testing of transformers 1

8/2/2019 Testing of Transformers 1

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High temperatures will damage the winding insulation. Small transformers do not generatesignificant heat and are cooled by air circulation and radiation of heat. Power transformers rated

up to several hundred KVA can be adequately cooled by natural convective air-cooling,

sometimes assisted by fans. In larger transformers, part of the design problem is removal of heat.Some power transformers are immersed in transformer oil that both cools and insulates the

windings. The oil is a highly refined mineral oil that remains stable at transformer operating

temperature. Indoor liquid-filled transformers are required by building regulations in manyjurisdictions to use a non-flammable liquid, or to be located in fire-resistant rooms. Air-cooled

dry transformers are preferred for indoor applications even at capacity ratings where oil-cooled

construction would be more economical, because their cost is offset by the reduced building

construction cost.

Construction of oil-filled transformers requires that the insulation covering the windings bethoroughly dried before the oil is introduced. There are several different methods of drying.

Common for all is that they are carried out in vacuum environment. For distribution

transformers, which are smaller and have a smaller insulation weight, resistance heating can beused. This is a method where current is injected in the windings to heat the insulation. The

benefit is that the heating can be controlled very well and it is energy efficient.

Very small transformers will have wire leads connected directly to the ends of the coils, and

brought out to the base of the unit for circuit connections. Larger transformers may have heavybolted terminals, bus bars or high-voltage insulatedbushings made of polymers or porcelain. Alarge bushing can be a complex structure since it must provide careful control of the electric field

gradient without letting the transformer leak oil.


5/15

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6/15

TRANS-TECH ENGINEERING

Testing of Transformer is done to determine their electrical, thermal and mechanical suitabilityfor the system where they will be used or applied. Most of the tests performed on thetransformers are defined in national or international standards created by IEEE, NEMA and

ANSI, whose purpose is to define uniform set of tests recognized by both the manufacturer and

user.

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Measurement of winding resistance Measurement of insulation resistance Separate source voltage withstand test (High Voltage tests on HV & LV) Induced Over voltage Withstand test (DVDF test) Measurement of voltage ratio Measurement of NO LOAD LOSS & current. Measurement of LOAD LOSS & IMPEDENCE.(EFFICIENCY & REGULATION) Oil BDV test.

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All routine tests Additionally following tests are included in type tests1. Lightening Impulse test.2. Temperature rise test


7/15

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8/15

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9/15

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10/15

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11/15

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Load Test helps to determine the total loss that takes place, when the transformer

is loaded. Unlike the tests described previously, in the present case nominal voltage is appliedacross the primary and rated current is drown from the secondary. Load test is used mainly

1. To determine the rated load of the machine and the temperature rise2. To determine the voltage regulation and efficiency of the transformer.

This test measures the power consumed by the transformer when the 433V winding is short

circuited and the rated current is passed through the 11KV winding.

85915!:

This test checks the insulation property between Primary to earth, Secondary to earth andbetween Primary & Secondary.

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85915!:High Voltage tester (100KV & 3KV)


12/15

This test is made to prove that the transformer insulation will withstand voltage surges whichmay be caused by lightning or switching; this includes insulation to ground, insulation between

turns and windings, and the flashover value of the associated bushings. A high-voltage wave of

standard values, and approximating a lightning surge, is imposed on the unit to be tested. Thesurge generator usually consists of a number of capacitors connected so that they be charged inparallel from a relatively low-voltage source and discharged in series to give a high voltage

across the test piece. A standard impulse wave is illustrated below;

The standard wave reaches its peak voltage value in 1.5 microseconds and reduces to half the

voltage value in 40 microseconds. The value of the voltage applied depends on the rating of the

insulation, and may vary from 5 to 30 times the voltage rating of the insulation. Since ratherelaborate and costly equipment is needed, impulse tests are usually performed only on large

station-type power transformers, and rarely made after they leave the factory.

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."This test setup is generally used to perform induced voltage test on the transformers. This set up

consists of the used motor and generator combination to develop the required voltage and

frequency. It is used to find the insulation breakdown of the conductors from turns to

turns and layers to layers. It is an insulation test for the transformers.For an 11KV/433V

transformer, 866 Volts are applied at the 433V winding with the help of a Generator for 1

minute. This induces 22KV on 11KV side. The frequency of the 866V supply is also increased to

100HZ.

85915!:


13/15

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14/15

#10'234#556*788340Resistance Test

H.T. Side

129.7

129.1

130.0

L.T. Side 55.3m

55.3m

56.7m

High Voltage Test

Side Voltage Given Time period of Observation

H.T. Side 21 KV (75% of 28 KV) 1 min

L.T. Side 2250 V (75% of 3 KV) 1 min.

DVDF Test

Voltage Given Frequency given Time period of Observation

866V (2x433 V) 100 HZ (2x50 HZ) 1 min.

Turns Ratio TestRatio=1:44

Deviation (0.5%) Centridian (0.1%)

-0.2 0.04

-0.29 0.07

-0.09 0.08

No-Load Test

V(Volt) I(Ampere) W1(Watt) W2(Watt) mf433 V 0.2 A 32 W 8 W 2x2

Load Test

V1(H.V.) V2(L.V) I1(H.V.) I2(L.V.) W1 W2 mf

500 V 1.31 V 0.65A 33.3 A 0 W 70 W 4x2


15/15

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testing of transformers 1

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