Power Transformers Test

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POWER

TRANSFORMERS

Eng. Ashraf Zaki

Eng. Hossam Samir

Testing & Acceptance Conditions

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Power Transformers

T E S T I N G & A C C E P T A N C E C O N D I T I O N S

INDEX

Part (1) ……… Reference International Standards Part (2) ……… Reference Database Documents Part (3) ……… General Test Requirements Part (4) ……… List of Tests applied

Part (5) ……… Detailed (Routine / Sample) Test procedure (FAT)

Part (6) ……… Reference Test Reports Part (7) ……… Informative Documents

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Part (1)

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 IEC 60076-1

(Power transformers –General)

 IEC 60076-2

(Power transformers –Temperature Rise)

 IEC 60076-3

(Power transformers –Insulation levels & Dielectric Test)

 IEC 60076-5

(Ability to withstand short circuit)

 IEC 60076-10

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Part (2)

Reference Database Documents

This Document is to be read in conjunction with the following Database Documents:

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Part (3)

General requirements for routine, type and special tests

(Preparatory Check List)

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1- Tests shall be made at any ambient temperature between 10 °C and 40 °C and with cooling water (if required) at any temperature not exceeding 25 °C. 2- Tests shall be made at the manufacturer's works, unless otherwise agreed

between the manufacturer and the purchaser.

3- All external components and fittings that are likely to affect the performance of the transformer during the test shall be in place.

4- Tapped windings shall be connected on their principal tapping, unless the relevant test clause requires otherwise or unless the manufacturer and the purchaser agree otherwise.

5- The test basis for all characteristics other than insulation is the rated condition, unless the test clause states otherwise.

6- All measuring systems used for the tests shall have certified traceable accuracy and be subjected to periodic calibration, according to the rules of 4.11 of ISO 9001.

7- Where it is required that test results are to be corrected to a reference temperature, this shall be:

For oil-immersed transformers: 75 °C;

For dry-type transformers: according to the general requirements for

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Part (4)

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• Routine Tests

1- Measurement of winding resistance.

2- Measurement of voltage ratio and check of phase displacement. 3- Measurement of short-circuit impedance and load loss.

4- Measurement of no-load loss and current.

5- Dielectric routine tests (IEC 60076-3) which includes:

a) Switching Impulse Test.

(Not Applicable for Um≤72.5kV)

b) Lightning Impulse Test.

(Originally a Type Test, in some cases it’s considered a Routine Test)

c) Separate Source AC withstand voltage test. d) Short duration induced AC withstand voltage test e) Long duration induced AC withstand voltage test

(Not Applicable for Um≤72.5kV)

6- Functional Checks on Marshalling & Remote control Panel. 7- Operation Tests on On-Load Tap Changer.

(Found under Tap changer test report)

• Type Tests

1- Temperature-rise test (IEC 60076-2). 2- Dielectric type tests (IEC 60076-3).

• Special Tests

1- Dielectric special tests (IEC 60076-3).

2- Determination of capacitances windings-to-earth, and between windings. 3- Determination of transient voltage transfer characteristics.

4- Measurement of zero-sequence impedance(s) on three-phase transformers (10.7).

5- Short-circuit withstand test (IEC 60076-5). 6- Determination of sound levels (IEC 60551).

7- Measurement of the harmonics of the no-load current (10.6). 8- Measurement of the power taken by the fan and oil pump motors. 9- Measurement of insulation resistance to earth of the windings, and/or

measurement of dissipation factor (tan d) of the insulation system capacitances. (These are reference values for comparison with later measurement in the field. No limitations for the values are given here.)

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Part (5)

Detailed Routine Test Procedure

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Test Purpose:

In this test, the resistance of each transformer winding is measured using DC current at a recorded ambient temp.

Test Precautions:

For the Dry Type- Power Transformers, the transformer shall be at rest in a

constant ambient temperature for at least 3h.

Winding resistance and winding temperature shall be measured at the same time. The winding temperature shall be measured by sensors placed at

representative positions, preferably inside the set of windings, for example, in a duct between the high-voltage and low voltage windings

For the Oil immersed- Power Transformers, after the transformer has been

under oil without excitation for at least 3h, the average oil temperature shall be determined and the temperature of the winding shall be considered the same as the average oil temperature. The average oil temperature is taken as the mean of

the top and bottom oil temperatures.

Test Description & Procedure:

1- Resistance to be measured between each main terminal pair (i.e. A-B , A-C & C-B) in case of Delta connected windings, and between each terminal to neutral (i.e. a-n , b-n, …etc,) in case of wye connected windings.

