Impact of Feed-forward and Decoupling Terms on Stability of Grid-Connected Inverters

Three-phase voltage source converter (VSC) is widely used for interfacing distributed energy resource (DER) units with the ac utility grid. Current vector control in synchronous reference frame is the common approach for these applications. In this frame, system model represents a multi-input multi-output (MIMO) control system. However, decoupling terms are used to split the system into two single-input single-output (SISO) subsystems. Moreover, feed-forward terms are also employed to achieve soft start. In this paper, it is shown that when the grid is weak, the feed-forward terms have stabilizing effect in addition to the soft-start advantage. Nevertheless, the decoupling terms have a destabilizing effect. The paper concludes that, if proportional-integrating (PI) compensators are used (which is a common practice), the decoupling terms can be avoided. However, the feed-forward terms must be taken from the phase-locked loop to have an improved stability. Detailed stability analysis of a grid-connected VSC considering the PLL dynamics and grid impedance is carried out and verified by means of simulations.