Testingof

Transformer

K.N.Srinivasan

TraceabllityAccuracyPrecisionProbabilityStd. deviationStandardsAccreditation

Insulation testsShort ckt testPower ratingOther tests

Overview

A MODERN TRANSFORMER IS A COMPLEX ELECTROMAGNETIC APPARATUS

TRANSFORMERS SERVE FOR TRANSMISSION AND DISTRIBUTION OF ELECTRICAL ENERGY

TRANSFORMERS (DISTRIBUTION) ARE THE LAST MAJOR LINK BETWEEN UTILITY & CONSUMER

Introduction

TRANSFORMERS ARE STATIC ALTERNATING CURRENT MACHINES

PRINCIPLE IS BASED ON MUTUAL INDUCTION BETWEEN WINDINGS THROUGH A COMMON CORE

OIL-IMMERSED AND DRY TYPESPOWER AND DISTRIBUTION TYPESSPECIAL TYPES

Transformer on load

ϕ

V1

I0

E2

I1I’2

I2

E = - 4.44 φmf N

Measurement of winding resistance Measurement of voltage ratio Polarity test Measurement of no-load loss & no-load current Measurement of load-loss & short-circuit impedance Measurement of insulation resistance Dielectric tests

Routine tests

Measurement of winding resistance Measurement of voltage ratio Polarity test Measurement of no-load loss & no-load current Measurement of load-loss & short-circuit impedance Measurement of insulation resistance Dielectric tests Temperature-rise test

Type tests

Dielectric test Short-circuit test Measurement of zero-sequence

impedance of three phase transformers Measurement of acoustic noise level Measurement of harmonics of the no-load

current Measurement of power taken by the fans

& oil pumps

Special tests

Dielectric test Short-circuit test Measurement of zero-sequence

impedance of three phase transformers Measurement of acoustic noise level Measurement of harmonics of the no-load

current Measurement of power taken by the fans

& oil pumps

Special tests

• Separate source voltage withstand test

• Induced over voltage withstand test

• Impulse voltage withstand test

Dielectric test

Terminals of windings under test connected to HV terminal of the source . All other windings, core, frame and tank are earthed.

Duration of test is 60 secs.Highest system voltage(kV) Power frequency voltage(kV)

1.1 33.6 107.2 2012 2824 5036 7052 95

72.5 140

Separate source voltage withstand test

This test is useful in determining layer insulation and inter turn insulation of windings.

HV is kept open & twice the rated voltage at twice the rated frequency is applied to the LV for a duration of 60 secs.

Induced over voltage withstand test

Testing facilities • Impulse generator• Discharge circuit• Recording system• Voltage divider for measurement

Standard Impulse wave

Impulse withstand test

Wave shape Standard wave shape of impulse voltage is

1.2/50µsec. t1 is the rise time = 1.2µsecs. t2 is the tail time = 50 µ secs. Tolerance on t1 is ±30% & ±20% on t2.

Impulse generator Impulse wave is generated by discharge of

capacitors charged in parallel into a wave shaping circuit.

Connection

Fig.7.3.1.Connection diagram

lmpulse Application : (Negative Polarity).

Oscillograms : 7.3.1 Reduced full wave 100 kV (Peak)

7.3.2. 100% full wave 170 kV (Peak)

7.3.3 Reduced Chopped wave 103 kV (Peak)

7.3.4. 100% chopped wave 170 kV (Peak)

7.3.5. 100% chopped wave 170 kV (Peak)

7.3.6. 100% full wave 170 kV (Peak)

7.2.7. 100% full wave 170 kV (Peak)

Oscilograms

Oscilograms contd.

Oscilograms contd.

Fig 7.3.3 Connection diagram

Generator capacitance 0.0625 uF

Series resistance 160 ohms

Tail resistance 1040 ohms

Voltage divider 1200 pF in parallel with 100 ohms

Impulse applications : (Negative Polarity)

Oscillogram No. 7.3.15. Reduced full wave 43 kV (Peak)

7.3.16. 100% full wave - 75 kV (Peak)

7.3.17. 100% full wave 75 kV (peak)

Comparison of oscillograms 15 & 16 indicates that the second voltage waveoscillogram has collapsed and the corresponding point of the neutral currentthere are high frequency oscillations. This indicates an arc over across majorportion of the transformer winding as a result of failure of the transformerinsulation.