2- In case of tapped windings, above readings are recorded at each tap. 3- Average oil temperature is to be recorded.

4- Measured values are to be corrected to required temperatures as mentioned under Part (2) (Preparatory Check List).

Acceptance Criteria:

Supplier’s guaranteed values of winding resistance to be taken as a reference. Any variation to this value is to be within accepted tolerance.

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Test Purpose:

The voltage ratio shall be measured on each tapping in the no-load condition. The polarity of single-phase transformers and the connection symbol of three-phase transformers shall be checked.

Part (1) Voltage Ratio

1- Turns Ratio measuring instrument is to be connected to each pair of coupled windings (i.e. A-B & a-n)

2- Values are recorded at each tap in case of tapped windings and then compared to calculated ratio at the same tap.

Part (2) Phase Displacement

1- 3phase voltage source with known value is applied and other generated voltages are measured.

2- By performing some arithmetic analysis on the transformers vector group, the phase displacement can be verified.

(For details, Refer to Part (5) “Reference Test Reports”)

*Range of measured ratio shall be equal to the calculated ratio ±0.5%.

*Phase displacement is identical to approved arrangement and transformer’s nameplate.

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Test Purpose:

In this test, the value of the short circuit impedance Z% and the load (copper) losses (I2_{R) are obtained.}

1- Test is performed at rated frequency.

2- Supplied current should be equal to the relevant rated current but shall not be less than 50% of the same.

3- When measuring the short circuit impedance for a transformer having a tapped winding, it will be as following:

a) If tapping range is less than ±5%, measurements will be carried out @ principal tape only.

b) If tapping range is Greater than ±5%, measurements will be carried out @ principal tape as well as the two extreme tappings.

4- When measuring the load loss for a transformer having a tapped winding, it will be as following:

a) If tapping range within±5% and rated power less than 2500kAV, load loss guarantee refer to the principal tap only.

b) If tapping range exceeds±5% or rated power above 2500kAV, it shall be stated for which tapping beside the principal tap the load losses will be guaranteed by the manufacturer.

1- Voltage will be applied to the terminals of one winding (usually the H.V.) with the other winding short circuited (usually the L.V.)

2- Voltage to be increased gradually till the current in the energized winding reaches the required value (50% to 100% rated current).

3- If transformer is equipped with a tap changer, above tapping regulations are applied.

4- Readings to be taken as quickly as possible as the windings warm up and the winding resistance increases. Hence, the losses value will increase accordingly.

5- Using appropriate instruments (conventional three wattmeter method or digital wattmeter with ammeters & voltmeters) measurements of voltage,

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6- If the measured current deviates from the rated, Power & Voltage measured will be corrected using the following relations:

Where, (r) refers to rated values & (m) refers to measured values.

7- All recorded data are taken at ambient Temp. (Ta) which need to be

corrected at the required standard Temp (75C for Oil immersed Transformers).

Circuit Diagram for the Short Circuit impedance & Load Losses Test

*Measured impedance to be within guaranteed value and nameplate value. *Load losses to be within guaranteed values.

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Test Purpose:

In this test, the value of No-Load power (Po) & the No-Load current (Io) are

measured at rated voltage & frequency. Test Precautions:

1- Test is performed at rated frequency.

2- Two voltmeters are connected to the energized winding, one is measuring the voltage mean value and the other is for the R.M.S value. Test results are considered satisfactory if the readings of the two are equal within 3%. if it’s more than 3%, the validity of the test is subjected to agreement.

3- This test should be performed before the impulse test-if the later will be performed as a routine test- in order to avoid readings errors

1- Voltage applied (usually to H.V. windings) will be in a range starting from 90% of winding rated voltage up to 110% of the same in steps, each of 5%

(i.e. for a 33/11kV transformer, applied voltage values will be 29.7kV, 31.35kV,….., 36.3kV)

2- Readings of watt meters, Voltmeters & Ammeters are recorded to obtain the values of Vr.m.s, Vmean, Po andIo at each voltage step.

3- Measured value of power loss is corrected according to the following formula:

Pc=Pm (1+d)

D= (Vmean-Vr.m.s) / Vmean

*No Load losses to be within guaranteed values.

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All Dielectric Routine tests requirements are summarized in the following Table (IEC 60076-3):

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Voltage levels required for all dielectric tests for transformers having windings with highest voltage (Um) of not more than 170kV are summarized in the following table:

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Dielectric Tests on Transformers with Tapped windings

When applying any of the dielectric routine tests to a transformer equipped with a tap changer (Tapped windings) the following guide lines will apply:

1- If the tapping range equals or less than ±5%, tests will be applied to principal tap only.

