Transformer Testing

A PROJECT REPORT ON “TRANSFORMERS’

TESTING”

AT

NTPC RAMAGUNDAM A thesis report submitted to the SREENIDHI INSTITUTE OF SCIENCE AND TECHNOLOGY YAMNAMPET, GHATKESAR, R.R DIST. AFFILIATED TO JNTU-H In partial fulfillment of the degree of TRAINING IN ELECTRICAL ENGINEERING SUBMITTED BY S.KASI VISHWANATH

13311A0279

V.ASHOK BABU

13311A0264

DEPARTMENT OF ELECTRICAL ENGINEERING

SREENIDHI INSTITUTE OF SCIENCE AND TECHNOLOGY YAMNAMPET, GHATKESAR, R.R. DIST. AFFILIATED TO JNTUH.

1 1

CERTIFICATE This is to certify that the project entitled “TRANSFORMERS TESTING” By S.KASI VISHWANATH

13311A0279

V.ASHOK BABU

13311A0264

Students of DEPARTMENT OF ELECTRICAL ENGINEERING

SREENIDHI INSTITUTE OF SCIENCE AND TECHNOLOGY AFFILIATED TO JNTU-H Has done “TRANSFORMERS TESTING” At NPTC Ramagundam and gained valuable knowledge along with industrial experience.

PROJECT GUIDE ORDINATOR

PROJECT CO-

SRI SUNDAR VADIVEL

SRI B.V.SUBRAMANYAM,

SUPERINTENDENT (EMD),

AGM (EMD),

NTPC RAMAGUNDAM.

NTPC RAMAGUNDAM .

2 2

ACKNOWLEGEMENT We take this opportunity to record our gratitude to all those who helped us in successful completion of the project.

The successful completion of my work is indeed practically incomplete without mentioning all of these encouraging people who genuinely supported and encouraged us the whole time. I would like to express my sincere thanks to Sri B. V. SUBRAMANYAM, AGM (EM) for the help and support in the EM Department for giving this wonderful opportunity at NTPC Ltd (RSTPS). I express my profound gratitude to Sri S. SUNDARAVADIVEL, DEPUTY SUPERINTETNEND (EM), for the consistent support and exceptional resourcefulness throughout my training period. Finally, I thank all the members who coordinated and extended their valuable help and support in this endeavor. we wish to express our profound thanks to all the employees, in charges and workmen without their support, completion of this project would have been impossible. We solicitly express our gratitude towards the MR.M.V.R.SARMA(DY.MGREDC),L.HARIDAS,SYED YOUSUF,and other staff of EDC for extending their valuable services towards us to enter in this organization.

3 3

INDEX Sl.no. 1. 2. 3. 4. 5.

Sl.no.

Name of the Topic

Pg.no.

NTPC OVERVIEW 07 TRANSFORMER 14 TRANSFORMERS IN RSTPS 17 TRANSFORMER TESTING 21 TEST PROCEDURES a)Oil test 31 b)Voltage ratio measurement 32 c) Vector group & polarity 33 d)3-Φ excitation at 415v no load 35 e)Winding resistance m/m 36 f) Insulation resistance m/m 37 g) Capacitance & tandelta m/m 39 h)Separate source applied voltage test 41 i) No load &magnetizing m/m 42 j) Harmonics of no load current 44 k)Lighting impulse test with transfer voltage measurements l) Switching impulse voltage withstand

45

test m) Load loss & impedance m/m n) Temp. rise test o) Cooler loss m/m

47 48 50 52

Name of the Topic p) Oil leakage

4 4

Pg.no. 53

q) Insulation test on core, frame &auxiliary wdg

54

r) ratio &polarity test

55

6.

CONCLUSION

56

7.

REFERENCES

57

NTPC POWER PLANT 5 5

NATIONAL THERMAL POWER CORPORATION

6 6

NTPC- OVERVIEW NTPC, the major power utility, generating over one fourth of the total thermal power in the country, the corporation established its credentials over a period of two decades and is maintaining its impeccable record by consistently generating reliable and quality power. . NTPC established in 1975 and its mission has been to construct commission and operate power projects most economically and efficiently. NTPC has chalked out a capacity addition program of adding ambitious 38,014 MW by installing a number of coal and gas based power plants in the ten years.

NTPC VISION TO BE THE WORLD'S LARGEST AND BEST POWER PRODUCER, POWERING INDIA'S GROWTH.

MISSION DEVELOP AND PROVIDE RELIABLE RELATD, PRODUCTS AND SERVIES AT COMPETITIVE PRICES, INTERGRATING MULTIPLE ENERGY SOURCES WITH INNOVATIVE AND ECO FRIENDLY TECHNOLOGIES & CONTRIBUTE TO SOCIETY.

CORE VALUES (BE COMMITTED) Business Ethics Environmentally & Economically Sustainable Customer Focus Organization & Professional Pride Mutual Respect and Trust Motivating Self & Others Innovation and Speed Total Quality for Excellence Transparent & Respected Organization Enterprising 7 7

Devoted

INTRODUCTION TO RSTPS APPROVAL OF RSTPS:  RSTPS was declared by government of India in 1978.  Late Sri Morarji Desai, the then PM laid the foundation stone of RSTPS.  RSTPS began commercial operation by 1983. Ramagundam Super Thermal Power Station spread over 10000 acres of land is situated in Karimnagar district of A.P. It has an installed capacity of 2600 MW having 3*200 MW three units in Stage-I, 4*500 MW four units in Stage-II & III. This is the biggest power station in Southern India.

UNIT WISE POWER GENERATION:The whole plant is divided into 3 Stages, each Stage being planned at one time.

STAGE-I (3*200 MW) This stage consists of three units [unit-1, unit-2 & unit-3] each with a generation capacity of 200 MW. The turbines for these three units were manufactured by The Ansaldo Energia Ltd. The construction began in the late 1970s and these units have performed well over a long period setting many records regarding maintenance and generation over the other two stages.

STAGE 2 (3*500 MW) This stage again consists of three units [unit-4, unit-5, unit-6] each with a generation capacity of 500 MW. The turbines for these three units were manufactured by Bharat Heavy Electricals Limited (BHEL).

STAGE 3 (1*500 MW) This stage comprises only one unit [unit-7]. This is a first of its kind in South India being a computer operated unit. A wide disparity may be seen between the control rooms of the other two stages and this computerized unit. To this day, many Power plant engineers train in this unit to upgrade themselves to this new mode of operation. This unit also has the tallest chimney in Asia.

DISTRIBUTION OF ELECTRICITY:8 8

Total Capacity of Ramagundam NTPC is 2600 MW of Stage 1, 2 & 3 (i.e. unit’s 1, 2, 3, 4, 5, 6 & 7) distributing electricity to the following states:

STATE

MW

% AGE

Andhra Pradesh

610 MW

29 %

Tamilnadu

470 MW

22 %

Karnataka

345 MW

16 %

Kerala

245 MW

12 %

Goa

100 MW

5%

50 MW

2%

Pondicherry

Remaining 250 MW is un-allocated and can be given to any State, but at present it is being sent to Andhra Pradesh.

SALIENT FEATURES OF RSTPS Installed capacity

:

2600MW

Unit Sizes

:

Stage I –

3*200 MW

Stage II – 3*500 MW Stage III – 1*500 MW Location

:

Ramagundam, Karimnagar, AP

Coal Source

:

Singareni Coal Mines

Water Source

:

Pochampad Dam

Coal Consumption

:

10 Million Tons per Annum

Water Consumption

:

250 Cusecs

Transmission Line

:

2475km of 400KV

Approved Investment

:

Rs. 1702.18 Crs

Coal Transportation

:

M.G.R System of 22.4km

Height of Chimney

:

9 9

225m Stage I

250m Stage II 275m Stage III Ultimate Manpower Total Land

: :

3200 10,000 acres

Recognizing its excellent performance and vast potential, Government of the India has identified NTPC as one of the jewels of Public Sector 'MAHARATNAS'- a potential global giant. Inspired by its glorious past and vibrant present, NTPC is well on its way to realize its vision of being "one of the world’s largest and best power utilities, powering India's growth".

