Transforer Test-methods.pdf

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KončarPowerTransformersLtd.

TEST METHODS FOR POWERTRANSFORMERS

QT No: xx-yyPage : 1 / 2

Prepared by:

F. Juraković Approved by:

I. Šulc

Issue: 08.2003. 09.2003. 06.2004. 11.2004. 03.2006. 07.2006.

KPT-QT.001E, izdanje 08.2003.

TEST METHODS FOR POWER TRANSFORMERS

Contract number: XXXXXXX Transformer type: 0 XXX 000 000 – 000

TESTING POWER TRANSFORMERS

Test procedures and equipment used for the testing of large power transformers at Končar Powertransformers are dealt with in the following Sections.The electrical characteristics and dielectric strength of the transformer are checked by means ofmeasurements and tests defined by standards.The tests are carried out in accordance with IEC Standard 60076, Power transformers, unless otherwisespecified in the contract documents.

CONTENTS

Item Title ID1. Summary of dielectric tests KPT-QTPT 001E issue 08.2003.

2. Measurement of voltage ratio and check ofconnection symbol

KPT-QTPT 002E issue 08.2003.

3. Measurement of winding resistance KPT-QTPT 003E issue 08.2003.

4. Impedance and load loss measurement KPT-QTPT 004E issue 08.2003.

5. Measurement of no-load loss and current KPT-QTPT 005E issue 08.2003.

6. Induced overvoltage withstand test KPT-QTPT 006E issue 08.2003.

7. Partial discharge measurement KPT-QTPT 007E issue 08.2003.

8. Separate-source voltage test KPT-QTPT 008E issue 08.2003.

9. Operation tests on on-load tap-changer KPT-QTPT 009E issue 08.2003.

10. Measurement of the zero-sequence impedance KPT-QTPT 010E issue 09.2003.

11. Capacitance and the insulation power factormeasurement


12. Insulation resistance measurement KPT-QTPT 012E issue 09.2003.

13. Measurement of the electric strength of theinsulating oil


14. Temperature rise test KPT-QTPT 014E issue 11.2004.

15. Lightning impulse test KPT-QTPT 015E issue 08.2003.

16. Test with the lightning impulse chopped on thetail


17. Switching impulse test KPT-QTPT 017E issue 09.2003.

18. Measurement of acoustic sound level KPT-QTPT 018E issue 09.2003.

19. Measurement of higher harmonics inmagnetizing current


20. Tightness (leakage) test KPT-QTPT 020E issue 09.2003.


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TEST METHODS FOR POWERTRANSFORMERS

QT No: xx-yyPage : 2 / 2

Issue : 08.2003. 09.2003. 06.2004. 11.2004. 03.2006. 07.2006.

KPT-QT.001E, izdanje 08.2003.

Item Title ID21. FRA measurement KPT-QTPT 021E issue 06.2004.

22. Core insulation measurement KPT-QTPT 022E issue 03.2006.23. Power consumption of cooling system KPT-QTPT 023E issue 03.2006.

24. Measurement of transferred surges KPT-QTPT 024E issue 07.2006.


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SUMMARY OF DIELECTRIC TESTS KPT-QTPT 001EPage: 1 / 4

1. SUMMARY OF DIELECTRIC TESTS

The Basic rules for insulation requirements and dielectric tests are summarized in table 1 (IEC 60076-3).Levels of standard withstand voltages, identified by highest voltage for equipment Um of winding are given intables 2, 3 and 4.

The choice between the different levels of standard withstand voltage in these tables depends on theseverity of over voltage conditions to be expected in the system and on the importance of the particularinstallation.

TestsCategoryofwinding

Highest voltageforequipment Um

kV

Lightningimpulse (LI)

Switchingimpulse (SI)

Long duration AC (ACLD)

Short duration AC (ACSD)

Separatesource AC

Uniforminsulation Um≤72,5 Type

(note 1)Not applicable Not applicable

(note 1)Routine Routine

72,5


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Highest voltage forEquipment um

Kv r.m.s.

Rated lightning impulsewithstand voltage

kV peak

Rated short duration induced or separate source AC withstand voltage

kV r.m.s.

203,6 10

407,2 20

6012 28

7517,5 38

9524 50

125

14536 70

170

52 250 9560 280 115

72,5 325 140

380 150

100 450 185123

550 230145

650 275

170

750 325

NOTE Dotted lines may require additional phase-to-phase withstand tests to prove thatthe required phase-to-phase withstand voltages are met.

Table 2 – Rated withstand voltages for transformer winding with highest voltage for

equipment Um≤170 kV Series I based on European practice

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Rated lightning impulsewithstand voltage

kV peak

Rated short-duration induced orseparate source AC

withstand voltagekV r.m.s. Highest voltage for

equipment Um Distribution (note 1)and class I

transformers(note 2)

CLASS IItransformers(note 3)

Distributionand class Itransformers

CLASS IItransformers

15

26,4

36,5

48,3

72,5121

145

169

95

125

150

200

250

350

110

-

150

200

250

350350

450

550

650

750

34

40

50

70

95

140

34

-

50

70

95

140140

185

230

275

325

NOTE 1 Distribution transformers transfer electrical energy from a primary distribution circuit to asecondary distribution circuit.

NOTE 2 Class I power transformers include high-voltage windings of Um≤72,5 kV.

NOTE 3 Class II power transformers include high-voltage windings of Um≥121 kV.

Table 3 – Rated withstand voltages for transformer windings with highest voltage for equipment

Um≤169 kV - Series II based on North American practice

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Highest voltage forequipment Um

kV r.m.s.

Rated switching impulsewithstand voltage phase-

to-earth

kV peak

Rated lightningimpulse withstand

voltage

kV peak

Rated short-durationinduced or separatesource AC withstand

voltage

kV r.m.s.

650550 325

750650 360

245850

750 395300 950

850 460362 1 050

950 510

1 175

1050850 460

1175950 510

420 13001050 570

550 14251175 630

15501300 680

16751300 note 3

1800800 1425 note 3

19501550 note 3

2100

NOTE 1 Dotted lines are not in line with IEC60071-1 but are current practice in some countries.

NOTE 2 For uniformly insulated transformers with extremely low values of rated AC insulation levels, special measures mayhave to be taken to perform the short-duration AC induced test.

NOTE 3 Note applicable, unless otherwise agreed.

NOTE 4 For voltages given in the last column, higher test voltages may be required to prove that the required phase-to-phasewithstand voltages are met. This is valid for the lower insulation levels assigned to the different U m in the table.

Table 4 – Rated withstand voltages for transformer windings with Um>170 kV

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Končar

PowerTransformers

Ltd.

MEASUREMENT OF VOLTAGE RATIO AND

CHECK OF CONNECTION SYMBOL

KPT-QTPT 002E

Page : 1 / 2

2. MEASUREMENT OF VOLTAGE RATIO AND CHECK OF CONNECTION SYMBOL

2.1 PURPOSE OF THE MEASUREMENT

The voltage ratio of a transformer is the ratio at no-load of rated voltage of one winding to the rated voltageof another winding (line to line voltage in a three-phase transformer).

The purpose of the measurement is to check that the deviation of the voltage ration does not exceed the limitof the transformer standard (generally 0,5%).The vector group is also checked at the same time.

2.2 PERFORMANCE OF THE MEASUREMENT

The voltage ratio measurements are carried out by means of a measuring bridge. The accuracy of the bridge

is ±0,1%.

The voltage supply used for the bridge is 400/230 (380/220) V, 50 Hz.The function of the bridge is shown in Fig. 2-1. The voltages of the transformer under test are compared tothe corresponding voltages of a regulating inductive divider, which is placed inside the bridge and equipped

with a decade display. When the zero indicator is equilibrated, the voltage ration of the inductive divider isthe same as that of the transformer under test. The result of deviations is shown directly on the display of thebridge.

U1

1U

1V

1W

1N

2U

2V

2W

2N

y

xU

2

Test object

~

x

y x

U

U +=

2

1

Fig. 2-1

Because the bridge measuring device works on the single-phase principle the voltage ratio is measuredphase by phase between two windings mounted on the same leg. The indication on the bridge displaydepends on the vector group of the transformers main voltages (See Fig. 2-2)

At the same time with the voltage ratio measurement the vector group symbol of the transformer is alsochecked. When the measuring conductors of the transformer are connected to the bridge according toFig. 2-1 and Fig. 2-2, the bridge can be balanced only if the vector group is correct.

Prepared by:J. Bujanović

Controlled by:

I. Šulc Approved by:

I. Šulc

Issue: 08.2003.

