A novel switching strategy for DC link voltage balance in parallel connected three-level diode clamped back-to-back converter systems serving to variable speed pumped storage plant

Abstract

Variable speed pumped storage plant (VSPSP) is a practical feasible solution to meet peak demand in power network. During recent era, doubly fed induction machine (DFIM) is preferred for the sites, where large variations in water head or surplus power available in the grid to increase the energy efficiency at part loads. In general, DFIM with parallel connected three-level diode clamped converter (3L-NPC) systems are employed in rotor circuit for large rated pumping applications (above 100 MW). However, parallel connected 3L-NPC suffers with DC link voltage imbalance due to neutral phase currents, which degrades the overall performance of the inverter. To overcome this issue, this paper proposes a novel switching strategy based on virtual switching vector selection scheme, which balances the DC link voltage for wide range of load variation without any additional circuitry. The effectiveness of proposed switching strategy is realized with MATLAB/Simulink environment and experimental validation is performed through 2.2 kW DFIM system.