Unit 1-2-a

Department of Electrical and
Electronics
Faculty Name: Mrs Veena Bhat
Designation: Assistant Professor
Subject: Testing and Commissioning
of Electrical equipment
Semester: VII

Testing & Commissioning of Electrical Equipment
Objectives:
 Verify proper functioning of the equipment/system after
installation.
 Verify that the performance of the installed equipment/systems
meet with the specified design intent through a series of tests and
adjustments.
 Capture and record performance data of the whole installation
as the baseline for future operation and maintenance.
 For the avoidance of doubt, depending on the specific demands
of individual installation, the additional requirements substitute
testing & commissioning works in regard to any elements in the
installation other than those indicated in the Procedure.

Syllabus-Testing & Commissioning of Electrical
Equipment
PART - A
UNIT - 1 & 2
TRANSFORMERS:
a. Specifications: Power and distribution transformers as per BIS
standards.
b. Installation: Location, site, selection, foundation details (like bolts size,
their number, etc), code of practice for terminal plates, polarity & phase
sequence, oil tanks, drying of windings and general inspection. 5 Hours
c. Commissioning tests: Following tests as per national & International
Standards, volt ratio test, earth resistance, oil strength, Bucholz & other
relays, tap changing gear, fans & pumps, insulation test, impulse test,
polarizing index, load & temperature rise test. 7 Hours
d. Specific Tests: Determination of performance curves like efficiency,
regulation etc, and determination of mechanical stress under normal
&abnormal conditions. 3 Hours

Equipment
UNIT - 3 & 4
SYNCHRONOUS MACHINES:
a. Specifications: As per BIS standards.
b. Installation: Physical inspection, foundation details, alignments, excitation
systems, cooling and control gear, drying out.
c. Commissioning Tests: Insulation, Resistance measurement of armature & field
windings, waveform & telephone interference tests, line charging capacitance.
4 Hours
d. Performance tests: Various tests to estimate the performance of generator
operations, slip test, maximum lagging current, maximum reluctance power
tests, sudden short circuit tests, transient & sub transient parameters,
measurements of sequence impedances, capacitive reactance, and separation of
losses, temperature rise test, and retardation tests. 6 Hours
e. Factory tests: Gap length, magnetic eccentricity, balancing vibrations, bearing
performance. 2 Hours

Equipment
PART - B
UNIT - 5, 6 & 7
INDUCTION MOTORS:
a. Specifications for different types of motors, Duty, I.P. protection.
2 Hours
b. Installation: Location of the motors (including the foundation details)
& its control apparatus, shaft & alignment for various coupling, fitting of
pulleys & coupling, drying of windings. 4 Hours
c. Commissioning Test: Mechanical tests for alignment, air gap
symmetry, tests for bearings, vibrations & balancing. 5 Hours
Electrical Tests: Insulation test, earth resistance, high voltage test,
starting up, failure to speed up to take the load, type of test, routine test,
factory test and site test (in accordance with ISI code 4 Hours
d. Specific Tests: Performance & temperature raise tests, stray load
losses, shaft alignment, and re-rating & special duty capability. 4 Hours

Equipment
UNIT - 8
SWITCH GEAR & PROTECTIVE DEVICES: Standards, types, specification,
installation, commissioning tests, maintenance schedule, type & routine
tests. 6 Hours
TEXT BOOKS:
1. Testing & Commissioning Of Electrical Equipment -S. Rao,Khanna
publishers,2004
2. Testing & Commissioning Of Electrical Equipment -B .V. S. Rao, Media
Promoters and Publication Pvt., Ltd.
REFERENCE BOOKS:
1. Relevant Bureau of Indian Standards
2. A Handbook on Operation and Maintenance of Transformers- H. N. S.
Gowda, Published by H. N. S. Gowda,2006
3. Handbook of SwitchGears,BHEL, TMH,2005.
4. J and P Transformer Book,Elsevier Publication.

Standards & Specifications
Objectives :
Meet generally recognized demand.
Develop confident.
Quality improvement.
More Demand leads to more sales.
More profit.
 Expansion :New establishment development.
Increase Employments opportunity
Guide for design, manufacture, testing , installation, operation
maintenance etc.

Standards & Specifications
Definition:
Documents that specify or recommend
• Minimum levels of performance ,quality of goods
& services.
• Optimal conditions & procedures for operations in
science, industry, and commerce .

Types of Standards
Category, dimension, structure, equipment, quality,
grade, component, performance, durability, or safety .
Methods of manufacturing, methods of designing,
methods of drawing, methods of using, or methods of
operation of safety condition of production.
Methods of testing, analyzing, appraising, verifying, or
measuring
Terms, abbreviations, symbols, marks, preferred
numbers, or units
Design, methods of execution, or safety conditions.

