Two degrees of freedom active damping technique for LCL filter-based grid connected PV Systems

In grid connected photovoltaic (PV) systems, lowpass filters are utilized to reduce injected current harmonics. $LCL$ filters have recently drawn attention for PV system grid interfaces due to their small size and they have shown better attenuation to switching harmonics than simple L filters. However, the $LCL$ filter causes resonance resulting in oscillation and instability issues. This paper proposes an effective active damping technique by introducing a two-degree-of-freedom (2DOF) PID control structure. The 2DOF control structure allows the independent action of PI and D terms giving two degrees of freedom. The design is based on a typical three-phase grid-tied PV system. The active damping control loop is formed by using the existing grid side inductor currents and thus eliminating the need of additional sensors. The relative stability is illustrated in frequency domain by using bode plots. A real-time hardware-in-loop study is performed to validate the performance of the proposed 2DOF technique to damp out the $LCL$ filter resonance.