizing in rush current which will be several times transformer full load current will flow. As the inrush current flow in the primary of the transformer only, it appears to the differential relay as an internal fault.

This relay is able to distinguish the difference between the magnetizing inrush current and short circuit current by the difference in wave shape. Magnetizing inrush current is characterized by large harmonic components and that are not noticeably present in the short circuit current. A harmonic analysis of typical magnetizing inrush current wave is shown in table below.HARMONIC COMPONENTS AMPLITUDE IN PERCENTAGE OF

FUNDAMENTAL2nd 63.03rd 26.84th 5.15th 4.16th 3.77th 2.4

As seen from the above the 2nd harmonic component is predominant in the magnetizing inrush current.A differential relay which extract the 2nd harmonic current and fed to the restraining coil to make relay inoperative due to magnetizing inrush current.

CHAPTER-7

TESTINGS

7.1 RATIO TEST:

Three phase AC supply voltage is applied to HV winding of the generator transformer and by changing the tap positions the corresponding changes in LV side voltage were observedTap No.

ActualRatio

RY YB BR ry Yb br MeasuredRatio

1 10.52 395 395 395 37.5 37.5 37.5 10.532 10.38 395 395 395 38 38 38 10.393 10.25 395 395 395 40 40 40 9.874 10.11 395 395 395 42 42 42 9.405 9.98 395 395 395 42.5 42.5 42.5 9.296 9.84 395 395 395 43 43 43 9.187 9.70 395 395 395 44 44 44 8.978 9.57 395 395 395 45 45 45 8.779 9.43 395 395 395 46 46 46 8.5810 9.29 395 395 395 47 47 47 8.4011 9.15 395 395 395 48 48 48 8.2212 9.02 395 395 395 49 49 49 8.0613 8.88 395 395 395 50 50 50 7.914 8.75 395 395 395 50.5 50.5 50.5 7.8215 8.61 395 395 395 51 51 51 7.74and are as shown below.

IR Value test:

IR values taken with 1 KV Megger at 25° C, by connecting the Megger terminal as fallow:

HV terminal to ground. LV terminal to ground HV to LV.

IR Values observed during test:HV side:

R-body 150/200 MΩY-body 150/200 MΩ

B-body 150/200 MΩ

LV side:r -body 200/500 MΩy -body 200/500 MΩb -body 200/500 MΩ

HV to LV:R-r 500/infinity Y-y 500/infinity B-b 500/infinity

7.2 Transformer Open Circuit and Short Circuit

S.C Test:

Short Circuited HV terminals and voltage applied to LV side through a Distribution transformer, so as to flow full load current (or certain percentage of full load current, say 20%) in LV winding.

Voltage and Ampere are measured on the secondary of the DTR (Across CT & PT).

V, W1, W2, A Copper losses will be calculated from the above

O.C TEST:C T ratio adopted: 10/5=2PT ratio adopted: 3300/110V=30

*Rated Voltage applied for LV winding with HV open from 11KV/440V, 100KVA DTR, and measured the following values, V, W1, W2, A Total Iron losses = (W1-W2) X MF

7.3 Transformer oil testing:

Transformer oil collected from the transformer form Top, Bottom and Middle sample points and send to the laboratory for testing Oil properties and Dissolved gases.

The following results are obtained:

132/3.3 kV10 MVADTR

100 KVA11kV/433

TEST RESULTS

Name of the test Limit ResultAppearance *Amber/Clear &

transparentAcidity (mg KOH/ g)

0.30 Max 0.005

B.D.V (KV) 40.0 Min *29/52Density (gm/cm3) 0.89 Max 0.86/Flash point (°C)Specific resistance (Ω-cm)

145 Min0.1E12 Min

1502.64E12

Tan δ 1 Max 0.0093Water content (ppm)

40 Max 5

* Before overhaul

7.4 Dissolved Gas Analysis:

Symbol Unit Result Total combustible gas

Ml -

Hydrogen H2 ppm 2.1Methane CH4 ppm 6.4Ethane C2H6 ppm 2.3Ethylene Acetylene

C2H4

C2H2

ppm ppm

3.8ND

Carbon Monoxide

CO ppm 165.6

Carbon Dioxide CO2 ppm 1126.0

PERMISSIBE GAS CONCENTRATION IN PPM

Service life of the equipment in years

H2 CH4 C2H6 C2H4 C2H2 CO CO2

Up to 4 yrs

100/150

50/70 30/50 100/150

20/30 200/300

3K/3.5K

4 to 10 yrs 200/300

100/150

100/150 150/200

30/50 400/500

4K/5K

Above 10yrs

200/300

200/300

800/1000

200/400

100/150

600/700

9K/12K

GASES INVOLVED IN DIFFERRENT FAULTS

S.No Type of fault Gases involved

1 ARCING H2,C2H2,CH42 HOT SPOT H2,C2H43 PARTIAL DISCHARGE H2,CH44 INSL.

DECOMPOSITIONCO,CO2

7.5 MAGNETISING CURRENT TEST:

With the generator transformer kept under no load condition (LV side open), three phase AC voltage applied to HV winding and the no load currents values are taken.

Applied voltage = 415 V

Tap no.1 R-10.5mAY-9.5mAB-10.5mA

7.6 MAGNETIC BALANCE TEST:

With the voltage applied across one phase of the winding, the voltages induced in the other phases are observed at different tap positions. The sum of the voltages (or fluxes) in other two phases should be approximately equal to the voltage applied to the one phase.Here any two phases acts as the return paths for the third path, for which the voltage applied, with this test any defect in the magnetic circuit can be easily identified.

RY YB BRTap no.1 400 240 150

230.5 400 160190 200 400

Tap no.8 400 210 190205 400 190195 200 400

Tap no.15 400 200 195200 400 200195 200 400

CONCLUSION:

In all industrial countries the electrical power demand is ever increasing, almost doubling its self approximately per decade. This automatically demands for design, development and construction of increasingly of high reliable in power transformers. Such large power transformers maintenance and protection plays a vital role in power system.

It is a well known fact that “the prevention is always better than cure”. Periodical maintenance and proper protection provides reliable and qualitative power to the power system hence prevents it from block outs, which in turn saves money and energy.

Now a days, with the invention of static relays/numerical the protection of a power transformer became simple and easy. And a single and small (small in size compared to earlier electromagnetic relays) relay can provide the entire range of protection schemes for the power transformer.

Last but not the least, the testing of power transformer, its auxiliaries like bucholtz relays, PRVs , CTs , PTs, Relays and oil will give early signal about the transformer healthiness and alerts the maintenance engineers to act immediately before the major problem took place.