Stability improvement of grid-connect inverter using combination of passive and active damping

the gird-connected converters with regulation of power factor to unity become to be more popular. These inverters are connected to the grid through L or LCL filters. An LCL filter is more effective and reduces harmonic distortion at lower switch frequency than an L filter. However, an LCL filter can bring stability problem. Even more, in practical engineering, there are some additional filters behind the LCL filter, which cause serious stability problem, this paper models a two-level-filter as an example. To improve the stability, one solution is to introduce a resistor in series/parallel with the filter capacitor or inductance which is called passive damping and another is to modify control algorithm to stabilize the system which is called active damping. This paper analyzes the reason why the effect of passive and active damping method is limited. To enhance the stability of the system, a method that combines the passive and active damping is proposed, this method possesses both the advantage of passive and active damping. Simulation and experimental results verify this combination.