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Testing of Power Transformer – Measurement of Zero-Sequence Impedance

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Testing of Power Transformer – Measurement of Zero-Sequence Impedance
Testing of Power Transformer – Measurement of Zero-Sequence Impedance (on photo: preparing the 400 kV transformer for the test field inside the ABB’s factory)

Earth-fault protection

Purpose of the measurement

The zero-sequence impedance is usually measured for all star-connected windings of the transformer. The measurement is carried out by supplying a current of rated frequency between the parallell 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 is needed for earth-fault protection and earth-fault current calculations.

Measuring circuit and performance of measurement

Circuit for zero-sequence impedance measurement in shown below, where:

  • G1 = supply generator
  • T1 = transformer to be tested
  • T2 = voltage transformer
  • T3 = current transformer
  • P2 = voltmeter
  • P3 = ammeter
  • I = test current
Figure 1 - Circuit for zero-sequence impedance measurement
Figure 1 – Circuit for zero-sequence impedance measurement

The zero-sequence impedance is dependent on the current flowing through the winding. Usually the value corresponding to rated current IN is stated. This implies that the measurement is carried out with a test current of 3 x IN.

However, this is not always possible in practice since the current must be limited to avoid excessive temperature of metallic constructional parts.

The zero-sequence impedance is measured as function of test current, and when necessary the final result is obtained by extrapolation.

Test Result

The zero-sequence impedance is usually given as a percentage of the rated phase impedance.

When the transformer has a three-limb core and no delta-connecter windings, the zero-sequence impedance is about 30…60 %. When the transformer has a delta-connected winding, the zero-sequence impedance is 0.8…1.0 times the corresponding short-circuit impedance.

In the test report the zero-sequence impedance values at the principal and extreme tappings are stated.

Reference: Testing power transformers – ABB

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Edvard Csanyi

Edvard - Electrical engineer, programmer and founder of EEP. Highly specialized for design of LV/MV switchgears and LV high power busbar trunking (<6300A) in power substations, commercial buildings and industry fascilities. Professional in AutoCAD programming. Present on