Stability Improvements of an LCL-filter based Three-phase Active Rectifier

Three-phase active rectifiers guarantee sinusoidal input currents and controllable dc voltage at the price of a high switching frequency ripple that can disturb and reduce efficiency of other EMI sensitive equipment connected to the grid. This problem could be solved choosing a high value for the ac inductors making them expensive and bulky. Moreover the dynamic of the system becomes poor with so high value of inductance. On the contrary to adopt an LCL-filter configuration allows to use reduced values of the inductances (preserving dynamic) and to reduce the switching frequency pollution emitted in the grid. However the stability of the system should be rigorously studied. A poor analysis made on qualitative considerations could lead to excessive damping (unnecessary increase of the losses) or insufficient damping (the system seems to be stable but it is not). In this paper the damping, both passive (based on the use of resistors) and active (based on the modification of the control algorithm), is studied using the z-plane root locus approach and looking to dynamic performances and losses. In fact it is necessary to verify the dynamic effects by the introduction of damping resistors or by the modification of the control algorithm to perform active damping. The analysis is validated both with simulation and experiments.