A Calculation Method of Field Current of Synchronous Machines at Sudden Short-circuit

Abstract

The mutual leakage reactance between d-axis damper and field windings is ignored in conventional d-axis equivalent circuits. It has been pointed out, however, that the calculated value of the field current differs considerably from the measured value if this reactance is not taken into account. This is due to the difficulty of determining the physically-correct damper winding impedance value. A method to determine the equivalent circuit constants using the mutual leakage reactance has been reported previously, where the d-axis damper winding time constant is measured from the upper and lower envelopes of field current at sudden three-phase short-circuit. Yet there are machines for which the upper and lower envelopes of field current are not readily established, in which case this method is unsatisfactory. The authors describe a method to accurately identify the equivalent circuit constants taking into account the mutual leakage reactance, using a standstill test with a small-capacity DC power supply (DC decay testing method). The field current at sudden short-circuit can be simulated accurately using these equivalent circuit constants. The validity of the proposed method is demonstrated by implementation results on two salient-pole synchronous machines with the same specifications (one with damper winding, the other without). Furthermore, the dependent relation between the armature leakage reactance and mutual leakage reactance, as well as its influence on the calculation of field transient currents, are made clear.