Active damping control for a three phase grid-connected inverter using sliding mode control

This paper proposes a new active damping method based on virtual resistors for three-phase grid connected voltage source inverters (VSI) with LCL filter. The use of this filters is a standard solution to provide the proper attenuation to the sideband voltage harmonics of the modulation signal. However these grid filters can introduce an unstable dynamics if a passive or active damping is not properly introduced. The passive damping solution causes a decrease of the overall efficiency due to the associated losses and reduces the filter effectiveness but in contrast no more extra sensors are needed. Active damping methods are more efficient because they don´t introduce losses but suffers from control and system complexity because in some cases a high number of sensors are needed. The proposed strategy employs a nonlinear sliding mode control (SMC) scheme in natural frame in combination with a Kalman filter to emulate a resistor in the LCL filter which damps the system actively without the use of extra sensors. Two current controllers are designed in order to control the active and reactive power independently and a voltage controller is designed to control the neutral point which is fixed with a third harmonic voltage in order to increase the control dynamic range.