Points to remember
 Some standards are government-mandated, and
others are voluntary.
 Penalties associated with not adhering to the
standard .
 Standards are updated frequently to keep pace
with changing technology .
Standard should be capable of achieving with the
reasonable effort and time.

Standards for Power Transformers
It covers the following data:
 Service conditions
 Ratings
 Tapping
 Cooling system identification
 Limits of temperature rise
Ability to withstand short-circuits and corresponding tests
for short-circuit withstand ability
 Information to be given on the rating plate and diagram
plate
 Terminal markings
 Vector diagrams, connection, procedures and interpretation

Standards for Power Transformers
 Tolerances
 Tests: Acceptance tests, type tests, routine tests, special tests
 Fitments
 Insulation Levels
 Information for tenders
Practice for maintenance
International standards published by IEC are accepted universally
 Each nation has its own national standards which are based on
particular requirements and practices of that nation
 Indian Standards Institution (Indian Bureau of Standards)
publishes IS standards
 Manufacturers and users in India recognize IS standards and
IEC(International Electrotechnical Commission) standards
Construction of power transformers wmv_(360p).mp4

Standard Specification of a Power Transformer
(IS 2026)
1. Type of Transformer – separate winding transformer, auto-
transformer or booster transformer.
2. Number of phases: single or poly phase.
3. Frequency .
4. Rated voltages for each winding .
5. Connection symbol .
6. Requirement of on-load/off-circuit tap-changers.
7. Impedance voltage at rated current.
8. Indoor or outdoor type.

(IS 2026)
9. Type of cooling and if different types of cooling is involved ,
rated power for each type of cooling .
10. Temperature rises and ambient temperature conditions
including altitude and in case of water cooling chemical
analysis of water .
11. Number of cooling banks, spare capacity if any and number of
standby cooling pumps & fans .
12. Highest system voltage for each winding .
13. Method of system earthing for each winding .
14. Insulation levels.
15. Over fluxing conditions.

(IS 2026)
16. Details of auxiliary supply voltage (for fans, pumps OLTC, motor
alarm, control).
17. Controls of tap changers.
18. Short circuit levels of the system.
19. Vacuum & pressure withstanding values of the transformer
tank.
20. Noise level requirement.
21. Number of rails and rail gauge for movement along shorter and
longer axes.
22. Fittings required with their vibrant description.
23. Any other appropriate information including special tests if any
& capitization formula for the losses.

Specification of outdoor type 3-phase distribution
transformer up to and including 100kVA, 11kV - IS
1180(Part 1) – 1989
Standard specifies the requirements and tests for oil immersed,
naturally air cooled , three phase, double wound non sealed type
outdoor distribution transformers.
1. kVA rating: The standard rating shall be 16,25,63 and 100kVA
2. Rated frequency : The rated frequency shall be 50Hz
3. Nominal system voltage : Nominal system voltage shall be
chosen from 3.3, 6.6 and 11kV
4. No load voltage ratio: 3300/433-250V, 6600/433-250V or
11000/433-250V
5. The winding connection and vectors: The primary winding
shall be connected in delta and secondary winding star
Dyn11 so as to produce a positive phase displacement of 30o
from the primary to the secondary vectors of the same phase

1180(Part 1) – 1989
6. Tapping ratings and tapping methods: The number of
tapping, windings tapped(primary or secondary) are to be
specified. The total change in voltage as percentage of total
voltage and percentage voltage change per tap change are
to be specified.
7. Ex : The total change in voltage is ±10% and is achieved by
changing taps with ±1.5% per tap
8. The transformer tank and the transformer oil shall comply
with the requirements (IS 335; 1983)

1180(Part 1) – 1989
9. Standard fittings: Details pertaining to the following standard
fittings shall be given :
a. Earthing terminals .
b. Oil level gauge .
c. Lifting lugs .
d. Rating and terminal marking plates .
e. Breather .
f. Drain cum sampling valve (3/4 nominal size threads)
preferably steel with plug.
g. Oil filling hole (1 ¼ nominal size thread) with cover ( for
transformers without conservator .

1180(Part 1) – 1989
10. Mounting arrangements.
11. Insulation levels .
12. Limits of temperature rise.
13. Losses and impedance values.
14. Ability of transformers to withstand external short-circuit .
15. Any other required relevant information to be provided .

Significance of Ratings
Rating denotes the assigned numerical value of
capabilities .
Manufacturer assigns certain definite ratings to the
transformer .
These assigned ratings are guaranteed by
manufacturer .
Ratings are confirmed by conducting the Acceptance
Tests or Type Tests to the satisfaction of user or his
consultants .
Ratings are marked on Rating Plate fixed on the
transformer tank .

