Investigation of reducing the influence of digital control delay to LCL-type grid-connected inverter

Digital control system usually introduces a beat of delay, which includes PWM upload delay and zero-order holder delay. The delay not only reduces the system phase margin, but also changes the impedance characteristics of active damping, which influences the system minimum phase property and threatens the stability of the injected current. In this paper, a PWM real-time upload method and the detailed digital implementation steps are proposed to eliminate PWM upload delay. Then the first-order high pass filter is proposed to compensate the zero-order holder delay of resonance frequency band. The combination of the two schemes increases the system phase margin and improves the compatibility between switch frequency and LCL resonance frequency, which makes system always maintain minimum phase property. A 3KW prototype is designed, and the proposed control method is verified by experiments.