PERFORMANCES AND ACHEIVEMENTS  NTPC, Ramagundam was accredited with ISO 14001 certification for confirming to International Standard of Environmental Management System. 

Up award in environmental protection from council of power utilities.

 It achieved Raj Bhasha award for the year 1998-1999 for the first time NTPC featured in 1997 edition of the “Limca Book of Records” as the largest thermal power plant suppliers and the first to construct and commission the HVDC transmission link in the country.

BASIC POWER PLANT PROCESS It is known for ages that when coal is burnt it releases heat energy. The same phenomenon chemically represented on C + O2 CO2+heat energy (395 KJ/mole)

10 10

ENERGY TRANSFER In the Boiler chemical energy in fuel is converted into thermal energy by heating water and converting it into steam. The steam produced in the boiler is expanded. In the turbine, the thermal energy is converted into kinetic energy. As the steam expands it rotates the turbine. This motion of the turbine is transmitted to generator in which the mechanical energy is converted into electrical energy, which is transmitted to various load centers through transmission line.

POWER GENERATION PROCEDURE IN A THERMAL POWER PLANT Coal is the chief fuel in thermal power plants. The generation of electricity from coal can be classified into the following stages:  Coal to Steam  Steam to Mechanical Energy  Mechanical Energy to Electrical Energy CHEMICAL ELECTRICAL ENERGY ENERGY COAL GENERATOR

THERMAL ENERGY BOILER

11 11

MECHANICAL ENERGY TURBINE

COAL TO STEAM COAL HANDLING Coal from the coal wagons is unloaded in the coal handling plant, here the coal is crushed. This coal is transported up to the raw coal bunkers with the help of belt conveyers. Coal is transported to Bowl Mills by coal feeders. These coal feeders are of two types, volumetric and gravimetric. The coal is pulverized in the Bowl mills, where it is ground to a powder form. The BOWL MILL is one of the most advanced designs of coal pulverizer presently manufactured. The advantages of this mill are:    

Lower power consumption. Reliability Minimum maintenance Wide capacity

The mill consists of a round metallic table on which coal particles fall. This table is rotated with the help of a motor. There are three large steel rollers, which are spaced 120 degrees apart. When there is no coal, these rollers do not rotate but when the coal is fed to the table it packs up between roller and the table and this force the rollers to rotate. Coal is crushed by the crushing action between the rollers and rotating table. This crushed coal is taken away to the furnace through coal pipes with the help of hot and cold air mixture from Primary Air Fan. The P.A. Fan takes atmospheric air from F.D Fan is heated in the air heaters and sent to the furnace as combustion air.

STEAM CIRCUIT Water from the boiler feed pump passes through the economizer and reaches the boiler drum. Water from the drum passes through down comers and goes to bottom ring header. Water from the bottom ring header is divided to all the four sides of the furnace. Due to heat and the density difference the water rises up in the water wall tubes. Water is partly converted to steam as it rises up in the furnace. This steam and water mixture is again taken to the boiler drum where the steam is separated from the water. Water follows the same path while the steam is sent to super heaters for super heating. The super heaters are located inside the furnace and the steam is superheated (540 deg.) and finally it goes to the turbine.

FLUE GASES AND ASH CIRCUIT 12 12

Flue gases from the furnace are extracted by I.D fan, which maintains balance draft in the furnace with F.D Fan. These flue gases emits their heat energy to various super heaters in the pant house and finally passes trough air pre-heaters and goes to electrostatic precipitators where the ash particles are extracted. Electrostatic precipitators consist of metal plates, which are electrically charges. Ash particles are attracted on to these plates, so that they do not pass through the chimney to pollute the atmosphere. Regular mechanical hammers blows cause the accumulation of ash to fall to the bottom of the precipitators where they are collected in a hopper for disposal. This ash is mixed with water to form slurry and is pumped to ash pond. But nowadays 80% of the ash is used in formation of ash bricks which is cheaper and stronger than the normal bricks other 20% is sent into the atmosphere through the chimney.

STEAM TO MECHANICAL POWER A steam pipe conveys steam to the turbine through a stop valve (which can be sued to shut off steam in an emergency) and through control valves that automatically regulate the supply of steam to the turbine. Stop valve and control valves are located in a steam chest and a governor, driven from the main turbine shaft, operates the control valves to regulate the amount of steam used. (This depends upon the speed of the turbine and the amount of electricity required from the generator). Steam from the control valves enters the high-pressure cylinder of the turbine, where it passes through a ring of stationary blades fixed to the cylinder wall. These act as nozzles and direct steam into a second ring of moving blades mounted on a disc secured to the turbine shaft. The turbine has three stages high pressure, intermediate pressure and low pressure stages .firstly, the steam enters H.P turbine then the steam is lead to the boiler to again increase its temperature and pressure and then led to the I.P turbine thereafter to L.P turbine where the system is double flow kind, where temperature and pressure are lost. The steam almost reduces to droplets thereby the system rotates the shaft of the turbine which is coupled to the generator. Finally, the steam is taken to the low pressure cylinders, each of which it enters at the centre. The turbine shaft usually rotates at 3,000 revolutions per minute. This speed is determined by the frequency of the electrical system used in this country and is the speed at which a 2-pole generator must be driven to generate alternating current at a frequency of 50 cycles per second.

MECHANICAL ENERGY TO ELECTRIC ENERGY The mechanical energy is converted in to the electrical energy in the generator of the power plant. This conversion takes place through the principle of dynamically induced EMF and in accordance with the Faraday’s laws of Electromagnetic Induction. 13 13

PLANT LAYOUT

TRANSFORMERS

A transformer is an energy transformation device that transforms alternating current (AC) or voltage at one level to AC and voltage at another level. A transformer can economically convert voltage or current from low to high levels, or from high to low levels. The transformer usually consists of two or more insulated windings on a common iron core. In industrial and commercial applications, transformers are used to step 14 14

down voltages from utility service voltage to lower distribution voltage levels or lower utilization voltages that may be required for a facility or a plant. Transformers are very reliable devices and can provide service for a long time if maintained and serviced regularly. Transformer failures, when they occur, are usually of a very serious nature, which may require costly repairs and long downtime. The best insurance against transformer failure is to ensure that they are properly installed and maintained.

Transformer Categories and Type For consideration of maintenance requirements, transformers can be divided into the following categories:  Insulating medium  Construction  Application and use 1) Insulating Medium:The transformer’s insulating medium can be subdivided into two types: dry and liquid filled. a) Dry Type Dry-type transformers are usually air cooled with winding insulation of class A, B, C, or H. The dry-type transformer can be either Self-cooled or Forced air cooled. Self-cooled: A self-cooled transformer of the dry type is cooled by natural circulation of air through the transformer case. The cooling class designation for this transformer is AA. Forced air cooled: A forced air-cooled transformer of dry type is cooled by means of forced circulation of air through the case. Transformers of this type have air-blast equipment such as fans with louvered or screened openings. These transformers are rated at 133% of the rating of the Selfcooled Dry-type Transformers. The cooling class designation for this transformer is FA. Dry-type Transformers can be obtained with both Selfcooled and Forced air-cooled rating. The designation for such a transformer is AA/FA. Dry-type transformers can also be cooled by gas instead of air. For such transformers, a sealed tank is required. B) Liquid-Filled Transformer In this type of transformer, the windings and core are totally immersed in a Liquid contained in the transformer tank. The tank is equipped with cooling fins for circulation of the transformer liquid. The transformer liquid provides an insulating medium for the coils as well as for dissipation of heat. Two liquids have been used extensively in the past for transformers: Mineral oils and Polychlorinated biphenyls (PCB), commonly known as Askarel. Askarel was extensively used in transformers for indoor applications because it is a 15Non-flammable synthetic insulating 15

fluid. Askarel is a non-biodegradable and toxic. Newer fluids have been introduced, such as Silicone, RTemp, Wecosal, and Alpha 1 for replacement of Askarel. Others are still in developmental stages. Several cooling methods are used for liquid-filled transformers. Self-cooled: A self-cooled transformer uses the natural circulation of the insulating liquid. Heat in the transformer tank is dissipated by convection currents set up in the liquid, which circulates through the tank and cooling fins. The cooling class designation for this transformer is oil natural, air natural OA. Forced air cooled: In this type of transformer, air is forced over the cooling surface of the tank to supplement the self-cooled rating. The supplemental air is provided by fans that are mounted on the transformer tank and which can be manually or automatically controlled. The cooling class designation for this type of transformer is OA/FA. Forced air cooled and forced oil cooled: This transformer uses a pump to circulate oil through a heat exchanger to increase heat dissipation, which supplements the self-cooling and forced air cooling. The cooling class designation for this transformer is OA/FA/FOA. Water cooled: This transformer uses water instead of air to provide the cooling. The cooling system consists of a heat exchange by means of water pumped through a pipe coil installed inside or outside the transformer tank. The cooling class designation for this transformer is FOW.

