DC decay test for estimating d-axis synchronous machine parameters: a two-transfer-function approach

A parameter estimation of a d-axis synchronous generator is presented, in which flux decay test data from the field and armature windings are simultaneously employed. The test is carried out while the machine is at standstill and a DC current is applied to the d-axis armature winding with the field winding in short circuit. The d-axis parameter estimation is achieved by simultaneously fitting the field and d-axis armature currents. The current patterns are approximated by exponential functions, the coefficients of which are related analytically to the d-axis characteristic parameters of the synchronous machine. It is a normal practice to relate the characteristic parameters to a d-axis transfer function, which is only related to the d-axis armature current. To use the field current data on the estimation process, a novel proposal of a transfer function that includes the field current is also used in the estimation process. To validate the proposed approach, a parameter estimation of a 7 kVA, 220 V, 60 Hz synchronous generator is presented.