A Precise Modeling and Performance Enhancement of the Pole Drop Test for Wound Rotor Synchronous Machines

The pole drop test is an offline diagnostic method commonly used to detect shorted turns (STs) in the field winding of wound rotor synchronous machines (WRSMs). The test operates on the principle of injecting an AC signal into the field winding and measuring the voltage drop across the series-connected coils of the rotor poles. A pole with a voltage drop lower than the average value suggests the presence of STs in its coil. Although easy to interpret, the pole drop test lacks adequate sensitivity to detect ST faults with a low number of turns or high contact resistance. Such faults may further deteriorate during machine operation if not detected by offline tests. Given the lack of a comprehensive analysis of the pole drop test, this paper first provides a thorough examination of this test and its performance. Next, it introduces an enhanced pole drop test that employs high-frequency signals to improve the conventional method. The efficiency of the proposed testing method is analyzed by modeling of STs in the rotor winding using equivalent electromagnetic and electrical circuits. Additionally, finite element (FE) modeling of a 6-pole, 50 Hz, 538 kW WRSM using Ansys Electronics software further evaluates the proposed pole drop test. Results confirm that the proposed technique effectively detects STs of varying degrees and is also robust against ST faults with high contact resistance.