This paper proposes a technique that compensates for unbalance and nonlinearity in microgrid inverters with power transformers operating in stand-alone mode. When a microgrid inverter is operating in stand-alone mode, providing high-quality power is very important. When an unbalanced, nonlinear load is connected, zero sequence current and negative sequence current occur, which leads to an unbalanced output voltage. This paper examines why the zero sequence component occurs differently depending on the structure of a three-phase transformer connected to the inverter output terminal, and it proposes a method for controlling the zero sequence component. It also uses a resonant controller to remove the harmonics that correspond to the negative sequence component and the nonlinear component. The proposed elements were verified by a Powersim (PSIM) simulation.
In this paper, we propose a voltage control technique for the seamless transfer algorithm of a microgrid inverter. When the microgrid inverter is switched from grid-connected mode to standalone operation mode, an output voltage transient may occur, and adversely affect the load. To solve this, seamless transfer algorithms that complete mode switching within 4 ms have been studied. However, previous control techniques have very complex structures. In this study, we find the cause of the voltage transient occurring in the mode conversion, through state average modeling of the inverter. Subsequently, we propose a feed-forward compensator for seamless transfer. The cause of the voltage transient is removed by the compensation, and smooth mode switching is achieved. The proposed controller is verified by Powersim (PSIM) simulations.
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