summer internship report on erection, commissioning and pre-comissioning of500MVA transformer at pgcil

POWER GRID CORPORATION OF INDIA LIMITED
INTERNSHIP REPORT
ERECTION, COMMISSIONING AND PRE-COMISSIONING
OF 500MVA TRANSFORMER
Produced by
Akinapelli Rahul

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AN INTERNSHIP ON
ERECTION, COMMISSIONING AND PRE-COMISSIONING
OF 500MVA TRANSFORMER
AT
POWER GRID CORPORATION OF INDIA LIMITED
400/220KV WARANGAL-SS
Submitted by: -
AKINAPELLI RAHUL
161230007(Batch 2016-2020)
As part of Bachelor of Technology (Electrical and Electronics
Engineering) Curriculum of
NATIONAL INSTITUTE OF TECHNOLOGY DELHI

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CERTIFICATE
This is to certify that AKINAPELLI RAHUL, a student of National Institute of
Technology Delhi (roll no.161230007, batch:2016-2020) from Electrical and
Electronics Engineering Department has successfully completed internship/training
programme on ERECTION, COMMISSIONING AND PRE-COMISSIONING OF
500MVA TRANSFORMER in Power Grid Corporation of India Limited, 400 KV
Sub station at Warangal for the period from 30.05.2018 to 30.06.2018 (one month).
MANAGER

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ACKNOWLEDGEMENT
The internship opportunity I had with POWER GRID CORPORATION OF INDIA
LIMITED was a great chance for learning and professional development. Therefore, I
consider myself as a very lucky individual as I was provided with an opportunity to be
a part of it. I am also grateful for having a chance to meet so many wonderful people
and professionals who led me though this internship period.
Bearing in mind previous I am using this opportunity to express my deepest gratitude
and special thanks to the Mr. J.Anjaneyulu, Asst. General Manager of POWER GRID
CORPORATION OF INDIA LIMITED who in spite of being extraordinarily busy with
his duties, took time out to hear, guide and keep me on the correct path and allowing
me to carry out my project at their esteemed organization and extending during the
training.
I express my deepest thanks to Mr. M.Chandrashekar Reddy, Manager for taking
part in useful decision & giving necessary advices and guidance and arranged all
facilities to make life easier. I choose this moment to acknowledge his/her contribution
gratefully.
It is my radiant sentiment to place on record my best regards, deepest sense of
gratitude to Mr. Ch.Sampath, Jr.Engineer for their careful and precious guidance
which were extremely valuable for my study both theoretically and practically.
Finally, I apologize all other unnamed who helped me in various ways to have
a good training
Sincerely,
AKINAPELLI RAHUL

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ABOUT POWER GRID CORPORATION OF INDIA LIMITED
The Power Grid Corporation of India Limited (POWERGRID), is an Indian state-
owned electric utilities company headquartered in Gurugram, India. POWERGRID
transmits about 50% of the total power generated in India on its transmission network.
Its former subsidiary company, Power System Operation Corporation Limited
(POSOCO) handles power management for National Grid and all state transmission
utilities. POWERGRID also operates a telecom business under the name
POWERTEL. I S Jha, an alumnus of National Institute of Technology, Jamshedpur
serves as the Chairman and Managing Director of the company.
Power Grid at Warangal is a 400/220KV Substation situated in Oglapur village,
Warangal Dist. The aim of this power grid is to connect Khammam, Bhopalpally,
Ramagundam power plants to share load &satisfy demands on the need basis. And
at this power grid 400KV voltage is step-downed to 220KV to supply power to Nagaram
and Durshed cities. For this the power grid equipped three ICT’s (Inter Connecting
Transformer) of 315MVA. Recently the third ICT is replaced with 500MVA transformer
keeping in view of future load demand across Nagaram and Durshed lines.

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TABLE OF CONTENTS
1. Introduction ………………………………………………………………………….1
1.1 Transformer……………………………………………………………………...1
1.2 Components of Transformer……………………………………………………1
2. Erection and unloading of 500MVA Transformer…………………………………7
2.1 Unloading of Transformer………………………………………………………7
2.2 Erection of Transformer…………………………………………………………8
3. Pre-Commissioning Checks/Tests of Transformer..……………………………18
3.1 SFRA Test …………………………………………………………………….20
3.2 Capacitance and Tan Delta Measurement Test………….……………..…..24
3.3 Voltage and Turns Ratio Measurement……………………………………...29
3.4 Magnetizing Current Test……………………………………………………..31
3.5 Magnetic Balance Test………………………………………………………..32
3.6 Verification of Vector Group and Polarity Test………………………………33
3.7 Short Circuit Impedance Test…………………………………………………34
3.8 Measurement of Winding Resistance………………………………………..35
3.9 Winding Insulation Resistance Measurement………………………………36
3.10 Core Insulation Test………………………………………………………..38
3.11 Oil characteristic Test………………………………………………………38
3.12 Tests on Bushing CTs……………………………………………………...39
3.13 Measurement of Earthing Pit Resistance………………………………...41
3.14 Contact Resistance Measurement………………………………………..42
3.15 Final Commissioning Checks……………………………………………..43
3.16 Energisation of Unit and Site Closing……………………………………..44

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ABSTRACT
As a student of B.Tech in National Institute of Technology Delhi, I got an opportunity
to do my summer internship in Power grid Corporation of India Limited. With this
internship my aim is to learn about the erection, commissioning and pre-
commissioning tests of the 500 MVA transformer under the guidance of the
authorities at power grid.
This involves the following points
❖ Erection of transformer involves unloading, making connections, internal
inspection, final tightness test (pressure test and vacuum test) and safety
measures taken to successfully erect the transformer after received at the site.
❖ Pre-Commissioning checks and tests of transformer involves the tests and
checks performed before energising a transformer. This section involves almost
15 tests which are compulsory needed to ensure the healthy performance of
transformer.
❖ Finally, the energising of transformer involves the closing of circuit breakers and
isolators and opening of earth switch according to the procedure and
successfully sharing the load on 220KV lines.

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1.INTRODUCTION
1.1TRANSFORMER:
Electrical power transformer is a static device which transforms electrical energy
from one circuit to another without any direct electrical connection and with the help of
mutual induction between two windings. It transforms power from one circuit to another
without changing its frequency but may be in different voltage level.
Since the invention of the first constant-potential transformer in 1885, transformers
have become essential for the transmission, distribution, and utilization of alternating
current electrical energy. A wide range of transformer designs is encountered in
electronic and electric power applications. Transformers range in size from RF
transformers less than a cubic centimetre in volume to units interconnecting the power
grid weighing hundreds of tons.

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Working Principle of Transformer:
The working principle of transformer is very simple. It depends upon Faradays law
of electromagnetic induction. Mutual induction between two or more winding is
responsible for transformation action in an electrical transformer. Faraday's law states
that "Rate of change of flux linkage with respect to time is directly proportional to the
induced EMF in a conductor or coil".
1.2 COMPONENTS OF TRANSFORMER:
These are the basic components of a transformer
1. Main Tank,
2. Laminated Core,
3. Windings,
4. Transformer Oil,
5. Conservator Tank,
6. Breather,
7. Radiator,
8. Buchholz Relay,
9. Pressure Relief Valve,
10.Tap Changer.
Main Tank:
It is a main part of transformer. It is steel made box. Transformer core is placed inside
this tank. Windings and other helpful devices are placed inside this tank. It is filled with
insulating oil (mineral oil). It is usually made of cylindrical or cubical shape depending
on transformer construction. It is coated internally and externally with colour for safety
point of view. Colour coating also provide protection in case of winding connection with
tank accidentally.
Laminated Core:
Core is made with laminated steel sheet in all type of transformers to provide
continuous magnetic path and also to provide minimum air gap. For this purpose
silicon enriched steel is used. Sometimes heat treatment is also used on steel to

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increase permeability of steel. hysteresis losses also decreased in core with increase
in permeability. By making core laminated eddy current losses also reduced in core.
Laminations of core also insulated from each other through varnish.Two phase
transformers consists of two legs and three-phase transformers are usually consists
of three legs. Cores are usually circular or rectangular in shape. laminated cores tight
with bolts to avoid vibration in core.
Windings:
Single phase transformer has one primary and one secondary winding. But three-
phase transformer consists of three primary and three secondary windings which
connects to each other with proper methods. Low voltage winding is always placed
inner side of core. High voltage is placed above the low voltage winding. Both windings
are electrically insulated from each other through insulation material. There is also a
proper distance between two windings for movement of oil. Oil acts as a cooling agent.
Because windings become hot with the flow of current in windings.
The windings must be able to withstand the large mechanical forces created by a
short-circuit. The winding insulation must be able to withstand the highest operating
temperature without excessive degradation. The cooling fluid must be able to flow
freely through spaces between the windings to remove the heat. The windings are
arranged concentrically. The highest voltage is located on the outside.
Transformer Oil:
Transformer oil or insulating oil is an oil that is stable at high temperatures and has
excellent electrical insulating properties. Transformer oil is most often based on
mineral oil, but alternative formulations with better engineering or environmental
properties are growing in popularity.
Functions and Properties of Transformer Oil:
Transformer oil's primary functions are to insulate and cool a transformer. It must
therefore have high dielectric strength, thermal conductivity, and chemical stability,
and must keep these properties when held at high temperatures for extended periods.

