test methods

Končar PowerTransformers Ltd.

TEST METHODS FOR POWER TRANSFORMERS

QT No: xx-yy Page : 1 / 2

Prepared by: F. Jurakovi ć

Approved by: I. Šulc

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

KPT-QT.001E, izdanje 08.2003.


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 Power transformers are dealt with in the following Sections. The electrical characteristics and dielectric strength of the transformer are checked by means of measurements and tests defined by standards. The tests are carried out in accordance with IEC Standard 60076, Power transformers, unless otherwise specified in the contract documents. CONTENTS

Item

Title

ID

1. Summary of dielectric tests KPT-QTPT 001E issue 08.2003.

2. Measurement of voltage ratio and check of connection 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 factor measurement


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

13. Measurement of the electric strength of the insulating 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 the tail


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 in magnetizing current


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



QT No: xx-yy Page : 2 / 2

Issue : 08.2003. 09.2003. 06.2004. 11.2004. 03.2006. 07.2006. KPT-QT.001E, izdanje 08.2003.

Item

Title

ID

21. 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.


SUMMARY OF DIELECTRIC TESTS KPT-QTPT 001E Page: 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 in tables 2, 3 and 4. The choice between the different levels of standard withstand voltage in these tables depends on the severity of over voltage conditions to be expected in the system and on the importance of the particular installation.

Tests Category of winding

Highest voltage for equipment Um

kV

Lightning impulse (LI)

Switching impulse (SI)

Long duration AC (ACLD)

Short duration AC (ACSD)

Separate source AC

Uniform insulation

Um≤72,5

Type (note 1)

Not applicable

Not applicable (note 1)

Routine

Routine

72,5<Um≤170 Routine Not applicable Special Routine Routine

170<Um<300 Routine Routine (note 2)

Routine Special (note 2)

Routine

Uniform and non uniform insulation Um≥300 Routine Routine Routine Special Routine NOTE 1 In some countries, for transformers with Um≤72,5 kV, LI tests are required as routine tests, and ACLD tests are required as routine or type tests.

NOTE 2 If the ACSD test is specified, the SI is not required. This should be clearly stated in the enquiry document.

Table 1 – Requirement and tests for different categories of winding

Prepared by: J. Bujanović

Controlled by: I. Šulc


Issue: 08.2003. KPT-QA.029E 1/2 izdanje 03.2002.



Highest voltage for Equipment um

Kv r.m.s.

Rated lightning impulse withstand voltage

kV peak

Rated short duration induced or separate source

AC withstand voltage

kV r.m.s.

20 3,6 10 40 7,2 20 60 12 28 75 17,5 38 95

24 50 125 145 36 70 170 52 250 95 60 280 115

72,5 325 140

380 150 100 450 185 123

550 230 145

650 275 170

750 325

NOTE Dotted lines may require additional phase-to-phase withstand tests to prove that

the 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

Issue : 08.2003. KPT-QA.029E. 2/2 izdanje 03.2002.



Rated lightning impulse

withstand voltage

kV peak

Rated short-duration induced or separate source AC

withstand voltage kV r.m.s. Highest voltage for

equipment Um Distribution (note 1) and class I

transformers (note 2)

CLASS II transformers (note 3)

Distribution and class I transformers

CLASS II transformers

15

26,4 36,5 48,3 72,5 121

145

169

95

125 150 200 250 350

110

-

150 200 250 350 350 450 550 650 750

34

40 50 70 95

140

34

-

50 70 95

140 140 185 230 275 325

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




Highest voltage for

equipment Um

kV r.m.s.

Rated switching impulse withstand voltage phase-

to-earth

kV peak

Rated lightning impulse withstand

voltage

kV peak

Rated short-duration induced or separate source AC withstand

voltage kV r.m.s.

650

550 325 750 650 360

245 850

750 395 300 950

850 460 362 1 050

950 510

1 175 1050 850 460 1175 950 510

420 1300 1050 570

550 1425 1175 630 1550 1300 680 1675 1300 note 3 1800

800 1425 note 3 1950 1550 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 may have 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-phase withstand voltages are met. This is valid for the lower insulation levels assigned to the different Um in the table.

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



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 voltage of 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 limit of 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 to the 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 is the same as that of the transformer under test. The result of deviations is shown directly on the display of the bridge.

U1

1U

1V

1W

1N

2U

2V

2W

2N

y

xU2

Test object

~

xyx

UU +

=2

1

Fig. 2-1 Because the bridge measuring device works on the single-phase principle the voltage ratio is measured phase by phase between two windings mounted on the same leg. The indication on the bridge display depends 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 also checked. When the measuring conductors of the transformer are connected to the bridge according to Fig. 2-1 and Fig. 2-2, the bridge can be balanced only if the vector group is correct.






MEASUREMENT OF VOLTAGE RATIO AND CHECK OF CONNECTION SYMBOL


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 specified tapping voltage ratios are stated, as well as the deviations of measured ratios from these values. The connection symbol is also stated in the test report.

Dd 0iii

i

ii

II

IIII

IIIIII

0

iDy Iiii ii

iDz 0iii ii

0

i iii8

Yz IIiiii

ii

II

Dz 1010

i iiiii

Yz 7

Dz 8 ii

7i

iii

Dz 6

6

ii

iiiiii

Yz 5i

5

ii iii

Dz 4 iii

ii i

2Dz 2iii i

ii

iiiii

Yz Ii

I

4

iDd 2

iii

ii

I

2

iDd 4 iii

ii 4

iiiii

Dy 5i

5

6

iii

iiiDd 6

Dy 7i

7

ii

iii

Dd 8iii

iii

8

10Dd 10

iii

i ii

iii ii

Dy II i

II

i

iii ii

Yy 0

0

iiiiii

Yd II

iiiYd 5i

ii

5

6

Yy 6 ii iii

i

7

Yd 7 ii iiii

Yd IIII

iiii

ii

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



MEASUREMENT OF WINDING RESISTANCE


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 direct current. The measurement is performed for each connection of connectable windings and for each tapping connection. Furthermore the corresponding winding temperature is measured. The measured resistances are needed in connection with the load loss measurement when the load losses are corrected to correspond to the reference temperature. The measurement will also show whether the winding joints are in order and the windings correctly connected.