Rated voltage of windings
It is the voltage assigned ( to be applied or developed at no-load) between
the line terminals of a winding.
The rated voltage for each winding (HV/MV/LV) should be specified
separately.
It refers to continuous rating.
The value refers to principal tapping.
The transformer should be capable of delivering the rated current at a
voltage 5% above the rated voltage.
Rated insulation level of the winding corresponds to respective rated
voltage of the winding.
Insulation level = withstand levels for tests.
The rated apparent power of a winding and the rated current refers to the
rated voltage .
The rated voltage of each winding should be mentioned on the rating plate
of the transformer.

System Voltage
Nominal system voltage: Nominal value of r.m.s line to line
voltage under normal operating conditions.
Highest system voltage: Highest r.m.s line to line voltage
permissible to be maintained under normal operating conditions .

Rated Frequency
It is the frequency at which the transformer is
designed to operate .

Rated Current of Transformer Winding
Rated current: It is the assigned value of current flowing through
the line terminals of the winding at rated frequency .
The assigned current rating is confirmed by conducting
Temperature Rise test( Type Test) and by measurement of load
losses (Routine Test.)
The rated current is for specified ambient temperature .
For higher ambient temperature, suitable de-rating factor is used.
Rated current refers to continuous rating unless specified
otherwise.
For higher operating currents, the permissible duration is
recommended by the manufacturer.
The design of current carrying conducting parts cooling system and
insulation system is based on permissible temperature rise of
windings.

Rated Insulation Level of Transformer
 Rated Insulation Level of a transformer denotes the combination
of voltage values which characterize the insulation of each
winding and associated parts with regard to their ability to
withstand the dielectric stresses and are as follows:
1. Rated voltage,
2. Highest system voltage,
3.Impulse withstand voltage,
4. Power frequency withstand voltage)

Rated Power of a Transformer
Rated Power ( kVA or MVA) - The power rating of a transformer is
given in terms of the conventional value of the apparent power (in kVA
or MVA) assigned to the transformer .
The transformer is designed, manufactured, guaranteed and tested
for the assigned rated power .
In case of two winding transformer each winding has the same rated
power which by definition is equal to the rated power of the
transformer .
In case of a three-winding transformer (having HV/MV/LV windings)
the rated power of each winding should be stated separately .
The rated power corresponds to continuous duty unless specified
otherwise .
The transformer should be designed to deliver rated current at an
applied voltage 5% above and below the rated voltage .

Impedance voltage
The impedance of a transformer is expressed in terms of
impedance voltage and is measured by conducting sustained short
circuit test .
Impedance voltage determines the regulation and efficiency of
transformer .
Impedance voltage should be mentioned in the tender as one of
the service parameters .
It is generally expressed as a percentage of rated voltage .
It is in the range of 4.5% to 15% .
Short circuit current is inversely proportional to the impedance
voltage .
Higher impedance voltage ensures reduced through short-circuit
current .

Voltage Regulation
Definition:
The algebraic difference between rated secondary voltage and
actual secondary voltage for given secondary current(load) at given
power factor, with primary applied voltage of rated value and rated
frequency.
Effect of lagging and leading power factor.

Rating and Terminal plates
Objective: List the details to be given on the rating plate and
terminal plate
IS 1180 (part 1): 1989 .
Non-detachable .
Weather proof material .
Visible position .
Entries shall be permanently marked.
 H.V. winding has been given a capital letter and the L.V. winding
on the same phase a corresponding small letter.

Rating and Terminal plates
For single phase transformers:
A: for the H.V.winding
3A: for the third winding
a: for the L.V.winding
For two phase transformers:
AB:for the H.V.windings
ab: for the L.V.windtnqs
For three phase transformers:
ABC:for the H.V.windings
3A 3B 3C: for the third windings (if any)
abc: for the L.V. winding
The three phase transformers when facing the h.v side, the terminals
are Located from left to right NABC. and when facing the L.v.side
cban.

Terminal Marking plates
 Each transformer shall be provided with a terminal marking
plate in accordance with Fig.
Typical marking plate for transformer with taps

Data collection for ordering a Power Transformer
1. Requirements of load, application aspects .
2. Location, environmental conditions, ambiant temperature, rain,
dust etc.
3. Sub-station layout, voltage levels and fault-levels .
4. Transport facilities, roads , between the manufacturer premises
and final site .
5. Road, rail permits, availability of suitable trailers , wagons etc.
6. Civil engineering requirements such as the room, foundation
trenches for cabling, ventilation aspects etc.

Data collection for ordering a Power Transformer
7. Data regarding transport requirements and handling
facilities .
8. Overall dimensions, weight, lifting arrangements,
transport facilities.
9. Special trailers, transport route, roads, bridges. Culverts,
arches etc. throughout the route .
10. Rail route, requirement of special wagons etc.
11. How a transformer is dispatched from manufacturer
works and transported up to site .