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To improve cooling of large power transformers, the oil-filled tank may have external
radiators through which the oil circulates by natural convection. Power transformers
with capacities of thousands of MVA may also have cooling fans, oil pumps, and even
oil-to-water heat exchangers.
Power transformers undergo prolonged drying processes, using electrical self-heating,
the application of a vacuum, or both to ensure that the transformer is completely free
of water vapor before the insulating oil is introduced. This helps prevent corona
formation and subsequent electrical breakdown under load.
Oil filled transformers with a conservator (oil reservoir) may have a gas detector relay
(Buchholz relay). These safety devices detect the build-up of gas inside the
transformer due to corona discharge, overheating, or an internal electric arc. On a slow
accumulation of gas, or rapid pressure rise, these devices can trip a protective circuit
breaker to remove power from the transformer. Transformers without conservators are
usually equipped with sudden pressure relays, which perform a similar function as the
Buchholz relay.
Conservator Tank:
This is a cylindrical tank mounted on supporting structure on the roof the transformer
main tank. The main function of conservator tank of transformer is to provide
adequate space for expansion of oil inside the transformer.

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Function of Conservator Tank of a Transformer:
When transformer is loaded and when ambient temperature rises, the volume of oil
inside transformer increases. A conservator tank of transformer provides adequate
space to this expanded transformer oil. It also acts as a reservoir for transformer
insulating oil.
Construction of Conservator Tank:
This is a cylindrical shaped oil container closed from both ends. One large inspection
cover is provided on either side of the container to facilitate maintenance and cleaning
inside of the conservator.
Conservator pipe, i.e. pipe comes from main transformer tank, is projected inside the
conservator from bottom portion. Head of the conservator pipe inside the conservator
is provided with a cap. This pipe is projected as well as provided with a cap because
this design prevents oil sludge and sediment to enter main tank from conservator.
Generally, silica gel breather fixing pipe enters into the conservator from top. If it enters
from bottom, it should be projected well above the level of oil inside the conservator.
This arrangement ensure that oil does not enter the silica gel breather even at highest
operating level.
Breather:
Whenever transformer is loaded, the temperature of the transformer insulating oil
increases, consequently the volume of the oil is increased. As the volume of the oil is
increased, the air above the oil level in conservator will come out. Again at low oil
temperature; the volume of the oil is decreased, which causes the volume of the oil to
be decreased which again causes air to enter into conservator tank. The natural air
always consists of more or less moisture in it and this moisture can be mixed up with
oil if it is allowed to enter into the transformer. The air moisture should be resisted
during entering of the air into the transformer, because moisture is very harmful for
transformer insulation. A silica gel breather is the most commonly used way of filtering
air from moisture. Silica gel breather for transformer is connected to conservator tank
by means of breathing pipe.

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Construction of Silica Gel Breather
The silica gel breather of transformer is very simple in the aspect of design. It is nothing
but a pot of silica gel through which, air passes during breathing of transformer. The
silica gel is a very good absorber of moisture. Freshly regenerated gel is very efficient,
it may dry down air to a dew point of below -40oC. A well-maintained silica gel breather
will generally operate with a dew point of -35oC as long as a large enough quantity of
gel has been used.
Radiator:
In 50KVA above transformers, radiators are used with main tank of transformer for
cooling purpose. It is like a pipes or tubes. It increases the surface area of
transformer. Radiator makes cooling in transformer more effective. This method of
cooling is called ONAN (oil natural air natural).

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Cooling fans:
In 26MVA and above transformers, cooling fans are also used on radiator. Oil
temperature gauge provide on or off signal for cooling fans. When temperature
becomes greater than 65ºC, temperature oil gauge turns on cooling fans. This method
of cooling is called ONAF (oil natural and air forced).
Oil pumps:
In 26 MVA above transformers oil pumps are also used along with cooling fans and
radiator. Oil pumps used to rotate oil in transformer. This method of cooling is called
OFAF (oil forced and air forced).
Buchholz Relay:
Buchholz relay is a safety device which is generally used in large oil immersed
transformers (rated more than 500 kVA). It is a type of oil and gas actuated protection
relay. It is used for the protection of a transformer from the faults occurring inside the
transformer, such as impulse breakdown of the insulating oil, insulation failure of turns
etc.
Working principle of Buchholz relay:
Whenever a fault occurs inside the transformer, such as insulation failure of turns,
breakdown of core or excess core heating, the fault is accompanied by production of
excess heat. This excess heat decomposes the transformer insulating oil which results
in production of gas. The generation of gases depend on intensity the of fault. Gas
bubbles tend to flow in upward direction towards conservator and hence they are
collected in the Buchholz relay which is placed on the pipe connecting the transformer
tank and conservator.

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Pressure Relief Valve:
It is a curve type mirror tube connected with main tank of transformer. It provides
protection to transformer from greater pressure. Sometime greater pressure is
developed inside a transformer due to decomposition of oil. It is necessary part of high
power transformer. Transformer can also burst without pressure relief vent. Valves are
used for filling and draining of transformer oil. It is also used for filtering and sampling.
Usually three valves are available in transformer.
Tap Changer:
In larger electrical power transformer, for proper voltage regulation of transformer, on
load tap changer is required. As there is no permission of switching off the
transformer during tap changing. The tapping arrangement is placed in separate
divertor tank attached to electrical power transformer main tank. Inside this tank, the
tap selectors are generally arranged in a circular form. The divertor switches have
contacts operating in rapid sequence with usually four separate make and break units.

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2. ERECTION AND UNLOADING OF
500MVA TRANSFORMER
2.1 Unloading of Transformer:
All the transformer unloading, and handling work should be carried out and supervised
by specialized people, following all safety rules and using supporting points indicated
on drawing. The use of any other points will result in severe damages to the
transformer. The transformer should be unloaded from trailer by using wooden
sleepers and rails for dragging the transformer to its plinth.
1) The following should be avoided during the unloading process
i. The transformer imbalance (Maximum 10 degree)
ii. Abrupt movements
iii. Impact against the ground
iv. Side Impact
2) Considerations before unloading
i. Availability of access road between unloading point and plinth.
ii. Ensure overhead crane capacity for weight of main unit.
iii. Readiness of foundation
iv. Keep under base of main unit at least 300 – 400 mm above ground level by
providing wooden slippers to facilitate jacking.
v. Remove lashing before unloading.
3) Unloading from Trailer
i. Unload main unit only on wooden slippers
ii. Jack the transformer at jacking pad only.
iii. Ensure simultaneous operation of all 4 jacks.
iv. Use only haulage lugs for hauling.
v. Ensure capacity of winches and wire ropes to be used for haulage.
vi. Do not use chain pulley block in place of winches.
4) After checking of exact position of transformer, the following sequence should
be followed.
i. Install all wheels to transformer using hydraulic jacks sized for at least 50% of
the units weight.
ii. Before resting the wheels into groove, make sure all of them properly adjusted.
iii. Lower the transformer with the help of the hydraulic jacks until it remains resting
on the bottom of the groove. Never allow the transformer to remain inclined.

