Test Methods

(1)

Konč_ar

PowerTransformers Ltd.

TEST METHODS FOR POWER

TRANSFORMERS

QT No: xx-yy

Page : 1 / 2 Prepared by: F. Juraković Approved by: I. Šulc 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 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.

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Konč_ar

TEST METHODS FOR POWER

TRANSFORMERS

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.

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Končar

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

Approved by:

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Končar

SUMMARY OF DIELECTRIC TESTS

KPT-QTPT 001E _{Page: 2 / 4}

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.

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Končar

SUMMARY OF DIELECTRIC TESTS

KPT-QTPT 001E _{Page: 3 / 4}

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

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Končar

SUMMARY OF DIELECTRIC TESTS

KPT-QTPT 001E _{Page: 4 / 4}

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

Issue : 08.2003.

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Končar

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 1 U 1V 1W 1N 2U 2V 2W 2N y x U₂ Test object

~

x

y

x

U

+

=

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.

Prepared by: J. Bujanović Controlled by: I. Šulc Approved by: I. Šulc

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Končar

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 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 0 iii i ii I I II II III III 0 i Dy I iii ii i Dz 0 iii ii 0 i _iii 8 Yz II i iii ii II Dz 10 10 i ii iii Yz 7 Dz 8 ii 7 i iii Dz 6 6 ii ii i iii Yz 5 i 5 ii iii Dz 4 iii ii i 2 Dz 2 iii i ii iii ii Yz I i I 4 i Dd 2 iii ii I 2 i Dd 4 iii ii 4 iii ii Dy 5 i 5 6 iii i ii Dd 6 Dy 7 i 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 i iii ii Yd I I iii Yd 5 i ii 5 6 Yy 6 ii iii i 7 Yd 7 ii _iii i Yd II II i iii ii

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

Issue : 08.2003.

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Končar

MEASUREMENT OF WINDING RESISTANCE KPT-QTPT 003E

Page : 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 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 Hg-thermometer pocket on the transformer cover. The required current is obtained from a battery 60V.

2U

2V

2W

1U

1V

1W

V

A

+

-battery

Test object

Re Fig. 3-1

The resistance value is then determined as

reading

ampermeter

reading

voltmeter

R

=

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.

Prepared by:

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Končar

MEASUREMENT OF WINDING RESISTANCE KPT-QTPT 003E

Page : 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 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.

Issue : 08.2003.

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Končar

PowerTransformers

Ltd. IMPEDANCE AND LOAD LOSS MEASUREMENT

KPT-QTPT 004E

Page : 1 / 5

4. IMPEDANCE AND LOAD LOSS 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.

Prepared by:

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Končar

KPT-QTPT 004E

Page : 2 / 5 G A W I_A A A I_B I_C W W f C V A B C U_A U_BU_C I_A I_B I_C U_A U_B U_C

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):

Issue : 08.2003.

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Končar

KPT-QTPT 004E

Page : 3 / 5

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 %

U

E

_u i.e.











 +

⋅

=

′

100

1 E

u

U

(4.3) _[% _]

´

⋅

100 %

−

=

I

E

i i.e.











 +

⋅

=

′

100

1 E

i

I

(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 ⋅

−

=

value

corrected

value

measured

value

corrected

c

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Končar

KPT-QTPT 004E

Page : 4 / 5

Applied on the measured power this yields:

%

100 cos

)

cos(

´

cos

⋅

+

⋅

−

⋅

=

ϕ

δ

ϕ

I

U

I

U

I

U

c

]

100 %

cos

sin

cos

)

100

1 (

)

100

1 (

1 ⋅







⋅

−

⋅

+

⋅

+

−

=

ϕ

δ

ϕ

δ

ϕ

i u

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:

] [ [ ] [ ]

100 )%

cos

(

_% _% %

=

+

−

_ϕ

⋅

δ

i u

E

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

(4.5) _{[ ]}

[

_{[ ]} _{[ ]}

(

)

]

%

cos

0291

,

0

% % %

=

+

E

u

+

E

i

−

δ

u

−

δ

i

⋅

ϕ

E

The power factor has to be computed:

(4.6)

´

)

I

U

P

⋅

=

+

δ

ϕ

cos(

and

)

´

cos

.

cos(

cos

ϕ

−

δ

⋅

=

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 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 (

E

%

P

=

⋅

−

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.

Issue : 08.2003.

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KPT-QTPT 004E

Page : 5 / 5

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 Pand 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

= _m m r

P

I

2













(4.9) _m m r

U

I

U

=

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)

(

₁ 22 ₂ ₎

)

2 2

5 ,

1 I

I

R

_m

I

R

_m

R

I

=

+

The additional losses Pam at the measuring temperature are

(4.11)

P

_am

=

P

_m

−

I

2

R

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

P

C

R

I

P

s m s am m s s c

°

+

°

+

=

75

2

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 the relating to the principal and extreme tappings.