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 using instruments 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 obtained from a battery 60V.

2U

2V

2W

1U

1V

1WV

A

+

-battery

Test object

Re

Fig. 3-1 The resistance value is then determined as

readingampermeterreadingvoltmeterR =

3.3 PERFORMANCE OF THE MEASUREMENT

Before the measurement starts, the transformer is standing for at least 3 hours filled with oil and without excitation. During this period the temperature differences of the transformer will equalize and the winding temperature will become equal to the oil temperature.

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






MEASUREMENT OF WINDING RESISTANCE


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

(3.1)

−=

−LRt

eREi 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 will be saturated and the inductance will be low. The measuring current is usually 5…10 times the no-load current 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 constant and 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, between which the resistances are measured, the connection, the tapping position and the average temperature of the windings during the measurement are stated.



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

4. IMPEDANCE AND LOAD LOSS MEASUREMENT

4.1 PURPOSE OF THE MEASUREMENT

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

4.2 IMPEDANCE

The measured impedance voltage depends on the voltage rating of the winding where the measurements are made. Consequently it is customary to express the impedance voltage as a percentage of the rated voltage 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 to which winding the current is supplied. The load losses are losses associated with the load current and the leakage flux and consist of losses in conductors as for DC, eddy-current losses in conductors caused by the leakage flux and hysteresis and eddy-current losses in the core, clumps and tank structure. From the total losses measured and the winding DC-resistances, the stray losses are computed. Separation of the loss components is necessary as information for prediction and control of losses. It is also necessary for converting 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 supply current to the high voltage winding. For large test objects the demand for reactive power will be considerable and is normally supplied by static condenser banks. As the impedance voltage will vary within wide limits, a step-up transformer is normally necessary between the generator and the test object. Voltages, currents and power are measured by instruments supplied from measuring transformers as given in fig. 4-1 as an example. Only high quality measuring transformers and instruments must be used. The impedances of transformers are linear and there is no need to take creation of harmonics into consideration. Especially large 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 are processed and a complete test record written out. In a traditional instrumentation the three-wattmeter method should always be used for three-phase measurements. Irrespective of the instrumentation for power measurement, the errors in ratio and phase displacement of the measuring transformers will introduce errors which have to be corrected. Correction of errors is discussed in clause 4.5. The temperature is an important factor in this test and is measured with thermometers in the oil system.







G

A

W

IA

AA

IB IC

W

Wf

C

V

AB

C

UA UB UC

IA IB IC UA 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 of transformers. Consequently it is imperative that the exact losses are established and known errors are stated and corrected. Determination of measuring errors and their correction is a complex matter. The present analysis will not cover 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 for equipment 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 voltmeters have 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 better resolution and synchronous recording of values. It is important that the corrections correspond to the actual ranges and deflections. Additionally, for wattmeters the corrections must cover the range of actual power factors. Measuring transformers introduce errors in ratio and phase displacement. The errors in phase displacement are 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 minutes respectively and their respective signs. According to definitions in standards ((for example IEC Publ. 60185 and 60186):




Ratio errors are considered as positive when the secondary value is greater than the nominal value when the primary value equals the rated primary value. Phase displacement errors are considered as positive when the secondary value leads the primary value in phase. The ratio errors Ei and Eu are then:

(4.2) ][ %100´% ⋅

−=

UUUEu i.e.

+⋅=′

1001 uEUU

(4.3) ][ %100´% ⋅

−=

IIIEi i.e.

+⋅=′

1001 iEII

(4.4) δ=δu-δi Signs of phase displacement of the current and voltage vectors and their combination into a total phase angle 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

valuemeasuredvaluecorrectedc




Applied on the measured power this yields:

%100cos

)cos(´´cos⋅

⋅⋅+⋅⋅−⋅

=ϕ

δϕϕIU

IUIUc

] %100cos

sinsincoscos)100

1()100

1(1 ⋅

⋅−⋅⋅+⋅+−=

ϕδϕδϕiu EE

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

δiu EEE

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

(4.5) [ ] [ ][ [ ] ( ) ]%cos0291,0

%%% ϕδδ ⋅−−++= iuiu EEE

The power factor has to be computed:

(4.6) ´´

´)IU

P⋅

=+ δϕcos( and )´´

´cos.cos(cos δϕ −⋅

=IU

Parc

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 of the measuring transformer. Consequently, care should be taken to apply the errors corresponding to both the burden in the measuring circuit and the actual deflection. The corrected value for the power is:

(4.7) ][ )100

1( %EPP −⋅=

The correction shall principally be performed on each phase value. That is easily accomplished using a computer 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 much within the phases. Analyses of load losses on several large units show that the power factors deviate only slightly within phases, but the phase displacement errors of measuring transformers can vary considerably even for units of 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, a capacitor bank C is used to compensate part of the inductive reactive power taken by the transformer.




The voltage of the supply generator is raised until the current has attained the required value (50…100% of the 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 current are obtained by applying corrections to the values P and U relating to the measuring current. The corrections are made as follows:

(4.8) P = mm

r PII

2

(4.9) mm

r UIIU =

Mean values are calculated of the values corrected to the rated current and the mean values are used in the following. According to the standards the measured value of the losses shall be corrected to a winding temperature 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) ( ))222

122 5,1 mmI RIRIRI +=

The additional losses Pam at the measuring temperature are

(4.11) RIPP mam2−=

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

(4.12) C

PCRIPs

msam

ms

sc °+

++

+°+

=75

752

ϑϑϑ

ϑϑϑ

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 the relating to the principal and extreme tappings.


Končar Power Transformers Ltd.