Oscillograms 7.3.15 to 7.3.17

MINOR INSULATION -conductor turns, layers ● enamel, paper ● oil ducts, paper cylinders

MAJOR INSULATION- winding, core ● press board cylinders, oil ducts

BUSHINGS - porcelain , condenser

Power & distribution transformers must be constructed to withstand mechanical stresses caused by external faults. These stresses are produced due to electromagnetic forces as a result of very high current in the windings during short-circuits.

Short-circuit test

Prior to the short-circuit, transformer is subjected to routine tests.

Asymmetrical current The peak current that transformer is

required to withstand = Isc(peak)=K Isc

Dynamic ability to withstand short-circuit

Value of K :x/r 1 1.5 2 3 4 5 6 8 10 >14

K 1.51 1.64 1.76 1.95 2.09 2.19 2.27 2.38 2.46 2.55

Short-circuit may be a) Preset orb) Post set

Duration of each test being 0.5 sec. ( > 5 MVA 0.25 sec)

For 1φ TransformersNo. of tests = Three

one test with tap at highest voltage ratio, one test in principal taping & one test in

lowest voltage ratio

Short-circuit testing procedure

3φ Transformers No. of tests = Nine

Three tests with tap at highest voltage ratio, three tests in principal taping & three tests in lowest voltage

ratio

Tolerances on Asymmetrical current = ±5 % Symmetrical current = ±10 % Test duration = ±10 %

Oscillogram of dynamic short-circuit test on 3Phase 11/0.433kV 500kVA Distribution

transformer

All the routine tests shall be repeated Dielectric test shall be at 75% Transformer shall be untanked

Reactance measured after s.c. test shall not differ by more than 2% for circular coils & 7.5%

for non-circular coils.

Analysis of the results

SC Test contd.

This is to ascertain that transformer & its cooling arrangements are effectively designed so that temp. rise of winding & cooling

medium does not exceed the permissible limits.Temp. rise limits for Dry type transformers

Class of insulation Temp. rise °CA 50E 65B 70F 90

H 115 C 140

Temperature rise test

Temp. rise limits for oil immersed transformers

Part Temp. rise °C External cooling medium

Air Water

Winding 55 60 (temp.rise by resistance method)

Top oil 50 55

(temp.rise by thermometer method)

a) Direct loadingb) Back-to-Back methodc) Short-circuit method

Loading methods

Duration of test Top oil temperature-rise does not vary more than 10 C/hour during 4 consecutive hourly readings

Partial Discharge Tests100pC500pC

Sound pressure level 20 log10 d/0.00002 dB

d Newton per square meter

Noise Level

meter

Highest voltagefor equipment

UmkV

(r.m.s. value)

Rated short durationpower frequencywithstand voltage

kV(r.m.s. value)

Ratedlightning impulsewithstand voltage

kV(peak value)

3.6 10 2040

7.2 20 4060

12 28 607595

17.5 38 7595

24 50 95125145

36 70 145170

52 95 250

72.5 140 325

123 (185)230

450550

145 (185)230275

(450)550650

170 (230)275325

(550)650750

245 (275)(325)360395460

(650)(750)850950

1050

Highestvoltage forequipment

Um kV

(r.m.s. value)

Standard switching impulse withstand voltage---------------------------------------------------------Longitudinal Phase-to-earth Phase-to-phaseInsulation (ratio to the (note 1) phase-to-earthkV kV peak value)(peak value) (peak value)

Standard lightning impulsewithstand voltage

kV(peak value)

300 750 750 1,50 850950

750 850 1,50 9501050

362 850 850 1,50 9501050

850 950 1,50 10501175

420 850 850 1,60 10501175

950 950 1,50 11751300

950 1050 1,50 13001425

525 950 950 1,70 11751300

950 1050 1,60 13001425

950 1175 1,50 14251550

765 1175 1300 1,70 16751800

1175 1425 1,70 18001950

1175 1550 1,60 19502100