Robust Current Controller based solar-inverter system used for voltage regulation at a substation

This paper is a continuation of the work conducted in Chhabra, M., Barnes, F., “Robust Current Controller Design using Mu-Synthesis for Grid-Connected Three Phase Inverter” [1]. High penetration of distributed photovoltaic generation on a distribution system can present several challenges and opportunities for utilities. Voltage sags and swells cannot always be compensated for by slowly responding utility equipment, resulting in degradation in power quality. Voltage variations are commonly caused by rapidly varying solar irradiance, and/or variations in the load. In this paper a robust current controller based solar-inverter system is used for voltage regulation at a substation. Conventional inverter current controllers based on proportional-integral (PI) control may not always offer the superior tracking performance, and harmonic rejection ability of robust controllers. We use the repetitive control strategy, in tandem with a mu-synthesis based controller, to attain optimal sinusoidal reference tracking and harmonic rejection. Musynthesis based control is chosen to attain optimal reference tracking in the presence of plant uncertainties. By applying the mu-synthesis principle, a feedback controller that simultaneously achieves robust stability and robust tracking performance is obtained. To test the proposed inverter current controller, the inverter is interconnected to a 500kW solar system model and operated in volt-var control mode. Ten such 500kW solar-inverter systems are paralleled and interconnected to a substation. The substation is modeled with multiple loads, tap changing transformers, and a 70MVar variable capacitor bank. Simulation performance is compared to an H∞ based optimal current controller, and a PI based current controller.