MEASUREMENT OF NO- LOAD LOSS AND CURRENT


5. MEASUREMENT OF NO- LOAD LOSS AND CURRENT


In the no-load measurement the no-load losses P0 and the no-load current I0 of the transformer are determined 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 are interpolated to correspond to the voltage values from 90 to 110% of UN at 5% intervals. APPARATUS AND MEASURING CIRCUITS

IA IB IC

AB

C

UA U B UC

IA IB IC UA UB UC

Data Acquisition & Wav e Analy ser

U = Ua v 1,11

U = U rms

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 the sinusoidal form. This is caused by the harmonics in the magnetizing current which cause additional voltage drops in the impedances of the supply. The readings of the mean value meter and r.m.s. meter will be different. The test voltage wave shape is satisfactory if the readings U´and U are equal within 3%.





Končar Power Transformers Ltd.

MEASUREMENT OF NO- LOAD LOSS 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 is sinusoidal. 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 the mean-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: ( )dPP m += 10

´

´U

UUd −= (usually negative)

The current and power readings of different phases are usually different (the power can be negative in some phase). This is due to the asymmetric construction of the 3-phase transformer; the mutual inductances between different 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 of all 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.



INDUCED OVERVOLTAGE WITHSTAND TEST KPT-QTPT 006E Page : 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, tapping leads and terminals, withstand the temporary overvoltages and switching overvoltages to which the transformer may be subjected during its lifetime. For non-uniformly insulated windings this test will also demonstrate the strength of insulation from windings to 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 generator through 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 duration test is routine test for transformer with highest voltages Um>170 kV. In other cases one of these two tests could be specified as a special test.

6.2.1 Short duration induced AC withstand voltage test [ACSD] for transformers with uniformly insulated high voltage windings

On transformer with uniformly insulated windings, only phase-to-phase tests are carried out. Phase-to-earth tests 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 with symmetrical three-phase voltage induced in the phase windings. If a transformer has a neutral, it should be earthed during the test. The test voltage is twice the rated voltage. However, the voltage developed between line terminals of any other windings shall not exceed the rated short duration power-frequency withstand voltage. The time of application of the full test voltage shall be:

Hzforfrequencytestfrequencyratedt 5030sec120

..

=×= ,

or 36 seconds for 60 Hz power frequency

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







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 are tested with symmetrical three-phase voltage. The phase-to-phase test voltages shall not exceed the specified 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 of the voltage as shown in Fig. 6-1.

U start

A

B

C

D

E

3/1,1 mU⋅3/1,1 mU⋅3/3,1 mU⋅

2U

voltagetestU1 2U

< 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 min B=5 min; D=5 min The Background noise level shall not exceed 100 pC The 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-uniformly insulated 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 measurement b) 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 test voltage with different points of the windings connected to earth at each time. There are few possible methods, which avoid excessive overvoltage between line terminals. For particular complicated winding arrangements, the test sequence and the test connections should be agreed 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).




b) For the partial discharge performance evaluation, during the phase-to-phase test, measurements should be 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 uniformly insulated high-voltage windings For the highest insulation levels (>170 kV) a long duration induced voltage test including observation of partial 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-voltage windings 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




A

B

C

D

E

3/1,1 mU⋅3/1,1 mU⋅ 3/5,12 mUU ⋅=

3/7,11 mUU ⋅=

3/5,12 mUU ⋅=< Ustart

U 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<300 kV E= 5 min; During the whole application of the test voltage, partial discharges shall be monitored. Further information, about purpose and methods may be obtained from enclosed application guide for partial discharge measurements. (KPT-QTPT 007E) The test is successful if:

- no collapse of the test voltage occurs - the continuous level of partial discharges does not exceed 500 pC during long duration test at U2 - the partial discharge behaviour shows no continuously rising tendency at U2.

In the case of failure to meet the partial discharge acceptance criteria, further investigation should be undertaken in accordance to IEC 60076-3, clause 12 and Annex A.



PARTIAL DISCHARGE MEASUREMENT KPT-QTPT 007E Page : 1 / 2

7. PARTIAL DISCHARGE MEASUREMENT

7.1 GENERAL

The partial discharge (PD) measurement is a method of observing the quality of the insulation without risk of breakdown. From the results of measurement conclusions can be drawn about the state of insulation, the quality of manufacture and possible concealed defects of insulation. Partial discharge of some magnitude can cause gassing premature aging or even destruction of the insulation after a short time. On the other hand, partial discharges occurring in certain materials and not exceeding 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 discharge measurement 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 equipment 3 Coupling quadripol 4 Measuring point selector 5 ERA discharge detector models: type 652, with discharge magnitude meter type 666 (band with 40-220 kHz or 3dB) 6 Oscilloscope for observation the pulse distribution over one cycle of the test voltage ratio 7 Feeding generator 200 Hz 8 Step up transformer 9 Compensating power reactors 10 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 indication on instruments constitute a Broad Band pass system determined by their frequency characteristics. In accordance 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 point selector give us opportunity to perform the measurement on several terminals simultaneously.






PARTIAL DISCHARGE MEASUREMENT KPT-QTPT 007E Page : 2 / 2

7.2 CALIBRATION MEASUREMENT

The purpose of the measurement is to determine the scale factor “K” for the measurement with the complete test and measuring circuit. The calibration is performed by injecting an apparent charge q0 between each HV terminals and earthed transformer 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 to read the applied charge directly on the pC meter multiplied by scale factor k.

V

W

1

4 5 6

F

U12

Zm

Fig. 7-2 Calibration measurement

7.3 PERFORMANCE OF THE MEASUREMENT To achieve the desired low PD level, it is necessary to perform a thorough preparation of the test transformer. The terminals should be shielded, the bushings must be cleaned and all foreign objects removed from the cover and tank because unearthed surface can give undesired discharges. The background level should be recorded with the complete test circuit connected, including the supply circuit, 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 U2 and held there for a duration of 5 min; raised to U1, held there for the test time as stated in instruction Part for 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 specified intervals (5 min) during the induced voltage test. If higher then prescribed or specified PD levels occur the inception and extinction voltages should be determined. The voltage should be increased and subsequently reduced until the discharges are decreased below the specified 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; applied calibration charge, applied voltage, time intervals and background level will put down on a form made for this purpose.


