Single-Loop All-Pass-Filter-based Active Damping for VSCs with LCL filters Connected to the Grid

LCL filters are commonly used to connect Voltage Sourced Converters (VSCs) to the grid. This type of filters are cheaper than a single inductor, but they can generate resonance problems if no active or passive damping method is used. Active damping methods are becoming popular in the literature because they improve efficiency, but they are sometimes difficult to implement and additional measurements are required. This paper proposes a method to provide active damping for VSCs connected to the grid that is based on making zero the open loop phase at the resonance frequency. It will shown that this strategy provides adequate damping of oscillations and that it can be achieved in two different ways: at the design stage (if the design constraints make it possible) or with an all-pass filter in series with the current controller. All the proposed control algorithms are verified by simulation and in a 15 kW prototype of a three-phase VSC connected to the grid with an LCL filter.