Slow-Scale and Fast-Scale Instabilities in Parallel-Connected Single-Phase H-Bridge Inverters: A Design-Oriented Study

This paper reports the slow- and fast-scale instabilities in the parallel-connected single-phase H-bridge inverters and discusses the two types of instabilities from the practical design viewpoint. Simulations show that the slow-scale instability which occurs in the whole line cycle is a type of global instability, whereas the fast-scale instability which occurs around the middle time of each half-line cycle is a type of local instability. In order to reveal the mechanisms of the slow- and fast-scale instabilities, theoretical analyses are carried out through the derived averaged model and discrete-time model, respectively. It is identified that the slow-scale instability is due to the occurrence of Hopf bifurcation, and the fast-scale instability manifests itself as period-doubling bifurcation. Furthermore, stability boundaries in various design parameter spaces considering the mismatches in different system parameters between inverter modules, as well as the effects of the current-sharing control loop on the slow- and fast-scale instabilities are also given. Besides, the influences of the nonlinear load and the control method for parallel system on the two types of instabilities are briefly discussed. These findings can be used to guide the tuning of the paralleled inverter system parameters to ensure stable operation in practice. Finally, experimental results are presented to verify the results of the simulation and theoretical analysis.