Construction: Transformers can be classified by tank construction and core construction. a) Tank Construction Several types of transformer tank construction are used to prevent exposing liquid to the atmosphere. These types are as follows: Free breathing: This type is open to the atmosphere (i.e., the airspace above the liquid is at atmospheric pressure). The transformer breathes as the air pressure and temperature change outside the tank. Some of these transformers can be equipped with dehydrating compounds in the breather. Conservator or expansion-tank: These transformers are equipped with small expansion tanks above the transformer tank. The transformer tank is completely filled with oil, and the transformer breathes by means of this small tank, usually through a dehydrating compound. The purpose of the small tank is to seal the transformer fluid from the atmosphere and to reduce oxidization and formation of sludge. Sealed tank: These transformers are equipped with an inert gas, such as nitrogen that is under pressure above the liquid in the transformer tank. Generally, the pressure range for this type of transformer is −8 to +8 lb/in2 Gas-oil sealed: These transformers have an auxiliary tank to completely seal the interior tank, containing transformer liquid, from the atmosphere. 16 16

Vaporization: This type of transformer uses a special non-flammable insulating fluid, such as fluorocarbon (General Electric R-113), which is non-flammable, and a special condenser assembly welded on top of the transformer tank. The cooling tube ends are swaged and welded to tube headers. This transformer uses the technique of sprayed liquid on core and coil assembly (i.e., vaporization cooling known as pool boiling). The purpose of the condenser is to cool the boiling vapor into liquid for continued circulation of the fluid. b) Core Construction Transformers employ basically two types of core construction techniques. Core type: In core-type construction, the transformer winding surrounds the laminated core. The coils can be cylindrical, flat, or disk shaped. They can be arranged to fit around the rectangle or square cross section. Coretype construction provides a single-path magnetic circuit through the magnetic core. Most small distribution transformers are of this construction. Shell type: In shell-type construction, the magnetic core surrounds the windings. The primary and secondary windings may be interspaced side by side or circularly stacked one above the other. Some large power transformers have this form of construction. One advantage of the shell type is that it offers a separate path for the zero-sequence currents through the core, as compared to the core type in which the zerosequence path exists only through the transformer tank and end connections.

Application and Use: Transformers used for converting energy can be classified into five categories according to their application and use. a) Distribution Transformers A Distribution transformer has a rating from 3 to 500 kVA. There are various types of Distribution transformers, depending upon the cooling and insulating medium, service application, and mounting method. Transformers with voltage ratings of as high as 34,500 V are available. Virtually all Distribution transformers are Self-cooled. b) Network Transformer It has special and severe requirements for network service, such as ventilation, vault size, submergibility, and short-circuit requirements. Network transformers can have kVA ratings in excess of 500 kVA and primary voltage up to 23 kV. c) Arc-Furnace Transformer The Arc-furnace transformer is a special purpose transformer used in process industries. It is a low-voltage and high-amperage transformer and is specially braced to withstand mechanical stresses caused by fluctuating current requirements. Due to distorted waveform because of arcs, it has extra winding insulation. 17 17

d) Rectifier Transformer The rectifier transformer is also a special purpose transformer used in the rectification of AC to direct current (DC) applications in the process industry. These transformers are specially braced to withstand mechanical stresses produced by high currents. e) Power Transformer The power transformer has a rating in excess of 500 kVA and is primarily used in transforming energy from generating stations to transmission lines, from transmission lines to distribution substations, or from utility service lines to plant distribution substations.

EHV TRANSFORMERS IN RSTPS: The transformers used in RSTPS are:a) GENERATOR TRANSFORMER (Unit-1 to Unit-7) b) TIE TRANSFORMER (TIE-1 to TIE-4) c) AUTO TRANSFORMER (AT-1 to AT-5) GENERATOR TRANSFORMER:-It allows energy supplied by the generator to be transferred to the network at the required voltage.

TIE TRANSFORMER:-It is a step-down transformer which is used to supply the electrical power to the station transformer and colony transformer.

AUTO TRANSFORMER:-It can be used to buck/boost transformers’ increasing or decreasing supply voltage by a small amount .They can be 18 18

used in place of full transformer where the ratio of primary to secondary is very small (less than ~4) [2:1] .

NAME PLATE DETAILS OF TRANSFORMERS IN RSTPS:GENERATOR TRANSFORMER (1-ɸ, 50 Hz)  Shell form Type of cooling HV side:-voltage current LV side:-voltage current Impedance voltage at 420/3/21 KV Ambient temperature Maximum temperature oil rise: Winding: -

-

OIL –transformer Tap changer

-

WEIGHT-core and winding Tank and fittings Oil Total

-

AUTO TRANSFORMER (3-ɸ, 50 Hz) Type of cooling OFAF HV IV

17000 liters 220 liters 90000Kg 30000Kg 15500Kg 135500Kg

YNa0d11 - ONAN

19 19

OFAF 420/3 KV 825 A 21KV 9250A 13.89% 50c max. 50c 55c

189 189

252 252

ONAF 315 315

Reactive (MVA) LV Reactive LV Active

63 3

MVA RATING HV side:-voltage LV side:-voltage Impedance voltage at 420/3/21 KV Ambient temperature Maximum temperature oil rise: Winding: -

-

Noise level

-

TIE TRANSFORMER (3-ɸ, 50 Hz)

84 4

105 5

315 -

420/3 KV 21KV 13.89% 50c max. 50c 55c

86.5dB Max.

YNyn0d11

Type of cooling

- ONAN / ONAF

Rated capacity (MVA) HV LV

-

60/100 60/100

HV side:-voltage LV side:-voltage Impedance voltage at 400/34.5 KV Ambient temperature Maximum temperature oil rise: Winding: -

-

OIL – transformer

-

WEIGHT- core and windings Tank and fittings Oil Total

-

-

400KV 34.5KV 15.89% 50c max. 40c 55c 23400 liters

72000Kg 31500Kg 21000Kg 124500Kg

TRANSFORMER RECTIFIER (1-Φ, 50 Hz) Type of cooling KVA RATING HV side:-voltage LV side:-voltage

20 20

ONAN 60

-

53.5 KV 373.5V

LINE Amperes

HV LV Ambient temperature Maximum temperature oil rise: Winding: DC Output Max.

-

1.12 160.6 -

-

50c max. 50c 55c 70KV peak 800mA

Why Maintain and Test? A well-organized and implemented program minimizes accidents, reduces unplanned shutdowns, and lengthens the mean time between failures (MTBF) of electrical equipment. Benefits of EPM can be categorized as direct and indirect. Direct benefits are derived from reduced cost of repairs, reduced Down-time of equipment, and improved safety of personnel and property. Indirect benefits can be related to improved morale of employees, better Workmanship, increased productivity, and the discovery of deficiencies in the system that were either designed into the original system or caused by later changes made in the system.