Dispatch instructions
1. Dried-out, filled with oil, ready for service ( small
transformer).
2. With oil covering core and coils only (Medium
transformer) .
3. Without oil in the tank, filled with by nitrogen at pressures
above atmospheric pressure (large transformer).
4. Tank cover is always bolted into its position prior to final
packing.
5. May be packed in a strong wooden packing case of
dispatch .

Dispatch instructions
6. May be sent without packing case depending upon
conditions of transport.
7. Terminals and fitments protruding out need careful
consideration.
8. Protective hoods may be specially provided.
9. EHV transformers, the HV bushings are packed
separately.
10. Temporary connections are brought out for insulation
resistance measurements.
11. Some of the accessories are also packed separately.

Transportation
Mode of transportation : rail, road or water depending on size of
transformer, destination, delivery time & route limitations.
Small and medium transformer: Truck .
Large transformer tanks: road trailers, rail wagons.
In case of large transformers, the transport route, handling
movement, wagons, trailers, necessary permits from railways and
road authorities should be pre planned before finalizing tank
dimensions .
Wagons should be ordered or booked in advance.
Installation of new transformer:Installing the new
transformer_(480p).mp4
Transformer Offload :Transformer Offload
Animation.wmv_(360p).mp4

Transportation
According to weight, the size & capacity of Trailer, plinth size and
mode of transport can be standardized as below

Installation
Instructions:
1. Persons getting inside the transformer must wear clean clothes and
clean synthetic-rubber-soled sandals or boots.
2. Never stand directly on any part of the insulation.
3. No one should be allowed on top of the transformer, unless he has
emptied his pockets .
4. All the tools and spanners used for erection should be securely tied
with taps so that these could be recovered if dropped in, by
accident.
5. All components should be carefully cleaned outside separately,
before erecting .
6. Fibrous material should not be used for cleaning. The presence of
suspended fibrous material will reduce the electrical properties of
transformer oil.

7. Interior of the transformer should not be exposed to damp
atmosphere as far as possible, to avoid condensation.
8. In the event of a sudden change in the weather bringing rain or
snow, provision must be made for closing the tank quickly and
pressurizing it with nitrogen so as to preserve the insulation .
9. Naked lights and flame should never be used near oil filled
transformer.
10. Smoking must not be allowed on the transformer cover when
the cover plates are open, nor in the vicinity of oil processing
plant.
11. Never allow anyone to enter the transformer if adequate supply
of air in the tank is not available .
Installation

Foundation
1. Simple, firm and leveled foundation is necessary for
transformers.
2. Base should be horizontal .
3. Level of concrete plinth for outdoor transformers should
be above the maximum flood level.
4. Suitable rail tracks should be provided where rollers are
used .
5. After bringing to final position, the wheels should be
locked to prevent movement .
6. Severe vibrations from earth should not be transmitted to
the body .

Facility for drainage of oil :
1. Indoor transformers having oil capacity of more than
2000 liters should be provided with soak pits.
2. Outdoor transformers having oil capacity of more
than 9000 liters should be provided with drainage
facility.
Foundation

Cabling
1. Power cables and control cables should never be run in
the same conduit or cable tray.
2. D.C control cables, a.c protection cables and a.c power
cables should be separated from each other.
3. Routing of a.c/d.c protection cables and a.c power cables
should be pre-planned so that crossover is minimum.
4. Cables may be carried along the walls, clamped on vertical
supports at regular intervals depending on the cable size.
5. Cable trays may be made of galvanized iron or aluminium
sheets .

Transformer Tank
1. The transformer tank to be of adequate strength to withstand
positive and negative pressures built up inside the tank while
the transformer is in operation.
2. The transformer tank cover to be welded with tank rim so as to
make a leak-proof joint.
3. The exterior of the transformer tank and other ferrous fittings
to be thoroughly cleaned, scraped and given a primary coat and
two finishing coats of durable ( a weather resisting paint or
enamel).
4. All steel screws, nuts and fasteners exposed to atmosphere
shall either be galvanized or cadmium plated.
5. The space above the oil level in the tank shall be filled with dry
air or nitrogen confirming to commercial grade of IS 1747:
1992.

6. Dry air / nitrogen plus oil volume inside the tank shall be such
that even under the extreme operating conditions the pressure
generated inside the tank does not exceed 40KPascal a positive
or negative.
7. The transformer cover and frame shall be such that it is
possible to remove the weld and rowel twice
Transformer Tank

Types of Transformer tank
Plain tanks:
• Rectangular box type in shape and are commonly used for small
and medium rating transformers.
Shaped tanks:
•The profile of the tank body is suitably shaped to make it more
economical.
•The shaping is decided by the electrical layout, considerations of
transformer windings and terminal gear/tap changers mounting
arrangements.
Bell shaped tanks:
•Tanks which are made into two separable parts are known as bell
type tanks.
•When the top portion is removed, the height of the lower portion
is such that there is accessibility to the core and winding for
inspection and maintenance.

