An adaptive PI control scheme to balance the neutral-point voltage in a solar PV fed grid connected neutral point clamped inverter

Abstract

In the context of current harmonics mitigation in inverter, neutral point clamped (NPC) inverter is the promising one because of its robustness. Owing to the advantages of NPC inverter, this paper considers a PV fed grid connected NPC inverter. However, the main concern with the NPC inverter is the unbalance in the DC link capacitor voltage due to non-idealities such as DC offset in modulating signals, dissimilarities in the DC link capacitors and asymmetric gating signals, which lead to non-zero neutral point voltage. To address this issue, this paper proposes a simple adaptive proportional integral (API) controller and operates in conjunction with the dq axes power controller for PV fed Grid connected NPC inverter. Comprehensive mathematical expressions are derived for the design of API controller and the control parameters are adjusted continuously based on maximum PV power, PV voltage and modulation index for achieving the faster dynamic response. The closed loop stability of the proposed API controller is verified under the deviation in neutral point voltage using root locus plot. The effectiveness of the API with dq axes power controller is analysed and validated through MATLAB simulations and experimentation under the various dynamic operation of the PV- grid interactive system.