All the electrical equipment need to be maintained safely from some of the failures. The transformers can fail from any combination of electrical, mechanical, and thermal factors. Actual transformer failures as listed above involve breakdown of the insulation system which may result from any of the factors (failure modes) just mentioned above. Electrically induced failures: These involve transient or sustained overvoltage conditions, lightning and switching surges, partial discharges, and static electrification. The partial discharges may be caused by poor insulation system design, by manufacturing defects or by contamination of the insulation system (both oil and solid insulation). Mechanically induced failures: A mechanically induced failure is due to deforming of a transformer’s windings that eventually results in the abrasion or rupturing of its paper insulation. Transformer winding deformation happens in either during shipping or during magnetically induced electromechanical forces. When a transformer experiences an internal or heavy through fault, the windings are subjected to electromechanical forces that are beyond their design capability. When this happens, it can cause hoop (inward radial) buckling of the innermost windings, conductor tipping, conductor telescoping, spiral tightening, end ring crushing, and/or failure of the coil clamping system. 21 21

Thermally induced failures: Thermal degradation causes the paper insulation of the windings to loose its physical strength to the point where it can no longer withstand the vibration and mechanical movement that occur inside a transformer. The thermally induced failures are due to overloading beyond its design capability for long period of time, failure of the cooling system to dissipate heat, blockage of axial oil duct spaces, operating the transformer in an overexcited condition, and/or excessive ambient temperature conditions.

TRANSFORMERS’ TESTING DC TESTING: - The DC testing of transformers involves testing of the solid winding insulation and the insulating fluids used in transformers. The testing of solid winding insulation complements other transformer testing. The solid winding insulation tests are not conclusive in themselves, but provide valuable information on winding conditions, such as moisture content, and carbonization. The DC tests are considered nondestructive even though at times they may cause a winding failure. It should be pointed out that a winding failure results from an incipient failure that the test was supposed to detect. If it had gone undetected, it might have occurred at an unplanned time. The DC tests conducted for transformer winding insulation are:Insulation Resistance Measurement This test is performed at or above rated voltage to determine if there are low resistance paths to ground or between winding to winding as a result of winding insulation deterioration. The test measurement values are affected by variables such as temperature, humidity, test voltage, and size of transformer. This test should be conducted before and after repair or when maintenance is performed. The test data should be recorded for future comparative purposes. The test values should be normalized to 20°C for comparison purposes. The general rule of thumb that is used for acceptable values for safe energization is 1 MΩ per 1000 V of applied test voltage plus 1 MΩ.

22 22

Electronic megohmmeter, 5000 V

Typical Insulation Resistance Values for Power and Distribution Transformers Transformer Winding Voltage (kV) 20°C 60° C 6.6 400 25 6.6–19 800 50 22–45 1000 65 ≥66 1200 75

30°C

Winding Ground (MΩ) 40°C 50°C

200

100

50

400

200

100

500 600

250 300

125

100

In the absence of more reliable data the following formula is suggested:

IR=CE / (KVA) Where IR is the minimum 1 min 500 V DC insulation resistance in megohms from winding to ground, with other winding or windings guarded, or from winding to winding with core guarded 23 23

C is a constant for 20°C measurements E is the voltage rating of winding under test KVA is the rated capacity of winding under test This formula is intended for single-phase transformers. If the transformers under test is one of the three-phase type, and the three individual windings are being tested as one, then E is the voltage rating of one of the single-phase windings (phase to phase for delta connected units and phase to neutral or star connected units) KVA is the rated capacity of the completed three-phase winding under test Values of C at 20°C 60 Hz Tanked oil-filled type 1.5 Un-tanked oil-filled type 30.0 Dry or compound-filled type 30.0

25 Hz 1.0 20.0 20.0

Dielectric Absorption Test The dielectric absorption test is an extension of the transformer winding insulation resistance measurement test. The test consists of applying voltage for 10 min and taking readings of resistance measurements at 1 min intervals. The resistance values measured during this test are plotted on log–log paper with coordinates of resistance versus time. The slope of the curve for a good insulation system is a straight line increasing with respect to time, whereas a poor insulation system will have a curve that flattens out with respect to time. There are two tests that are conducted under dielectric absorption test which are PI and DAR tests.

15 24 kV DC dielectric test set 24

. Dielectric Test Values for Routine Maintenance of Liquid-Filled Transformers Transformer Winding Rated Voltage (kV) Maintenance DC Voltage (kV)

Factory Test AC

Routine

Voltage (kV)

1.2 2.4 4.8 8.7 15.0 18.0 25.0 34.5

10 15 19 26 34 40 50 70

10.40 15.60 19.76 27.04 35.36 41.60 52.00 72.80

DC High-Potential Test The DC hi-pot test is applied at above the rated voltage of a transformer to evaluate the condition of winding insulation. The DC high-voltage test is not recommended on power transformers above 34.5 kV; instead the AC hi-pot test should be used. Generally, for routine maintenance of transformers, this test is not employed because of the possibility of damage to the winding insulation. However, this test is made for acceptance and after repair of transformers. If the hi-pot test is to be conducted for routine maintenance, the AC test values should not exceed 65% of factory AC test value. The routine maintenance AC voltage value should be converted to an equivalent DC voltage value by multiplying it by 1.6, that is, 1.6 times the AC value for periodic testing (i.e., 1.6 × 65 = 104% of AC factory test value). The DC hi-pot test can be applied as a step-voltage test where readings of leakage current are taken for each step. If excessive leakage current is noticed, voltage can be backed off before further damage takes place. For this reason, the DC hi-pot test is considered to be a nondestructive test. Some companies conduct the AC hi-pot test at rated voltage for 3 min for periodic testing instead of the 65% of factory test voltage. The procedure for conducting this test is as follows: Transformer must have passed the insulation resistance test immediately prior to starting this test. • Make sure transformer case and core are grounded. • Disconnect all high-voltage, low-voltage, and neutral connections, lowvoltage control systems, fan systems, and meters connected to the transformer winding and core. • Short-circuit with jumpers together all high-voltage bushings and all lowvoltage bushings to ground. If a hi-pot test is to be conducted for routine maintenance, consider the following in advance: (1) assume that a breakdown will occur, (2) have replacement or parts on hand, (3) have personnel available to 25 25

perform work, and (4) is the loss of the transformer until repairs are made beyond the original routine outage.

AC VOLTAGE TESTS The AC voltage tests can be classified into the categories as listed below: 1. PF and DF 2. AC high potential tests 3. Very low frequency (VLF) 4. AC series resonant 5. Induced frequency 6. Partial discharge (PD) 7. Impulse tests The AC tests may be classified as destructive and nondestructive tests. The PF and DF tests are considered nondestructive since the test voltages used in performing these tests do not exceed line-to-neutral voltages of the equipment being tested. The basic principle of the nondestructive testing is the detection of a change in the measurable characteristics of an insulation that can be associated with the effects of contaminants and destructive agents without overstressing the insulation. The AC high potential, VLF, and AC series resonant tests may be classified as destructive since the test voltages associated with these tests are higher than normal operating voltages which may overstress the insulation. The effect of repeated high voltage (HV) tests on insulation are cumulative and therefore thoughtful consideration should be given on the benefits of these tests for routine field and maintenance testing, except for special investigations or for acceptance testing. The induced frequency, PD, and impulse tests are primarily conducted at the factory during manufacturing of electrical apparatus and equipment.

PF/DF Testing

The PF/DF tests measure insulation capacitance, AC dielectric losses, and the ratio of the measured quantities. When insulation is energized with an AC voltage, the insulation draws a charging current. This charging current comprises of two components called capacitive current and resistive current. The capacitive current leads the applied test voltage by 90°, whereas the resistive current is in phase with the voltage. The capacitive current is directly proportional to the dielectric constant, area, and voltage and inversely proportional to the thickness of the insulation under test. The capacitive current may calculated by the following formula: Icap= E/ Xc = E x ω x C Icap= E x ω x Єo x Єr x (A/d) Where E is the test voltage C = Єo x Єr x (A/d) Єo is the dielectric constant of vacuum (0.08854 × 10-12 F/cm) 26 26

Єr is the dielectric constant of the insulation A is the area (cm2) d is the thickness of insulation Changes in the capacitive current indicate degradation in the insulation, such as wetness or shorted layers, or change in the geometry of the insulation. The resistive current supplies the energy lost due to dielectric losses such as carbon tracking, volumetric leakage, surface conduction, and corona. Dielectric losses due to water contamination or carbon tracking or other forms of deterioration increase by the square of the voltage, where as dielectric losses due to corona increase exponentially as the voltage increases. PF/DF testing is sensitive enough to detect a deteriorated moisture problem in the insulation compared to an insulation resistance test.