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2.2 Erection of Transformer :
Erection of power transformer requires great deal of planning and arrangement of
resources. It is essential to have erection agency with skilled manpower having
experience of EHV class power transformer. Each and every unit is to be treated like
a project, so that cost, quality and time are controlled and monitored through a

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process. It will ensure that erection activities are carried uninterrupted with safety and
without any damage to transformer parts / items.
It is suggested to have kick off meeting with following main agenda:
1. Competency of erection agency and their manpower skills.
2. To confirm receipt of transformer as per BOQ in full shape
3. To confirm availability of T&P as per requirement of unit, size and rating.
4. To confirm readiness of plinth and radiator foundation as per requirement (Physical
check of dimensions)
5. To confirm overhead conductor take off or power cable terminations arrangement.
6. To confirm safety measures adopted and location hazards, if any.
7. Organization reporting structure, data recording, responsibility and clearances.
8. To confirm insurance, workmen compensation and labour related statutory
requirements.
9. Comply to specific requirements agreed in Design Review.
Safety Measures & Precautions:
1. Keep recommended fire extinguishers at site.
2. During hot oil circulation, keep fire extinguisher ready near transformer.
3. Carry out all pre-commissioning Test and final commissioning check as elaborated
in this Manual before energizing transformer.
4. Take precaution while handling PRV devices having heavy springs in compression
to safeguard person and system.
5. Replace N2 filled tank by breathable dry air of dew point less than (-40˚C) at least
for 24 hours.
6. Provide adequately rated cables & fuses.
7. Never apply voltage when transformer is under vacuum.
8. Oil spillage shall be inspected regularly and attended if any. Oil shall not be allowed
to fall on ground.
9. Keep all combustible items away at safe distance to reduce risk of fire.
10. Welding on oil filled transformer may be avoided or done as per instruction of
manufacturer only.
11. All erection personnel must use Personal Protective Equipment’s like, helmet,
safety shoe, boiler suit, etc.
12. No welding work shall be taken up near transformer.
13. Electrical equipment like filter machine, dry air generator etc., must be earthed.
14. First Aid box shall be kept ready at site.
15. Adequate lighting must be available for clear visibility
16. Cordon off the working area, particularly when transformer augmentation work in
a switchyard is taken up.
17. All major erection activity like bushing, conservator and radiators must be carried
out with crane of adequate capacity and boom size.
18. Never carry out work with unskilled workers.
19. Safety posters, like “No Smoking”, “Wear Helmet”, etc., must be displayed.

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20. Testing circuit and procedures are important to follow as per manual to avoid any
induction effect before and after the Test. Approved and tested Earth rods are
essential for this purpose.
Receipt of Transformer at Site:
1) When a transformer arrives at site a careful external inspection must be made
of the unit, its cooling system and all sealed components, referring to the
general arrangement drawing and the shipping list.
2) Inspect all packing cases and loose components for damage or missing items.
3) Check whether the transformer has arrived at site with a positive gas pressure
in case of dispatch without oil. In case of dispatch of main body in oil filled
condition, check oil level and leakages if any.
4) Should the transformer arrive at site without pressure (owing to gas leakage),
it must be assumed that moisture has entered the tank and that the moisture
will have to be driven out. In such cases, the manufacturer’s advice must be
sought.
5) In case of any oil leakage or damage is discovered, the transportation company,
the transport insurer and manufacturer shall be informed immediately.
6) A record of damage must be prepared in conjunction with other participants and
supplier representative. Minor damage which may appear unimportant should
also be recorded.
7) Confirm that case numbers match with the packing list. Check their contents
tally with the packing list if the packing case is damaged.
8) Fill in the check list for external as well as internal inspections.
9) For oil filled transformers a sample of oil should be taken from the bottom of the
tank and tested for BDV and moisture content. If the values do not meet the
relevant standards the matter should be taken up with the manufacturer.
10)Down load impacts recorded by impact recorder and analyse the same in
consultation with supplier.
Insulating Oil:
When oil is dispatched to site separately it is usually in sealed steel drums. In
some of the cases, oil is supplied in tankers also. The oil to be used to filling
and topping up must comply with oil specification given in POWERGRID
Technical Specification for acceptance criteria. Oil Samples shall be taken from
oil drums/ tanker received at site and sent to our nearest oil Lab for oil
parameter testing (BDV > 50 kV, ppm<40, Resistivity > 150 x 1012 Ohm-cm &
Tan delta < 0.0025 @ 90 °C). The later is important since dirty transportation
vessels can significantly contaminate the oil. High dielectric losses cannot be
removed by filter treatment, such lots must be rejected. If the oil is supplied in
railroad or trailer tanks, one or two samples are sufficient. If the oil is delivered
in 200 litres drums, the following scheme for checking is recommended.

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Number of drums delivered No. of drums to be checked
2 to 5 2
6 to 20 3
21 to 50 4
51 to 10 7
101 to 200 10
201 to 400 15
In case any doubt arises, number of drums to be checked needs to be
increased. However, before filling oil, each drum has to be physically checked
for free moisture and appearance. A register needs to be maintained indicating
the number of drums supplied in each lot as per LOA and number of drums of
each lot used in filling a particular Transformer/ Reactor. The oil test results
carried out as above should also be recorded.
The copy of test certificate of routine testing at oil refinery should be available
at site for comparison of test results.
Check the seals on the drums. The drum should first be allowed to stand with
bung (lid) vertically upwards for at least 24 hours. The area around the bung
should be cleaned & clean glass or brass tube long enough to reach to within
10mm of the lowermost part of the drum should be inserted, keeping the
uppermost end of the tube sealed with the thumb while doing so. Remove the
Thumb thereby allowing oil to enter the bottom of the tube. Reseal the tube and
withdraw an oil sample. The first two samples should be discarded.
Thereafter, the sample should be released into a suitable receptacle. Samples
to be collected preferably in clean glass bottles. The bottles are to be rinsed
with the same oil and to be without any air bubble.
Internal Inspection:
Before starting erection, thorough internal inspection of Transformer/ Reactor is to be
carried out by POWERGRID engineer along with manufacturer's representative.
Internal inspection should be preferred in dry and sunny weather and should be
finished as quickly as possible to avoid ingress of moisture admitting dry air.
Prior to making any entry into the transformer tank, establish a foreign material
exclusion programme to avoid the danger of any foreign objects falling into the
transformer. Loose articles should be removed from the pockets of anyone working
on the transformer cover.
i. All jewellery, watches, pens, coins and knives should be removed from pockets.
ii. Protective clothing and clean shoe covers are recommended.
iii. Tools should be tied with clean cotton tape or cord securely fastened.
iv. Plated tools or tools with parts that may become detached should be avoided.

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An inventory of all parts taken into transformer should be recorded and checked before
closing inspection cover to assure all items were removed.
If any object is dropped into the transformer and cannot be retrieved, the manufacturer
should be notified.
❖ The inspection should include:
i. Removal of any shipping blocking or temporary support.
ii. Examination for indication of core shifting.
iii. Tests for unintentional core or core clamp grounds.
iv. Visual inspection of windings, leads, and connections including clamping,
bracing, blocking, spacer alignment, phase barriers, oil boxes, and coil wraps.
v. Inspection of DETC and in-tank LTCs including contact alignment and pressure.
vi. Inspection of current transformers, including supports and wiring harness.
vii. Checks for dirt, metal particles, moisture, or other foreign material.
viii.In case of any abnormality noticed during internal inspection, same to be
referred to manufacturer, CC-Engg. & CC-OS immediately before starting
erection activities.
ix. Detailed photographs of all visible parts/ components as per above are to be
taken during internal inspection and to be attached with pre-commissioning
report.
Final tightness test with vacuum (i.e. leakage test or Vacuum Drop
Test):
Before oil filling is started, a final check is made for the tightness of the transformer
tank by applying vacuum. When vacuum is applied to a transformer without oil, a
leakage test must be carried out to ensure that there are no leaks in the tank which
would result in wet air being drawn into the transformer. The following procedure is to
be adopted:
1. Connect the vacuum gauge to a suitable valve of the tank. (Vacuum application
and measurement should be performed only on top of the main tank) - A
vacuum gauge of Mc Load type or electronic type, with a reading range
covering the interval - 1 kPa (0.1 - 10 mm mercury) to be used
2. Connect the vacuum pump to another opening.
3. Evacuate the transformer until the pressure is below 50 mbar (5 kPa).
4. Shut the vacuum valve and stop the pump.
5. Wait for an hour and take a first vacuum reading – say P1
6. Take a second reading 30 minutes later- say P2.
7. Note the volume of the tank (quantity of oil required according to the rating
plate) and express as volume, V, in m3
8. Take the difference between P2 and P1, and multiply this with the oil quantity
V. If the pressures are expressed in kPa, and the oil quantity in m3, then the
product shall be less than 3.6
9. (P2 – P1) x V < 3.6

