The (Transformer) Short-Circuit Impedance Test

The short-circuit impedance (%Z) of power transformers is sometimes measured on site and it can be compared to the nameplate of factory test values. It is used to detect winding movement that may have occurred since the factory tests were performed. Winding movement usually occurred due to heavy fault or mechanical damage during transportation or insulation.

The measurements are usually performed on one phase at a time. Changes of more than +/-3% of the short-circuit impedance should be considered significant.

Test Methods
A convenient method to measure the short-circuit impedance of a transformer is the voltmeter-ammeter method. This method is applicable to testing either single-phase or three-phase transformers. An AC power source, with a rated frequency of the transformer being tested, is used to drive a current through the impedance. The current and the voltage across the impedance are measured simultaneously. The impedance is then given by the ratio of the measured voltage and current.

Conductors used for short-circuiting the transformer windings should be low-impedance conductor having a cross-section equal or greater than No.1 AWG. They should be as short as possible and be kept away from magnetic masses. This is to minimize the “loading” effect of the connecting wires to the impedance the transformer being measured.

Ref: IEEE Std. 69-1995. IEEE guide for Diagnostic Field testing of Electric Power Apparatus

Category Filed in: Tests Conducted On Power/Main Transformers -