KPT-QA.029E 1/2 izdanje 03.2002.


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Končar

PowerTransformers

Ltd.

MEASUREMENT OF VOLTAGE RATIO AND

CHECK OF CONNECTION SYMBOL

KPT-QTPT 002E

Page : 2 / 2

The ratio measurement is performed with the test object in no-load condition.The voltage ratios are measured for each tapping connection of the transformer. In the report the specifiedtapping voltage ratios are stated, as well as the deviations of measured ratios from these values. Theconnection symbol is also stated in the test report.

Dd 0

iii

i

ii

I

I

II

II

III

III

0

iDy I

iii ii

iDz 0

iii ii

0

i

iii8

Yz IIi

iii

ii

II

Dz 1010

iii

iii

Yz 7

Dz 8 ii

7

iiii

Dz 6

6

ii

iii

iii

Yz 5i

5

iiiii

Dz 4 iiiii i

2Dz 2 iii iii

iiiii

Yz Ii

I

4

iDd 2 iiiii

I

2

i

Dd 4 iiiii

4

iiiii

Dy 5i

5

6

iii

i

iiDd 6

Dy 7i

7

ii

iii

Dd 8

iii

ii

i

8

10

Dd 10

iii

i ii

iii ii

Dy II i

II

i

iii ii

Yy 0

0

iiii

ii

Yd I

I

iiiYd 5

i

ii

5

6

Yy 6 ii iii

i

7

Yd 7 ii iiii

Yd II

II

i

iii

ii

Fig. 2-2 Designation of symbols for three-phase transformers

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MEASUREMENT OF WINDING RESISTANCE KPT-QTPT 003EPage : 1 / 2

3. MEASUREMENT OF WINDING RESISTANCE

3.1 PURPOSE OF THE MEASUREMENT

The resistance between all pairs of phase terminals of each transformer winding are measured using directcurrent. The measurement is performed for each connection of connectable windings and for each tappingconnection. Furthermore the corresponding winding temperature is measured.The measured resistances are needed in connection with the load loss measurement when the load lossesare corrected to correspond to the reference temperature.

The measurement will also show whether the winding joints are in order and the windings correctlyconnected.

3.2 APPARATUS AND MEASURING CIRCUIT

Winding resistance between corresponding terminals is measured by means of U-I method.The measurement is performed in all OLTC tappings. DC current and voltage drop are measured by usinginstruments of 0,2 class according to Fig. 3-1. A ration between voltage drop and current gives the measured resistance. Temperature is measured by Hg-thermometer placed in the thermometer pocket on the transformer cover. The required current is obtainedfrom a battery 60V.

2U

2V

2W

1U

1V1W

V

A

+

-

battery

Test object

Re

Fig. 3-1

The resistance value is then determined as

reading ampermeter

reading voltmeter

R =


Before the measurement starts, the transformer is standing for at least 3 hours filled with oil and withoutexcitation. During this period the temperature differences of the transformer will equalize and the windingtemperature will become equal to the oil temperature.

The average winding temperature is obtained by determining the average oil temperature. The average oiltemperature is obtained by measuring the top oil temperature in an oil-filled thermometer pocket situated incover, and the bottom oil temperature in the drain valve and taking the average of these two. When switchingon the supply voltage E to the measuring circuit the winding inductance L tends to resist the increase of thecurrent.

Prepared by:

J. Bujanović Controlled by:


I. Šulc

Issue: 08.2003.



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MEASUREMENT OF WINDING RESISTANCE KPT-QTPT 003EPage : 2 / 2

The rate of increase depends on the time constant of the circuit:

(3.1)

−=

− L

Rt

e

R

E i 1

t = time from switching on

L/R = time constant of the circuit

R = total resistance of the circuit

To shorten the time for the current to become steady so high a measuring current is used that the core willbe saturated and the inductance will be low. The measuring current is usually 5…10 times the no-loadcurrent of the winding. However, the current should be less than 10% of the rated current of the winding,otherwise the temperature rise of the winding caused by measuring current will give rise to measuring errors.Furthermore the time constant can be reduced by using as high a supply voltage as possible enabling an

increased series resistance in the circuit. When using a battery, the supply voltage is approximately constantand the current is adjusted by means of the series resistance Re.

3.4 TEST RESULT

The resistance values and the average temperature are calculated. In the report the terminals, betweenwhich the resistances are measured, the connection, the tapping position and the average temperature ofthe windings during the measurement are stated.

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IMPEDANCE AND LOAD LOSS MEASUREMENT KPT-QTPT 004EPage : 1 / 5

4. IMPEDANCE AND LOAD LOSS MEASUREMENT

4.1 PURPOSE OF THE MEASUREMENTThe measurement of impedance and load loss of transformers is a routine test performed on all units. It isonly possible to do in a proper way on the complete unit at the final testing. It serves to verify properties thatare of great importance to the transformer operation. The impedance is decisive for the distribution ofcurrents and voltages within the power system and load losses are important for an economic operation ofthe network. It is not practical to carry out these measurements with the test object in normal operationtransmitting its rated power. The tests are made at short-circuit condition with one winding short-circuitedand current at rated frequency supplied to another winding. For multi-winding transformers the test has to berepeated for each combination of two windings.

4.2 IMPEDANCE

The measured impedance voltage depends on the voltage rating of the winding where the measurementsare made. Consequently it is customary to express the impedance voltage as a percentage of the ratedvoltage of the corresponding winding.

4.3 LOAD LOSSES

The measured load losses will be practically the same independent of which winding is short-circuited and towhich winding the current is supplied. The load losses are losses associated with the load current and theleakage flux and consist of losses in conductors as for DC, eddy-current losses in conductors caused by theleakage flux and hysteresis and eddy-current losses in the core, clumps and tank structure. From the totallosses measured and the winding DC-resistances, the stray losses are computed. Separation of the losscomponents is necessary as information for prediction and control of losses. It is also necessary forconverting the losses from the temperature at the measurement to the reference temperature as the loss

components are affected differently by a change in temperature.

4.4 APPARATUS AND MEASURING CIRCUIT

On account of the test room facilities it is customary to short-circuit the low voltage winding and supplycurrent to the high voltage winding. For large test objects the demand for reactive power will be considerableand is normally supplied by static condenser banks. As the impedance voltage will vary within wide limits, astep-up transformer is normally necessary between the generator and the test object. Voltages, currents andpower are measured by instruments supplied from measuring transformers as given in fig. 4-1 as anexample. Only high quality measuring transformers and instruments must be used. The impedances oftransformers are linear and there is no need to take creation of harmonics into consideration. Especiallylarge test objects have low power factors and this imposes severe demands on the measurements of power.In the state-of-the-art testing installation digital wattmeters will be utilized which have superior ability for data

recording. With a data acquisition system and a suitable computer program the measured data areprocessed and a complete test record written out.In a traditional instrumentation the three-wattmeter method should always be used for three-phasemeasurements.Irrespective of the instrumentation for power measurement, the errors in ratio and phase displacement of themeasuring transformers will introduce errors which have to be corrected. Correction of errors is discussed inclause 4.5.The temperature is an important factor in this test and is measured with thermometers in the oil system.

Prepared by:



I. Šulc

Issue: 08.2003.



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G

A

W

I A

A A

IB

IC

W

W

f

C

V

A

BC

U A

UB

UC

I A IB IC U A UB UC

Data Acquisition & Wave Analyser

Fig. 4-1 Circuity for measuring load loss and impedance voltage

4.5 ERROR ANALYSIS

With increasing cost of energy, the loss evaluation has become an important factor in appraisals oftransformers. Consequently it is imperative that the exact losses are established and known errors are statedand corrected.

Determination of measuring errors and their correction is a complex matter. The present analysis will notcover the complete subject but deal only with errors in the measuring equipment.

We denote measured values as P´, U´, I´, cosϕ´ and corrected values as P,U, I and cosφ respectively.Consequently, the losses to be measured are:

P´=U´ • I´ • cosϕ´

Corrections for errors introduced by the recording instruments should be available from calibration sheets forequipment in question. In a traditional analogue instrumentation the best available instruments should be

used. At present it is customary to use watt meters of class 0,5 with cosφ=0,1. Ampermeters and voltmetershave class 0,1 or 0,2. The actual instrument errors are normally only a fraction of their nominal classes.

Digital wattmeters have accuracies of the same order as the best of available analogue instruments, but their

reading will result in greatly improved accuracy because random errors are virtually eliminated due to betterresolution and synchronous recording of values.