Basic Test Connections (Test Modes) Basic test configurations that simplify testing on complicated insulation systems inside HV apparatus.

Grounded-Specimen Test Mode

In grounded-specimen test (GST) mode, all current between the AC source and ground (through CX) is measured by the bridge. GST is used when one terminal of the insulation to be measured is permanently connected to ground, such as a bushing flange, transformer tank, or grounded apparatus frame. GST mode also connects the LV lead(s) directly to ground. This enables the lead (s) to be used to ground a specimen terminal that is not normally grounded.

GST Mode with Guard (GST-G)

In Double test sets this connection is referred to as guard-specimen test mode. In this mode, all current between the AC source and ground (through CX) is measured by the bridge. The LV lead(s) may be connected to the test circuit guard. Any current present on the LV lead(s) during the test are bypassed directly to the AC source return, and are eliminated from the measurement. GST-G mode is used to isolate an individual section of insulation and test it without measuring other connected insulation.

Ungrounded-Specimen Test Mode (UST)

In ungrounded-specimen test (UST) Mode, only current between the AC source and the LV lead (through CX) is measured. Any current fl owing to a grounded terminal is bypassed directly to the AC source return, and is eliminated from the measurement. UST mode is only used to measure insulation between two ungrounded terminals of the apparatus. In UST mode, ground is considered guard since grounded terminals are not 27 27

measured. UST mode is used to isolate an individual section of insulation and test it without measuring other connected insulation. The PF test as applied to transformers is the most comprehensive test for detecting insulation degradation, usually caused by moisture, carbonization, and other forms of contamination. Depending on the type, size, and voltage rating of transformer, the PF test may be performed as an overall transformer PF test, or on individual components of the transformer to localize the dielectric circuit for effective analysis of the test results; that is deterioration in the solid winding, bushing, and liquid insulation can be localized by separate tests on these components. Generally, it is common practice to perform PF tests of the bushing and the solid winding together on medium-voltage transformers that have solid porcelain-type bushings. On HV transformers with condenser- type bushings, the PF tests are performed on the individual bushings by the UST method. On all other bushings, hot-collar tests are performed by the GST method.

Hot-Collar Tests of Non-condenser-Type Bushings The hot-collar tests may be performed on compound-type, porcelain drytype bushing, oil-filled bushings, and cable pot heads. The collar is energized by the test voltage (thus the term hot collar), while the center conductor is grounded. It is well-established fact that the compound and dry type bushing fail from leaks that develop in the top end of the bushing allowing moisture to enter the bushing chamber. As a result, leakage paths are established which lead to bushing failure. By applying collar test in the upper region of the bushing, moisture, or deterioration can be detected in the early stages. The collar tests are also useful in detecting low levels of oil or compound in bushing and pot heads. The collar can be made of conductive rubber or metallic foil, braid or wire. When performing collar tests, care should be used to ensure that the collar makes intimate contact with the surface of the bushing or pothead. Hot-collar tests may be made as single collar tests or multiple collar tests .A single hot-collar test consists of a measurement between an externally applied collar and the bushing, while the center conductor is grounded (GST mode). In this test mode all currents passing between the energized collar and ground are measured. In the UST mode, current between the energized collar and the center conductor are measured including the surface leakage currents flowing over the upper portion of the bushing whereas the surface leakage currents flowing in the lower portion of the bushing (grounded flange mounting) are not measured. In the guard mode, the currents between the energized collar and the center conductor are measured and the surface leakage currents fl owing over the upper and lower portion of the bushing are guarded, whereas the surface leakage current over the upper portion of the bushing only are guarded.

PF/DF TEST FOR BUSHINGS

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The problems found in bushing are • Cracks • Dirty bushings • Loss of oil or compound • Short-circuited condenser bushings • Wet or deteriorated bushings or tap insulation • Dirty tap insulation • Corona in bushing insulation system The PF tests including the hot-collar tests performed on similar types of bushing under the same test and weather conditions should test similarly, and be within acceptable limits. When PF of a clean bushing increases significantly from its initial value, it is usually due to the effect of contamination, such as moisture which lowers the dielectric strength of the bushing. The possibility of the failure of the bushing in service increases as its dielectric strength decreases due to the effect of contamination. PF tests, made on a regular basis, have been used in assessing the serviceability of the bushing over the years. To decide whether a bushing should be removed from service because it has a slightly higher PF than normal depends upon the magnitude of the overall PF and hot-collar test results. However, a bushing that shows a substantial increase in PF each year is an indication of potential failure hazard. It is recommended that the bushing insulation should be evaluated based on the results of PF, capacitance, and hot-collar test results. With regard to hot-collar test, higher than normal losses are indicative of contamination or deterioration of bushing insulation. Any bushing differing significantly from others by few milli-watts (up to one-tenth of a watt for the 10 kV test) should be investigated. The watts loss limit in bushing for the 2.5 KV test is approximately 0.15 W. The loss of oil or compound may be detected by comparing the hot-collar test current rather than the PF value. Abnormally low test current (10%–15%) may indicate absence of compound or oil. Testing under successively lower petticoats (skirts) will show normal current reading when compound or oil is reached.

Transformer Excitation Current Test Transformer excitation current test is another test that can be performed with the PF test equipment. Excitation current is also known as the NOLOAD OR MAGNETIZING CURRENT of the transformer. In this test, voltage is applied to the primary windings one at time with all other windings left open. The excitation current of a transformer is the current the transformer draws when voltage is applied to its primary terminals with the secondary terminal open. The excitation current test, when used in routine preventive maintenance or field acceptance testing of transformers, provides means of detection for winding problems, such as short-circuited or open turns, poor joints or contacts, core problems, etc. The excitation current test is conducted on each phase winding at a time, that is only one winding is under test with the other winding including the secondary winding are floating. When performing this test, the bushings are not shorted together like they are 29when conducting the PF tests. The 29

test is performed by applying voltage to one end of the transformer winding and connecting the other end with the LV switch of the PF test set in the UST position. Three measurements are routinely made (H1–H0, H2–H0, and H3–H0) at voltages generally below rated voltage—not exceeding 2.5 or 10 kV depending upon the PF test set equipment. The LV winding is isolated from its source or load and is left floating during the test. The neutral is left grounded, as is in normal service. The exciting current flows in the core such that two high and one low readings are obtained because the middle leg carries a lower amount of current compared to the two outer core legs. This is because when the middle winding H2–H0 is energized, the current flows only in the middle leg and does not involve the joints and yokes of the core, where as when the H1–H0 and H3–H0 windings are energized, the current flow path involves the respective core leg, two joints and two yokes which gives the higher current.

WINDING RESISTANCE It helps in determining the following: I2 R losses in transformer  Winding temperature at the end of temperature rise test of transformer  As a bench mark for assessing possible damages in the field.

It is done at site in order to check for abnormalities due to loose connections, broken strands of conductor, high contact resistance in tap changers, high voltage leads and bushings .The resistance is measured at ambient temperature and then converted to resistance at 75c for all practical purposes of comparison with specified design values, previous results and diagnostics. Winding resistance at a standard temperature of 75c is

R75=Rt [(273+75) / (273+t) ] MAGNETIC BALANCE TEST It is used to find the flux distribution (magnetic), core assembly condition and defect in winding (if any).It may check at LV side or HV side as per site condition. For star connection, apply the supply at u & n measure the value at v & n, w & n and repeat the same with other terminals. For delta connection, apply voltage must be in phase-phase live u & v, v & w, w & u. Left side phase AN Voltage applied at left side phase 230 V Voltage applied at central phase 115 V Voltage applied at right side 50 V phase 30 30

Central phase BN 180 V 230 V

Right side phase CN 50 V 115 V

180 V

230 V

Procedure for magnetic balance test for transformer:1. First keep the tap changer of the transformer in normal position. 2. Now disconnect the neutral from the ground. 3. Apply the single phase AC supply 230volts across HV and Neutral terminal. 4. Now measure the voltages in the other terminals with respect to neutral. 5. Repeat the test for each other three phases.