Končar Power transformers Ltd.

SEPARATE – SOURCE VOLTAGE TEST KPT-QTPT 008E Page: 1/1

8. SEPARATE-SOURCE VOLTAGE TEST

8.1 PURPOSE OF THE TEST The object of the test is to secure that the insulation between the windings and the insulation between windings and the earthed parts withstand temporary overvoltages and switching overvoltages which may occur in service. 8.2 TEST CIRCUIT

T2

b

cGS

G1

L

AR

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 is adjusted 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 seconds between 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 neutral terminal. 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.


OPERATION TESTS ON ON-LOAD TAP 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 (with exception 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 supply

voltage ratio 1 complete operating cycle with the transformer energized at rated voltage and frequency at no load

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






MEASUREMENT OF THE ZERO-SEQUENCE IMPEDANCE


10. MEASUREMENT OF THE ZERO-SEQUENCE IMPEDANCE

10.1 PURPOSE OF THE MEASUREMENT The zero sequence impedance is the impedance, which a three-phase circuit gives to a set of currents that are 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 of unsymmetrical three-phase vectors are resolved into three symmetrical component sets: the positive, the negative and the zero sequence phase system. The relation of the symmetrical sets of voltage and currents is for each system given by corresponding impedances. Measurement of zero-sequence impedances is a special test that is carried out only when specified in the contract.

10.2 MEASURING CIRCUIT AND METHOD The zero-sequence impedance is normally measured in connection with the load-loss and impedance voltage test. The circuitry used is the same as for this test, but modified for one phase measurement as shown in fig. 10.1.

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




Issue: 08.2003. 09.2003. KPT-QA.029E 1/2 izdanje 03.2002.


MEASUREMENT OF THE ZERO-SEQUENCE IMPEDANCE


The phase terminals of the Y-connected winding are short-circuited and the voltage is applied between this connection and the neutral point. For units where the current-carrying windings are equipped with tap changers, the measurements should be performed 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 current capacities 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 which is:

[ ]ohmsIUZ ⋅= 30

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

%1003%100 20

0 ⋅⋅⋅=⋅=r

r

r UP

IU

ZZZ

Issue : 08.2003. 09.2003. KPT-QA.029E. 2/2 izdanje 03.2002.


CAPACITANCE AND THE INSULATION POWER FACTOR MEASUREMENT


Prepared by: F. Juraković




11. CAPACITANCE AND THE INSULATION POWER FACTOR MEASUREMENT


The purpose of the measurement is to determine the capacitances and power factor (tanδ) between the windings and the earthed parts and between the different windings of the transformer. The capacitance values are needed when planning transformer overvoltage protection and calculating the overvoltages affecting the transformer. In addition the results are used by the manufacturer for design purposes. The power factor (tanδ) is used as an indicator of the general condition of the insulation. The power factor value 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 winding capacitances 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

C10C12 C20

C10C12

C20

C13

C23 C30

1 2 3

Fig. 11-1 Transformer capacitances


CAPACITANCE AND THE INSULATION POWER FACTOR MEASUREMENT



The term “guard” signifies one or more conducting elements arranged and connected on an electrical instrument 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 -Measuring instrument

(Tettex» Type5281/2805)

C1-0 C1-2 C2-0

VN-HV

NN-LV

Ispitivani transformator - Tranformer under test

RC

CN

v

High

Low

Test circuit for measurement C1-2+C1-0 (GST or GND). (Capacitance of HV-winding to LV-windingand a tank. Tank and LV-winding to ground.)



C1-0 C1-2 C2-0

VN-HV

NN-LV


RC

CN

v

High

Low

Test circuit for measurement C1-2 (UST). (Capacitance of HV-winding to LV-winding)



C1-0 C1-2 C2-0

VN-HV

NN-LV


RC

CN

v

High

Low

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

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


INSULATION RESISTANCE MEASUREMENT KPT-QTPT 012E Page : 1 / 2

12. INSULATION RESISTANCE MEASUREMENT


The purpose of the measurement is to determine the insulation resistance from individual winding to ground or between individual windings. The insulation resistance measured in the factory afford a useful indication as to whether the transformers are in suitable condition for application of dielectric test. Furthermore, results obtained in such tests are useful as reference values for later measurement at site. The absolute 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 a general allowable minimum insulation resistance value for transformers of different ratings. 12.2 PERFORMANCE OF THE MEASUREMENT 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 circuit High 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 after connecting the voltage. The type of meter used, the measuring voltage, and temperature, R15, R60 and R60/R15 are stated in the report. The basic diagrams of the test circuits for one three-winding transformer is shown on Fig. 12-1.






INSULATION RESISTANCE MEASUREMENT KPT-QTPT 012E Page : 2 / 2

E G L (-) (+)

Transformer under test m

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 R 1-0 = 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 R 1-2 = 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



MEASUREMENT OF THE ELECTRIC STRENGTH 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 in accordance 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 electric strength 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.






TEMPERATURE RISE TEST KPT-QTPT 014E Page : 1 / 4

Prepared by: S. Maroš




14. TEMPERATURE RISE TEST 14.1 THE PURPOSE OF THE MEASUREMENT The purpose of the measurement is to check that the temperature rises of the oil and the windings do not exceed 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 loss measurement (KPT-QTPT 004E) and in the resistance measurement (KPT-QTPT 0003E). In addition thermometers are used for the measurement of the temperature of oil, cooling medium and the ambient temperature and further a temperature recorder and thermocouples are used for the measurement of certain temperatures and for equilibrium control. 14.3 PERFORMANCE OF THE MEASUREMENT The test is performed by using the short-circuit method. The temperature rise of the windings is determined by the resistance method. The test is performed in line with IEC 60076-2 as follows: Cold resistance measurement The resistances and the corresponding oil temperature are measured. Resistance is measured by means of standardized U-I method or when the resistance cannot be accurately measured, due to the long duration of transient phenomenon by means of inductive voltage correction method. The winding temperature is the same as the oil temperature. Determination of the temperature rise of oil The power to be supplied to the transformer is the sum of the no-load losses and the load losses on the tapping on which the temperature-rise test is to be performed (generally the maximum loss tapping). With this 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 the cooling medium temperature can be calculated from the equilibrium temperatures. Determination of the temperature rise of windings Without interrupting the supply the current is reduced to rated current for 1h. The supply values and the temperatures are recorded as above.

