Improved DPWM scheme for improvement of grid current quality in a large-scale grid-connected inverter system with a LCL-filter

This paper proposes an improved discontinuous PWM (DPWM) method in a grid-connected inverter system. When the conventional 60° DPWM is used in the switching method, the output voltage of the inverter has many harmonics compared to the continuous PWM (CPWM) switching method. In large-scale system, a LCL-filter cannot sufficiency reduce the harmonics of the inverter output voltage. If these harmonics and the resonance frequency band caused by a grid-connected LCL-filter are overlapped, the grid current oscillates. To solve this problem, the proposed method continuously changes the discontinuous modulation signal by injecting triangular wave to modulation signal. The proposed algorithm can reduce the harmonics of inverter output voltage. The proposed DPWM is verified from PSIM simulation with a 600-kW 2-level grid-connected inverter system.