Transient state analysis of a doubly-fed induction generator under three phase short circuit

Abstract

The Doubly Fed Induction Generator (DFG) is a Variable-Speed Constant-Frequency Generator operating in either subsynchronous or supersynchronous mode. In this paper the transient behavior of the Doubly Fed Induction Generator, symmetrically loaded, after a three phase short circuit is presented. Both speed and rotor excitation voltage and frequency remain unchanged during short circuit. The complete mathematical model of the transient state and experimental results are given as well as the transient state equivalent circuit.The Doubly Fed Induction -chine, is known since the beginning of the century. Its performance was investigated for either motor or generator operation, but mainly in the steady state. Transient state was investigated in [l], [a] and [3] but only for the single and double cage induction motor. In this paper the authors, following procedures similar to those of [l], [2] and [3], present a generalized analysis of the DFG in the transient state during the three phase short circuit from load condition. A detailed mathematical analysis is presented from which the transient DFG parameters, the transient equivalent circuit, as well as the DFG transient response are obtained. The theoretical results were verified by experiments. ANALYSISTo determine the DFG short circuit stator and rotor currents, the DFG is assumed to rotate at a constant speed wn = (1-s)w and is considered to be excited from the rotor dide by a frequency converter with a voltage phasor Vz. During the period of short circuit, the speed as well as the rotor excitation voltage and frequency remain unchanged.The stator of the DFG may, either be connected to the infinite bus of voltage VI, or supply an isolated passive load RotjwLo at constant voltage VI. Stator and rotor voltages, currents as well as DFG parameters are expressed in terms of d, q , y -axis' quantities, with the d,q axes rotating at synchronous speed [l] , Fig. 1 90 SM 414-3 EC the IEEE Electric Machinery Committee of the IEEE Power Engineering Society for presentation at the IE!%E/ \ W t + O Fig. 1 The 3-phase DFG and the d,q axes.The self inductance d,q components for either stator (index 1) or rotor (index 2) are considered equal due to the fact that the stator and rotor balanced windings are distributed around a smooth uniform air gap. The same applies also to the mutual inductances [l] . Thus:The stator and rotor flux linkage d, q components (4) (5) (6) (7)are :The d,q voltage components for stator ma rotor may be derived from (4) through (7).where w is the synchronous angular speed, s is the slip and p is the differential operator (d/dt).Substituting (4) through (7) in ( 8 ) through (11) appropriately, the DFG equations in the transient state are deduced: 63In the case where the stator is short circuited, the stator voltages Vld and VIg become zero. The rotor excitation voltage V2 is divided into the components VZd and VZq along the direct and quadrature axes. The short circuit equations for the DFG may be obtained by applying Laplace transformation to (12) through ...

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