Lecture # 2, Measurment of Impedance Voltage & Load Loss

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TRANSFORMERTRANSFORMERPRESENTED BY PRESENTED BY

RAZA AFTABRAZA AFTAB

SUPPORTED BY SUPPORTED BY JAHANZEB AHMED & JAHANZEB AHMED &

MUHAMMAD SHAFIQUEMUHAMMAD SHAFIQUE

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CONTENTSCONTENTS

1. Measurement of No-Load Loss & Acoustic Sound Level √

2. Measurement of Impedance Voltage & Load Loss

3. Separate Source & Induce Over-voltage withstand test (with measurement of Partial Discharges)

4. Lightning impulse over-voltage withstand test

5. Heat run (temperature rise test)

6. Measurement of zero-sequence impedance

7. Measurement of voltage & current harmonics

8. Tests on OLTC & Insulating Oil

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Measurement of Load Loss & Impedance voltage

DEFINITIONIt is the absorbed active power at rated frequency and reference temperature associated with a pair of windings when rated current is flowing through the line terminals of one of the windings and the terminals of the other winding are short-circuited.

Load losses has an importance from the economical point of view. .

Load loss = ohmic loss of windings and internal connections + stray losses (eddy current losses due to leakage field)

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Measurement of Load Loss & Impedance voltage

PURPOSEThe purpose of this test is to determine the power losses (I2R) in the windings for the capitalization of losses and for determining the required loading during the temperature rise test. The test is performed at principal and extreme tap positions if the taps provided are > + 5%.

IEC declare it as a routine test

AFFECTED BYTemperature, I2, frequency, instrument accuracy at low power factorCLASS

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Measurement of Load Loss & Impedance voltage

STANDARDS

IEC 60076-1, Clause 10.1 & 10.4 and IEC 60076-8, Clause 10TOLERANCE

S PL: +15% of guaranteed value provided total loss does

not exceed guaranteed value by more than 10%.

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Measurement of Load Loss & Impedance voltage

EQUIPMENTS/INSTRUMENTS1. Motor-Generator Set2. Adapter Transformer3. Power Analyzer4. Precision Current Transformer5. Precision Potential Transformer6. Compensation Capacitors

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Measurement of Load Loss & Impedance voltage

MEASURING CIRCUIT

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Measurement of Load Loss & Impedance voltage

PROCEDURERated current is generally applied to the HV windings with the LV short circuited. The test is usually carried out at the principal and extreme taps. Test current should be close to the value of rated current as much as possible and the voltage waveform should be in sinusoidal form with rated frequency. If rated current is not available, then current greater than 50% is preferable. The voltage, current and load losses for each phase should be measured during the test. In case that the generator could not supply the system, the reactive power is encountered by using the capacitor banks.

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Measurement of Load Loss & Impedance voltage

PROCEDURE - continued…..The readings have to be taken as quickly as possible to prevent the temperature changes in the windings and the applied current should be sustained 25% - 100% of rated current. This increases the accuracy of the reading.

Now according to IEC the measured value of the losses shall be referred to a winding temperature 75ºC. The transformer is at ambient (tm) temperature when the measurements are carried out, the loss value are corrected to the reference temperature (75ºC) according to the standards as follows:

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Measurement of Load Loss & Impedance voltage

PROCEDURE - continued…..The d.c. losses Pac at the measuring temperature tm are

calculated using the resistance values RHV and RLV obtained in the resistance measurement: RHV and RLV between the line terminals.

DC losses at tm measuring temperature Pdc = 1.5(I2.RHV + I2.RLV)

Additional losses at tm measuring temperature Pac = Pkm - Pdc

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Measurement of Load Loss & Impedance voltage

PROCEDURE - continued…..If the measuring current is different than the rated current, then the load losses are calculated referring the rated current as follows:

Pk = Pkm. (IN/Im)2 where: Im = Measured currentPkm = Measured load lossesPk = Load losses at rated

current

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Measurement of Load Loss & Impedance voltage

PROCEDURE - continued…..The load losses at reference temperature:

Pk = Pdc. t1 + 75ºC + Pac. t1 + 75ºC where: Im = Measured current ts + tm ts + tm Pkm = Measured load losses

Pk = Load losses at rated It1 = 235ºC for Copper = 225ºC for Aluminum

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Measurement of Load Loss & Impedance voltage

TEST RESULTS

Following items are recorded in the test report:

1. Impedance voltage2. Measured current3. Load losses at the measured current4. Ambient temperature5. Load losses at rated current & reference temperature6. Percentage impedance at rated current & reference temperature

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Measurement of Load Loss & Impedance voltage

PRECAUTIONS

1. CT secondary should be shorted.2. Condenser bushing taps to be earthed.3. Readings should be taken quickly to keep the temperature almost constant.4. Voltage should be measured at the terminals to avoid addition of cable losses.5. Applied current should be controlled for gradual increase from zero to prevent inrush current.

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Measurement of Load Loss & Impedance voltage

DEFINITIONIt is the equivalent series impedance Z = R + jX in (a.c.) ohms at rated frequency and reference temperature, across the terminals of one winding of a pair, when the terminals of the other winding are short circuited.

For a 3-phase transformer the impedance is expressed as phase impedance, in a transformer having a tapped winding, the short circuit impedance is referred to a particular tapping unless other specified, the principal tapping applies.

Short circuit impedance voltage is an important parameter for the parallel operation of the transformers and fault calculations.

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Measurement of Load Loss & Impedance voltage

PURPOSEThe purpose of determining the short circuit impedance voltage is the parallel operation of the transformer and fault calculations. The test is performed at principal and extreme tap positions if the taps provided are > + 5%.

IEC declare it as a routine test

AFFECTED BYTemperature, I2, frequency, instrument accuracy at low power factorCLASS

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Measurement of Load Loss & Impedance voltage

STANDARDS

IEC 60076-1, Clause 10.1 & 10.4 and IEC 60076-8, Clause 10TOLERANCE

S For a 2-winding or specified 1st winding pair of multi-winding :- principle tapping ± 7.5% (if Z ≥ 10%), ± 10% (if Z < 10%); any other tapping: ± 10% (if Z ≥ 10%), ± 15% (if Z < 10%). Auto connected winding pair or specified 2nd winding pair of multi-winding :- principal tapping: ± 10% of declared value; any other tapping of the pair: ± 15%. Further winding pairs: to be agreed but ≥ ± 15% of declared value.

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Measurement of Load Loss & Impedance voltage

PROCEDURE

If the measuring current is different than the rated current, then the short circuit impedance is calculated referring the rated current as follows:

Uk = Ukm. (IN/Im)2 where: Im = Measured currentUkm = Measured short circuit impedanceUk = Short circuit impedance at rated current

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Measurement of Load Loss & Impedance voltage

PROCEDURE

The short circuit impedance at measuring temperature tm can be calculated as follows:

Ukm = 100. (Ukm/Un) %

URM = 100. (Pkm/SN) % Ohmic component

Ukm = √U2km – U2

RM % Inductive component

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Measurement of Load Loss & Impedance voltage

PROCEDURE

The short circuit impedance at reference temperature of 75ºC can be calculated as follows:

UR = 100. (Pk/Sn) %

Uk = √U2R – U2

km %

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