Types of Transformer tank
Corrugated tanks:
•An alternative for providing vertical ribs welded to the plates is to
form corrugation on the plates by suitably folding the plates.
•The merits are additional cooling area on the tank walls and
reduction in tank weight.
Stub-end wagon type tanks:
•Special construction and designed to withstand dynamic loading
during transit besides the static load.
•Such large size transformers are not supported on girders thereby
reducing the height during transport.
•The design is such that these tanks are supported from either end
stub-end wagons and the transformer hangs in the vertical
position, with minimum clearance between the bottom of the tank
and railway track.

Testing of transformer tank
1. Vacuum test
Procedure:
1. The oil is completely drained.
2. After ensuring all the fitting, the vacuum pump is started and
the required vacuum is measured by a vacuum gauge.
3. During vacuum testing the air leakage points are detected by
air leakage detecting instrument. If the leakages are found in
casketed rims, the bolts are tightened.
4. The deflection reading at the starting and after maintaining full
vacuum for one hour and releasing the vacuum are taken to
find out the permanent deflection.
5. This should be within specified allowable limits of deflection,
depending on the size of the tanks.

Testing of tanks
2. Oil pressure test
Procedure:
1. The oil is filled up to tank cover and the required pressure is
applied using pump.
2. The pressure is maintained for few hours and all the wildings
are checked for leakages.
3. If leakage found, rectification is done by draining out the oil.
4. The tank deflection readings are measured before the starting
of oil pressure, at full oil pressure and after releasing the oil
pressure.

Testing of tanks
3. Measurement of stresses:
1. At various locations stresses are required to be measured.
2. The strain gauges are fixed to the tank structure with proper
adhesive.
3. A gauge consists of a fine wire suitably fixed to the body of the
structure.
4. Under load, strains are developed on the body which results in
displacements of the points to which the ends of the gauges
are fixed.
5. The readings are simultaneously recorded. The tank is
subjected to full vacuum and readings are taken. The strain
gauges are fixed inside and outside the tank wall to compare
the top and bottom principal stresses on the surface.
6. The strain gauge gives values of strains in the direction of
Rosettes from which the two principal stresses and their
directions are calculated

Importance of Transformer Oil
1. Used as electrical insulation.
2. Used as coolant.
3. The basic raw material to get transformer oil is a low viscosity
lube called as Transformer Oil Base Stock (TOBS).
4. This is obtained by fractional distillation and series of treatment
of crude petroleum.
5. TOBS characteristics are kept within permissible limits in order
to produce good insulating oils.
6. TOBS is further refined by acid treatment to get transformer oil.
7. Transformer oil consists of four major generic class of organic
compounds as paraffin’s, naphthenic, aromatics and olefins.
8. All these are hydrocarbons and hence insulating oil is called a
pure hydrocarbon mineral oil.

Transformer Oil
Qualities of good insulating oil:
1. The fresh dielectric oil has pale yellow color.
2. Dark or cloud color indicates deterioration.
3. The oil should never contain suspended particles, water soluble
acids and bases, and active sculpture of colloidal carbon and
these impurities accelerate deterioration rate.
4. It should be free from dust particles, carbon particles and
sludge.
5. It should have high dielectric strength.
6. It should have low viscosity.
7. It should have high flash point.
8. It should posses good electrical characteristics.

Transformer Oil Characteristics
The characteristics of transformer oil as per IS335: t-oil-
characteristics.docx

Procedure of filling oil in the transformer tank
1. Before filling with oil, transformer should be fitted with all
accessories including valve gauges, thermometers and plugs and
made oil tight.
2. Oil sample is tested before filling.
3. It should be ensured that no air packets are left in the tank and no
dust or moisture is present.
4. All air vents should be opened.
5. Oil should be filled from the filtering plant.
6. To prevent aeration of oil, the transformer tank should be filled
through the bottom drain valve.
7. Enough time should be allowed (16 to 24 hours) for the oil to settle in
the transformer and also the bubbles to escape.
8. Vacuum filling may be used for large transformers.
9. A vacuum pump may be connected to the top valve of the
transformer and oil hose to the top filter press valve.
10. The tank shall be tightly sealed.

Maintenance of Transformer Oil
1. Oil maintenance is carried out in accordance with standards titled
"Code, practices of maintenance of insulating oil".
2. The code refers to the contamination of oil and determination of
suitability of oil for further service.
3. It also gives the procedure of treatment of oil at site.
4. The transformer oil is tested for the desired qualities giving more
emphasis to moisture and dielectric strength during periodic
maintenance.
5. A sample of insulating oil is taken from the bottom of the transformer
tank.
6. The fresh dielectric oil has pale clear yellow color and dark brown and
cloudy appearance indicate deterioration.
7. Filters are used to remove solid impurities.
8. Moisture level is to be less than the specified value which otherwise
cause internal flashover.
9. For satisfactory use of insulating oil for the desired and specified
qualities, periodic checking of the oil is to be carried out.

