Transformer Testing Notes

K.N.Srinivasan

•A MODERN TRANSFORMER IS ACOMPLEX ELECTROMAGNETICAPPARATUS

•TRANSFORMERS SERVE FOR•TRANSFORMERS SERVE FORTRANSMISSION AND DISTRIBUTIONOF ELECTRICAL ENERGY

•TRANSFORMERS (DISTRIBUTION)ARE THE LAST MAJOR LINKBETWEEN UTILITY & CONSUMER

•TRANSFORMERS ARE STATICALTERNATING CURRENT MACHINES

•PRINCIPLE IS BASED ON MUTUALINDUCTION BETWEEN WINDINGSINDUCTION BETWEEN WINDINGSTHROUGH A COMMON CORE

•OIL-IMMERSED AND DRY TYPES

•POWER AND DISTRIBUTION TYPES

•SPECIAL TYPES

V1

I1I ’2

ϕ

I0

E2

I2

E = - 4.44 φφφφmf N

I0

R1 + R’2

I ’2

X1 + X’2

IµIw

X0

R0

E’2=E1V1

X1 + X’2

IEC-76 & IS 2026 specify the routine, type IEC-76 & IS 2026 specify the routine, type & special tests for power transformers

� Measurement of winding resistance� Measurement of voltage ratio� Polarity test� Measurement of no-load loss & no-load current

Routine Tests

� Measurement of no-load loss & no-load current� Measurement of load-loss & short-circuit

impedance� Measurement of insulation resistance� Dielectric tests

� Measurement of winding resistance� Measurement of voltage ratio� Polarity test� Measurement of no-load loss & no-load

Type Tests

� Measurement of no-load loss & no-load current

� Measurement of load-loss & short-circuit impedance

� Measurement of insulation resistance� Dielectric tests� Temperature-rise test

� Dielectric test� Short-circuit test� Measurement of zero-sequence

impedance of three phase transformers

Special Tests

� Measurement of acoustic noise level� Measurement of harmonics of the no-

load current� Measurement of power taken by the

fans & oil pumps

• Separate source voltage withstand test

• Induced over voltage withstand test

Dielectric Tests

• Induced over voltage withstand test

• Impulse voltage withstand 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 3

3.6 10

Separate Source Voltage Withstand Test

3.6 107.2 2012 2824 5036 7052 9572.5 140

• This test is useful in determininglayer insulation and inter turninsulation of windings.

Induced Over voltage Withstand Test

• HV is kept open & twice therated voltage at twice the ratedfrequency is applied to the LV fora duration of 60 secs.

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

Impulse Withstand Test

Standard Impulse wave

Wave shape• Standard wave shape of impulse voltageis 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.•Tolerance on t1 is ±30% & ±20% on t2.

Impulse generator• Impulse wave is generated by dischargeof capacitors charged in parallel into awave 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)

Oscillograms

Oscillograms contd.

• 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 beconstructed to withstand mechanicalstresses caused by external faults. Thesestresses are produced due to

Short-Circuit Test

electromagnetic forces as a result of veryhigh current in the windings during short-circuits.

• Prior to the short-circuit, transformeris subjected to routine tests.Asymmetrical current• The peak current that transformer is• The peak current that transformer isrequired to withstand = Isc(peak)=KIsc

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)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

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%

SC Test contd.

� 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.

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 90H 115

C 140

Temp. rise limits for oil immersed transformers

Part Temp. rise °CExternal cooling medium

Air WaterWinding 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

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

Partial Discharge Tests100pC

500pC

Noise contd.

Noise Level

Sound pressure level 20 log10 d/0.00002 dB

d Newton per square meter

meter

IEC 76-1 (1993) Power Transformers, GeneralIEC 76-2 (1993) Power Transformers, Temperature riseIEC 76-3 (1980) Power Transformers, Insulation levels & Dielectric testsIEC 76-3-1 (1980) Power Transformers, Insulation levels & Dielectric tests,

External Clearances in Air,IEC 76-4 (1976) Power Transformers, Tapping & ConnectionsIEC 76-5 (1976) Power Transformers, Ability to withstand short circuit

Standards

IEC 76-5 (1976) Power Transformers, Ability to withstand short circuitIEC 606 (1978) Application Guide for Power TransformersIEC 616 (1978) Terminal & tapping markings for Power TransformersIEC 214 (1976) On-load tap changersIEC 722 (1982) Guide to the Lighting Impulse & Switching Impulse

Testing of Power Transformers & ReactorsIEC 726 (1982) Dry type Power TransformersIEC 742 (1983) Isolating transformers & Safety isolating transformers

Requirements

IEEE C57.12.00-1993 IEEE Standard General Requirements for LiquidImmersed Distribution, Power & Regulating Transformers

Highest voltagefor equipment

Um

kV(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

Standard insulation levels

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)8509501050

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

contd

1300

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

NOTES1.Value of the impulse component of the relevant combined test.

The introduction of Um=550kV (instead of 525 kV), 800 kV (instead of 765 kV),

1,200 kV, of a value between 765 kV and 1200 kV, and of the associated standard withstand voltages, is under consideration.