Control strategies of current-source inverters for distributed generation under unbalanced grid conditions

The purpose of this paper is to propose and design proper control strategies for the current-source inverter (CSI) based distributed generation under unbalanced grid conditions. The key is to apply two power schemes, namely the flexible positive and negative sequence control (FPNSC) and the positive and negative sequence control (PNSC) to generate current references for CSI grid inverters. The FPNSC offers the symmetrical current references but suffers from the power oscillations inevitable. The PNSC offers the smoother active or reactive power but needs to inject asymmetrical grid currents. The closed-loop grid current controllers are designed in both positive and negative-sequence frames for accurate regulation. The feedforward capacitor current compensation and active damping are designed to improve the system dynamics and suppress the LC resonance. The frequency lock loop (FLL) is used to track the grid angle for the controller under unbalanced grid conditions. Based on simulation, the performance of the proposed control strategies on the 4.16-kV CSI fed renewable energy system (CSI-RE) and CSI fed superconducting magnetic energy storage (CSI-SMES) are verified in this paper.