It is important that the corrections correspond to the actual ranges and deflections. Additionally, for watt-meters the corrections must cover the range of actual power factors.

Measuring transformers introduce errors in ratio and phase displacement. The errors in phase displacementare especially important in consideration of the low power factors for load-losses in power transformers.

Ratio and phase displacement errors are given in calibration records as deviations in percent and minutesrespectively and their respective signs. According to definitions in standards ((for example IEC Publ. 60185 and 60186):

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Ratio errors are considered as positive when the secondary value is greater than the nominal value when theprimary value equals the rated primary value.Phase displacement errors are considered as positive when the secondary value leads the primary value inphase.

The ratio errors Ei and Eu are then:

(4.2) ][ %100´

% ⋅−

=U

U U E

u i.e.

+⋅=′100

1 u E

U U

(4.3) ][ %100´

% ⋅−

= I

I I E

i i.e.

+⋅=′100

1 i E

I I

(4.4) δ=δu-δi

Signs of phase displacement of the current and voltage vectors and their combination into a total phaseangle error, valid for inductive conditions.

Fig 4.2

From the definition of error it follows that a relative correction factor c can be expressed as:

%100⋅−

=valuecorrected

valuemeasured valuecorrected c

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Applied on the measured power this yields:

%100cos

)cos(´´cos

⋅⋅⋅+⋅⋅−⋅

= ϕ δ ϕ ϕ

I U

I U I U

c

] %100cos

sinsincoscos)

1001()

1001(1 ⋅

⋅−⋅⋅+⋅+−=

ϕ

δ ϕ δ ϕ iu

E E

The following simplifications can be made:

cosδ ≈ 1; sinδ ≈ δ ; sinϕ ≈ 1 ( δ in radians)

By neglecting products of errors, the total error will be:

][ ][ ][ )%100cos

( %%% ⋅−++=ϕ

δ iu

E E E

Phase displacement errors are normally given in minutes and the correction formula is then:

(4.5) [ ] [ ][ [ ] ( ) ]%cos

0291,0%%%

ϕ δ δ ⋅−−++=

iuiu E E E

The power factor has to be computed:

(4.6)´´

´)

I U

P

⋅=+ δ ϕ cos( and )

´´

´cos.cos(cos δ ϕ −

⋅=

I U

P arc

In all the formulas (4.2) to (4.6) the errors are to be introduced with their respective signs.The ratio errors are normally only a small fraction of a percent and can in most cases be neglected.

The phase displacement errors are predominantly dependent on the burden and the degree of excitation ofthe measuring transformer.Consequently, care should be taken to apply the errors corresponding to both the burden in the measuringcircuit and the actual deflection. The corrected value for the power is:

(4.7)][)

100

1( % E

P P −⋅=

The correction shall principally be performed on each phase value. That is easily accomplished using acomputer program. When having to do manual correction, this is more convenient to do on the total three-phase losses using average values of currents, voltage and errors, provided these do not deviate muchwithin the phases.

Analyses of load losses on several large units show that the power factors deviate only slightly withinphases, but the phase displacement errors of measuring transformers can vary considerably even for unitsof the same make and with the best available class.


If the reactive power supplied by the generator G is not sufficient when measuring large transformers, acapacitor bank C is used to compensate part of the inductive reactive power taken by the transformer.

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The voltage of the supply generator is raised until the current has attained the required value (50…100% ofthe rated current according to the standard). If a winding in the pair to be measured is equipped with an off-circuit or on-load tap-changer, the measurements are carried out on the principal and extreme tappings.The readings have to be taken as quickly as possible, because the windings tend to warm up due to the

current and the loss values obtained in measurement are higher accordingly.If the transformer has more than two windings all winding pairs are measured separately.If the measuring current I deviates from the rated current I, the power P and the voltage U at rated currentare obtained by applying corrections to the values P and U relating to the measuring current.The corrections are made as follows:

(4.8) P =m

m

r P

I

I 2

(4.9)m

m

r U

I

I U =

Mean values are calculated of the values corrected to the rated current and the mean values are used in thefollowing. According to the standards the measured value of the losses shall be corrected to a windingtemperature of 75°C. The transformer is at ambient temperature when the measurements are carried out,and the loss values are corrected to the reference temperature 75°C according to the standards as follows.

The d.c. losses I2R at the measuring temperature ϑm are calculated using the resistance values R1m and R2m

obtained in the resistance measurement (for windings 1 and 2 between line terminals):

(4.10) ))222122 5,1 mm I R I R I R I +=

The additional losses Pam at the measuring temperature are

(4.11) R I P P mam

2−=

When the losses are corrected to 75°C, it is assumed that the d.c. losses vary directly with resistance andthe additional losses inversely with resistance. The losses corrected to 75°C are obtained as follows:

(4.12)C

P C

R I P

s

m s

am

m s

s

c °++

++

°+=

75

752

ϑ

ϑ ϑ

ϑ ϑ

ϑ

ϑs= 235°C for Copper

ϑs= 225°C for Aluminium

4.7 RESULTS

The report indicates for each winding pair the power SN and the following values corrected to 75°C and therelating to the principal and extreme tappings.

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KončarPowerTransformers Ltd.

MEASUREMENT OF NO- LOADLOSS AND CURRENT

KPT-QTPT 005EPage : 1 / 2

5. MEASUREMENT OF NO- LOAD LOSS AND CURRENT

5.1 PURPOSE OF THE MEASUREMENTIn the no-load measurement the no-load losses P0 and the no-load current I0 of the transformer aredetermined at rated voltage and rated frequency.The test is usually carried out at several voltages bellow and above the rated voltage UN , and the results areinterpolated to correspond to the voltage values from 90 to 110% of UN at 5% intervals.

APPARATUS AND MEASURING CIRCUITS

I A

IB

IC

A

B

C

U A

UB

UC

I A IB IC U A UB UC

Data Acquisition & Wav e Analyser

U = Ua v

1,11

U = Urms

W

W

W

A A A V

V

V

G

Fig. 5-1 Circuit for the no-load loss measurement

5.2 PERFORMANCE

The following losses occur at no-load:

- iron losses in the transformer core and other constructional parts- dielectric losses in the insulations- load losses caused by the no-load current

While two last mentioned losses are small, they are generally ignored.

When carrying out the no-load measurement, the voltage wave shape may somewhat differ from thesinusoidal form. This is caused by the harmonics in the magnetizing current which cause additional voltagedrops in the impedances of the supply. The readings of the mean value meter and r.m.s. meter will bedifferent. The test voltage wave shape is satisfactory if the readings U´and U are equal within 3%.

Prepared by: J. Bujanović

Controlled by:


I. Šulc

Issue: 08.2003.



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KončarPowerTransformers Ltd.

MEASUREMENT OF NO- LOADLOSS AND CURRENT


Because the losses are to be determined under standard conditions, it is necessary to apply a wave shape

correction whereby the losses are corrected to correspond to test conditions where the supply voltage issinusoidal.In the test the voltage is adjusted so that the mean value voltmeter indicates the required voltage value (U´). At the same time, a voltmeter responsive to the r.m.s. value of voltage shall be connected in parallel with themean-value voltmeter and its indicated voltage U shall be recorded.

The following formula is valid for the iron losses.

The measured no-load loss is Pm and the corrected no load is taken as:

( )d P P m += 1

0

´

´

U

U U d

−= (usually negative)

The current and power readings of different phases are usually different (the power can be negative in somephase).This is due to the asymmetric construction of the 3-phase transformer; the mutual inductances betweendifferent phases are not equal.

5.3 RESULTS

The report shows the corrected readings at each voltage value, as well as the mean values of the currents ofall three phases. A regression analysis is carried out on the corrected readings. From the no-load curve thus obtained the no-load losses and no-load apparent power corresponding to voltage values from 90 to 110% of UN at 5%intervals are determined and stated.

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INDUCED OVERVOLTAGE WITHSTAND TEST KPT-QTPT 006EPage : 1 / 4

6. INDUCED OVERVOLTAGE WITHSTAND TEST

6.1 PURPOSE OF THE TEST

The object of the test is to secure that the insulation terminals between the phase windings, turns, tappingleads and terminals, withstand the temporary overvoltages and switching overvoltages to which thetransformer may be subjected during its lifetime.For non-uniformly insulated windings this test will also demonstrate the strength of insulation from windingsto earth and between phases of multi-phased units.The induced voltage test is a routine test for all units and it is specified as the last dielectric test.

6.2 PERFORMANCE

The transformer is excited to the terminals of the low-voltage windings and all other windings are left open-circuited. Voltages are then induced in all windings according to the turn ratio.