2- If the tapping range is larger than ±5%, normally the tests will be performed with the two extreme taps in addition to the principal tap.

Repetition of Dielectric Tests

Repeating a Dielectric test on any transformer will be in two cases:

1- If the transformer has been in service and was serviced or refurbished, tests will be carried out with 80% of the original test levels.

2- If the transformer is new and, then all tests will be repeated with 100% of the original test levels.

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Test Purpose:

The purpose of the impulse voltage test is to secure that the transformer insulation withstand the lightning overvoltage which may occur in service. Test Precautions:

1- For Oil immersed transformers, test voltage should be of negative polarity.

2- Bushing spark gaps shall be removed or gap may be increased to prevent possible flashovers during test.

3- The test impulse wave shall be a standard full impulse wave

(1.2µs ±30% / 50µs ± 20%) where 1.2 is the Front time (T1) and 50 is the Tail time (T2).

4- The Peak Value of the voltage applied during test is obtained from Table (2) for the value of the highest voltage (Um) for the winding under test. 5- In case of flash over on spark gaps or test circuit during the test, this trial

will be disregarded and repeated. Test Procedure & Description:

1- Test is normally performed on both windings of the transformer.

2- When testing one of the windings, the tested line terminal is connected to the impulse generator and the remaining line terminals of the same

winding are earthed directly or through low impedance. (See Connection Fig. below)

3- The line terminals of the other winding are together earthed directly or through low impedance. (See Connection Fig. below)

4- The Neutral point, as well as the transformer tank will be earthed. 5- If the winding under test is tapped (Normally the H.V.) with a range

larger than ±5%, tapping is changed when testing each line terminal. 6- Impulse waveform is applied in the following sequence

a) One reduced full wave (50% ˜ 75%) of the rated test voltage peak value is applied.

b) Three (3) successive full waveforms are then applied.

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7- If the Neutral Terminal of the transformer has a rated insulation level, it can be verified by applying a full wave impulse related to this insulation level to the neutral terminal with all other line terminals earthed.

Connection Diagram for the Lightning Impulse (LI) Test

* Test is considered successful if the transformer withstood the test and if there’s no significant difference between both current and voltage transients recorded by the OCR’s or the digital recorders during the reduced or the full impulse wave application.

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Test Purpose:

The object of the test is to make sure 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.

1- The R.M.S value of the voltage applied during test is obtained from Table (2) for the value of the highest voltage (Um) for the winding under test. 2- Voltage supplied during the test shall be of a frequency not less than 80%

of the transformer rated frequency.

Test Procedure & Description:

1- Single phase voltage is applied to all line terminals of the winding under test (line terminals are shorted together and connected to source.

2- The line terminals of the other winding are all earthed with the neutral terminal and the tank body.

3- Test voltage is applied for 60sec.

* Test is considered successful if the transformer withstood the test and no collapse of the test voltage occurs.

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Test Purpose:

The object of the test is to secure that the insulation terminals between the phase windings, turns, tapping leads and terminals, withstand the temporary overvoltage and switching overvoltage to which the transformer may be subjected during its lifetime.

1- The R.M.S value of the voltage applied during test is obtained from Table (2) for the value of the highest voltage (Um) for the winding under test. If the winding under test is untapped, the voltage shall be as close as possible to twice the rated voltage.

2- Test is carried out with a frequency higher than the rated frequency to minimize magnetizing currents.

3- Test duration is obtained as following

a) If the test frequency is up to and including twice the rated frequency, the test duration will be 60sec.

b) If the frequency is higher than twice the rated value, test duration can be calculated according to the formula

Test Procedure & Description:

1- 3phase AC supply with high frequency is applied to the line terminals of one winding with the line terminals of the other winding open circuited.

2- If the transformer has a neutral, it should be earthed during the test.

* Test is considered satisfactory if the transformer withstood the test and no collapse of the test voltage occurs.

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Following, the list of functional checks that may be performed to Marshalling panel, as well as the remote tap changer control panel (RTCC):

1- Operation of cooling control circuit from marshalling box. 2- Operation of WTI / OTI.

3- Interconnection between the RTCC & the OLTC box mounted on transformer. 4- Indication circuits of RTCC.

5- Electrical operation of OLTC.

6- 2kV Megger test on all Control panel’s wiring.

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For details about operation tests performed on tap changer mounted on transformer, refer to:

“Tap Changers – Testing and acceptance condition Report / Part 1 /

On load Tap Changers”

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Part (6)

Reference Test Reports

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