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10.The transformer is then considered to be holding sufficient vacuum and is tight.
Continue reading (at least 2 to 3) at successive 30 min intervals to confirm the
result.
11.If the leak test is successful, the pumping will be continued, until the pressure
has come down to 0.13 kPa (1 Torr) or less. The vacuum shall then be held for
the time given in Table-3 before the oil filling starts.
12.If the specified vacuum cannot be reached, or if it does not hold, the leak in the
transformer system shall be located and corrected.
13.In case the transformer is provided with an On Load Tap Changer (OLTC),
while evacuating the main transformer tank, the diverter switch compartment
may also be evacuated simultaneously so that no undue pressure is allowed
on the tap changer chamber. While releasing vacuum, the tap changer
chamber vacuum should also be released simultaneously. For this one
pressure equalizer pipe should be connected between main tank and tap
changer. Manufacturer's instruction manual should be referred to protect the
air cell/diaphragm in the conservator during evacuation.
14.This vacuum must be maintained for the time specified as per the voltage class
in Table-3 before and should also be maintained during the subsequent oil
filling operations by continuous running of the vacuum pumps
Oil Filling:
Once the oil is tested from the drums and found meeting the requirements, the oil is
transferred to oil storage tank for oil filtration before filling inside the transformer. The
drums or trailer tanks shall not be emptied to the last drop - a sump of an inch or so is
left, to avoid possible solid dirt or water in the bottom. Before being used, the tanks
and hoses are visually inspected inside for cleanness. Any liquid residue from earlier
use will be carefully removed, and the container flushed with a small quantity of new
oil, which is then discarded. After filtration, oil sample is tested for meeting
POWERGRID specification for new oil.
Prior to filling in main tank at site and shall be tested for:
1 Break Down voltage (BDV) 70kV (min.)
2 Moisture content 5 ppm (max.)
3 Tan-delta at 90 °C < 0.01
4 Interfacial tension > 0.035 N/m
For transformer dispatched with gas (N2) filled from the works, the filling of oil inside
the tank is done under vacuum. Transformer of high voltage ratings and their tanks
are designed to withstand full vacuum. Manufacturer's instructions should be followed
regarding application of full vacuum during filling the oil in the tank.
When filling a transformer with oil it is preferable that the oil be pumped into the
bottom of the tank through a filter press or other reliable oil drying and cleaning
device should be interposed between the pump and the tank

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The oil flow at the entry valve must be controlled to maintain a positive pressure above
atmospheric and to limit the flow rate if necessary to 5000 litres / hour, or a rise in oil
level in the tank not exceeding one meter / hour (as measured on the oil level indicator)
Continue oil filling until the level reaches approximately 200 mm above the ambient oil
level indicated on the magnetic oil level gauge in the expansion vessel. Then, release
the vacuum, with dry air of dew point -40deg C or better.
The diverter tank can now be topped up at atmospheric pressure. Reconnect oil outlet
hose to valve on flange on tap changer diverter head. Reinstate breather and very
slowly top up the diverter switch such that the correct level is reached in the diverter
expansion vessel. In the event the expansion vessel is overfull drain oil from flange
into a suitable container until the correct level is reached.
When the vacuum filling of the transformer and diverter tank is complete, the cooling
system/ Radiator bank can be filled (WITHOUT VACUUM) at atmospheric pressure,
via an oil processing plant. Oil must be admitted, very slowly, through the bottom
cooler filter valve, with the cooler vented at the top and the top cooler filter valve
unblanked and open to atmosphere. As the oil level reaches the top vent, then top
valve to be closed and the processing plant can be shut down.
Note: Care must be taken not to pressurize the coolers/ radiators.
Upon completion, open the top cooler isolating valve in order to equalize the pressure
in the cooler with the transformer tank. This will also allow contraction or expansion of
the oil as the ambient temperature changes.
Before filling oil into the conservator, the air cell/ bellow to be inflated to 0.5 PSIG i.e.
0.035kg/cm2 max. by applying pressure (N2/Compressed dry air) so that it can take
shape. After releasing pressure, breather pipe is to be fitted however it is
recommended not to fit breather in position, instead a wire mesh guard over and flange
of the pipe to prevent entry of any insect inside the pipe. This will ensure free air
movement from the air cell to the atmosphere.
Use flow meter / indicator on outlet of filter machine and regulate the flow using the
valve to limit oil filling rate to 2000 litres per hour (max.) in case filter capacity is more
Oil to be pushed slowly into conservator through the transformer via valve No. 5 (valve
2,3,4 to remain open) till the oil comes out first through valve Nos. 2 & 3 (close these
valves) and then through valve No. 4. Allow some oil to come out through valve
No.4. Oil should come out freely into the atmosphere. This will ensure that air inside
the conservator is expelled out and the space surrounding the air cell is full of oil.
(Close valve No. 4). During all these operations valve No.1 shall be in closed
position.
Excess oil from the conservator is to be drained by gravity only through valve No. 1 or
through drain valve of the transformer via valve No. 5. Do not use filter machine for
draining oil from the conservator. Also, do not remove buchholz relay and its

P a g e | 23
associated pipe work, fitted between the conservator and the transformer tank while
draining oil.
Stop draining oil till indicator of magnetic oil level gauge reaches position-2 on the dial,
which is corresponding to 30 °C reading on the oil temperature indicator. Fill the
conservator according to the oil temperature and not the atmospheric temperature
After Oil filling, Hot Oil Circulation has to be applied to all the Transformers/ Reactors
except under the circumstances when active part of Transformer/ Reactor gets wet.
Following conditions can be considered to define the Transformer/ Reactor wet:
If Transformer/ Reactor received at site without positive N2 pressure.
If Dry air not used during exposure while doing erection activities
Overexposure of active part of Transformer/ Reactor during erection
(Overexposure when exposure > 12 Hrs)
Hot Oil Circulation Using Oil Flter Machine:
The circulation procedure for the main tank is as follows.
1. The Transformer/ Reactor is connected to the oil filter machine in a loop through
the upper and lower filter valves. The direction of circulation shall be from the
filter to the transformer at the top and from the transformer to the filter at the
bottom. (Please note that at the initial oil filling time the direction is reverse to
avoid air bubble formation).
2. The temperature of the oil from the filter to the Transformer should be around
60 ° C and in no case, it should go beyond 70 ° C otherwise this may cause
oxidation of oil.
3. The circulation shall proceed until a volume of oil has passed through the loop
corresponding to 2 times the total oil volume in the tank. (At freezing ambient
temperature, the circulation time is increased, circulate 3 times the volume at
temperature down to minus 20 ° C, increase to 4 times below that temperature).
1. Break Down voltage (BDV) 70 kV (min.)
2. Moisture content 5 ppm (max.)
3. Tan-delta at 90 °C 0.01 (max.)
4. Total Gas Content < 1%
5. Resistivity at 90 °C 6 X 1012 ohm-cm (min.)
6. Interfacial tension 0.035 N/m (min.)
7 Acidity 0.3 (mg KOH /g) (max.)
8 Sludge 0.05 % (max.)

P a g e | 24
3. PRECOMMISSIONING CHECKS &TESTS
Preparation for SAT (Site Acceptance Tests):
1. Site study
2. Collection of Factory Acceptance Test reports
3. Finalization of action plan for carrying out SAT
4. Prepare testing program schedule.
5. Check whether the transformer under Test had been isolated from other
electrical equipments and from induction using earth switch or local earthing
arrangement.
6. Make the Test procedure
7. Make the Test formats
8. Get the guidance of Dos and Don’ts from the experts in the field
9. Ensure for all safety assessments of Helmets, Gloves and Safety shoes.
10.Execute the tests according to the program schedule.
11.Compile the Test reports.
12.Do analysis of the Test results and ensure for healthiness of transformer.
➢ Once oil filling is completed, various pre-commissioning checks/ tests are
performed to ensure the healthiness of the Transformer/ Reactor prior to its
energization.
Following checks should be carried out before commencing the pre-commissioning
Test of the Power Transformer.
1. Ensure that Power Transformer and its auxiliaries should be free from visible
defects on physical inspection.
2. Ensure that all fittings should be as per out line General Arrangement Drawing
3. Ensure that bushings should be clean and free from physical damages
4. Ensure that oil level is correct in all bushings
5. Ensure that oil level in Main / OLTC Conservator tank in MOG is as desired.
6. Ensure gear box oil level in OLTC
7. Ensure that OTI and WTI pockets are filled with transformer oil
8. Ensure that cap in the tan delta measurement point in the bushing is grounded
9. Ensure unused secondary cores of Bushing CT’s, if any, has been shorted
10.Ensure CT secondary star point has been formed properly and grounded at one
end only as per scheme
11.Ensure that Buchholz Relay is correctly mounted with arrow pointing towards
conservator
12.Ensure all power and control cable terminals are tightened
13.Ensure all cables and ferrules are provided with number as per cable schedule
14.Ensure that external cabling from junction box to relay / control panel is
completed

