Control of Three Phase Inverters for Renewable Energy Systems Under Unbalanced Grid Voltages

This paper presents two control algorithms for grid-tied three-phase inverters for renewable energy systems under unbalanced grid voltages. The algorithms are focused on the quality of the injected currents, so they are used to inject balanced currents despite the voltage unbalances. Each algorithm is composed of three control loops: the reference signal generation, the reference signal synchronization and the curent controller.  The first control loop generates the reference currents in order to regulate the active and the reactive powers delivered to the grid by the renewable energy system.  The second loop is used to estimate the positive sequence component of the grid voltage. The last control loop generates the power inverter firing pulses in order to follow the reference current. In this paper are considered two algorithms, one algorithm in the  abc  coordinates and the other in the  dq0  coordinates. In both cases, it is considered a Kalman filter as the synchronization algorithm. The algorithms performance is evaluated by some simulations in PSIM and a comparative analysis is made between them. The results showed that the two algorithms present an outstanding performance in the injection of balanced currents with low total harmonic distortion despite the voltage unbalance. Also, the Kalman filter gets the desired estimation in less than a quarter of the grid period.