When the current has been switched off the hot-resistance measurement is performed. The test connection is changed for carrying out the resistance measurement and after the inductive effects have disappeared the resistance-time-curve are measured for suitable period of time (zero time is the instant of switching off the supply). 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 the instant of switching off. The temperature rises of the windings above the oil temperature are calculated on the basis of the “hot” and “cold” resistance values and the oil temperature. The temperature rises of the windings above the cooling medium temperature are found by adding the temperature rise of the oil above the 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 several times 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 as the 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 standards approximately half-way up the transformer. For forced-air cooled transformers the temperature of the ingoing air is measured. If water is used as cooling medium, the water temperature at the intake of the cooler is the reference temperature.




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 the temperature 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 top oil temperature rise above the temperature of the cooling medium has to be corrected according to the formula (14.2).

∆θt= ( )at

xr

PP θθ −′⋅

(14.2)

Pr= sum of referenced load loss (at maximum loss tapping) and no-load loss P = power supplied during the test x = 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=

−+

abt θθθ

2 (14.3)

θb = temperature of oil entering the windings (i.e. oil returning from cooling equipment), bottom oil temperature or as above but corrected in similar way acc. to the formula (14.2)

∆θe =

−

′+′⋅

a

bt

x

r

PP θθθ

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




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

2

222

btoil

θθθ += (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 the oil temperature as follows:

( ) 235235 11

22 −+= θθ

RR

(14.6)

R1= cold resistance R2= hot resistance θ1= the average temperature of oil during cold resistance measurement The average temperature rise ∆θw-oil of the winding above oil temperature is:

∆θw-oil = θ2 – θoil2 (14.7) If the current in the rated current period has deviated from the rated value (less than 10% of the rated current) the temperature rise of the winding above oil temperature has to be corrected according to:

∆θw-oil = ( )22 ´´ oilII y

r θθ −⋅

(14.8)

Ir = rated current of winding I = test current y = 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: ∆θw = ∆θw-oil + ∆θe (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.




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 to one of the windings and short circuiting one or more of the rest of the windings. After reaching steady state conditions oil temperature and oil temperature rise is determined in the same way as explained in chapters 14.3 and 14.4. The temperature rise for an individual winding above oil is obtained afterwards with rated current in the winding 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 in the 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 combination is given.


LIGHTNING IMPULSE TEST KPT-QTPT 015E Page : 1 / 5

15. LIGHTNING IMPULSE TEST

15.1 PURPOSE OF THE TEST The purpose of the impulse voltage test is to secure that the transformer insulation withstand the lightning overvoltages which may occur in service.

15.2 TESTING EQUIPMENT Impulse generator

FnRb Ra

FanC3

Rc Rs

F2 R b Ra

Fa2C

Cs

Rc Rs

F1Rb Ra

CsFa1

Rs

Cs impulse capacitor Rc charging resistor Rs series resistor Ra low-ohmic discharging resistor for lightning impulse Rb high-ohmic discharging resistor for switching impulse F1 … 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).







When the charging voltage has reached the required value, breakdown of the sparkgap F1 is initiated by an external 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 the breakdown 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 are dimensioned 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 delay of a few hundred nano-seconds. The arrangement is necessary in order to secure the functioning of the generator. The wave shape and the peak value of the impulse voltage are measured by means of an Impulse Analysing System (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 adjusted the charging voltage. In order to obtain the necessary discharge energy parallel or series-parallel connections of the generator can be used. In these cases some of the capacitors are connected in parallel during the discharge.

C1

V=U=R1

R2

Impulse generator

C2

C3

1U

1V

1W

N

2U

2V

2W

2N

3U1

3V2

3V

3W

Voltagedivider

Transformerunder test

Voltagerecorder

Channel 1 (U)of digitaltranzient rec.

Current recorderChannel 2(I) ofdigital tranzient rec.

S1

Fig. 15-2 Equivalent diagram of the impulse test circuit C1 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 the generator. The front time can be calculated approximately from the equation: For R1 >> R2 and Cg >> C (15.1) T tCR ⋅⋅= 21 3 and the half time to half value from the equation (15.2) 112 7,0 CRT ⋅⋅≈ In practice the testing circuit is dimensioned according to experience.




15.3 CONNECTION OF THE TEST OBJECT The testing impulse test is normally applied to all windings. The impulse test-sequence is applied successively 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 of the 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 testing c and d neutral terminal testing When the low voltage winding in service cannot be subjected to lightning overvoltages from the low voltage system (e.g. step-up transformers, tertiary windings) the low voltage winding may be impulse tested simultaneously with the impulse tests on the high voltage winding with surges transferred from the high voltage 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 earth through resistances of such value (resistances Ra in Fig. 15-4e) that the amplitude of transferred impulse voltage between line terminal and earth or between different line terminals or across a phase winding will be as high as possible but not exceeding the rated impulse withstand voltage. The resistance shall not exceed 5000 Ω.




The neutral terminal is normally tested directly or indirectly by connecting a high-ohmic resistor between the neutral and earth (voltage divider Ra, Ru) and by applying the impulse (Fig. 15-4c and d) to the line terminals connected 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 the applied 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 to half-value (T2) are defined in accordance with the standard.

