Control of three-level NPC converters in DFIG based off-grid wind energy conversion systems

This paper deals with control of three-level neutral point clamped (NPC) voltage source converters (VSCs) for medium rating doubly fed induction generator (DFIG) based off-grid wind energy conversion system. A three-level three-leg insulated gate bipolar transistors (IGBTs) based NPC-VSC is used as a rotor converter (RC). Four-leg three-level IGBTs based NPC-VSC is used as a load converter (LC). A battery energy storage system (BESS) is employed at the DC bus of back to back connected LC and RC in the rotor circuit. The BESS facilitates establishment of off-grid supply using RC control and absorb or supply deficit load power under wind speed variations. A vector-oriented control scheme for RC and LC establishes functions of a load leveler, a load balancer, a harmonic eliminator, a zero sequence load current compensator, a maximum power tracker and a capacitor voltage balancer under dynamic conditions either due to consumer loads or wind. The proposed control scheme for a DFIG based off-grid system is developed and its performance is found satisfactory under steady state and dynamic conditions.