Effect of Field-Voltage Source Impedance on Load-Rejection Test Results of Large-Rating Synchronous Generators

This paper presents a technique to determine the effect of the field-voltage circuit during the load-rejection test of a large-rating salient-pole synchronous generator. Once the circuit powered by a six-pulse converter connected to the excitation transformer of the generator unit corresponds to a power-supply voltage not constant immediately after the load rejection, a careful evaluation of the resulting signals is necessary to extract rotor and stator parameters from the comparison with theoretical expressions derived under the assumption of constant-field terminal voltage and excluding external effects. The results obtained in the load-rejection test of a 179-MVA, 13.8-kV generating unit at Nova Ponte power plant show the parameters that are more affected by the external impedance in series with the rotor circuit and how to compensate them.