P a g e | 25
15.Ensure operation of OLTC manually, electrically at local and electrically by
RTCC
16.Ensure indication of tap position on Diverter switch, Drive mechanism & RTCC
are same.
17.Ensure working of numerical AVR
Sr.no. Test / Checks Name Testing Equipments
1 SFRA Test Automatic SFRA kit
2 Capacitance and Tan delta
measurement Test
Automatic Capacitance & Tan delta
measurement kit
3 Transformer turns ratio Test Digital Ratio meter
4 Magnetizing current Test Digital multi meter
5 Magnetic balance Test Digital multi meter
6 Verification of vector group and polarity
test Digital multi meter
7 Short circuit impedance test Digital multi meter
8 Measurement of winding resistance test Digital winding resistance meter
9 Winding Insulation Resistance
Measurement
Digital insulation resistance meter
10 Core Insulation Resistance
measurement
Digital insulation resistance meter
11 Oil characteristic test Oil BDV test kit
12 Tests on bushing CT s Digital multi meter, CT primary
current injection kit, Knee point
voltage measurement kit, Insulation
resistance tester
13 Measurement of earthing pit resistance Earth resistance measurement kit
14 protection and alarms As per scheme
15 Contact resistance measurement Contact resistance measurement kit
3.1 SFRA Test:
(a) Purpose of the test
The transformer is considered to be a complex network of RLC components. The
contribution to this complex mesh of RLC circuit are from the resistance of the copper
winding, inductance of the winding coils and capacitance from the insulation layers
between coils, between winding, between winding and core, between core and tank,
between tank and winding etc.
(b) Principle of the test
Any form of physical damage to the transformer results in the changes of the RLC
network. These changes are looking for and employ frequency response to highlight
these small changes in the RLC within the transformer.

P a g e | 26
The test involves measuring the frequency response of each individual winding. The
frequency is measured by injecting a sine wave signal with respect to earth at one end
of winding to be tested and measuring the signal amplitude there and at other end of
winding. The attenuation (in db.) of the transmitted signal relative to reference signal
at the input terminal is measured over a frequency range from 20 Hz to 2 MHz SFRA
is used to check the eventual change in the internal geometry of the active part of the
transformer whether displacement of deformation i.e. the mechanical integrity of the
transformer. Transformers while experiencing severity of short circuit current looses
its mechanical property by way of deformation of the winding or core. During pre-
commissioning, this test is required to ascertain that Transformer active part has not
suffered any severe impact/ jerk during transportation.
(c) Procedure for the test
i. This test is carried out after completion of all commissioning activities.
ii. Factory FRA test report in soft form should be available at site.
iii. FRA signatures will be taken after assembly and oil filling and compared with
factory testing to ensure the healthiness of core/coil assembly during
transportation.
iv. Interpretation of test results carried out
v. Test results matching with the factory results
vi. 10 V AC is applied at variable frequency (20Hz to 2 MHz) to the winding for all
possible connections of the winding
vii. These signatures will be the benchmark for future reference.

P a g e | 27
viii. The FRA signatures should be analysed in conjunction with Impact Recorder
readings.
ix. Report of Impact recorder readings is to be obtained from manufacturer.
x. It is recommended to follow the standard procedure for the SFRA measurement
as per the standard test procedure recommended by the manufacturer.
xi. It should be done on maximum, normal and minimum tap of the transformer.
Type of connections for the test and typical waveform.
A.HV Phase to Neutral with LV open :
1. Feed the frequency signal from 20 Hz -2 MHz in the transformer R phase of the HV
winding with respect to the neutral (for star winding) and R phase of HV winding with
Y phase of HV winding (for delta winding).
2. Both the reference leads should be earthed properly.
3. Keep LV open (including core)
4. Kit will receive the response of the impedance characteristic in the transformer.
5. Response will be plotted in logarithmic scaled graph.
6. Repeat the all procedures for other phases
B. HV Phase to Neutral with LV shorted
1. Feed the frequency signal from 20 Hz -2 MHz in the transformer R phase of the HV
Y phase of HV winding (for delta winding).

P a g e | 28
3. Keep LV winding shorted (core is avoided)
6. Repeat all the procedures for other phases
C. LV Phase to Neutral with HV open
1. Feed the frequency signal from 20 Hz -2 MHz in the transformer R phase of the LV
Y phase of LV winding (for delta winding).
3. Keep LV winding open
D. Between HV and LV winding
1. Feed the frequency signal from 20 Hz -2 MHz in the transformer R ph of the HV
winding with respect to the LV winding

P a g e | 29
3. Keep LV winding open
Maximum possible combinations of connections for SFRA test
Test Type Test 3 Ô 1 Ô
Series Winding (OC)
All Other Terminals
Floating
Test 1 H1-X1
H1-X1
Test 2 H2-X2
Test 3 H3-X3
Common Winding (OC)
All Other Terminals
Floating
Test 4 X1-H0X0
X1-H0X0
Test 5 X2-H0X0
Test 6 X3-H0X0
Tertiary Winding (OC)
All Other Terminals
Floating
Test 7 Y1-Y3
Y1-Y2
(Y1-Y0)
Test 8 Y2-Y1
Test 9 Y3-Y2
Short Circuit (SC) Test 10 H1-H0X0
H1-H0X0
Short (X1-H0X0)
High (H) to Low (L) Test 11 H2-H0X0
Short (X1-X2-X3) Test 12 H3-H0X0
Short Circuit (SC) Test 13 H1-H0X0
H1-H0X0
Short (Y1-Y2)
High (H) to Tertiary (Y) Test 14 H2-H0X0
Short (Y1-Y2-Y3) Test 15 H3-H0X0
Short Circuit (SC)
Low (L) to Tertiary (Y)
Test 16 X1-H0X0
X1-H0X0
Short (Y1-Y2)
Test 17 X2-H0X0
Short (Y1-Y2-Y3) Test 18 X3-H0X0

P a g e | 30
3.2 Capacitance and Tan Delta Measurement Test:
a) Purpose of the test
Dissipation factor / loss factor/ Tan delta is defined as the ratio of resistive components
to that of capacitive current flowing in an insulating material. Dissipation factor (tan
delta) and capacitance measurement of bushing/winding provides an indication of the
quality and soundness of the insulation in the bushing/winding.
Changes in the normal capacitance of an insulator indicate abnormal conditions such
as the presence of moisture layer, short -circuits or open circuits in the capacitance
network.
b) Principle of the test
The capacitance and dissipation/loss factor (Tan δ / Cos φ) measurement are made
to determine the insulating condition of the transformer’s both winding to earth and
between the windings, and to form a reference for future measurements during
operating the transformer. There is a small amount of insulating loss in all insulators
used in transformer applications at normal operating voltage and frequency. In
appropriate insulators, this loss is very small. This loss changes in direct proportion
with the “square “of the applied voltage. The insulator and equivalent diagrams are
given below.
As seen in figure, the angle delta ’between the total current “Ir” and capacitive current
“IC” allows to make evaluation about the loss properties of the insulator. The loss angle
delta, depends heavily on the thick ness of the insulating material and surface
condition, structural property of the insulator, type of the material, (humidity, foreign
materials/ particles, air gaps, etc. which cause ionization the insulating material). The
conditions which increase the power losses of the insulator also decrease the
insulation strength. For this reason, loss angle measurement is a very valuable
criterion for evaluating the insulation material at a defined operating frequency.
Periodical measurements made during operating are also important to show the
general condition of the insulating material. In this way, it is possible to gather
information about aging of the solid insulating materials and degradation of the oil.

