lecture # 3, separate source & induce test

CERTIFIED QUALITY CONTROL PROFESSIONALS

MODULE-1 TRANSFORMER TESTING

Presented ByPresented ByRAZA AFTABRAZA AFTAB

Supported BySupported ByJAHANZEB JAHANZEB

AHMEDAHMED

Special Thanks toSpecial Thanks toHR DepartmentHR Department



CONTENTS

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



Separate Source Voltage Withstand Test

PURPOSE

The purpose of the test is testing the insulation between the windings and the insulation between the windings and earthed parts (tank, pressing beams etc.) under the temporary and switching overvoltages conditions which may occurred during the service.

The transformer is not magnetized. Every part of the test winding is exposed to the full test voltage between the winding and earth.

CLASS

IEC declare it as a routine test.




APPLICABLE STANDARD

IEC 60076-3, Clause 11

ACCEPTANCE CRITERIA

The test shall be successful if no collapse of the voltage occurs.




EQUIPMENTS/INSTRUMENTS

• Voltage regulating transformer• HV Transformer• Current Transformer• Ammeters & Voltmeters• Capacitive Voltage Divider

OR

• High voltage step-up transformer Phenix –USA make or equivalent with complete set of input control module.




TEST CIRCUIT




PROCEDURE

The full test voltage shall be applied for 60s between all terminals of the winding under test connected together and all terminals of the remaining windings, core, frame and tank or casing of the transformer, connected together to earth. The separate source AC voltage test shall be made with single-phase alternating voltage as nearly as possible of sine-wave form and not less than 80 % of the rated frequency. The peak value of voltage shall be measured. The peak value divided by √2 shall be equal to the test value. The test shall commence at a voltage not greater than one-third of the specified test value, and the voltage shall be increased to the test value as rapidly as is consistent with measurement.




PROCEDURE….Continued

In case of repeating the test, after the transformer has successfully withstood the test, the test voltage levels shall be reduced to 80% of the original value as mentioned in IEC 60076-3 clause: 9 unless otherwise agreed and provided that the internal insulation has not been modified in the mean time.




TEST RESULTS

Following items are recorded in the test reports.

1. Applied voltage

2. Leakage Current

3. Duration of the test




PRECAUTIONS

1. All connections should be tight.

2. Proper clearances should be observed.

3. Earthing should be proper.



Induce Overvoltage Withstand Test (with measurement of partial discharges)

PURPOSE

The purpose of the test is to ensure that the insulation between the phases, windings, turns, coils, tapping leads and line terminals withstand the temporary and switching voltages conditions to which the transformer may be subjected during its lifetime. Measurement of partial discharges gives comprehensive information about the quality of the insulating material.

CLASS

IEC declare it as a routine test.




APPLICABLE STANDARD

IEC 60076-3, Clause 12 for Induce Test and Clause 12.2.2, 12.3, 12.4 & Appendix A for partial discharge test.

AFFECTED BY

Partial Discharges

Partial Discharges

It is an electric discharge that only partially bridges the insulation between the conductors




EQUIPMENTS/INSTRUMENTS

• Motor Generator Set• Adaptor Transformer• Current Transformers• Voltage Transformers• Partial Discharge measuring test set• Calibrator




TEST CIRCUIT




PROCEDURE

Measurement of Partial Discharges

For the measurement of the partial discharges the BUSHINGTAP method is used. According to this method the calibration of the circuit has to be made before starting measuring. For this reason priority calibrator is required.

After the transformer has been energized, the value of partial discharge that was observed from the measuring equipment is multiplied by the correction factor (K) of every terminal and the apparent partial discharge value is found.




PROCEDURE…Continued

The partial discharges are measured during the increasing of test voltages at 110%, 130% of Um, where Um is the max. operating voltage and at exact test voltages, and the reducing of the test voltages.




PROCEDURE

Induced overvoltage test

This test is similar to no-load test. All the other windings, other than that being tested, are open circuited. If the winding has a neutral terminal, this may be earthed during the test. Twice the rated voltage is applied to the winding, preferably on LT side. The form of the voltage shall be as nearly as possible sinusoidal and its frequency sufficiently above the rated frequency to avoid excessive magnetizing current during the test. The peak value of the induced test voltage shall be measured.






The test shall be commenced at a voltage not greater than one third of the test voltage and shall be increased to the test value as rapidly as is consistent with the measurement. At the end of the test, the voltage shall be reduced rapidly to less than one third of the test value before switching off.

The duration of the test at full test voltage shall be 60s for any test frequency up to and including twice the rated frequency (i.e. 100Hz for the rated frequency of 50 Hz).






The test is successful is no collapse of the test voltage occurs. When the test frequency exceeds twice the rated frequency, the duration of the test shall be as follows:

Duration(s) = 120 x rate frequency / test frequency




CALCULATIONS

For partial discharge the correction factor (K) can be calculated as follows:

K = qo/qm Where:

qo :charge reproduced by the calibratorqm :charge observed at measuring equipment

The partial discharge q can then measured as:

q = qm. K




TOLERANCES

According to IEC the level of partial discharges should be less than:

• 300 pC @ 130%Um

• 500 pC @ 150% Um

• Continuous level < 100 pC @ 110% Um




TEST RESULTS

Following items are recorded in the test reports.

1. Partial Discharges

2. Test Voltage

3. Duration




PRECAUTIONS

1. For the measurement of partial discharges, calibration process has to be made at the de-energized state of the transformer.

2. No one should be present in the test bay but all other equipment (MG Set) must be running.

3. Connections should be checked strictly.

4. Voltage should be increased at a rapid and smooth pace.




PRECAUTIONS…Continued

5. A poor quality test connection can cause unexpected voltage rises. External flashovers can over unexpectedly. Keep well clear.

6. Saturation current for the core should be as much avoided.

7. All points of the transformers should be earthed through proper earthing.



THANK THANK YOUYOU

lecture # 3, separate source & induce test

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