To avoid excessive magnetizing current during the test, the test object is supplied from 200 Hz generatorthrough a step-up transformer.Induced voltage tests are specified as short duration or long duration tests.

Standard short duration test is routine test for transformer with highest voltage Um≤170 kV and long durationtest is routine test for transformer with highest voltages Um>170 kV. In other cases one of these two testscould be specified as a special test.

6.2.1 Short duration induced AC withstand voltage test ACSD for transformers withuniformly insulated high voltage windings

On transformer with uniformly insulated windings, only phase-to-phase tests are carried out. Phase-to-earthtests are covered by separate source AC test according to IEC 60076-3, clause 11.Dependent on the highest voltage for equipment Um, the test shall be carried out with or without partial

discharge measurements.

6.2.1.a Transformers without specified partial discharge measurement at ACSD

The test voltage connection is quite same as in service. A three-phase transformers are tested withsymmetrical three-phase voltage induced in the phase windings. If a transformer has a neutral, it should beearthed during the test.The test voltage is twice the rated voltage. However, the voltage developed between line terminals of anyother windings shall not exceed the rated short duration power-frequency withstand voltage.The time of application of the full test voltage shall be:

Hz for

frequencytest

frequencyrated t 5030sec120

.

.=×= ,

or 36 seconds for 60 Hz power frequency

The test is successful if no collapse of the test voltage occurs.

Prepared by:



I. Šulc

Issue: 08.2003.



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6.2.1.b Transformers with specified partial discharge measurement at ACSD

These transformers shall be tested with partial discharge measurement. The three-phase transformers aretested with symmetrical three-phase voltage. The phase-to-phase test voltages shall not exceed thespecified withstand voltage for the winding in question. The full test voltage is twice the rated voltage.The partial discharge performance shall be controlled according to the time sequence, for the application ofthe voltage as shown in Fig. 6-1.

U start

A

B

C

D

E

3/1,1m

U ⋅3/1,1 mU ⋅3/3,1

mU ⋅

2U

voltage

test

U 1 2

U

< U start

Fig. 6-1 Time sequence for the application of test voltage with respect to earth

A=5 min; C=test time (30 or 36 s) E=5 minB=5 min; D=5 min

The Background noise level shall not exceed 100 pCThe test is successful if:

- no collapse of the test voltage occurs;- apparent charge at U2 does not exceed 300 pC on all measuring terminals- the partial discharge behaviour does not show a continuing rising tendency

6.2.2. Short duration AC withstand voltage test (ACSD) for transformers with non-uniformlyinsulated high-voltage windings

For three-phase transformers, two sets of tests are required namely:a) A phase-to-earth test with specified withstand voltages between phase and earth, with partial

discharge measurementb) A phase-to-phase with earthed neutral and with rated specified withstand voltage between phases

with partial discharge measurement

a) The test sequence for a three-phase transformer consists of three single-phase applications of testvoltage with different points of the windings connected to earth at each time. There are few possiblemethods, which avoid excessive overvoltage between line terminals.For particular complicated winding arrangements, the test sequence and the test connections should beagreed upon before test and test diagram should be enclosed to the test report.

The test time and the time sequence for the application of test voltage shall be as shown in Fig. 6-1.U1 is the specified test voltage and U2=1,5Um/√3 (acc. to the table 2,3 or 4 from KPT-QTPT 001E).

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b) For the partial discharge performance evaluation, during the phase-to-phase test, measurements shouldbe taken at U2=1,3 Um.The test time and the time sequence for the application of test voltage shall be as described in 6.2.1.b.

6.2.3 Long duration induced AC voltage test (ACLD) with non-uniformly and/or uniformlyinsulated high-voltage windings

For the highest insulation levels (>170 kV) a long duration induced voltage test including observation ofpartial discharges, should be specified as a routine test (see table 1 in KPT-QTPT 001E). A three-phase transformer shall be tested preferably in a symmetrical three-phase connection (see Fig. 6-2a)or in some cases in a single-phase connection that gives voltages in the line terminals according to Fig.6-2b(successively applied to all three phases).

U

U U U -0,5U -0,5U

a) b)

G

G

Fig. 6-2

A three-phase transformer supplied from the low-voltage winding side with a delta-connected high-voltagewindings can receive the proper test voltages only in a three phase test with a floating high-voltage winding.The neutral terminal, if present, of the winding under test and/or other separate windings shall be earthed.Tapped windings shall be connected to the principal tapping, unless otherwise agreed.The test time and the time sequence for the application of test voltage shall be as shown on Fig. 6-3.

The voltage to earth shall be: U1=1,7Um/√3

U2=1,5 Um/√3

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A

B

C

D

E

3/1,1 mU ⋅3/1,1 mU ⋅ 3/5,12 mU U ⋅=

3/7,11 mU U ⋅=

3/5,12 mU U ⋅=

< UstartU start

Fig. 6-3 Time sequence for the application of test voltage for induced AC long-duration tests (ACLD)

A= 5 min; C= test time (30 or 36 s)

B= 5 min; D= 60 min for Um≥300 kV or 30 min for Um


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PARTIAL DISCHARGE MEASUREMENT KPT-QTPT 007EPage : 1 / 2

7. PARTIAL DISCHARGE MEASUREMENT

7.1 GENERALThe partial discharge (PD) measurement is a method of observing the quality of the insulation without risk ofbreakdown. From the results of measurement conclusions can be drawn about the state of insulation, thequality of manufacture and possible concealed defects of insulation.Partial discharge of some magnitude can cause gassing premature aging or even destruction of theinsulation after a short time. On the other hand, partial discharges occurring in certain materials and notexceeding certain intensity are harmless.PD measurements have become an important aid to quality control in transformer construction.The criteria for assessment are the apparent charge q in pC.

7.1 TEST AND MEASURING CIRCUIT

For power transformers the PD measurement is normally performed during the induced overvoltage test as it

was described in KPT-QTPT 006E.Figure 7-1 shows the connection of test and measuring equipment used during partial dischargemeasurement of one three phase transformer.

G F V

V

V

2U

2V

2W

1V

1W

32

1

4 5 6

7

8

9

11

Zm

1U

Fig. 7-1 Measuring basic circuit

1 Transformer to be tested (Test object)2. Bushing taps for connection the pd-measuring equipment3 Coupling quadripol4 Measuring point selector5 ERA discharge detector models: type 652, with discharge magnitude meter type 666 (band with 40-220kHz or 3dB)6 Oscilloscope for observation the pulse distribution over one cycle of the test voltage ratio7 Feeding generator 200 Hz

8 Step up transformer9 Compensating power reactors10 Selective low-pass filters (for 200Hz)11 Potential transformers plus measuring circuit

The scheme is generally adapted for testing high-voltage transformers. The measuring circuit and indicationon instruments constitute a Broad Band pass system determined by their frequency characteristics. Inaccordance to IEC 60270, the frequency characteristics are determined by lower and upper cut-off

frequencies f1 and f2, which is at 3 dB for wide band (∆f=f2-f1) or in this case 40-220 KHz.The measuring impedance Zm is connected to the test tap of the condenser Bushing. Using measuring pointselector give us opportunity to perform the measurement on several terminals simultaneously.

Prepared by:



I. Šulc

Issue: 08.2003.



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PARTIAL DISCHARGE MEASUREMENT KPT-QTPT 007EPage : 2 / 2

7.2 CALIBRATION MEASUREMENT

The purpose of the measurement is to determine the scale factor “K” for the measurement with the completetest and measuring circuit.The calibration is performed by injecting an apparent charge q0 between each HV terminals and earthedtransformer tank using measuring point selector, as it is shown in Fig. 7-2.

The ratio of q0 to reading of the pC meter gives the scale factor of the pC meter . (

omq

qk = )

Because ERA discharge detector is equipped with suitable variable amplifier the signal can be adjusted toread the applied charge directly on the pC meter multiplied by scale factor k.

V

W

1

4 5 6

F

U

12Zm

Fig. 7-2 Calibration measurement


To achieve the desired low PD level, it is necessary to perform a thorough preparation of the testtransformer.The terminals should be shielded, the bushings must be cleaned and all foreign objects removed from thecover and tank because unearthed surface can give undesired discharges.The background level should be recorded with the complete test circuit connected, including the supplycircuit, but at nearly zero voltage.