Testing of Transformer Oil
The condition of the oil should be checked before commissioning,
during maintenance and re-commissioning after overhaul
(refitting).
Sampling:
1. Sample of oil from the transformer should be taken from the
bottom of the tank.
2. The sample should be collected only after the oil is allowed to
settle for 24 hours.
Containers of sample:
1. Containers for samples should be bottles of plain glass, clean
and dry with one liter capacity.
2. The samples should not be filled up to the top.

Samples from transformer tank:
1. When taking sample remove dirt from the sampling valve plug.
2. Sample should not be taken immediately after opening the
valve.
3. Allow some oil to flow first, then collect sample in the bottle.
4. Oil sample from the transformer in service may be taken
preferably under switched off or no-load condition.
Oil sample from drums:
1. Before taking oil for testing, allows it to settle for 24 hours
without any disturbance so that the heavy contents will settle
down.
2. Then oil will be collected from the bottom of the drum.

Testing of dielectric strength:
BDV test:
1. The test sample from the bottom of the drum or transformer tank
is collected in the standard test cup (80mm x 60mm x 100cm) size.
2. Electrodes are polished brass spheres of 12.5 to 13 mm diameter
mounted horizontally.
3. The gap is 2.5 mm to 4 mm ± 0.2 mm depending upon the
magnitude of voltage available for breakdown test.
4. Allow the sample in the cup for 20 minutes for air bubbles to
vanish, apply ac voltage gradually and steadily till the breakdown
occurs between electrodes.
5. Six breakdown tests are conducted at an interval of one or five
minutes.
6. After each breakdown test, the oil is gently stirred with clean, dry
glass rod.
7. Average of five subsequent tests is considered as the BDV
(breakdown value) of oil sample is carried out.

Crackle test:
1. This test is performed to determine free water.
2. A sample is heated rapidly over silent flame.
3. The presence of moisture above 50-60 pap of water will give
typical crackling sound.
4. The Karl Fisher Solution test(The Water Determination Test ) is
used for determining the moisture more accurately (up to 2
pap).
5. During periodic maintenance crackle test, dielectric test, acidity
test and moisture measurement is carried out.

Drying-out of Power Transformers
Objective: Explain different methods of drying of
transformers
1. The transformer oil and insulation is hygroscopic .
2. If the transformer is dispatched without oil or is left
idle for a long period, the oil and insulation absorbs
moisture.
3. Drying out may be necessary prior to commissioning .
4. Purpose is to expel the moisture from the oil, the
winding insulation and the other internal parts .
5. Otherwise, transformer cannot withstand service
voltage for a long duration and its insulation may fail
prematurely .

Procedure for Drying-out
1. The transformer oil/winding is heated by one of the following
methods for a prolonged period( ten hours to four weeks).
2. Periodic readings of
3. (1) oil and winding temperature.
4. (2) power input.
5. (3) insulation resistance.
6. Temperature of oil is maintained at 80oC and that of windings at
90oC .
7. The values of insulation resistance start falling in the beginning
of drying out process.

Drying Out Curves of a Transformer
8. This indicates that the moisture
drops are getting distributed in the
winding and oil in the form of vapor.
9. After several hours, the insulation
resistance becomes steady.
10.This indicates that water vapors are
distributed in the insulation and oil.
11.On further continuation drying-out,
the insulation resistance values start
rising indicates moisture is expelled
from the windings and oil.
12.The drying-out process is stopped
when the insulation resistance value
(hot) is more than the specified value.
9. During rising mode of
the drying-out process
the polarization index
& dielectric strength
of the oil are
satisfactory (P.I – 1.3,
BDV – 45kV for 4mm
gap) .

Drying out methods
1. Drying of core and coils with oil by oven.
2. Drying of core and coils with oil by short circuit
method.
3. Drying with oil removed by using external heat.
4. Drying with oil removed by using both external and
internal heat.

1.Drying of core and coils with oil by using oven
1. The suitable oven is available, the core and coils can be
effectively dried in it by raising the temperature to a level not
exceeding 80oC.
2. A large volume of air should pass through the oven to remove
moisture and vapors.
3. Insulation resistance check will indicate when the coils are
dry.
4. Core and coils can also be dried in its own tank in an oven.
5. Transformer tank should be suitable for full vacuum.
6. Full vacuum is kept in the tank and a temperature of 75oC is
maintained.
7. Dry nitrogen is used for breathing the vacuum.

