Control Strategy of Single-Phase Hybrid-Mode Cuk Inverter for LVRT Capability

Abstract

This article proposes a control strategy for a Cuk module-integrated inverter (MII) with hybrid operation mode for low-voltage ride-through (LVRT) capability. The hybrid-mode Cuk MII operates in unfolding-type power conversion (UPC) mode during normal grid conditions and in two-stage power conversion (TPC) mode during grid faults. It also has the advantages of highly efficient power transfer and LVRT capability but is difficult to control because UPC and TPC modes have distinct system dynamics and suffer from grid voltage disturbances that have different magnitudes. To overcome this control problem, this article proposes a control strategy that corresponds to the operating mode of the hybrid-mode Cuk inverter. To achieve zero steady-state tracking error during the grid disturbance, a repetitive controller (RC) is used in the proposed control scheme. Different phase-lead compensators in the RC compensate for different phase lags caused by distinct system dynamics. To reduce the burden from the RC, different nominal duty ratios are used as the feedforward control input. To minimize the tracking error during grid faults, a proportional-resonant controller is used in parallel with the RC controller in the TPC mode. To analyze the stability of different control systems, a unified control system model is presented for the proposed inverter. The procedure to select control parameters is also presented in detail. Simulation results validate the proposed control scheme in the hybrid-mode Cuk inverter, and experiments are conducted using the 400-VA MII prototype to demonstrate its validity.