Positive Feed-Forward control of three-phase voltage source inverter for DC input bus stabilization

A Positive Feed-Forward (PFF) controller is proposed as a new active approach to improve stability of a three-phase DC/AC switching inverter. In particular, the proposed approach solves the source subsystem interaction problem, i.e., a system stability degradation which is commonly observed when the inverter is connected to a DC voltage source subsystem that presents finite Thévenin impedance. The implemented strategy is to combine the PFF control with the conventional Negative Feedback (NFB) control. When properly designed, the PFF control modifies the inverter input impedance in the frequency range where the subsystem interaction occurs. As a result, the PFF control stabilizes the DC bus voltage ensuring overall system stability, while allowing the NFB control to maintain good output voltage regulation performance. The approach results in greatly improved stability and damping of the DC bus voltage with a slight reduction of output feedback control bandwidth. Complete small-signal models are presented on a dq rotating frame using g-parameter representation. The design criteria for the PFF controller as well as the trade-off between dynamic output performance and stability improvement are discussed in detail. An extended analysis of stability based on minor loop gain and passivity concept at the DC input bus is also presented. The approach is validated by simulation results.