TRANSFORMERS’ TURNS RATIO TEST The performance of a transformer largely depends upon perfection of specific turns or voltage ratio of transformer. So it is an essential test for transformers. It is performed as a routine test for transformer. So for ensuring proper performance of electrical power transformer, voltage and turns ratio test of transformer is one the vital tests. Procedure for TTR:  

Apply three phase 415 volts to HV winding, keeping LV open Measure induced voltages both on LV and HV sides Repeat the same for different tap positions separately.

The TTR detects high resistance connections in the lead circuitry or high contact resistance in tap changer by higher excitation current and difficulty in balancing the bridge.

VECTOR GROUP This test is done to conform whether the vector group provided in the name plate is correct or not and also to confirm that the winding has not been damaged during transport, installation and erecting. A vector group is a means of identifying which combination of three phase connection (wye-delta, delta-delta, delta-wye etc.) will allow three phase transformer to be paralleled with each other as some combination cannot be paralleled.

OIL BREAK DOWN VOLTAGE TEST The dielectric BDV test is an important test to determine with standing capacity of any insulating oil or liquid. There is a degradation of transformer oil or ingress of moisture and it is necessary to test the insulating oils periodically.BDV test is one of the most common test done 31 31

on all insulating fluids but a very critical one.BDV is test of choice because it takes very less time to conduct and is a precursor to the condition of the insulating liquids, before carrying out extensive series of tests.

SWEEP FREQUENCY RESPOSE ANALYSIS SFRA is a powerful and sensitive method to evaluate the mechanical integrity of core, winding and clamping structure within power transformer by measuring their electrical transfer function over a wide frequency range. SFRA is a proven method for frequency measurement. SFRA Analysis can detect problems in transformers, such as:       

Winding deformation-axial, radial like hoop buckling, tilting, spiraling. Displacement between HV and LV winding Partial winding collapse Shorted open turns Faulty grounding of core (or) screens Core movement Broken clamping structures Problematic internal connection

DISSOLVED GAS ANALYSIS DGA is the single most important test performed on oil from transformers. As the insulating materials in a transformer break down due to thermal and electrical stresses, gaseous by-products are formed. The by-products are characteristic of the type of incipient fault conditions the materials involved and the severity of the condition. Indeed it is the ability to detect such as a variety of problems that makes this test such a powerful tool for detecting incipient fault condition and for root cause investigations are detectable in low concentration(ppm level) which usually permits early intervention before failure of the electrical apparatus occurs and allow for planned maintenance. DGA Technique involves extracting the gases from the oil and injecting them in to GAS CHROMATOGRAPH (GC).detection of gas concentration involves the use of a FLAME IONIZATION DETECTOR (FID) and THERMAL CONDUCTING DETECTOR (TCD). Most of the system also employs a methanizer which converts any CO and CO2 present into CH4. So that it can be burned and detected on the FID, a very sensitive sensor. The severity of an incipient-fault condition is ascertained by the total amount of combustible gases present (CO, H2, CH4, etc.) and their rate of generation. 32 32

Test Procedure for Oil Test 1. Purpose: A) BDV - To check the di-electric strength of oil B) PPM - To check the moisture content in oil

2. Standard: IEC 422 3. Method: A) BDV : IEC 156 B) PPM : IEC 814 4. Procedure: A) BDV - A Sample of oil has to be taken from the sampling valve of the transformer under test. The oil has to be put in the BDV container & the distance between the 2 electrodes must be set to 2.5 mm. B) PPM - A Sample of oil has to be taken from the sampling valve of the transformer under test. Mistubishi Moisturemeter is used to 33 33

measurement water content in oil. It uses the Carl Fischer titration method. 1 ml of oil has to be injected in the meter.

Test Procedure for voltage ratio Measurement 1. Purpose: - To measure the voltage ratio of one winding to another associated with a lower or equal voltage. - To check that the deviation of the voltage ratio from the specified value does not exceed the limit given in the related transformer standard. 2. Standard: IEC – 60076-1, CL 10.3 3. Method: The voltage ratio test is done phase by phase between the pairs of windings. Ratio is measured by using a ratio meter. 1-ɸ low voltage supply is given to HV windings with respective polarity from ratio meter & voltage induced on the corresponding phase of other winding (i.e. low voltage winding) is brought to ratio meter. Ratio meter directly indicates the ratio of the particular tap connected. Similarly ratio is checked at all taps & all the phases. Measurement shall be done at principle tapping for all three phases in case of polyphase transformer. During ratio measurement phase angle error & correctness of polarity is also checked. 4. Acceptance Criteria: a) % Ratio Error: ±0.5 % of declared ratio on all taps. b) Phase Angle error : 0.5 % radian (No positive phase angle error) 34 34

% Ratio error (Deviation) = [(measured ratio – calculated ratio) / calculated ratio] x 100 Calculated voltage ratio = HV winding voltage / LV winding voltage 5. Instrument used: Digital Ratio meter: Tinsley Make

6. Connection Diagram:

Test procedure for vector group & Polarity 1. Purpose: To check the phase displacement or connection symbol of the transformer & to check the phase relationship between the instantaneous induced voltages in the primary and secondary windings relative to the terminal markings. 2. Standard: IEC – 60076-1, CL 10.3 3. Method: a) The phase displacement or connection symbol of three phase transformers can be checked automatically with the “Transformer Ratio Meter “. b) Vector group is also checked with the help of voltage measurements as below. 4. Procedure:The confirmation of vector group of HV with respect to LV is done. In this case a three phase supply is given to HV terminals i.e. 1U11V1-1W1 and one terminal of HV is shorted with the corresponding terminal of LV i.e. 1U1 & 2U1 shorted, then voltages are measured between different terminals 1V1-2W1, 1W1-2W1, 1V1-2W1, 1W1-2V1. The values so obtained should satisfy that particular vector group. Since this test is carried out to establish the vector group and it depends on the available supply voltage, so theoretical values cannot be given. 35 35

YNd11:-

5. Instrument used: Digital Multimeter (Make – Motwane )

36 36

Test procedure for 3-ɸ Excitation at 415 volt at No load 1. Purpose: To measure the excitation current at 415V in order to cross check the results at site before commissioning from both HV & LV sides. 2. Procedure: 3 phase 415V, 50Hz supply is given to HV & LV winding individually. The other winding is kept open. Excitation current is measured of supplied winding. 3. Connection diagram:

4. Instrument used: a) Digital multimeter (MAKE- Rishabh) b) Digital Clamp-meter 37 37

Test procedure for Winding Resistance measurement 1. Purpose: a) To measure the winding resistance & Calculation of I R component of conductor losses. b) To check faulty joints. c) To check loose connection. 2. Standard: IEC - 60076-1, CL 10.2 3. Method: The average oil temperature is taken as the mean of the top and bottom oil temperature. The difference in temperature between the top and bottom oil shall be small. The resistances between all pairs of phase terminals of each transformer winding are measured using Digital Microohm meter (resistance meter). The measurement is performed for each connectable winding and for each tapping connection. 4. Calculation of Full winding resistance (FWR): In case of star connected 3 phase winding Ru, Rv, Rw = Phase resistance of each phase in ohms. FWR (R) = (Ru+Rv+Rw)/3*3 in ohms In case of delta connected 3 phase winding Ru, Rv, Rw = Line to Line resistance of each phase in ohms. FWR (R) = (Ru+Rv+Rw)/3*3*1.5 in ohms 5. Temperature correction for resistance: The Winding resistance measurement is made at a temperature θ1 and the measured value is R. Let θm is the reference temperature and R has to be corrected to θm by using the formula: R(θm)= R{(235+θm)/(235+θ1)} in ohm 6. Instrumentation used: a) Digital winding resistance meter for resistance measurement (MAKETETEX). b) RTD’s with temperature scanner for temp measurement (MAKERADIX). 38 38