2.Drying by Short-circuit method
1. The transformer can also be dried by heating the coils by short-
circuiting the low voltage winding and supplying a reduced
voltage at high voltage terminals.
2. Current should not exceed 70% of normal rated current and oil
temperature should be of the order of 75oC.
3. Winding temperature in no case should exceed 90oC.
4. Winding temperature can be monitored by measuring winding
resistance.
5. Method is more efficient in drying the insulation at site.
https://www.youtube.com/watch?v=3Jt_HShnJFQ

3.Drying out by Streamline Filter machine (BHEL)
Most practical method of drying out is by circulation of
hot oil through streamline filter machine incorporating oil
heater and vacuum chamber.
The vacuum pump of the filter machine should have
the capacity of creating vacuum as high as possible but
not less than 710mm of mercury.
Drying out process can be made faster by creating
vacuum in the transformer tank by lagging the
transformer tank to prevent loss of heat.

3.Drying out by Streamline Filter machine
(BHEL)

3.Drying out by Streamline Filter machine
(BHEL)
The oil temperature in transformer should be of the
order of 75oC.
It should be seen that the oil temperature at the filter
machine in no case exceeds 85oC.
Drying process can be terminated when transformer oil
characteristics are achieved within permissible limits and
insulation resistance of winding shows a constant or
rising trend.

Preparations of drying-out
Cover the tank with fire resistance mat such as asbestos-
cloth, glass sheet provide external shields to prevent drought
of cold air.
Connecting thermocouples, placing thermometer,
calibration, arranging measuring instruments .
To bring-out the well-insulated leads from the windings
from the terminal bushings for the measurement of insulation
resistance .
To bring-out thermocouple leads through one of the
opening in the tank.
To prepare a log book.

Precautions while drying-out
Never leave the transformer unattended during any
part of the process. The transformer should be watched
and observed.
Transformer to oil temperature should never exceed
85oC. The maximum temperature of anything in contact
with the oil should never exceed 90oC.
Maintain log sheet.
Use lagging to prevent loss of heat through the tank.
walls and effect of cold draughts.
Use proper ventilation to remove the moisture given
off by transformer oil.

Duration of drying-out
1 to 6 days for 11kV transformer.
10 days to 30 days for 220kV transformer.
15 days to 40 days for 400kV transformer.

Details about method of drying
1. In the first stage of drying-out, the insulation resistance
reduces.
2. This indicates release of moisture within insulation and oil.
3. In the second stage, the insulation resistance is steady.
4. In the third stage, insulation resistance starts increasing
indicating that moisture is being expelled.
5. Drying out process is stopped when sufficient insulation
resistance and Polarization index is reached during third phase.
6. BDV of oil samples is measured after every four hours.

Steps in drying out of a Power Transformer
Preliminary preparation of the machine, source of heat,
measurement etc.
Arrange the set-up.
Apply heat by one of the suitable means gradually.
Take periodic reading of
• Clock time
• Temperature of windings, body and air, ambient
• Insulation resistance values of 15 second Megger
reading and 60second Megger reading
• Winding resistance (At the beginning and at the end)

1. Maintain steady temperature or specified value (winding
temperature not to exceed 60oC or 70oC depending upon
the insulation class). Measure periodically the insulation
resistance values.
2. Initially, during the first few hours, the values of insulation
resistance reduces even though the heat is being applied
for drying-out.
3. During initial heating period, the moisture trapped in the
insulation in the form of small particles gets released
within the insulation. Hence the insulation resistance
value starts reducing.

4. Intermediate stage. After a span of a few hours or a few days,
the insulation resistance reaches a steady value. This indicates
that moisture has spread all over the insulation.
5. The input power is reduced to reduce the temperature rise
6. Rising stage. After a few hours of steady value, the insulation
resistance starts rising
7. This indicates that the moisture has vaporized and is being
expelled from the winding. The input power is reduced further
8. The drying out process is stopped when the desired value if
insulation resistance(hot) and polarization index is reached. The
input power is switched off.

Procedure of Inspection, Storage and Handling
1. Inspection on arrival at site – on arrival at the site, the packing
cases should be checked against the dispatch particulars.
2. Any loss of packages in the transit should be intimated to the
manufacturer and insurance company.
3. The transformer should be unpacked and inspected for any
signs, fittings, bushings etc.
4. Oil leakage should be checked along the valves, tank welds,
gasified flanges etc.
5. Gas preserve (if gas filled) should be checked.

Procedure of inspection, storage and handling
6. Transformer transit damage must be immediately
reported to the manufacturer and insurance
company.
7. Thorough inspection should be carried out jointly.
8. Storage – transformer to be installed immediately do
not need elaborate storage .
9. Otherwise, proper storage is needed.
10. To avoid entry of moisture, effect of rains/dust etc.
11. Preferable to store transformer indoors, with proper
coverings and proper flooring.

