AC-Voltage Harmonic Control for Stand-Alone and Weak-Grid-Tied Converter

This article presents an ac voltage controller that can operate under a wide range of loads, thanks to its high robustness to plant-model variations. A robust voltage controller is required in a droop-controlled inverter that operates connected to a grid or in island mode with other generators. In such modes of operation, the voltage source converter can experience large load-impedance variations depending on the number of parallel generators or the grid impedance value. Concerning the controller performance, the transient response is improved by selectively minimizing the output impedance of the system at the frequencies that typically contain the largest components of the output current. Complete harmonic control is achieved due to its zero output impedance at a set of design-selected frequencies. The proposed controller only measures the output voltage, and it contains a single-loop structure that uses all available bandwidth. In addition, a fast converter-current estimator provides protection against faults and inrush output currents. The design process only requires, as input parameters, the LC filter values, the sampling frequency, and a set of harmonics where load-current circulation is expected.