Test procedure for Insulation Resistance Measurement 1. Purpose : To determine the insulation resistance from individual windings to ground or between individual windings. 2. Standards : CGL standard 3. Method: Insulation resistance is measured using a megger make “Megohmeter” with voltage range 0.5-5kV. 4. Procedure : a) Measure top and bottom oil temp for average oil temp. b) Connect or short all phase terminals (and neutral if applicable) of each winding (eg. HV, LV) of the transformer independently. c) Connect positive terminal of megger to winding to be tested with respect to ground or w.r.t. other winding and connect negative terminal to ground or other winding to be tested. d) Apply test voltage by starting the instrument. e) Measure insulation resistance reading in mega-ohm meter after 10 sec and 60 sec and 600 sec for polarization index. f) Insulation resistance will be measured between  (HV+N)/(LV+TANK+EARTH)  LV/(HV+N+TANK+EARTH)  (HV+N)/LV(TANK+ EARTH) g) Calculate the polarization index (P.I) which is equal to ratio of insulation resistance after 600s to insulation resistance after 60s. Note that P.I value should be greater than or equal to 1.3. This test will be conducted in cold and hot condition. Hot shall be taken as an avg oil temp of 600C. Cold shall be taken as an avg oil temp of 15-30oC. Insulation resistance between core to earth avg core to tank will also be measured at 2kV. 5. Acceptance criteria: P.I shouldn’t be greater than39or equal to 1.3. 39

6. Instrument used: a) Digital insulation resistance meter (Megger) with time measurement. b) RTD’s with temp scanner for temp measurement (MAKE-RADIX)

7. Connection diagram: For eg: (HV+N)/LV (TANK+EARTH) Combination

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Test procedure for Capacitance and Tan Delta Measurement 1. Purpose : a) To check the tan delta and capacitance of the transformer and bushings. b) For comparison with field measurements in order to assess the probable conditions of the insulation when good judgment is used. 2. Standard : C57.12.90 3. Method: Capacitance and tan δ is measured by “DOBLE INSTRUMENTS” which directly give capacitance and tan δ with voltage range from 0.5 to 12kV and frequency of 50 Hz. Capacitance and insulation power factor (Tan δ) measurements are made of windings to ground, between windings. 4. Procedure for windings and bushings : a) Measure top oil and bottom oil temp for average oil temp. b) Connect or short all phase terminals (and neutral if available) of each winding (eg. HV, LV) of the transformer independently. c) Connect HV arm cable of the instrument to winding to be tested (or to the bushings to be tested) and LV arm to the other winding or earth (or test tap of the bushing to be tested). d) Record the readings of capacitance and tan δ. e) Repeat the test for different combinations. Combination: 1) HV+N/LV; 2) HV+N/ (LV+EARTH); 3) LV/ (HV+N+EARTH) 5. Acceptance criteria : a) Capacitance: record purpose only. b) Tan δ of winding @ 20oC = 0.5% Max. c) Tan δ of bushings = 0.5% Max. Tan δ @ 20oC = Tan δ @ Ambient temp / K Where K = 0.6428 x e(0.0222 x Amb. temp) 6. Instrument used :  DOBLE make capacitance and tan δ measurement kit.  Radix make RTD’s with temp scanner for temp measurement. 41 41

7. Connection diagram :

42 42

Test procedure for Separate Source Applied Voltage test 1. Purpose: To verify the power frequency withstand strength of the line and neutral terminals and their connected windings to earth and other windings. 2. Standard: IEC – 60076-3, CL 10 3. Method: The separate source voltage test is made with a single – phase alternating voltage as nearly as possible to the sine wave form and of any convenient frequency not less than 80% of the rated frequency. The peak value of the voltage is measured. The peak value divided by √2 shall be equal to the test voltage value. The test is commenced at a voltage not greater than (1/3)rd of the specified test value and the voltage is increased to the test value as rapidly as is consistent with measurement. At the end of the test, the voltage is reduced rapidly to less than (1/3) rd of test value before switching off. The full test voltage is applied for 60s between the winding under test and all terminals of the remaining windings, core, frame and tank or casing of the transformer, connected together to earth. Voltage to applied HV/E = 38kV; LV/E = 50kV. 4. Instrument used:  Voltage divider with peak voltmeter for voltage measurement.  Stopwatch for time measurement. 5. Connection diagram: For eg. Winding to be tested: (HV+N)/ (LV+E)

43 43

Test procedure for No load and Magnetizing current Measurement 1. Purpose: a) To check the guaranteed losses and % of excitation current. b) Interturn short in winding and be noticed in this test. c) Performance of the core material. 2. Reference standard: IEC – 60076-1, CL 10.5 3. Method: The No load loss and no load current shall be measured on one of the windings at rated frequency and at a voltage corresponding to rated voltage since the test is performed on the principal tapping. The remaining winding or windings shall be left open circuited and any windings can be connected in open delta shall have delta closed. A symmetrical and sinusoidal voltage is applied across the terminals and adjusted according to a voltmeter responsive to mean value of the voltage but scaled to read rms voltage of a sinusoidal wave having the same mean value. The reading of this voltmeter is U’. At the same time, a voltmeter responsive to the rms value of voltage shall be connected in parallel with the mean value voltmeter and its indicated voltage is U. 4. Procedure: a) Connect 3-ɸ supply to low voltage winding or tertiary winding terminals, in case of three winding transformer keep all terminals of remaining windings open. b) Ensure the tap position of the transformer under test should be at normal tap. c) Test is said to be passed if the voltage applied is withstood by the transformer and the measured no load losses, excitation current at specified voltage are within the guaranteed parameters. d) Repeat the test for different 90%, 100%, 110% of rated voltage of LV winding. e) On load tap changer is to be operated for one cycle at rated voltage. 5. No load loss correction factor: The no load loss Po = Pm = (W1+W2+W3) in kW And the no load loss current Iavg = Io = (I1+I2+I3)/3 in Amp 44 44

If the voltage indicated by both voltmeter is not the same, the no load loss is corrected by: Po = Pm / (P1+K*P2) in kW Where K = (U/U’)2 Where Po = corrected no load loss Pm = measured no load loss U’ = mean voltage reading U = rms voltage reading P1 = ratio of hysteresis losses to total iron losses 0.5 for oriented steel P2 = ratio of eddy current losses to total iron losses 0.5 for oriented steel % excitation current = (avg excitation current measured) / (rated current of excited winding)*100 6. Instrument used: a) Yokogawa power analyser for measurement of voltage, current, frequency, power and harmonics. b) PT (potential transformer) for step down of voltage applied for voltage measurement purpose. c) CT (current transformer) for step down of current circulated for current measurement purpose. 7. Connection diagram:

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Test procedure for harmonics of no load current 1. Purpose : To measure harmonics of no load current. 2. Standard : IEC – 60076-1, CL 10.6 3. Procedure : The harmonics of the no load current in all the phases are measured by means of harmonic analyzer and magnitude of the harmonics is expressed as a percentage of the fundamental component. 3rd, 7th, 9th and 11th harmonics of no load current at 100% of rated voltage shall be measured. 4. Connection diagram :

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Test procedure for Lightning Impulse test with Transfer Voltage measurements 1. Purpose : This test is intended to verify the impulse voltage withstand capacity of each winding to earth and other. 2. Standard : IEC 60076-3, CL 13 3. Method : a) Lightning impulse: this test is carried out to ascertain withstand capability of the insulation system with respect to lightning impulse waveform that is a standard wave of wave shape 1.2/50 μs. The transformer is subjected to 1.2(+/-30%)/ 50(+/-20%) μs impulse voltage. The voltage applied is a function of the insulation level in accordance with IC 600076-PART III. Each phase terminal is impulse with negative polarity while the other two phases are earthed through current shunt in case of delta connected winding or earthed directly in case of star connected winding and neutral is earthed through current shunt for current measurement. In case of star connected winding, the remaining terminals are earthed during the test. Standard requirements for impulse test are as below: Parameters

IEC 60076-PART III

Front time (T1)

1.2μsec +/-30% Tol

Tail time (T2)

50μsec +/-20% Tol

Chop time

2 to 6μsec

BIL (kV)