Fig. 15-5 In practice thigh rated pThe impulseinsulation anWaveform asimilar unitscircuit. The test seqnumber of vThe equipmcomputer, pThe recordinFor regulatinthe two othe

Issue : KPT-QA.029E. 2

T1 =1,67 T

he imowe tesd tedjus or e

uenoltagent lottegs g trr ph

08.2/2 izd

T

Standard lightning impulse Front time T1 = 1,2µs ± 30%Time to half-value T2 = 50 µs

pulse shape may deviate frr and windings of high input t is performed with negative st circuit. tments are necessary for moventual precalculation can g

ce consists of one referencee applications at full amplitu

for voltage and current signar and printer. at the two levels can be comansformers one phase is tesases are tested in each of th

003. anje 03.2002.

T2

± 20%

om the standard impulse when testing low-voltagcapacitance. polarity voltages to avoid erratic flashovers in the

st test objects. Experience gained from results oive guidance for selecting components for the wa

impulse (RW) at 75% of full amplitude followed de (FW) (according to IEC 60076-3 three full im

l recording consists of digital transient recorder,

pared directly for failure indication. ted with the on-load tap changer set for the ratede extreme positions.

t

U

0,3

0

0,5

0,9

1,0

e windings of

external

f tests on ve shaping

by the specified pulses). monitor,

voltage and



Detection of correctness at impulse testing is based on comparison of voltage and current records obtained at 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 REPORT The detailed test record cover setting of impulse generator, values for all components in the impulse shaping and measuring circuits, connection of the test object, parameters for the wave-shape and oscillogram records for each voltage application.



TEST WITH THE LIGHTNING IMPULSE CHOPPED ON THE TAIL


16. TEST WITH THE LIGHTNING IMPULSE CHOPPED ON THE TAIL 16.1 PURPOSE OF THE TEST The purpose of the chopped lightning test is to secure that the transformer insulation withstand the voltage stresses 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 and fault detection connections are used as for the standard lightning impulse test. The impulse is chopped by means of triggered-type chopping gap connected to the terminal to which the impulse is applied. The delay of the chopping–gap ignition impulse in relation to the impulse generation is adjustable, thus the time Tc from the 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 to the chopping can vary within the range of 2...6µs (Fig. 16-1) According to the standard IEC 60076-3 the amount of overswing to opposite polarity shall be limited to not more than 30% of the amplitude of the chopped impulse (Fig. 16-1). If necessary the overswing amplitude will be limited to the value mentioned by means of damping resistor inserted in the chopping circuit.

1,0

0,9

0,3

1

Fig 16

Prepared

Issue: KPT-QA.02

T

c

-1

by: J. B

08.9E 1/2

T

Chopped lightning impulse Front time Time to chopping

30%100 <=αβ

cY

ujanović Controlled

2003. 09.2003. izdanje 03.2002.

β

α

T1 = 1,2 µs ± 30% Tc = 2....6 µs

%

by: I. Šulc



TEST WITH THE LIGHTNING IMPULSE CHOPPED ON THE 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 winding currents obtained at 75% and 100% test voltages. At high test voltages there is a small delay in the ignitions of 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 initial high-frequency oscillations in the first part of the voltage front. In this case the fault detection must be based primarily 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 the tests on the three phases of a three-phase transformer, usually the two extreme tappings and principal tapping.

16.5 TEST REPORT

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



SWITCHING IMPULSE TEST KPT-QTPT 017E Page : 1 / 2

17. SWITCHING IMPULSE TEST

17.1 PURPOSE OF THE TEST The purpose of the switching impulse test is to secure that insulation between windings, between windings and earth, between line terminals and earth and between different terminals withstand the switching overvoltages, 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 a three-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 voltage between 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 remanent flux can amount to even 70 to 80 % of the saturation flux, the initial remanence of the core has a great influence on the voltage duration. By introducing remanent flux of opposite polarity in relation to the flux caused by the switching impulse, the maximum possible switching impulse duration can be increased. The remanence of opposite polarity is introduced in the core by applying low voltage current impulses to LV winding of opposite polarity to the transformer before each full voltage test impulse.

U

current recorder

C2

C1

voltage recorder

S1

Fig. 17-1 Transformer switc The test is performed with impulsgiven in the standard IEC 60076-3



-0,5U - 0,5 U

Loading resistor

hing impulse testing and fault detection connections

es of negative polarity. The requirements on the switching impulse shape are summarized in Fig. 17-2.




SWITCHING IMPULSE TEST KPT-QTPT 017E Page : 2 / 2

The voltage measurement is based on the peak voltmeter indication.

0

1.0 0.9

d T

Tp = 1,67 T

Fig. 17-2 Switching imp Front time Time above 90% Time to the first zero passage At full test voltage each phaseWhen comparing the wave shvoltage and increase in windapplied voltage. Thus voltagewill deviate from each other inThe fault detection is mainly bof voltage caused by flashovsound effects are observed. When the core reaches satransformer. Test report The test voltage values, impureport. The oscillographic reco

Issue : 08.2003. 09.20KPT-QA.029E. 2/2 izdanje 03.2002.

T
T
0,3

ulse

Tp> 100µs Td> 200µs T0> 500µs ( preferably 1000 µs )

will be tested with the number of impulses required by the relevant standard. ape it is to be noticed that the magnetic saturation causes drastic reduction of ing current and the time to saturation is dependent on the amplitude of the and current oscillograms obtained at full test voltage and at 75% voltage level this respect. ased on the voltage oscillograms. The test is successful if no sudden collapse er or breakdown is indicated on the voltage oscillograms and no abnormal

turation a slight noise caused by magnetosriction can be heard from the

lse shapes, and number of impulses at different voltage levels are stated in the rds are stored in the archives, where they are available when required.

03.


