ANALYSIS OF DOUBLY OUTPUT INDUCTION MACHINE

Abstract

In this report, an attempt to develop a vectorized dynamic model of induction machine which can be simulated as both motoring and generating mode when testing control strategies. Through the model developed in this report can be used for simulating all types of induction generator configurations. The choice of synchronous rotating reference frame makes it particularly favorable for the simulation of double-output configuration in transient conditions. The induction machine is modeled in vectorized form in the synchronous frame. The injected rotor voltages (at slip frequency) are derived from PI controllers that regulate the active and reactive powers delivered by the generator. The speed is adjusted by the turbine pitch control to maximize the power generated at a given wind speed at varying shaft speed in the sub synchronous and super synchronous regions. A modified equivalent circuit is used in the analysis in which core losses and harmonics are ignored. The machine representations are based on the motor convention; consequently in the generator mode of operation, such quantities as stator active power (Ps), reactive power (Qs), wind turbine torque( T,, ), electromagnetic torque (Te) are negative (with W,n >0) while stator power (Pr) is positive for sub synchronous operation and negative for over synchronous mode The machine model is based on the synchronous rotating reference frame with vectorized dynamic model and control unit is based on the line-voltage oriented reference frame. The machine side converter provides good decoupling between the active and reactive power and the network side converter controls the power flow between the DC bus and the AC side and allows the system to be operated in sub-synchronous and super synchronous speed. The induction machine runs at a specific speed with the stator disconnected from the grid (Is=O), the rotor is suddenly excited with slip-frequency voltages derived from voltage regulators so as_ to produce commended open-circuit stator terminal voltage. Behavior under varying input power typically observed in wind turbines is also reported. A MATLAB computer simulation study was undertaken and results on 1.5 kW wind turbine are presented indicating grid abnormalities and varying input power.