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
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.
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