MEASUREMENT OF ACOUSTIC SOUND LEVEL


18. MEASUREMENT OF ACOUSTIC SOUND LEVEL


The purpose of the sound level measurement is to check that the sound level of the transformer meets the specification requirements given in relevant standards e.g. IEC 60076-10 or guarantee values given by the transformers manufacturer. A sound spectrum analyses are carried out for the transformer at the customer's request. 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 level measurements. The measurements are performed using the weighing curve A. The sound spectrum analysis of the transformer is carried out by recording the sound band levels as a function of frequency. This is done with sound level meter Brűel & Kjaer type 2236 and calibrator type 4231. 18.3 PERFORMANCE OF THE MEASUREMENT The A-weighted sound pressure level of the Background noise shall be measured at points on the prescribed counter immediately before and after the measurements on the transformer. Power is supplied to the transformer under no load condition at the rated voltage and the frequency with the tapping selector on the principal tapping. The sound pressure level is the measured at various points around the transformer as detailed in the standards: at a distance (D) of 30 cm for ONAN or 2 m for ONAF cooling system spaced at an interval (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 of one 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 height. Preferably the background sound level should be at least 9 dB(A) below the measured combined sound level. If the difference is less than 9 dB(A) bat not less than 3 dB(A) a correction for background level will be applied according to standards. 9

x

19

30 3122

LV

STV HV

D=2m; x=1m

D 36

1

15

40

38

Fig. 18-1 Basic layout of measuring points






MEASUREMENT OF ACOUSTIC SOUND LEVEL


18.4 CALCULATION OF AVERAGE SOUND PRESSURE LEVEL

The uncorrected average A-weighted sound pressure level shall be calculated from sound pressure levels, LpAi, measured with the test object energized by using equation:

Σ=

=

−LpAi

i

NpA

NL 1,0

1101lg10

or when the range of values of LpAi does not exceed 5dB, a simple arithmical average will be used. Corrections for background level and environmental correction, in case of need or as circumstances require, should be done in accordance with relevant standard. 18.5 CALCULATION OF SOUND POWER LEVEL The A-weight sound power level of the transformer LwA shall be calculated from average A-weight sound pressure level LpA according to equation:

SLL pAWA lg10+=−

[ ]dB

S = the equivalent surface area in m2, defined by equation : a) for ONAN system S = 1,25 h ⋅ lm or b) for ONAF system S = (h+2) ⋅ lm where: h= height in meters of transformer tank lm = the length in meters of the prescribed counter More details can be found in relevant standard.



MEASUREMENT OF HIGHER HARMONICS IN MAGNETISING CURRENT


19. MEASUREMENT OF HIGHER HARMONICS IN MAGNETIZING CURRENT

19. 1 GENERAL

At imposed sinus voltage on a transformer, because of non-linear magnetic curve of the core, magnetizing current at no-load contains besides basic harmonics also higher harmonics. Higher harmonics in the current can cause in electric grid voltage distortion, that is, they can cause even higher harmonics. Such current and voltage harmonics can cause disturbances in electric grid or in connected appliances. However, since the portion of higher harmonics in relation to transformer rated current is smaller than 1%, they are insignificant for a user. 19.2 MEASURING EQUIPMENT In the Test Station, the measurement of higher harmonic contents, as a rule, is carried out during the measurement of no-load losses and in the same connection (see in KPT-QTPT 005E Fig 5-1). The test generator and intermediate transformer are used, as a rule, only in the linear range of their characteristics. The test circle is carried out without a feedback line so that the third degree harmonics cannot flow. The transmitting ratio of voltage transformers as well as the load of current transformers are selected in such a way that their working points lie in the linear range of the magnetizing characteristics. The measurement of higher harmonics in magnetizing currents is carried out with a Wide Band Power Analyser, producer NORMA, type D 6000. 19.3 PERFORMANCE OF THE MEASUREMENT For the measurement, first, a required voltage is adjusted, usually 100% of rated voltage, gradually increasing the value from zero to higher values. The measurement of voltage is carried out with a mean-value voltmeter. During the measurement of higher harmonics the power voltage should be maintained so that it has a constant value. Therefore, in the Test Station, during this period, the above mentioned fast-registering analyser with the memory in real-time procedure is used. 19.4 PROCESSING OF THE MEASUREMENT RESULTS By using this registering analyser, the final measured values of higher harmonics are immediately obtained for the test protocol. Higher harmonics are expressed in percentage of the fundamental one.






TIGHTNESS (LEAKAGE) TEST KPT-QTPT 020E Page : 1 / 1

20. TIGHTNESS (LEAKAGE) TEST

20.1 PURPOSE The purpose of the test is to prove tightness of transformer tank and accessories assembled on the transformer.

20.2 PERFORMANCE OF THE TEST Transformer is assembled and filled with oil. Overpressure of 35 kPa is applied on the tank cover and kept for 12 hours. Welds and joints on the tank are checked on leak. If requirements in the contract differ from those stated procedure and values as per contract should apply.

20.3. TEST REPORT Value of overpressure and elapsed time are recorded with confirmation of tightness.






FRA MEASUREMENT KPT-QTPT 021E Page : 1 / 2

Prepared by: R. Gardijan




21. FRA MEASUREMENT

21.1 PURPOSE The purpose of the FRA (Frequency Response Analysis) measurement is to detect displacement (or movement) of windings in the transformer. Usually the first measurement in the factory is used as a fingerprint. Results of later measurements are compared with the first one in the factory.

21.2 MEASURING EQUIPMENT TrafTek – B&C Diagnostics, Budapest, Hungary

Test connection - 20 m long measuring cable (triple coax cable)

The TRAFTEK equipment is designed for scanning the geometrical and mechanical movements and distortions of transformer windings using the swept frequency measuring methods. It is known that a transformer winding with its stray capacitances and inductances form a complicated RLC network. If we apply small AC voltage (about 4Vrms) with frequency range of 50 Hz to 1 MHz we shall get a typical voltage attenuation or winding impedance curve as a function of frequency.

21.3 PERFORMANCE OF THE MEASUREMENT The transformer under the test and measuring equipment are connected acc. to the Fig. 21-1.

A

B

C

a

b

c

N

75Ω

75Ω

75Ω

SW G, 50Hz-1MHz

AD Conversion

486 DX 100 CPU

Display

TrafTek

≈

Transformer

Fig. 21-1 Connection of the transformer and measuring equipment The software controlled sine wave generator produces output voltage of max. 4 Vrms with frequency range of 50 Hz to 1 MHz. It has 75 Ω output impedance. Input impedance is 75 Ω. Voltage from the generator is applied to the one transformer terminal (one winding end) and response voltage is measured on another terminal (the other winding end).