The voltage is increased stepwise, first up to 1,1 Um/√3 and held there for a duration of 5 min; raised to U2and held there for a duration of 5 min; raised to U1, held there for the test time as stated in instruction Partfor induced voltage (KPT-QTPT 006E)Immediately after test time, reduced to U2 and held thee for a specified duration for 5, 30 or 60 min (KPT-

QTPT 006E); reduced to 1,1 Um/√3 and held there for a duration of 5 min, reduced to a value below one-third of U2 before switching off.The standard PD measuring sequence is reading of the PD levels at specified voltage levels at specifiedintervals (5 min) during the induced voltage test.

If higher then prescribed or specified PD levels occur the inception and extinction voltages should bedetermined.

The voltage should be increased and subsequently reduced until the discharges are decreased below thespecified level and the voltage are recorded as inception / extinction voltage.In such case further investigations have to be performed to check the severity of the PD.For example: From the distribution of discharged pulses which appear an ellipse (on oscilloscope)conclusions can be drawn as a to the type of defect.

7.4 TEST REPORT

A summary of test results which include measurement of PD for each terminal or measuring channel; appliedcalibration charge, applied voltage, time intervals and background level will put down on a form made for thispurpose.

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Končar

Power

transformers Ltd.

SEPARATE – SOURCE VOLTAGE TESTKPT-QTPT 008E

Page: 1/1

8. SEPARATE-SOURCE VOLTAGE TEST


The object of the test is to secure that the insulation between the windings and the insulation betweenwindings and the earthed parts withstand temporary overvoltages and switching overvoltages which mayoccur in service.

8.2 TEST CIRCUIT

T2

b

c

GS

G1

L

A

R

P2

P3

VV

E

N

A

B

C

aT3

T1

P1

Fig. 8-1 Test circuit for separate-source voltage withstand test

G1 supply generator, T1 test transformer, T2 transformer under test, T3 current transformer,L compensating reactor, E voltage divider, P1 ammeter, P2 voltmeter (r.m.s. value), P3 voltmeter(peak value).

The voltage is measured using a capacitive voltage divider in conjunction with voltmeters responsive to

r.m.s. and peak values. The peak-voltmeter indicates the peak value divided by √2. The test voltage isadjusted according to this meter.

8.3 PERFORMANCE

The test is made with single-phase voltage of rated frequency. The test voltage is applied for 60 secondsbetween all terminals of the winding under test and all terminals of the remaining windings, core and tank of

the transformer, connected together to earth. (Fig. 8-1)On windings with non-uniform insulation the test is carried out with the test voltage specified for the neutralterminal.The test is successful if no collapse of the test voltage occurs.

8.4 TEST REPORT

The test voltage, frequency and test duration are stated in the report.

Made:J. Bujanović

Controlled:I. Šulc

Approved:I. Šulc

Issue: 08.2003

KPT-QA.029E.1/2 izdanje 03.2002.


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OPERATION TESTS ON ON-LOADTAP CHANGER


9. OPERATION TESTS ON ON-LOAD TAP CHANGER

After the tap-changer is fully assembled on the transformer, the following tests are performed at (withexception of b) 100% of the rated auxiliary supply voltage:

• 8 complete operating cycles with the transformer not energized

• 1 complete operating cycle with the transformer not energized, with 85% of the rated auxiliary supplyvoltage ratio 1 complete operating cycle with the transformer energized at rated voltage andfrequency at no load

• 10 tap-change operations with ± two steps on either side of the principal tapping with as far aspossible the rated current of the transformer, with one winding short-circuited


Controlled by:


I. Šulc

Issue: 08.2003.



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MEASUREMENT OF THE ZERO-SEQUENCEIMPEDANCE


10. MEASUREMENT OF THE ZERO-SEQUENCE IMPEDANCE

10.1 PURPOSE OF THE MEASUREMENTThe zero sequence impedance is the impedance, which a three-phase circuit gives to a set of currents thatare equal to and in phase with each other in all phases.The zero-sequence impedance is of interest for calculating loads and currents at unsymmetrical conditions. At such calculations the method of symmetrical components is applied. By this method any set ofunsymmetrical three-phase vectors are resolved into three symmetrical component sets: the positive, thenegative and the zero sequence phase system.The relation of the symmetrical sets of voltage and currents is for each system given by correspondingimpedances.Measurement of zero-sequence impedances is a special test that is carried out only when specified in thecontract.

10.2 MEASURING CIRCUIT AND METHOD

The zero-sequence impedance is normally measured in connection with the load-loss and impedancevoltage test. The circuitry used is the same as for this test, but modified for one phase measurement asshown infig. 10.1.

Fig. 10-1 Circuitry for measuring zero-sequence impedance

Prepared by:



I. Šulc

Issue: 08.2003. 09.2003.



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MEASUREMENT OF THE ZERO-SEQUENCEIMPEDANCE


The phase terminals of the Y-connected winding are short-circuited and the voltage is applied between thisconnection and the neutral point.

For units where the current-carrying windings are equipped with tap changers, the measurements should beperformed on the three main taps. Any tests on other tap positions should be specified in the contract.For test objects with auxiliary or stabilizing windings, care should be taken to control that the currentcapacities of these are not exceeded. When necessary the final result is obtained by extrapolation.The applied voltage and current are recorded.

10.3 PRESENTATION OF RESULTS

The zero-sequence impedance Z0 is the quotient of the voltage and the current on the per phase basis whichis:

[ ]ohms I

U Z ⋅= 3

0

Like short-circuit impedances the zero-sequence impedance is normally expressed in percent of the per-unitvalue:

%1003%1002

0

0 ⋅⋅⋅=⋅=

r

r

r U

P

I

U

Z

Z Z

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CAPACITANCE AND THE INSULATION POWERFACTOR MEASUREMENT


Prepared by:

F. Juraković Controlled by:


I. Šulc

Issue: 08.2003. 11.2004.


11. CAPACITANCE AND THE INSULATION POWER FACTOR MEASUREMENT

11.1 PURPOSE OF THE MEASUREMENTThe purpose of the measurement is to determine the capacitances and power factor (tanδ) between thewindings and the earthed parts and between the different windings of the transformer.The capacitance values are needed when planning transformer overvoltage protection and calculating theovervoltages affecting the transformer. In addition the results are used by the manufacturer for designpurposes.

The power factor (tanδ) is used as an indicator of the general condition of the insulation. The power factorvalue is useful for evaluation the dryness of the insulation or aging and any oil contamination.


All line terminals of each winding are connected together during the measurement. The windingcapacitances of two-and three winding transformers are shown on Fig. 11-1.

Transformer capacitances

a. two-winding transformer (tests with guard circuit)b. three-winding transformer (tests with guard circuit)

Measurements of capacitances is performed together with insulation power factor measurement.

a) Two-winding transformers (tests with guard circuit)

High to low, guard on ground (C12)High to ground, guard on low (C10)Low to ground, guard on high (C20)

b) Three-winding transformers (tests with guard circuit)High to ground, guard on low and tertiary (C10)High to low, guard on tertiary and ground (C12 High to tertiary, guard on low and ground (C13)Low to ground, guard on high and tertiary (C20)Low to tertiary, guard on high and ground (C23)Tertiary to ground, guard on high and low (C30)

ba

1 2

C10 C12 C20C10 C12

C20

C13

C23 C30

1 2 3

Fig. 11-1 Transformer capacitances


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CAPACITANCE AND THE INSULATION POWERFACTOR MEASUREMENT


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KPT-QA.029E. 2/2 izdanje 03.2002.

The term “guard” signifies one or more conducting elements arranged and connected on an electricalinstrument or measuring circuit so as to divert unwanted currents from measuring means.The basic diagrams of the test circuits are shown on Fig. 11-2.The capacitance, power factor and the average temperature values are stated in the test report.

Mjerni instrument-

Measuringinstrument

(Tettex»Type

5281/2805)

C1-0 C1-2 C2-0

VN-

HVNN-L

V

Ispitivani transformator - Tranformer un der test

RC

C

N

v

High

Low

Test circuit for measurement C1-2+C1-0 (GST or GND). (Capacitance of HV-windi ng to LV-windi ng

and a tank. Tan k and LV-winding to ground .)

Mjerni instrument-

Measuringinstrument

(Tettex»Type

5281/2805)

C1-0 C1-2 C2-0

VN-

HVNN-L

V

Ispitivani transformator - Tranformer un der test

RC

C

N

v

High

Low

Test circuit for mea surement C1-2 (UST). (Capacitance of HV-windin g to LV-winding )

Mjerni instrument-

Measuringinstrument

(Tettex»Type5281/2805)

C1-0 C1-2 C2-0

VN-

HVNN-L

V

Ispitivani transformator - Tranformer und er test

RC

C

N

v

High

Low

Test circuit for m easurement C1-0 (GSTg or GRD). (Capacitance of HV-winding to tank.)

