Interaction Dynamics and Active Suppression of Instability in Parallel Photovoltaic Voltage-and Current-Source Converters Connected to a Weak Grid

Parallel operation of power electronic converters is becoming popular in utility-scale photovoltaic (PV) systems. However, the literature does not cover the interaction dynamics and stabilization of parallel PV-based voltage- and current-source converters (VSC and CSC) connected to a weak grid. To fill in this gap, this article characterizes the dynamic interactions among the parallel PV-based VSC and CSC systems considering the effects of the PV source dynamics, grid strength, operating point variation, and control parameters. A detailed small-signal model of the parallel system is developed and used to characterize the dynamic interactions using eigenvalue and input/output impedance-based analyses. The study showed that undesirable converters interactions are yielded, the dc-links stability is reduced, and the parallel system cannot inject 1.0 p.u. of active power at unity short-circuit ratio (SCR). Therefore, an active stabilization approach is proposed for the parallel VSC-CSC system to reduce the interactions, improve stability, and facilitate 1.0 p.u. of active power injection at unity SCR. Detailed nonlinear time-domain simulations and real-time simulation results verified the accuracy of the analytical results and the effectiveness of the proposed stabilization method under a wide range of operating conditions.