FRA MEASUREMENT KPT-QTPT 021E Page : 2 / 2


21.4 TEST REPORT Impedance value Z in kΩ versus frequency or attenuation A (or damping) in dB (20 log (Uoutput / Uinput ) versus frequency can be plotted on the diagram with indication of terminals with applied and response voltage. Examples are given below.

0,10

1,00

10,00

1000 10000 100000 1000000

frequency (Hz)

Z (k

Ω)

-50,00

-40,00

-30,00

-20,00

-10,00

0,00

1000 10000 100000 1000000

frequency (Hz)

A (d

B)


CORE INSULATION MEASUREMENT KPT-QTPT 022E Page : 1 / 1

22. CORE INSULATION MEASUREMENT

22.1 PURPOSE The purpose of the measurement is to check and prove that the transformer core is insulated from the tank and core frame.

22.2 PERFORMANCE OF THE MEASUREMENT Earthing links from core to earth and from core frame to earth (if the latter one exists) are removed (disconnected) in earthing connection box (terminal box). Several combinations of measurement of insulation resistance are possible depending upon the performance of core and frame earthing : core to frame; core to tank; core frame to tank; core to (tank + core frame); (core + core frame) to tank. At least combination core to (tank + core frame) or core to tank is to be measured. In all measurements tank is assumed as earth potential. The measurement is performed by by means of an insulation resistance meter (“Megger”). For each measurement a DC voltage of at least 500 V (but not greater than 2500 V) is applied between pair of terminal bushings in earthing terminal box for a measuring period ≥ 1 min. or until the measuring insulation resistance become stable. Measured values for each combination shall be above 50 MΩ.

22.3 TEST REPORT Measured values with indication of measured combination are documented in transformer routine test report.


Controlled by: F. Juraković




POWER CONSUMPTION OF COOLING SYSTEM


23. POWER CONSUMPTION OF COOLING SYSTEM

23.1 PURPOSE The purpose of the test is to measure power consumption of transformer cooling plant or saying by another words to measure losses consumed by transformer cooling system. Depending upon the transformer cooling system this power can be consumed by fans and oil pumps. This measurement is performed only if required by the contract or transformer specification.

23.2 PERFORMANCE OF THE TEST The measuring circuit and used equipment is in principle the same as for load and/or no-load measurement (described in KPT-QTPT 04E and KPT-QTPT 05E). The transformer cooling system is supplied from voltage adjustable power source. The voltage is adjusted to rated value for motors or acc. to specified value in transformer specification. Values of current, voltage and loss are measured and recorded. Power consumption for each cooling group is measured if the transformer cooling system is divided into several groups.

23.3 TEST REPORT Measured values of current, voltage and loss at specified frequency are recorded in transformer test report.


Controlled by: F. Juraković




MEASUREMENT OF TRANSFERRED SURGES


Prepared by: R. Gardijan



Issue: 07.2006 KPT-QA.029E 1/2 izdanje 03.2002.

24. MEASUREMENT OF TRANSFERRED SURGES – determination of transient voltage transfer characteristics

24.1 PURPOSE

Measurements of lightning or switching surges transferred from the HV winding to the LV winding on power transformer – determination of transient voltage transfer characteristics

24.2 MEASURING EQUIPMENT

Recurrent surge generator, Haefelly, type 481, 400 Vpp

Digital oscilloscope, Tektronix, Type: TDS 544A, 1Gs/sec. The reccurrent surge generator is the low voltage equipment and equivalent of a high voltage impulse generator. Its wide range of applications includes the testing of models, the study of voltage distribution on high voltage windings during the impulse voltage stresses and the predetermination of the circuit parameters of impulse test plants. The components in the impulse circuit (impulse and load capacitances, front and tail resistors as well as inductances) are adjustable in steps. The wave form can thus be adjusted over a wide range with a high degree of precision.


The transformer under the test and measuring equipment are connected acc. to the Fig. 24-1.

A

B

C RSG

TDS

N

a1 b1

c1

a2

b2

c2

Fig. 24-1 Connection of the transformer and measuring equipment The manual adjustable impulse wave (lightning, chopped and switching) up to 400 Vpp, shape T1 = 0,06µs up to 500µs and T2 = 2µs up to 5000µs. Voltage from the generator is applied to the one transformer terminal (one winding end) and response (or distribution) voltage is measured on another terminal (the other winding end).


MEASUREMENT OF TRANSFERRED SURGES


Issue : 07.2006 KPT-QA.029E. 2/2 izdanje 03.2002.

24.4 TEST REPORT

a) Example of test report for lightning impulse for the transformer 48,4 MVA, 237 / 3,6 / 3,6 kV :

C…….………………..….− applied voltage

A,B…………………..….. − earthed through 450 Ω

N.…………………………− isolated

a1,b1,c1,a2,b2,c2.…….. − isolated

A

B

C

N

a1 b1 c1

a2

b2

c2

450Ω

450Ω

Voltage

% Transferred voltage

on the terminal Oscillogram

No Remark

+100 C − ⊥⊥⊥⊥ 01 front wave

+100 C − ⊥⊥⊥⊥ 02 tail wave

-5,35 a1 − ⊥⊥⊥⊥ 03

-5,80 b1 − ⊥⊥⊥⊥ 04

-20

0

20

40

60

80

100

120

0 1 2 3 4 5µµµµs

%

01 – Front of wave

-20

0

20

40

60

80

100

120

0 20 40 60 80 100µµµµs

%

02 – tail of wave

-8

-6

-4

-2

0

2

4

6

0 10 20 30 40 50µµµµs

%

03 – transferred voltage on terminal a1

-8

-6

-4

-2

0

2

4

6

0 10 20 30 40 50µµµµs

%

04- transferred voltage on the terminal b1

test methods

Documents