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Končar Power

Transformers Ltd.

MEASUREMENT OF NO- LOAD

LOSS AND CURRENT

KPT-QTPT 005E

Page : 1 / 2

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

I_A I_B I_C A B C U_A U_BU_C I_A _I_B _I_C _U_A_U_B_U_C

Data Acquisition & Wav e Analy ser

U = U_{a 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%.

Prepared by:

J. Bujanović Controlled by: I. Šulc Approved by: I. Šulc

Issue: 08.2003.

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Končar Power

Transformers Ltd.

MEASUREMENT OF NO- LOAD

LOSS AND CURRENT

KPT-QTPT 005E

Page : 2 / 2

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:

(

d

)

P

₀

=

_m

1 +

´

U

d

=

−

(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 load curve thus obtained the no-load losses and no-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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Končar

Ltd. 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:

for

Hz

frequency

test

frequency

rated

t

120 sec

30

50 .

.

_×

₌

=

,

or 36 seconds for 60 Hz power frequency

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

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KPT-QTPT 006E

Page : 2 / 4

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 ⋅U_m 3 / 1 , 1 ⋅U_m 3 / 3 , 1 ⋅U_m 2 U voltage test U₁ 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 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).

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Končar

KPT-QTPT 006E

Page : 3 / 4

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 -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

Issue : 08.2003.

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Končar

KPT-QTPT 006E

Page : 4 / 4 A B C D E 3 / 1 , 1 ⋅U_m 3 / 1 , 1 ⋅Um 1,5 / 3 2 Um U = ⋅ 3 / 7 , 1 1 Um U = ⋅ 3 / 5 , 1 2 Um U = ⋅ < U_start 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.

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Ltd.

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 3 2 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.

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Ltd.

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. (

om

q

k

=

)

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 U 12 Zm

Fig. 7-2 Calibration 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.

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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 c GS G₁ L A R P₂ P₃ V V E N A B C a T₃ 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.

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OPERATION TESTS ON ON-LOAD

TAP CHANGER

KPT-QTPT 009E

Page : 1 / 1

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

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MEASUREMENT OF THE ZERO-SEQUENCE

IMPEDANCE

KPT-QTPT 010E

Page : 1 / 2

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

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MEASUREMENT OF THE ZERO-SEQUENCE

IMPEDANCE

KPT-QTPT 010E

Page : 2 / 2

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:

[

ohms

]

I

U

Z

₀

= 3

⋅

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

%

100

3 %

100

₂ 0 0

=

⋅

=

⋅

r r r

U

P

I

U

Z

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CAPACITANCE AND THE INSULATION POWER

FACTOR MEASUREMENT

KPT-QTPT 011E

Page : 1 / 2

Prepared by:

F. Juraković Controlled by: I. Šulc Approved by: I. Šulc

Issue: 08.2003. 11.2004.

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

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)

b a 1 ₂ C₁₀ C₁₂ C₂₀ C₁₀ C₁₂ C₂₀ C₁₃ C₂₃ _C 30 1 2 3

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CAPACITANCE AND THE INSULATION POWER

FACTOR MEASUREMENT

KPT-QTPT 011E

Page : 2 / 2

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 (T ettex» T ype 5281/2805) C1-0 C1-2 C2-0 VN-HV NN-L V

Ispitivani transformator - T ranformer under test

R C C N v High Low

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

Mjerni instrument -Measuring instrument (T ettex» T ype 5281/2805) C1-0 C1-2 C2-0 VN-HV NN-L V

Ispitivani transformator - T ranformer under test

R C C N v High Low

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

Mjerni instrument -Measuring instrument (Tettex» T ype 5281/2805) C1-0 C1-2 C2-0 VN-HV NN-L V

Ispitivani transformator - Tranformer under test

R C C N v High Low

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

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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.

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.

Prepared by:

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Issue: 08.2003. 09.2003.

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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 R1-3 NN-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 R1-3

NN-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 )

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MEASUREMENT OF THE ELECTRIC

STRENGTH OF THE INSULATING OIL

KPT-QTPT 013E Page : 1 / 1

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.

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TEMPERATURE RISE TEST

KPT-QTPT 014E

Page : 1 / 4 Prepared by: S. Maroš Controlled by: I. Šulc Approved by: I. Šulc

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.

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.

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TEMPERATURE RISE TEST

KPT-QTPT 014E

Page : 2 / 4

Issue : 08.2003. 11.2004.

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 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=

(

t a

)

x r

P

θ

′

−

⋅













(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

θ

e of the oil is

∆θ e=













+

₋

a b t

θ

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 b t x r

P

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