lecture # 6, measurment of zero sequence impedance, harmonics of no-load current & oltc tests

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 impedance7. Measurement of voltage & current harmonics8. Tests on OLTC & Insulating Oil



Measurement of Zero Sequence ImpedancePURPOSE

Applicable to star or zigzag interconnected star connections with neutral brought out.

The purpose of this test is to measure the zero sequence impedance for all star connected windings of the transformer needed for earth fault protection and earth fault current calculations

CLASS

IEC declare it as a special test.



APPLICABLE STANDARD

IEC 60076-1, Clause 10.7, Measurement of zero sequence impedance on three phase transformers.

DEFINITION OF ZERO SEQUENCE IMPEDANCE

The impedance expressed in Ω/Phase at rated frequency between the line terminals of a three phase star connected or zigzag connected winding connected together and its neutral terminal.

Zo = 3 V/I Ω/Phase

Measurement of Zero Sequence Impedance



TOLERANCES

According to IEC 60076-8, Clause # 4, “Zero Sequence Properties”.

CALCULATIONS

Net impedance (Zo/3 ohms) = measured voltage / measured current

Zero sequence impedance per phase (Zo) = 3 * net impedance




EQUIPMENTS/INSTRUMENTS

Following equipments/instruments are required to perform this test:

• Motor-Generator Set• Power Analyzer• Current Transformer• Potential Transformer




TEST CIRCUIT




PROCEDURE

The measurement is carried out by supplying a current of rated frequency between parallel connected phase terminals and the neutral terminal. The zero sequence impedance per phase is three times the impedance measured in this way.

The zero sequence impedance for a star-star connected transformer, having a tertiary winding in open delta connection, can be measured in the following four ways.

• Supply on HV side, with LV open and tertiary short circuited.




PROCEDURE….Continued


• Supply on HV side, with LV shorted and tertiary open circuited.





• Supply on LV side, with HV open and tertiary short circuited.





• Supply on LV side, with HV shorted and tertiary open circuited.



TEST RESULTS

Following items are recorded in the test reports:

1. Zero sequence impedance value.

2. Corresponding test condition.




PRECAUTIONS

1. Current flowing in the windings should be according to their current carrying capacity.

2. Measurements should be performed quickly.




Measurement of Harmonics of the no-load currentPURPOSE

The purpose of this test is to determine the extent of distortion of the no-load current waveform.

CLASS

IEC declare it as a special test.

APPLICABLE STANDARD

IEC 60076-1, Clause 10.6, Measurement of harmonics of the no-load current.



BACKGROUND

The magnetizing current of transformers contains harmonics due to the non-linear magnetizing characteristic. The magnetizing current is in normal operation small, in the order of 1% of the rated current of the transformers, and the influence of harmonics is negligible. However, if the transformer core is saturated due to an applied voltage higher than the rated voltage, the magnetizing current and its harmonics will dramatically increase.

Measurement of Harmonics of the no-load current



CALCULATIONS

VTHD (%) = 100 * SQRT [(V22 + V3

2 + V42 + ... + Vn

2)] / Vt

ITHD (%) = 100 * SQRT [(I22 + I32 + I42 + ... + In2)] / It






• Motor-Generator Set• Step-up Adapter Transformer• Power Analyzer• Current Transformer• Potential Transformer




TEST CIRCUIT




PROCEDURE

Rated voltage is generally supplied to the low voltage winding and all the other windings are open circuited. Other windings, which can be connected in open delta, should have the delta closed.

The voltage across the windings should be symmetrical and sinusoidal, as far as possible. The test voltage should be adjusted according to a voltmeter responsive to mean value of voltage.

The harmonic analyzer directly gives the harmonics and their corresponding magnitudes in volts, from which percentage harmonic voltages can be found out, with reference to the fundamental component.




TEST RESULTS

Following items are recorded in the test reports:

1. Applied voltage.

2. Fundamental and harmonic components in volts.

3. Fundamental and harmonic components in percentages




PRECAUTIONS

Readings of current and power should be taken at the exact rated voltage of the winding




TESTS ON ON-LOAD TAP CHANGERPURPOSEThe purpose of this test is to verify the operation of the on-load tap changer at various conditions of the transformer i.e. un-energized, unloaded, loaded & reduced conditions.

CLASS

IEC declare it as a Routine test.

APPLICABLE STANDARD

IEC 60076-1, Clause 10.8, “Tests on OLTC”.



DEFINITION

On-Load Tap Changer

A device for changing the tapping connections of a winding, suitable for operation while the transformer is energized or on load.

TESTS ON ON-LOAD TAP CHANGER





• Motor-Generator Set• Step-up Adaptor Transformer• Power Analyzer• Current Transformer• Potential Transformer • Compensation Capacitors




PROCEDURE

With the tap changer fully assembled on the transformer the following sequence of operations shall be performed without failure.

1. With the transformer un-energized, eight complete cycles of operation (a cycle of operation goes from one end of the tapping range to the other, and back again).

2. With the transformer un-energized, and with the auxiliary voltage reduced to 85% of its rated value, one complete cycle of operation.




PROCEDURE

3. With the transformer energized at rated voltage and frequency at no load, one complete cycle of operation. This test may be performed after performing the no load test.

4. With one winding short circuited and, as far as practicable, rated current in the tapped winding, 10 tap change operations across the range of two steps on each side from where a coarse or reversing changeover selector operates, or otherwise from the middle tapping.




PRECAUTIONS

1. All connections should be tight.

2. Proper clearances should be observed.

3. Proper alignment between the tap changer indicator and the motor drive indicator should be ascertained.




THANK THANK YOUYOU

lecture # 6, measurment of zero sequence impedance, harmonics of no-load current & oltc tests

Documents

sequence impedance supply

currentzero sequence

sequence properties

sequence impedance7

transformer testingmeasurement

transformer testingcontents1

phase star

measured voltage