An optimal power flow control method for grid-connected inverters

Most of the modern renewable energy technologies generate DC electric power, which is incompatible with AC grids. In order to control the power flow from DC bus of such generating units to AC grids, grid-connected inverters are essential. This paper proposes an optimization-based control method for single-phase grid-connected inverters which could be considered as a mixture of Model Predictive Control (MPC) and Direct Power Control (DPC). The proposed method minimizes switching frequency and, simultaneously, takes into account power quality constraints such as standard output current harmonics and Total Harmonic Distortion (THD). Due to long computational time required for running optimization problem, solutions for a set of outputs are computed off-line and stored in a look-up table. An open-loop control method based on the look-up table is proposed. Furthermore, in order to provide small signal regulation and disturbance compensation, an on-line algorithm is implemented. Simulation results assure that, the method works accurately in an ideal environment and dramatically lower switching frequencies are achieved. In addition, the closed-loop method not only provides continuous control on output power, but also compensates possible disturbances.