P a g e | 31
The active loss of the measurement circuit can be calculated according to below
equation:
P= U. I. Cos delta = U 2. C. ω. tan delta
(It is accepted that in very small angles, Cos delta will be equal to tan delta)
Capacitance, tan delta, active loss and Cos delta can be measured by bridge methods
at defined voltages or by a “power factor” (Cos delta) measuring instrument. The
measurement is made between windings and between the windings and the tank.
During the test, the temperature of the transformer should also be recorded and
corrected in accordance with the reference temperature. The loss factor depends
heavily on temperature. For this reason, in order to make comparisons later, it has to
be converted to reference temperature (for example 20 deg reference temperature)
by a coefficient.
c) Correction equation:
F20 = Ft / K
F20: loss factor at 20˚C temperature
Ft: loss factor value at t measuring temperature
K: correction factor
d) Equipment for the test:
10 KV or 12 KV fully automatic Capacitance and Tan delta test kit to be used for
accurate measurement and repeatability of test results
Transformer winding insulation test
a) Procedure for the test
1. In a two-winding transformer, there are three measurements of capacitance
i. HV to ground
ii. LV to ground
iii. HV to LV
2. These values of capacitance and their respective values of insulation factor (tan
delta) are to be measured.
3. All HV line terminals connected and labelled (H); all LV line terminals connected
and labelled (L); and a connection to a ground terminal, usually connected to
transformer tank labelled (G).
4. Leads from the instrument or bridge are connected to one or both terminals and
ground.
5. Either grounded specimen measurement of guarded measurements are possible,
so that all capacitance values and dissipation factor values can be determined.
6. These measurements are usually made at voltage of 10 kV or less, at power
frequency.
b) Connection diagram
For tan delta of bushings, connections are to be carried out in UST mode.
For tan delta between windings, connections are to be carried out in UST mode.
For tan delta of windings with earth, connections are to be carried out in GST mode

P a g e | 32
(i) HV winding measurement
1. Short all the three phases of HV winding and make the zero-sequence impedance.
In other words, make the current flow only in capacitance region (Omit inductance) in
the impedance network of the transformer.
2. Make HV cable connection at HV terminals and LV cable at neutral (ground should
be isolated). If it is delta connection, then LV cable connection to be made in next
phase.
3. Select the test mode GST-G for capacitance measurement in between windings
and GST-YG for capacitance measurement in between windings.
3. Apply 10 kV voltage in the stepwise manner and cross check the values in all step
voltages.
4. Note the measurement values of applied voltage, leakage current, power factor and
dissipation factor, capacitance, humidity and ambient temperature.
(ii) HV-LV connection

P a g e | 33
1. Short all the three phases of HV and LV winding and make the zero-sequence
impedance. In other words, make the current flow only in capacitance region (Omit
inductance) in the impedance network of the transformer.
2. Make HV cable connection at HV terminals and LV cable at LV terminals.
3. Select the test mode UST-YG for capacitance measurement in between tank and
winding.
voltages.
5. Connections of the leads to be carried out as per diagram.
(iii) LV- Earth connection
1. Short all the three phases of LV winding and make the zero sequence impedance.
In other words, make the current flow only in capacitance region (Omit inductance) in
the impedance network of the transformer.
2. Make HV cable connection at LV terminals and LV cable at Neutral terminals.
(Ground should be isolated). If it is delta connection then LV cable connection to be
made in next phase.
3. Select the test mode GST-G for capacitance measurement in between windings
and GST-YG for capacitance measurement in between windings.
voltages.
5. Connections of the leads to be carried out as per diagram
(iv) LV-HV connection
1. Short the all three phases of LV and HV winding and make the zero-sequence
impedance. In other words, make the current flow only in capacitance region (Omit
inductance) in the impedance network of the transformer.
2. Make HV cable connection at LV terminals and LV cable at HV terminals.

P a g e | 34
3. Select the test mode UST-YG for capacitance measurement in between tank and
windings
4. Apply 10 kV in the stepwise manner and cross check the values in all step voltages.
5. Connections of the leads to be carried out as per diagram
Transformer bushing insulation test
a) Procedure for the test
Measurement of C1 Capacitance and Tan delta
1. Connect the crocodile clip of the HV cable to the top terminal of the shorted HV/IV
bushings.
2. Unscrew the test tap cover, insert a pin in the hole of the central test tap stud by
pressing the surrounding contact plug in case of 245 kV OIP Bushing and remove
the earthing strip from the flange by unscrewing the screw (holding earth strip to the
flange body) in case of 420 kV OIP Bushing.
3. Connect the LV cable to the test tap (strip/central stud) of the bushing under test
to the kit through a screened cable and earth the flange body.
4. Repeat the test for all Bushings by changing only LV lead connection of the kit to
test tap of the Bushing which is to be tested
Measurement of C2 Capacitance and Tan delta
1. HV lead to be connected to the test tap of the bushing under test (if required
additional crocodile type clip may be used) and LV of the kit to be connected to the
ground. HV of the bushing is to be connected to the Guard terminal of the test kit.
2. Test to be carried out in GSTg mode at 1.0kV.
3. Repeat the test for all Bushings by changing only LV lead connection of the kit to
test tap of the Bushing which is to be tested

P a g e | 35
3.3 Voltage and Turns Ratio Measurement:
3.3.1 Voltage Ratio Measurement
To determine
a. Any abnormality in tapings in the winding
b. Any abnormality in the winding
c. Result ensures the inductance property of the transformer
b) Equipment for the test
Digital multi meter
c) Principle for the Test
The total voltage induced into the secondary winding of a transformer is proportional
to the number of turns in the primary to the number of turns in the secondary, and by
the amount of voltage applied to the primary
.
d) Procedure for the Test
1. Keep the tap position of the transformer in lowest position and LV in open
2. The voltage should be applied in the high voltage winding in order to avoid unsafe
voltage
3. Apply 3 phase 415 V on HV terminals
4. Measure the voltage on each phase (ph-ph) on HV and LV terminals
simultaneously.
5. Ratio measurement must be made on all the taps to confirm the proper alignment
and operation of the tap changer.
6. Calculate the turns ratio in each tap position of tap: Vp/Vs
3.3.2 Transformer Turns Ratio Test
To determine the turns ratio of transformers to identify any abnormality in tap
changers/shorted or open turns etc.
b) Equipment for the test
Automatic Transformer turns ratio (TTR) meter

P a g e | 36
c) Principle for the test
The total voltage induced into the secondary winding of a transformer is
proportional to the number of turns in the primary to the number of turns in the
secondary, and by the amount of voltage applied to the primary.
1. Connect H1 and H2 leads on HV winding and X1 and X2 leads on LV winding.
2. Apply 110 V AC from the kit.
3. Adjust the phase angle error on zero.
4. Adjust % error knob such that null detector shows zero.
5. Carry out the test for all taps.
3.4 Magnetizing Current Test
a) Purpose of the Test
Excitation/ Magnetizing current test is performed to locate defect in magnetic core
structure, shifting of windings, failure in turn to turn insulation or problem in tap
changer.
b) Principle of the Test
Excitation/ Magnetizing current is the current required to force a given flux through the
core. It is the RMS value of the current flowing through a line terminal of a winding
when voltage is applied at rated frequency, the other winding being open circuited.
c) Equipments for the Test
Digital multi meter
d) Circuit for the Test
e) Procedure for the Test
i. Apply 3 phase 440 V AC supply on HV terminals and keep LV open.
ii. Measure current in all the three phases.
iii. Carry out the test on max., normal and min. tap position.
iv. Repeat the test for LV side.

P a g e | 37
f) Acceptance Criteria
i. Excitation current < 50 milli-Amperes, then difference between two higher
currents should be less than 10%.
ii. Excitation current > 50 milli-Amperes, then difference between two higher
currents should be less than 15 %.
iii. Value of center leg should not be more than either outside for a three-phase
reactor.
iv. Results between similar single-phase units should not vary more than 10%.
3.5 Magnetic Balance Test
To check the balance in the magnetic circuit (core balance) in three phase
transformers. It verifies core balance.
The voltage should be applied in one phase and measured in the other two phase of
the winding. The sharing of the voltage will be maximum in the next phase of the
winding (clock wise) more than 60% of the injected voltage and minimum voltage
appear in another phase of the winding (clock wise) less than 40% of the injected
voltage.
c) Equipments for the Test
Digital multi meter

P a g e | 38
i. For delta connected winding, apply 3 phase 440 V between phase to phase of
a winding and measure the voltage induced in other two phases of the same
winding.
ii. For star winding, apply 1 phase 230 V between phase and neutral and measure
the voltage induced in other two phases of the same winding.
iii. Similarly, all the phase should be checked with reference to other two phases
F) Acceptance Criteria
i. The identical results confirm no damage due to transposition.
ii. Zero voltage or very negligible voltage induced in any of the other two phases
shall be investigated.
iii. The applied voltage may be expressed as 100% voltage and the induced
voltage may be expressed as percentage of the applied voltage. This will help
in comparison of the two results when the applied voltages are different.
iv. The voltage induced in the centre phase shall be 50 to 90% of the applied
voltage.
v. However, when the centre phase is excited then the voltage induced in the
outer phases shall be 30 to 70% of the applied voltage.
vi. Zero voltage or very negligible voltage induced in the other two windings should
be investigated.
3.6 Verification of Vector Group and Polarity Test
To verify the phase angle relationship in the winding and polarity of transformer
By shorting the R phase of HV and LV terminals, the magnitude of the phases by using
the phase angle relationship is obtained
c) Equipment for the Test
Digital multi meter
i. Keep the tap position of transformer at normal
ii. Short 1U of HV and 2U of LV
iii. Apply 440 V 3 phase supply to HV terminals
iv. Measure the voltage across the following terminals as below
v. Make conditions in the way of arithmetical and logical for verifying the phase
angle difference
For Yy0 transformer
Rn+Nn=RN
Bb=Yy
By=Yb
For Yna0d11 transformer
2R1-N=2Y1-N=2B1-N=Constant

P a g e | 39
2R1-1B1=3R1-N>3Y1-N>3B1-N
3Y1-1B1>3Y1-1Y1
Yb
3.7 Short Circuit Impedance Test
This test is used to detect winding movement that usually occurs due to heavy fault
current or mechanical damage during transportation or installation since dispatch from
the factory. It is expressed as a percentage of the rated voltage of former winding. In
this case current flowing through secondary is the full load current and is indicative of
copper losses.
Rated full load current to flow through this winding when secondary winding is shorted,
is known as impedance voltage
Digital ampere meter
1. Connect the 3 phase 440 V supply to the HV winding and short the 3 phase of LV
winding.

