The DC decay test for determining synchronous machine parameters: measurement and simulation

The DC flux decay test has been proposed as an attractive method of extracting synchronous machine parameters. This test is an attractive alternative to the variable-frequency static impedance test. It is quicker and easier to perform and with carefully chosen short-circuit resistance can give good values for transient and subtransient parameters (although ideally a test is required for each). With the machine stationary, direct current is passed through two phases and the decay of current is observed following the short-circuiting of the supply. Tests on two large turbine-generators performed and simulated by finite-element electromagnetic calculations are reported. The combination of tests, calculations, and analysis has enabled the cause of various difficulties associated with the test to be isolated. Recommendations are made to improve the accuracy of measurements