Procedure of Inspection, Storage and Handling
12. Oil should not be drained unless the provision of filling
inert gas is available.
13. Oil is stored separately in drums or tanks.
14. Handling – transformers are provided with lugs and
shackles for the purpose of lifting.
15. Before lifting the transformer, all the bolts of the cover
should be tightened.
16. When Jacks are used for lifting, the projections provided
for lifting should be used.
17. Proper balance should be maintained while lifting the
transformer .

Inspection upon arrival at site
1. Immediately after transformer is received at site, it
should be thoroughly examined externally for possible
damages which may have occurred during transit.
2. Nitrogen gas pressure ( when filled during dispatch)
should be checked .
3. Positive pressure if not found indicates that there is a
leakage, and there is a possibility of moisture entering
the tank during transit.
4. This can be measured by measuring the dew point.

Inspection upon arrival at site
5. The dew point measurement indicates the amount of
surface moisture content in transformer insulation.
6. As the insulation temperature and transformer gas
pressure vary, the acceptable dew point will vary.
7. The various packages must also be checked.
8. Internal inspection should be carried out to the extent
possible through inspection covers .
9. Particular attention should be paid to the connections,
bolts, links, coil clamping bolts, tap changers, current
transformers and the general insulation.
10. Breakdown strength of transformer oil should be
examined carefully.

Handling
Following means are normally used for lifting operations.
Overhead travelling crane or gantry crane.
Jib crane.
Derricks
Jacks and winches.
Note: The overhead travelling crane and jib cranes are
obviously the most convenient and safe means.

Precautions
1. Transformer (main package) should be lifted only
through lifting points provided for attaching the
slings.
2. Cover must always be bolted in position.
3. Transformer should be jacked up using the jacking
pads specially provided for that purpose.
4. Jacks should never be placed under any valves.

Cooling Methods of a Transformer
The excess temperature in transformer may cause serious
problems like insulation failure.
Dry type transformers :
• Air Natural (AN) (up to 3 MVA).
• Air Blast : Above 3MVA up to 15 MVA : Air is forced on the
core and windings with the help of fans or blowers. The air
supply must be filtered to prevent the accumulation of dust
particles in ventilation ducts.
Oil immersed transformers.
•Oil Natural Air Natural (ONAN)
•Oil Natural Air Forced (ONAF)
•Oil Forced Air Forced (OFAF)
•Oil Forced Water Forced (OFWF)

Oil Natural Air Natural (ONAF):
•By the principle of convection, the
heated oil flows in the upward
direction and then in the radiator.
•The vacant place is filled up by
cooled oil from the radiator.
•The heat from the oil will dissipate
in the atmosphere due to the
natural air flow around the
transformer.
•This method can be used for
transformers up to about 30 MVA.

Oil Forced Air Forced (OFAF)
•Oil is circulated with the help of a
pump.
•The oil circulation is forced through
the heat exchangers.
•Then compressed air is forced to
flow on the heat exchanger with the
help of fans.
•The heat exchangers may be
mounted separately from the
transformer tank and connected
through pipes at top and bottom. This
type of cooling is provided for higher
rating transformers at substations or
power stations.

Oil Forced Water Forced (OFWF)
•Forced water flow is used to
dissipate heat from the heat
exchangers.
•The oil is forced to flow through the
heat exchanger with the help of a
pump, where the heat is dissipated in
the water which is also forced to flow.
• The heated water is taken away to
cool in separate coolers.
•This type of cooling is used in very
large transformers having rating of
several hundreds MVA.

Phasor Diagram and Phasor Groups
Phasor are used to represent the induced emfs in phasor diagram
of a transformer.
The direction employed for the rotation of phase is counter clock
wise.
In three phase transformers polarity alone is insufficient to
represent the relation between HV and LV windings.
The terminal markings on HV and LV side voltage, phasor
diagrams are required to show the angular displacement between
HV and LV winding.
The angular difference between phases representing the voltages
induced between HV and LV terminals having the same marking
letters and the corresponding neutral point (real or fictitious)
expressed with respect to the side is termed as the displacement.

The following standard phasor diagrams are frequently
encountered in practice.
Group Phase
displacement
Connections
I 00 Yy0,Dd0,Dz0
II 1800 Yy6,Dd6,Dz6
III 300 lag Dy1,Yd1,Yz1
IV 300 lead Dy11,Yd11,Yz11

Polarity
Definition:
Induced relative voltage direction in HV & LV winding.
Induced voltage direction is same for both- Subtractive.
Induced voltage direction is different in both winding - Additive.

Phase Sequence
Definition:
 In poly phase system phasor reaches maximum values during a
sequence of time.
Note:
1. In parallel operation, phase sequence should be same.
2. Phase sequence of the supply is decided or fixed by the
generating plant.

Necessary figure to be sketched by
referring suitable book.

Unit 1-2-a

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