P a g e | 40
2. Measure primary voltage and current on HV and LV.
3. Carry out the test on min., normal and max. Tap positions.
f) Acceptable Criteria
The measured impedance voltage should be within 3 percent of impedance specified
in rating and diagram nameplate of the transformer Variation in impedance voltage of
more than 3% should be considered significant and further Investigated
3.8 Measurement of Winding Resistance
To check for any abnormalities like loose connections, broken strands and high
contact resistance in tap changers due to vibr ations, fault occurred due to poor
design, assembly, handling, poor environment, over loading or poor maintenance and
gross difference between the windings and for openness in the connections.
Kelvin bridge technique was adopted in the way of modern digital micro processor
method for measuring the winding resistance. Voltage drop is proportional to the
winding resistance by injecting the DC current. Voltage drop across the winding
terminal in the phase is in the following manner
U= R*I + (d0/dt); 0-flux
L= 0/t
When DC current is applied, (d0/dt) =0
So, U=R*I
Automatic winding resistance measurement kit which work on the principle of Kelvin
Bridge.
1. Connect the current and potential leads to the transformer winding.
2. The potential leads must be connected between the current leads.
3. Do not clip the potential leads to the current leads.
4. Do not use additional extension cable leads.
5. Observe the polarity.

P a g e | 41
6. Select the test current range, which should be more than 1 % of rated current.
7. Test current should not be more than 10% of rated current.
8. If less than 1%, measured resistance is not consistent.
9. If more than 10%, it could cause erroneous readings due to overheating of the
winding.
10. If possible, always inject test current to HV and LV simultaneously (two channel
measurement).
11. This will magnetize the core more efficiently and shorten the time to get stable
readings.
12. Measure the readings in both positive and negative polarities (tap position in
ascending and descending position)
13. During tap changing operation, continuity checks between HV to neutral to be
carried out by analog multi-meter while changing tap.
14. For delta connected windings, such as tertiary winding of auto-transformers,
measurement shall be done between pairs of line terminals and resistance per winding
shall be calculated as per the following formula
a. Resistance per winding = 1.5 x Measured value
15. Take the winding temperature reading while doing the resistance measurement.
16. Calculate the resistance at 75°C as per the following formula
17. R = R (235+75)/(235+t ),Where R = Resistance measured at winding temperature
(t)
f) Acceptance criteria
All readings should be within +/- 1% of each other. There should be even increase or
decrease of resistance value for the winding having tapping winding for all three
phase.
3.9 Winding Insulation Resistance Measurement
This test reveals the condition of voids in the dielectric insulations like solid insulation
in the winding due to heat or moisture, any dampness solubility in the oil, presence of
any foreign objects which is having the corrosive characteristic present in the bushing.
If a test voltage is applied across a piece of insulation, then by measuring the resultant
current and applying ohm’s law (R=E/I), the resistance of the insulation can be
calculated. Effect of temperature in measurement. For every 10 deg increase in
temperature, half the resistance; or for every 10 deg increase in temperature, double
the resistance. For example, a 100 Gohm resistance at 20 deg becomes 25 Gohm at
40 deg. Cen.

P a g e | 42
Motorized or battery-operated insulation tester (0-5 KV range)
a. HV/E+LV Shorted
b. LV/E + HV Shorted
c. HV /LV + Ground Open
1. Connect leads to HV winding and LV winding to measure IR between windings.
2. Apply 5 KV and take measurement at interval of 15 sec, 60 sec and 600 sec

P a g e | 43
3. Connect leads between winding and earth to measure IR between winding and
earth.
4. Keep the other windings shorted and earthed.
5. Repeat the test.
3.10 Core Insulation Test
To check that core is not earthed other than the specific earth point.
The ground connection terminals for the transformer core are located at top plate of
transformer in a box. The terminals are protected by a cover. The terminal box contains
a terminal block with three terminals
1. Terminal marked CL is connected to core lamination
2. Terminal marked CC is connected to core clamps
3. Terminal marked G is connected to ground
c) Equipment for the Test:
Motorized or battery operated insulation tester (0-5 KV range)
d) Procedure for the Test:
1. Shorting link between Core, frame and earth to be removed
2. Apply 2.5 KV and take measurement at interval of 60 sec
3. After completion of Test, provide shorting link between core, frame and earth
Acceptance Criteria
IR should be > 1000 MΩ
3.11 Oil Characteristic Test
To check the electrical, mechanical & chemical property of the oil. The dielectric break
down voltage Test is an important Test to determine the withstanding capacity of any
insulating oil. There is a degradation of transformer oil or ingress of moisture and it is
necessary to Test the insulating oil periodically.
b) Equipment for the Test
Motorized oil BDV Test kit
c) Procedure for the Test:
1. Take oil sample from the bottom of the main tank in the oil cup. Always flush drain
valve before taking sample
2. Also flush the oil cup
3. Let the bubbles be settled down

P a g e | 44
4. Carry out the Test and take reading at which oil insulation break down
5. Take 6 readings at time interval of 10 min.
6. Average of the 6 readings is final BDV of oil
7. Oil sample to be collected from bottom of the main tank and to be sent to ERDA to
carry out tests as per IS 1866
8. Oil sample to be collected from bottom of the main tank and to be sent to ERDA to
carry out tests for DGA
Acceptance Criteria
Appearance Clear, free from sediment and suspended matter Visual
1. Break Down voltage (BDV) : 70kV (min.)
2. Moisture content : 5 ppm (max.)
3. Tan-delta at 90 °C : Less than 0.01
4. Interfacial tension :
More than 0.035
N/m
3.12 Tests on Bushing CTs
These tests are carried out to identify the healthiness of bushing CT s and verifying
the measuring and protection systems
b) Equipment for the Test
i. Digital multi meter
ii. Insulation tester
iii. Knee point measurement kit
iv. Current injection kit
c) Procedure for the Test
While testing, the other windings on the same phase of the transformer may have to
short circuited in order to obtain a stable reading. It is better to demagnetize any CT
that is tested by impressing DC voltage across winding.
A. Polarity Test
The polarity should show in accordance with the terminal markings
b) Procedure for the Test
1. The polarity test can be done by inductive kick of direct current

P a g e | 45
2. In this test, a 1.5 V battery is connected to the primary P1-P2 of CT in such a manner
that +ve terminal of batter to be connected to P1 and –ve terminal to be connected to
P2.
3 Connect +ve lead of voltmeter in S1 and –ve terminal in S2
4. Close the switch and apply 1.5 V DC, check the deflection
5. It should be in +ve deflection
c) Acceptance Criteria
All CT secondary polarity should be as per name plate
B. Current ratio Test
To measure the ratio of a primary to secondary current in the bushing CT and find the
current ratio error
1. Connect the current injection kit to the primary of the bushing CT and measure the
current through tong tester having the range of 300A
2. Measure the current through tong tester having the range of 300mA in secondary
CT
3. Apply 20% of rated current to he primary side
4. Measure the corresponding primary and secondary current.
5. % error of CT ratio = (Measured current ratio- Theoretical ratio)/Theoretical ratio
C. Excitation Current
The magnetization test is conducted in order to see the condition of the turns of the
secondary. This test gives the indications regarding the shorting of turns CT secondary
winding and to establish CT characteristics as well as capability of CT.
b) Procedure of the test
1. Apply AC voltage to the secondary winding of the CT with primary open circuit
2. Vary the applied voltage from 25% of Vk to 110 % of Vk
3. Measure the current drawn by the winding at each selected value is recorded
4. Verify that, exciting current is less than specified at Vk/2
5. This test should not be performed for metering core
6. If Knee Point Voltage is not mentioned then Knee Point Current may be taken into
consideration.
D. Insulation Resistance Measurement
To check any shorting of any CT secondary core with earth or between cores

