Stability enhancement of multi-parallel inverter systems in distributed power grids with unconventional control delay

As the scale of distributed generation systems continues to expand, the capacity requirements for multi-parallel inverter systems (MPIS) are also increasing. The greater the number of grid-connected inverters (GCIs) operating in parallel, the higher the requirements for the stability and the dynamic performance of the entire system. Adjusting the control delay can enhance the dynamic response capability of GCIs, but it may impose additional impacts on their stability. In most existing studies, control delay is assumed to be one-sample delay, while few studies have explored the impact of unconventional control delays on MPIS. To address this issue, this paper simplifies the analysis of MPIS using the passivity analysis method. By modeling the output admittance of the GCIs, it is pointed out that changes in control delay will alter the negative region of the real part of the output admittance, thereby affecting system stability. Therefore, this paper derives in detail the constraint conditions on the gain and phase of the controller imposed by control delay, aiming to eliminate the negative real part region of the output admittance. Based on these constraint conditions, a controller design method using a second-order Finite Impulse Response (FIR) filter is proposed. The proposed design method ensures the passivity of GCI with unconventional control delays, thereby guaranteeing the stability of MPIS. Finally, a parallel experiment with two inverters is conducted, and the proposed method ensures the stable operation of the system and achieves plug-and-play characteristics. • The passivity theory is adopted to simplify the analysis of the parallel inverter system. • The controller gain and phase constraints based on control delay are derived. • A simple controller design method considering control delay is provided. • Experimental verification was carried out on a dual-inverter experimental platform.