Fig. 11.2 TEST CIRCUIT FOR MEASUREMENT OF CAPACITANCE AND INSULATION POWER FACTOR


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INSULATION RESISTANCE MEASUREMENT KPT-QTPT 012EPage : 1 / 2

12. INSULATION RESISTANCE MEASUREMENT

12.1 PURPOSE OF THE MEASUREMENTThe purpose of the measurement is to determine the insulation resistance from individual winding to groundor between individual windings.The insulation resistance measured in the factory afford a useful indication as to whether the transformersare in suitable condition for application of dielectric test.Furthermore, results obtained in such tests are useful as reference values for later measurement at site. Theabsolute insulation resistance values depend on the transformer rated power, temperature, dryness,cleanliness and some other properties of the parts. That is why it is impossible to nominate or define ageneral allowable minimum insulation resistance value for transformers of different ratings.


The insulation resistance is expresssed in megohms and measured by means of an insulation resistance

meter with three line terminals at a voltage of 2500 or 5000 V d.c. All line terminals of each winding are connected together during the measurement.The resistance readings R15 and R60 are taken 15 sec and 60 sec after connecting the voltage.Measurement to be made in insulation resistance tests are:

a) Two winding transformer tests with guard circuit:High to low, guard on ground (R12)High to ground, guard on low (R10)Low to ground, guard on high (R20)

b) Three winding transformer tests with guard circuitHigh to ground, guard on low and tertiary (R10)High to low, guard on tertiary and ground (R12)High on tertiary, guard on low and ground (R13)

Low to tertiary, guard on high and ground (R23)Low to ground, guard on high and tertiary (R20)Tertiary to ground, guard on high and low (R30)

Each winding is measured separately by connecting the voltage between the winding to be tested and earth,while the other windings are earthed. The resistance readings R15 and R60 are taken 15s and 60s afterconnecting the voltage.The type of meter used, the measuring voltage, and temperature, R15, R60 and R60/R15 are stated in thereport.The basic diagrams of the test circuits for one three-winding transformer is shown on Fig. 12-1.

Prepared by:



I. Šulc

Issue: 08.2003. 09.2003.



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INSULATION RESISTANCE MEASUREMENT KPT-QTPT 012EPage : 2 / 2

E G L (-) (+)

Transformer under test

m

VN-HV

R1-0 R1-2 R2-3 R3-0

R2-0

R1-3

NN-LV STN-STW

Measuring instrument (Megger)

Basic test circuit for insulation resistance measurement, using «GUARD» - G terminal

( Measurement R1-0 = HV - m (LV + STW) ; HV - winding to tank (m) LV and STW winding to ''G'' terminal )

m

E G L (-) (+)

Transformer under test

VN-HV

R1-0 R1-2 R2-3 R3-0

R2-0

R1-3NN-LV STN-STW

Measuring instrument (Megger)

Basic test circuit for insulation resistance measurement, using «GUARD» - G terminal

( Measurement R1-2 = HV - LV (STW+m) - HV - winding to LV winding, STW-winding and tank (m), to «G» terminal )

Fig. 12-1 Basic diagrams for three-winding transformer

Issue : 08.2003. 09.2003.

KPT-QA.029E. 2/2 izdanje 03.2002.


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MEASUREMENT OF THE ELECTRICSTRENGTH OF THE INSULATING OIL


13. MEASUREMENT OF ELECTRIC STRENGTH OF THE INSULATING OIL

The electric strength of the oil is given by the breakdown voltage, measured using an electrode system inaccordance with IEC 60156. The electrodes are spherical surfaced with 25 mm radius and are 2,5 mm apart.The measurement is carried out at 50 Hz, the rate of increase of the voltage being 2 kV/s. The electricstrength is the average of five break-down voltage values.

The electric strength of new treated oil should be at least 60 kV. Oil, which does not withstand this voltage,may contain air bubbles, dust or moisture.


Controlled by:


I. Šulc

Issue: 08.2003.



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TEMPERATURE RISE TEST KPT-QTPT 014EPage : 1 / 4

Prepared by:

S. MarošControlled by:


I. Šulc

Issue: 08.2003. 11.2004.


14. TEMPERATURE RISE TEST

14.1 THE PURPOSE OF THE MEASUREMENTThe purpose of the measurement is to check that the temperature rises of the oil and the windings do notexceed the limits agreed on or specified by the standards.

14.2 THE MEASURING CIRCUIT

The supply and measuring facilities as well as the measuring circuit are the same as in the load lossmeasurement (KPT-QTPT 004E) and in the resistance measurement (KPT-QTPT 0003E). In additionthermometers are used for the measurement of the temperature of oil, cooling medium and the ambienttemperature and further a temperature recorder and thermocouples are used for the measurement of certaintemperatures and for equilibrium control.


The test is performed by using the short-circuit method. The temperature rise of the windings is determinedby the resistance method. The test is performed in line with IEC 60076-2 as follows:

Cold resistance measurementThe resistances and the corresponding oil temperature are measured. Resistance is measured by means ofstandardized U-I method or when the resistance cannot be accurately measured, due to the long duration oftransient phenomenon by means of inductive voltage correction method.The winding temperature is the same as the oil temperature.

Determination of the temperature rise of oilThe power to be supplied to the transformer is the sum of the no-load losses and the load losses on thetapping on which the temperature-rise test is to be performed (generally the maximum loss tapping). Withthis power the transformer is being heated (warmed up) to thermal equilibrium. The supply values and the

temperatures of different points are recorded at suitable time intervals. The oil temperature rise above thecooling medium temperature can be calculated from the equilibrium temperatures.

Determination of the temperature rise of windingsWithout interrupting the supply the current is reduced to rated current for 1h. The supply values and thetemperatures are recorded as above.

When the current has been switched off the hot-resistance measurement is performed. The test connectionis changed for carrying out the resistance measurement and after the inductive effects have disappeared theresistance-time-curve are measured for suitable period of time (zero time is the instant of switching off thesupply). The resistance is measured between same line terminals as in the cold resistance measurement.The resistances of the windings at shut-down are obtained by extrapolating the resistance-time-curves to theinstant of switching off. The temperature rises of the windings above the oil temperature are calculated onthe basis of the “hot” and “cold” resistance values and the oil temperature. The temperature rises of thewindings above the cooling medium temperature are found by adding the temperature rise of the oil abovethe cooling medium temperature to the before mentioned winding temperature rises.

For multi-winding transformers the latter part of the temperature rise test is generally carried out severaltimes in order to determine the individual winding temperature rises at the specified loading combination.

For air-cooled transformers with natural air circulation the temperature of the cooling medium is the same asthe ambient temperature. The ambient temperature is measured by means of at least three thermometers,which are placed at different points around the transformer at a distance defined by the standardsapproximately half-way up the transformer.For forced-air cooled transformers the temperature of the ingoing air is measured. If water is used as coolingmedium, the water temperature at the intake of the cooler is the reference temperature.


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KPT-QA.029E. 2/2 izdanje 03.2002.

The top oil temperature is measured by thermometer placed in an oil-filled thermometer pocket on the cover.In addition the temperatures of oil coming in and going out of the cooler and the surface temperatures at

different points are measured by means of thermocouples and a chart recorder.The readings of the thermometers mounted on the transformer are checked in connection with thetemperature rise test.

14.4 RESULTS

The temperature rises are calculated as follows:

Oil temperature rise

The temperature rise of top oil ∆θt is ∆θt = θt - θa (14.1)

θt = top oil temperature as mean value measured by sensors immersed in top oil

θa= external cooling medium temperature (ambient air or water)

When the test has been performed with applied test losses different from actual total losses the recorded topoil temperature rise above the temperature of the cooling medium has to be corrected according to theformula (14.2).

∆θt= ( )at

x

r

P

P θ θ −′⋅

(14.2)

Pr = sum of referenced load loss (at maximum loss tapping) and no-load loss P = power supplied during the testx = exponent according to actual standard (x=0,9 for ON and x=1,0 for OF and OD cooling)

θt’= recorded top oil temperature

The average temperature rise θe of the oil is

∆θ e=

−

+a

bt θ

θ θ

2 (14.3)

θb = temperature of oil entering the windings (i.e. oil returning from cooling equipment), bottom oiltemperature

or as above but corrected in similar way acc. to the formula (14.2)

∆θ e =

−

′+

′

⋅

a

bt

x

r

P

P θ

θ θ

2 (14.4)

θt´ = top oil temperature measured at supplied power P θb´ = bottom oil temperature measured at supplied power P

∆θe= the average temperature rise of the oil


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Issue : 08.2003. 11.2004.