P a g e | 46
1. IR measurement secondary core to earth
2. Connect insulation tester leads between CT secondary and earth
3. Apply 500 V DC and measure IR value
4. Carry out test for all cores of all HV and IV CTs
5. IR measurement secondary core to core
6. Connect insulation tester leads between CT secondary cores
7. Apply 500 V DC and measure IR value
8. Carry out test for all combinations of core to core for all HV and IV CTs
c)Acceptance Criteria
Insulation resistance should be more than 50 Mega ohm
F. Continuity test of Bushing CT secondary winding
To check any open of any CT secondary core including pilot wires up to C & R panel
1. Connect multi meter across each core and check continuity up to Tr MK box and up
to remote protection panel end.
G. Winding resistance measurement of bushing CT secondary
winding
To check healthiness of winding of CT secondary up to MK Box of Transformer
1. Connect multi meter across each core and measure winding resistance up to Tr
MK box.
3.13 Measurement of Earthing Pit Resistance
To measure value of earthing pit resistance and verify that, fault current has
minimum resistance to ground
There is hand operated D.C.generator. While feeding current to spike, D.C. current is
converted into A.C. current by the converter and A.C. current received from spike is

P a g e | 47
again converted in D.C. current by the help of rectifier, while going to generator. A.C.
current is fed to the spike driven in earth because there should not be electrolytic
effect.
c)Procedure for the Test
1. Earth tester is used for measurement of Earth resistance.
2. For measurement of earth pit resistance, pit earthing connection should be
disconnected from main grid.
3. Earth tester terminals C1 & P1 are shorted to each other and connected to the earth
electrode (pipe) under test.
4. Terminals P2 & C2 are connected to the two separate spikes driven in earth.
5. These two spikes are kept in same line at the distance of 25 meters and 50 meters
due to which there will not be mutual interference in the field of individual spikes.
6. If we rotate generator handle with specific speed we get directly earth resistance on
scale.
7. If earth resistance is more, proper treatment is to be given.
d) Acceptance Criteria
Value of earth pit resistance should be less than or equal to 1 ohm.
3.14 Contact Resistance Measurement
To determine the firmness of torque level in between the bushing jumper and
transmission line If torque level is more than or less then the standard position, the
heat will dissipated in the joints. It leads to corrosion in the joints.
Voltage drop is proportional to the contact resistance by injecting the DC current. It
depends on the voltage drop across the contact terminal in between the transmission
line and the jumper of the bushing.

P a g e | 48
c) Procedure
Direct measurement of resistance by using micro ohm meter
d)Acceptance Criteria
The value of contact resistance should not be more than 5 micro ohm per joint/
connector
3.15 FINAL COMMISSIONING CHECKS
It is important to ensure seamless, full and final integration of power transformer in a
substation after commissioning tests specified above. These checks are related to
functional and operational conditions within transformer elements as well as external
interfaces so that transformer can perform successfully in a transmission system.
1. All the test results of unit are verified and compared with factory results before
commissioning.
2. No leakage of oil in any part of unit.
3. Ensure external electrical clearance of conductor jumpers in the switchyard with
transformer body, gantry, column, jumpers, fire wall etc.
4. Ensure that tertiary winding terminals are insulated, when they are not used /
connected to any system.
5. Ensure earthing of Neutral, main tank body, radiator frame structure, fans and
motor.
6. Neutral earthing flat of suitable size must run through support insulator and
connected to two separate earthing pits which are in turn connected to main
earth mat of switchyard.
7. Ensure that conductor jumpers connected to HV, LV and tertiary terminals are
not tight and should have the allowance for contraction. Also ensure that
connectors are properly erected with tightness at bushing terminal.
8. Ensure that R.Y.B designated terminals of transformer are matching with R,Y,B
buses of switchyards on HV and LV side.
9. Ensure oil level in the Bushings.
10.Ensure continuity of OLTC operation at all taps.
11.In a transformer bank of three single phase units, ensure master- slave OLTC
scheme.
12.In a transformer bank of three single phase units, ensure tertiary connection
and protection scheme.
13.Ensure oil filling in conservator tank according to temperature scale in MOG
and also ensure oil level in prismatic glass.
14.Ensure that all valves between main tank and radiator banks are opened.
15.Ensure those radiator valves connected to header are open.
16.Ensure that valve to conservator tank via Buchholz relay is open.
17.Ensure physical operation of local protections like Buchholz, PRV, Surge relay
of OLTC etc.

P a g e | 49
18.Ensure OTI and WTI settings of fan & pumps operation, Alarm and Trip as per
approved drawings. Fan and pump operation shall be ensured locally and
remotely.
19.Review and ensure protection scheme of power transformer with over all
protection scheme at remote end in control room.
i. Differential Protection
ii. Restricted Earth Fault Protection.
iii. Over current and Earth fault protection.
iv. Over fluxing Protection.
v. OTI & WTI- alarm and trap
vi. RTCC panel interface with protection system
vii. Local protection like Buchholz, PRV etc.
viii. Integration of on-line condition monitoring equipments.
20.Ensure the common earthing of tank, frame and core provided in transformer.
21.Ensure the shorting of spare cores of bushing CT’s.
22.Ensure that cap in the tan delta measurement point in the bushing is put back.
23.Ensure Fire Protection System and oil drain valve operation before charging
and commissioning.
24.Oil test results after filtration must be within specified limit.
25.Spares like bushings shall be tested and kept ready before charging and
commissioning.
26.Allow minimum period of 24 hrs after filtration for oil temperature to settle down.
27.Ensure release of air from plugs provided on top of main tank, conservator and
radiator headers.
28.Take charging clearance certificate from all erection agencies for removal of
man, material and T&P from site.
29.Ensure healthiness of Air Cell.
30.Ensure availability of oil in the breather cup in main tank/ OLTC tank.
31.Ensure all rollers are locked with rails.
32.Ensure door seals of Marshalling Box are intact and all cable gland plate’s
unused holes are sealed.
33.Ensure change over operation of AC supply from source- I to source-II in local
master control cubicle.

P a g e | 50
3.16 ENERGISATION OF UNIT AND SITE CLOSING
Commissioning of transformer is not complete unless it is put into regular service.
Following activities to follow:-
1. Initially charge the transformer under no load and keep it energize for 24 hrs.
2. Gradually load the transformer observing the noise, vibration, temperature rise,
oil leakage etc.
3. Check OLTC operation.
4. Carry out Thermo vision scanning of HV/LV terminals and tank body.
5. Carry out DGA test of oil as per schedule given in flow chart of this manual
6. Hand over testing and commissioning records to operation staff along with O&M
manual of OEM.
7. Ensure closure of project by clearing site in all respect particularly removal of
temporary sheds, T&P, Oil and handing over spares to customer as per
contract.

P a g e | 51
CONCLUSION
The past months of my training have been very instructive for me. POWER GRID
CORPORATION OF INDIA LIMITED has given me opportunities to learn and develop
myself in many areas. I gained a lot of experience, especially in the erection,
commissioning and pre-commissioning of transformer. A lot of the tasks and activities
that I have worked on during my internship are familiar with what I’m studying at the
moment. I worked in many areas where I did different work.
There is a big difference in the college projects and the tasks and activities during the
actual work. In college we learn how to describe the work in projects, where in work
you learn how to implement them in reality. This internship was definitely an
introduction to the actual work field for me.
My mentor during my internship was M.Chandrashekar Reddy, Manager from whom
I have also learned a lot from during my internship. As a Manager, he has lots of
knowledge of the working area. He was very helpful and always willing to give me
advice and feedback which I appreciate. He had always time to answer all my
questions concerning my internship.

P a g e | 52
REFERENCES
[1] Manual on Transformer and Reactor [Online]. Available:
http://www.powergridindia.com/company-overview-0.
[2] Power Transformer – standardisation. Available: ieema, https://ieema.org/about-
ieema/services/power-transformer-standardisation-manual/ [Accessed August 2,
2018].
[3] "Transformer," Wikipedia, Available: https://en.wikipedia.org/wiki/Transformer .
[4] Technical Specification for “400/220/33kV, 500MVA, 3 Phase Auto Transformer,”
Gujarat Energy Transmission Corp. ltd.
[5] Rahul Mehra, “400/220KV SCADA Controlled Substation,” Rajasthan Technical
University, Kota(Raj), July 2015.

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