Identification of generalised models of synchronous machines from time-domain tests

Abstract
Recent developments have shown that numerical models of synchronous machines are, in many cases, improved by introducing an increased number of dampers or dynamic constants. The major issues affecting the model identification by such an extension, which in fact becomes necessary when the machine possesses a solid rotor are discussed from a time-domain point of view. The first result is that all methods which determine the equivalent circuit from test data in an indirect manner, by using the time constants or operational impedances as intermediate tools, give rise, along the d-axis, to systematic errors in the sub-transient and sub-subtransient time constants of the estimated circuits. Unfortunately, when the identification experiment is the standard short circuit, even the direct method suffers from the drawback of weak identifiability of the model, with the result that the uniqueness of the equivalent circuit, directly estimated from test data, is not at all guaranteed. In fact, from an information content point of view, the richer tests are those made at standstill, with a perfect decoupling between the two axes. It is demonstrated by simulation that if the direct approach is applied to such a test, with pulse wave modulated (PWM) voltages as the excitation signals, a numerically cheap and accurate one-step procedure for time-domain identification of circuit models is obtained.

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