Enhanced Control of Large Rated Variable Speed Hydrogenerating Unit Under Unbalanced Grid Voltage Conditions

Abstract

This paper analyses the operation of a large rated variable speed hydrogenerating unit (VSHGU) under unbalanced grid voltage conditions. During unbalanced grid voltage conditions, electromagnetic torque pulsations and dc link voltage ripples become severe problems, which reduce the reliability of the system and lifespan of the dc link capacitor. To mitigate the adverse effects of unbalanced grid voltage conditions, a dual sequence proportional integral (PI) controller with synchronous reference frame-phase locked loop (SRF-PLL) is designed for a 250 MW hydrogenerating unit to control positive and negative sequence components of rotor current. The performances of single and dual sequence controllers are compared in terms of DFIM characteristics. Torque pulsations and dc-link voltage ripples have been reduced by 75%, and 50% by dual sequence controller in comparison with single sequence controller. In addition, a dual sequence controller with cascaded delay signal cancellation–phase locked loop (CDSC-PLL) is designed, further, it improves the responses of DFIM characteristics in terms of reduced overshoot and reduced settling time during speed/toque perturbations. The simulation results demonstrate the effectiveness of the designed control technique.