The mutual leakage reactance between the d-axis damper and field windings is ignored in the conventional d-axis equivalent circuit. It has been pointed out that the calculated value of the field transient current differs considerably from the measured value when this reactance is not taken into account. A method to determine this reactance has been reported previously, but this method has problems concerning the measurement precision. The authors have previously presented a calculation method of equivalent circuits, adapting a DC decay testing method, using two synchronous machines of the same specifications (one with damper winding, the other without). Yet, this method is not practical because of the use of two machines.
This paper presents a calculation method of equivalent circuit constants taking into account the mutual leakage reactance to accurately represent the field transient current using a single machine. The proposed method determines equivalent circuit constants by calculating the physically-correct d-axis transient reactance from the operational impedances when the field winding is shorted and when the field winding is shorted with an external resistance. The validity of the proposed method is demonstrated by comparing the measured values with the calculated values of field and armature currents at sudden three-phase short-circuit.