MEASUREMENT OF ZERO-SEQUENCE IMPEDANCE
The zero-sequence impedance is usually measured for star or zig-zag connected windings of the transformer. The measurement is carried out by supplying a current of rated frequency between the parallel connected phase terminal. The zero-sequence impedance per phase is three times the impedance measured in this way. The zero-sequence impedance is needed for earth-fault protection and earth-fault current calculations.
MEASURING CIRCUIT AND PERFORMANCE OF MEASUREMENT
Fig. 9.1
G1 supply regulator, T1 transformer to be tested, T2 current transformer, P2 voltmeter, P3 ammeter, I test current.
The zero-sequence impedance is dependent on the current flowing through the winding. It is measured as a 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.
In the test report the zero-sequence impedance value at the principal tapping is stated.