Re-examination of the Doubly Fed Induction Machine (DFIM) Model Taking Into Account Rotor Reactive Power

Modelling electric machines is crucial for analysing their behaviour and designing controllers. It is of the utmost importance to make use of a consistent equivalent circuit of the Doubly Fed Induction Machine (DFIM) that is applicable to a variety of operating modes. This is because it helps in the calculation of the machine's steady-state performance, converter ratings and controller set-points. Traditional models of doubly fed induction machines employ the steady-state equivalent circuit of a wound-rotor induction machine with all rotor parameters referred to the stator through a frequency conversion. The present study investigates the validity of the traditional steady-state circuit model by taking into account the sequence change in rotor voltages and currents at super-synchronous speeds. The validity of phasor diagrams constructed using the traditional circuit is assessed, with particular focus on super-synchronous operation in both motoring and generating modes. It has been demonstrated that the existing model is applicable to all rotor speeds (whether sub-synchronous or super-synchronous). However, caution should be exercised when utilising expressions of rotor reactive power that involve dynamic dq</em> and steady-state phasor models. Therefore, modified expressions are developed for rotor reactive power that are applicable regardless of the operating speed. The accuracy of the proposed method for different operating modes is confirmed by comprehensive simulation results developed with Matlab® Simulink. An investigation is also conducted into the sensitivity of rotor reactive power direction to parameter changes, and it is shown that machine parameter changes have a negligible effect on rotor reactive power direction.