An Improved Rotor PLL (R-PLL) for Enhanced Operation of Doubly Fed Induction Machine

A versatile method to compute the rotor position of a three-phase induction machine under balanced or unbalanced grid voltage is proposed in this paper. The rotor position of a doubly fed induction machine (DFIM) is computed in closed loop using R-PLL with the q-axis stator flux as the reference signal and the computed rotor speed as feed forward input. A PI controller is used to reduce the phase error while the updated feed forward input (rotor speed) eliminates the frequency error, thereby facilitating a quick and accurate response. Furthermore, the rotor speed is computed directly using only the measured stator voltages and currents, independent of any measurement of rotor current or position. Also, the speed computation process is free from voltage or flux integration or heuristic mathematical calculations. Starting on the fly and accurate computation near and through the synchronous speed even under unbalanced grid voltage are the attractive features. The effectiveness of the scheme is validated through PSCAD/EMTDC simulations and laboratory experiments. Excellent results demonstrate the efficacy of the proposed scheme.