KPT-QA.029E. 2/2 izdanje 03.2002.

Temperature rise of the winding

The average temperature of oil θoil 2 during the hot-resistance measurement is:

2

22

2

bt oil

θ θ θ

+= (14.5)

where θt2 and θb2 are top and bottom oil temperatures during hot-resistance measurement.

The average winding temperatures are calculated on the basis of the hot and cold resistance values and theoil temperature as follows:

( ) 2352351

1

2

2 −+= θ θ

R

R (14.6)

R1= cold resistanceR2= hot resistance

θ1= the average temperature of oil during cold resistance measurement

The average temperature rise ∆θ w-oil of the winding above oil temperature is:

∆θ w-oil = θ2 – θoil2 (14.7)

If the current in the rated current period has deviated from the rated value (less than 10% of the ratedcurrent) the temperature rise of the winding above oil temperature has to be corrected according to:

∆θ w-oil = ( )22 ´´ oil I I

y

r θ θ −⋅

(14.8)

Ir = rated current of windingI = test currenty = exponent according to the standard (y=1,6 for ON and OF cooled transformers;y =2,0 for OD cooled transformers)

The winding temperature rise is then, finally:

∆θ w = ∆θ w-oil + ∆θ e (14.9)

The temperature rise of the hot spot ∆θh.spt of the winding above the ambient temperature is:

∆θh.spt = ∆θt + H ∆θw (14.10)

where H is hot-spot factor and is taken 1.1 if different value has not been specified.


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Issue : 08.2003. 11.2004.

KPT-QA.029E. 2/2 izdanje 03.2002.

14.5 TEST MODIFICATION FOR MULTI-WINDING TRANSFORMER

Total losses for the specified load combination will be developed in short-circuit method by applying power toone of the windings and short circuiting one or more of the rest of the windings. After reaching steady stateconditions oil temperature and oil temperature rise is determined in the same way as explained in chapters14.3 and 14.4.

The temperature rise for an individual winding above oil is obtained afterwards with rated current in thewinding as described in chapters 14.3 and 14.4. This can be achieved usually in two-winding combination(one supplied, one short circuited) or more than one can be short circuited in order to reach rated current inthe winding with highest rated power.

14.6 TEST REPORT

The report indicates:- cold resistance values and the corresponding oil temperature- temperature rises of oil for corresponding losses- winding temperature rises calculated from the measuring results

In addition, information on the tapping position, the cooling and short-circuit method and winding combinationis given.


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LIGHTNING IMPULSE TEST KPT-QTPT 015EPage : 1 / 5

15. LIGHTNING IMPULSE TEST

15.1 PURPOSE OF THE TESTThe purpose of the impulse voltage test is to secure that the transformer insulation withstand the lightningovervoltages which may occur in service.

15.2 TESTING EQUIPMENT

Impulse generator

FnR

b Ra

FanC3

Rc

Rs

F2 Rb Ra

Fa2

C

Cs

Rc

Rs

F1

Rb Ra

Cs

Fa1

Rs

Cs impulse capacitorRc charging resistorRs series resistorRa low-ohmic discharging resistor for

lightning impulseRb high-ohmic discharging resistor for

switching impulseF1 … Fn main spark gaps

Fal …Fan auxiliary spark-gaps

Fig. 15-1 Basic circuit diagram of the impulse generator

The impulse generator design is based on the Marx circuit.The basic circuit diagram is shown on Fig. 15-1. The impulse capacitors Cs (12 capacitors of 750nF) are

charged in parallel through the charging resistors Rc (28kΩ) (highest permissible charging voltage 200 kV).

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When the charging voltage has reached the required value, breakdown of the sparkgap F 1 is initiated by anexternal triggering pulse. When F1 breaks down, the potential of the following stage (point B and C) rises.

Because the series resistors Rs is of low-ohmic value compared with the discharging resistors Rb (4,5 kΩ)and the charging resistor Rc, and since the low-ohmic discharging resistor Ra is separated from the circuit by

the auxiliary spark-gap Fal, the potential difference across the spark-gap F2 rises considerably and thebreakdown of F2 is initiated.Thus the spark-gaps are caused to break down in sequence.

Consequently the capacitors are discharged in series-connection. The high-ohmic discharge resistors Rb aredimensioned for switching impulses and the low-ohmic resistors Ra for lightning impulses. The resistors Ra are connected in parallel with the resistors Rb, when the auxiliary spark-gaps break down, with a time delayof a few hundred nano-seconds. The arrangement is necessary in order to secure the functioning of thegenerator.The wave shape and the peak value of the impulse voltage are measured by means of an Impulse AnalysingSystem (DIAS 733) which are connected to the voltage divider.The required voltage is obtained by selecting a suitable number of series-connected stages and by adjustedthe charging voltage. In order to obtain the necessary discharge energy parallel or series-parallelconnections of the generator can be used. In these cases some of the capacitors are connected in parallelduring the discharge.

C1

V=U=R1

R2

Impulse generator

C2

C3

1U

1V

1W

N

2U

2V

2W

2N

3U1

3V2

3V

3W

Voltagedivider

Transformer

under test

Voltage

recorder

Channel 1 (U)

of digitaltranzient rec.

Current recorder Channel 2(I) of digital tranzient rec.

S1

Fig. 15-2 Equivalent diagram of the impulse test circuitC1 resulting impulse capacitance, R2 resulting series resistance, R1 resulting discharge

resistance, C2 and C3 capacitances of the voltage divider

The required impulse shape is obtained by suitable selection of the series and discharge resistors of thegenerator.

The front time can be calculated approximately from the equation:For R1 >> R2 and Cg >> C (15.1)

T t

C R ⋅⋅=21

3

and the half time to half value from the equation

(15.2)112 7,0 C RT

⋅⋅≈ In practice the testing circuit is dimensioned according to experience.

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15.3 CONNECTION OF THE TEST OBJECT

The testing impulse test is normally applied to all windings. The impulse test-sequence is appliedsuccessively to each of the line terminals of the tested winding. The other line terminals and the neutral

terminal are earthed (single-terminal test, Fig. 15-4a and b).When testing low voltage windings of high power, the time to half-value obtained is often too short ( Fig.15-5). However, the time to half value can be increased by connecting suitable resistors ( Ra in Fig.15-4b) between the adjacent terminals and earth. According to the IEC 60076-3 standard the resistances ofthe resistors must be selected so that the voltages at the adjacent terminals do not exceed 75% of the test

voltage and the resistance does not exceed 500Ω.

RECORDER

S1

Ra

Ra

b

RECORDER

c

S1

RECORDER

a

S1

RECORDER

Ra

RECORDER

Ru

d

S 1

RECORDER

e

S1

S 1Ra R b

RECORDER

Fig. 15-4 Transformer impulse and fault detection connections.

a and b l-terminal testingc and d neutral terminal testing

When the low voltage winding in service cannot be subjected to lightning overvoltages from the low voltagesystem (e.g. step-up transformers, tertiary windings) the low voltage winding may be impulse testedsimultaneously with the impulse tests on the high voltage winding with surges transferred from the highvoltage winding to the low voltage winding (Fig. 15-4e, test with transferred voltages).

According to the standard IEC 60076-3 the line terminals of the low voltage winding are connected to earththrough resistances of such value (resistances Ra in Fig. 15-4e) that the amplitude of transferred impulsevoltage between line terminal and earth or between different line terminals or across a phase winding will beas high as possible but not exceeding the rated impulse withstand voltage. The resistance shall not exceed

5000 Ω.

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The neutral terminal is normally tested directly or indirectly by connecting a high-ohmic resistor between theneutral and earth (voltage divider Ra, Ru) and by applying the impulse (Fig. 15-4c and d) to the line terminalsconnected together.The impulse test of a neutral terminal is performed only if requested by a customer. The front time is allowed

to be up to 13 µsec. The failure detection is normally accompolished by exmination of the oscillograms of theapplied test voltage, the neutral current and / or the capacitively transferred current.

15.4 PERFORMANCE OF THE IMPULSE TEST

The test is performed with standard lightning impulses of negative polarity. The front time (T1) and the time tohalf-value (T2) are defined in accordance with the standard.