+/- 3%

Sequence of application of impulses for line terminals are as follows: For impulse test as per IEC:  One reduced full wave (50-75%)  One full wave (100%)  One reduced chop wave (50-75%)  Two full chop wave (110%)  Two full waves (100%) 47 47

Sequence of application of impulses for neutral terminal is as follows:  

One reduced full wave. Three full wave

b) Transfer surge voltage measurement: This test is carried out to ascertain withstand voltage transferred on LV side when impulse strikes on HV side. The HV side any one phase is applied one reduced and one full wave, correspondingly on LV side same phase measurement is done of the voltage transferred during impulse. 4. Instrument used: a) High volt make impulse generator with impulse analyzer. b) High volt make impulse voltage divider for voltage measurement. c) Current shunts. 5. Connection diagram: For eg: Impulse test on HV

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Test procedure for Switching Impulse Voltage Withstand Test 1. Purpose: This test is carried out to ascertain the withstand capability of the insulation system w.r.t. the switching impulse voltage. 2. Standard: IEC 60076-3, CL 14 3. Procedure: The transformer is subjected to switching impulse voltage the voltage applied, is a function of insulation level in accordance with IEC 60076. During the test, the transformer is connected as shown. Each phase terminal is impulse with negative polarity while the applied voltage waveforms are taken on HIGH VOLT MAKE IMPULSE ANALYSER. The transformer is subjected to the following impulses:  One reduced full wave  Three 100% full waves Test shall be connected on all the phases of HV windings at different taps i.e. normal tap and both extreme taps (Phase A-Min ratio tap; Phase B-Mid ratio tap; Phase C-Max ratio tap). During the test considerable flux is developed in the magnetic circuit. In order to avoid core saturation, after every negative polarity shots two 50% positive polarity impulses shall be applied.

4. Connection diagram:

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Test procedure for Load loss and Impedance Measurement 1. Purpose: To check guaranteed load losses and % impedance. 2. Standard: IEC 60076-1, CL 10.4 3. Procedure: a) Measure top and bottom oil temp for average oil temp. b) Measurement is done by applying 3-ɸ voltage to HV terminals at power frequency and LV terminals short circuited. The impedance voltage applied and the rated current passed are measured on power analyser. Measurement of load loss is done by three wattmeter method. The load losses include:  I2R loss in windings and  Stray losses in the transformer c) Measurement will be done at principle, maximum and minimum taps for losses and % impedance voltage. Note that each measurement will be performed quickly and intervals between them will be small enough to ensure that temp rise don’t cause significant errors. d) The measured values of load loss and impedance shall be corrected to rated base MVA and reference temp. e) Test is said to be passed if the measured load loss, % impedance are within the guaranteed parameters. Load loss correction to 75oC as follows: Calculate I2R loss @ 75oC for HV and LV windings. I = Resistance winding rated current R = Resistance value at 75oC taken from resistance readings Total I2R loss @ 75oC = HV I2R loss @ 75oC + LV I2R loss @ 75oC Total I2R loss @ to C = (235+t) / (235+75) x Total I2R loss @ 75oC Total measured loss @ to C = Addition of three wattmeter readings Total measured loss @ to C at rated current = (rated current/ measured50 current)2 x Total measured loss @ to C 50

Stray loss @ to C = Total measured loss @ to C at rated current Total I2R loss @ to CStray loss @ 75oC = (235+t) / (235+75) x Stray loss @ to C Load loss @ 75oC = Total I2R loss @ 75oC + Stray loss @ 75oC % impedance @ to C = (impedance voltage at rated current/ rated voltage) x 100 Impedance voltage at rated current = (rated current/ measured current) x measured impedance voltage @ to C 4. Instrument used: a) Power analyzer for measurement of voltage, current, frequency and power. b) PT (Potential transformer) c) CT (Current transformer) d) RTD’s for measurement of temp. 5. Connection diagram:

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Test procedure for Temperature rise test 1. Purpose: The temperature of the oil and winding of a transformer loaded continuously with a certain power tends to take on value at which a balance is created between the heat generated due to losses and the heat dissipated by the cooling arrangement. This temperature which depends on the load as well as the surrounding temp, should not exceed a certain prescribed value. Hence to check these values, this test is performed. 2. Standard: IEC 60076 PART II 3. Procedure: To determine the temp rise of oil and winding, the transformer is connected similar to load loss test and total losses (i.e. no load loss + load loss) or 80% of total losses or 90% of rated current are supplied to it. They are converted into heat that must be dissipated by the cooling arrangement. Readings of the top oil, cooler top and bottom and the ambient temp (location for ambient temp measurement shall be agreed during the test) are recorded at one hour intervals. The test is continued till the top oil rise is stabilized. If the top oil rise is less than one degree for four consecutive readings, it is taken to be stabilizes. Then the rated current is circulated for one hour. Immediately, after that shut down is taken and the resistances of HV and LV windings is measured at regular time intervals of about half a minute. These values are then plotted on a graph versus time to determine the resistance values at instant of shut down, by extrapolation. The final temp rise of winding is determined from the resistance values at the instant of shut down. Temperature rise test will be carried out at maximum los tap position. Same sequence is repeated for all cooling mode. The temperature inside the transformer local control cubical shall be measured by thermometer and recorded throughout the test. The temperature rise shall not be exceed the 15oC above ambient temp. The temperature inside the cable box to be measured during temperature rise test. 4. Acceptance criteria: Temperature for oil rise and winding rise should not exceed the guaranteed values. 52 52

5. Instrument used:  Yokogawa power analyzer  Current transformer  Potential transformer  RTDs  Temperature scanner  Resistance meter 6. Connection diagram:

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Test procedure for Cooler loss measurement 1. Purpose: This is a special test to measure the losses due to the auxiliary equipment’s such as fans, pumps. 2. Standard: IEC 60076-1, CL 10.1.3 h 3. Procedure: Three wattmeter method is used to measure the power taken by fans/ pumps. Three phase supply be applied to all the regular fans/ pumps. Proper direction of rotation shall be ensured before measuring the power taken by fans.

Test procedure for Oil Leakage 54 54

1. Purpose: To check the oil tightness of the transformer. 2. Standard: CBIP Clause No. 17.3.1 3. Procedure: All oil-filled compartments shall be tested for oil tightness by completely filling oil of a viscosity not greater than that of insulating oil conforming at ambient temp and applying a pressure as per design and specification measured at the base of the tank. This pressure shall be maintained for a period of not less than 12hrs for oil, during which time no leakage shall occur. 4. Acceptance criteria: No leakage shall occur.

Test procedure for Insulation test on Core, Frame And Auxiliary winding 55 55

1. Purpose: To check the insulation withstand capability of core, frame and auxiliary winding. 2. Standard: As per R & D Plate drawing 3. Procedure: Auxiliary circuit’s insulation test: The wiring for auxiliary power and control circuitry subjected to a 1 min AC separate source test of 2kV. Core to Frame insulation test: The core to frame insulation will be subjected to a 1 min AC separate source test of 10kV. Frame to tank insulation test: The frame to tank insulation will be subjected to a 1 min AC separate source test of 10kV. 4. Acceptance criteria: The test is passed if voltage is withstood for 1 min.

Test procedure for Ratio and Polarity test on Current transformer 1. Purpose: To measure Ratio and polarity of current transformer for proper connection. 2. Standard: As per R & D Plate drawing 3. Procedure:

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Measurements in Ratiometer Direct ratio readings and polarity indication will be observed. 4. Acceptance criteria:

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The ratio and polarity readings should match as mentioned in R and D diagram.Conclusion:The transformers’ have to be carefully maintained under better supervision. The transformers’ have to be tested for a due course of time to know the condition of the transformer whether it is in a better condition or have to be replace any of the faulty one. The above mentioned tests are conducted on different types of transformers as per requirement. All the test are provided with well-defined kits so that the testing results can be easily evaluated and compared as required. The insulation strength , winding resistance , oil BDV etc; gives the overall view of the condition of transformer so that the loads connected to it will not get damaged or effected due to the failure of the transformer.

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