U

T2

0,3

T

T1 =1,67 T

1,0

0,9

0,5

0t

Fig. 15-5 Standard lightning impulse

Front time T1 = 1,2µs ± 30%

Time to half-value T2 = 50 µs ± 20%

In practice the impulse shape may deviate from the standard impulse when testing low-voltage windings ofhigh rated power and windings of high input capacitance.The impulse test is performed with negative polarity voltages to avoid erratic flashovers in the externalinsulation and test circuit.Waveform adjustments are necessary for most test objects. Experience gained from results of tests onsimilar units or eventual precalculation can give guidance for selecting components for the wave shapingcircuit.The test sequence consists of one reference impulse (RW) at 75% of full amplitude followed by the specifiednumber of voltage applications at full amplitude (FW) (according to IEC 60076-3 three full impulses).The equipment for voltage and current signal recording consists of digital transient recorder, monitor,computer, plotter and printer.The recordings at the two levels can be compared directly for failure indication.

For regulating transformers one phase is tested with the on-load tap changer set for the rated voltage andthe two other phases are tested in each of the extreme positions.

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Detection of correctness at impulse testing is based on comparison of voltage and current records obtainedat reduced and full amplitudes.The two traces should have a perfect match to constitute evidence that the insulation has passed the test.

15.5 TEST REPORTThe detailed test record cover setting of impulse generator, values for all components in the impulse shapingand measuring circuits, connection of the test object, parameters for the wave-shape and oscillogramrecords for each voltage application.

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TEST WITH THE LIGHTNING IMPULSE CHOPPED ONTHE TAIL


16. TEST WITH THE LIGHTNING IMPULSE CHOPPED ON THE TAIL


The purpose of the chopped lightning test is to secure that the transformer insulation withstand the voltagestresses caused by chopped lightning impulses, which may occur in service.

16.2 TEST EQUIPMENT

For the chopped lightning impulse test the same testing and measuring equipment and the same testing andfault detection connections are used as for the standard lightning impulse test. The impulse is chopped bymeans of triggered-type chopping gap connected to the terminal to which the impulse is applied. The delayof the chopping–gap ignition impulse in relation to the impulse generation is adjustable, thus the time Tc fromthe start of the impulse to the chopping can be adjusted (Fig. 16-1).

16.2 PERFORMANCE OF THE TEST

The test is performed with impulses of negative polarity. The duration Tc from the beginning of the impulse tothe chopping can vary within the range of 2...6µs (Fig. 16-1) According to the standard IEC 60076-3 theamount of overswing to opposite polarity shall be limited to not more than 30% of the amplitude of thechopped impulse (Fig. 16-1). If necessary the overswing amplitude will be limited to the value mentioned bymeans of damping resistor inserted in the chopping circuit.

1,0

0,9

0,3

α

Tc

T1

Fig 16-1 Chopped lightning impulseFront time T1 = 1,2 µs ± 30%

Time to chopping Tc = 2....6 µs

%30%100


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TEST WITH THE LIGHTNING IMPULSE CHOPPED ONTHE TAIL


The voltage measurement is based on the peak voltmeter indication.The test with chopped lightning impulse is combined with the test carried out with standard impulse.

The following order of pulse application is recommended by the standard IEC 60076-3- one 75% full impulse- one 100% full impulse- one or more 75% chopped impulses- two 100% chopped impulses- two 100% full impulses

16.4 THE FAILURE INDICATION

The fault detection is also for chopped impulses primarily based on the comparison of voltages and windingcurrents obtained at 75% and 100% test voltages. At high test voltages there is a small delay in the ignitionsof the chopping-gap, which causes differences in the fault detection of voltages and winding currents.Furthermore differences in the instant of firing of the stages in the impulse generator may give rise to initialhigh-frequency oscillations in the first part of the voltage front. In this case the fault detection must be basedprimarily on the recordings obtained at the application of full impulses.When carrying out the chopped-impulse test, unless otherwise agreed, different tappings are selected for thetests on the three phases of a three-phase transformer, usually the two extreme tappings and principaltapping.

16.5 TEST REPORT

The test voltage values, impulse shapes, tappings and the number of impulses at different voltage levels arestated in the report.

The oscillographic records and measurement records are stored in the archives, where they are availablewhen required.

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SWITCHING IMPULSE TEST KPT-QTPT 017EPage : 1 / 2

17. SWITCHING IMPULSE TEST

17.1 PURPOSE OF THE TESTThe purpose of the switching impulse test is to secure that insulation between windings, between windingsand earth, between line terminals and earth and between different terminals withstand the switchingovervoltages, which may occur in service.

17.2 PERFORMANCE OF THE TEST

The same testing and measuring equipment as for the lightning impulse test are used here. According to the IEC 60076-3 the switching impulse test is carried out on each high voltage line terminal of athree-phase winding in sequence. A single-phase no-load test connection is used in accordance with Fig. 17-1. The voltage developed between line terminals during the test is approximately 1,5 times the test voltagebetween line and neutral terminals.The flux density in the magnetic circuit increases considerably during the test. When the core reaches

saturation the winding impedance is drastically reduced and a chopping of the applied voltage takes place(Fig. 17-2). The time to saturation determines the duration of the switching impulse. Because the remanentflux can amount to even 70 to 80 % of the saturation flux, the initial remanence of the core has a greatinfluence on the voltage duration. By introducing remanent flux of opposite polarity in relation to the fluxcaused by the switching impulse, the maximum possible switching impulse duration can be increased. Theremanence of opposite polarity is introduced in the core by applying low voltage current impulses to LVwinding of opposite polarity to the transformer before each full voltage test impulse.

U -0,5U - 0,5 U

currentrecorder

C2

C1

voltagerecorder

S1

Loading resistor

Fig. 17-1 Transformer switching impulse testing and fault detection connections

The test is performed with impulses of negative polarity. The requirements on the switching impulse shapegiven in the standard IEC 60076-3 are summarized in Fig. 17-2.

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SWITCHING IMPULSE TEST KPT-QTPT 017EPage : 2 / 2

The voltage measurement is based on the peak voltmeter indication.

T0

1.00.9

Td T

Tp = 1,67 T

0,3

Fig. 17-2 Switching impulse

Front time Tp> 100µs

Time above 90% Td> 200µs

Time to the first zero passage T0> 500µs ( preferably 1000 µs )

At full test voltage each phase will be tested with the number of impulses required by the relevant standard.When comparing the wave shape it is to be noticed that the magnetic saturation causes drastic reduction ofvoltage and increase in winding current and the time to saturation is dependent on the amplitude of the

applied voltage. Thus voltage and current oscillograms obtained at full test voltage and at 75% voltage levelwill deviate from each other in this respect.The fault detection is mainly based on the voltage oscillograms. The test is successful if no sudden collapseof voltage caused by flashover or breakdown is indicated on the voltage oscillograms and no abnormalsound effects are observed.When the core reaches saturation a slight noise caused by magnetosriction can be heard from thetransformer.

Test report

The test voltage values, impulse shapes, and number of impulses at different voltage levels are stated in thereport. The oscillographic records are stored in the archives, where they are available when required.

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MEASUREMENT OF ACOUSTIC SOUNDLEVEL


18. MEASUREMENT OF ACOUSTIC SOUND LEVEL

18.1 PURPOSE OF THE MEASUREMENTThe purpose of the sound level measurement is to check that the sound level of the transformer meets thespecification requirements given in relevant standards e.g. IEC 60076-10 or guarantee values given by thetransformers manufacturer. A sound spectrum analyses are carried out for the transformer at the customer'srequest. The sound spectrum indicates the magnitude of sound components as a function of frequency.

18.2 MEASURING EQUIPMENT

A precision sound pressure level meter type 1 complying with IEC 60651 is used in the sound levelmeasurements. The measurements are performed using the weighing curve A. The sound spectrum analysisof the transformer is carried out by recording the sound band levels as a function of frequency.This is done with sound level meter Br űel & Kjaer type 2236 and calibrator type 4231.


The A-weighted sound pressure level of the Background noise shall be measured at points on the prescribedcounter immediately before and after the measurements on the transformer. Power is supplied to thetransformer under no load condition at the rated voltage and the frequency with the tapping selector on theprincipal tapping. The sound pressure level is the measured at various points around the transformer asdetailed in the standards: at a distance (D) of 30 cm for ONAN or 2 m for ONAF cooling system spaced at aninterval (X) of 1 meter; as it is shown on Fig. 18-1.The microphone position in the vertical direction shall be on horizontal planes at one third and two thirds ofone transformer tank height when the height of the tank is equal to or greater than 2,5 m.When the tank height is less than 2,5m, the measurement plane is located at half the tank h

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