Improving off-grid PV system power quality, and comparing with grid power quality

Abstract

Developments in power electronics have enabled widespread application of Pulse Width Modulation (PWM) inverters, notably for connecting renewable systems to the grid. This study demonstrates a high quality power of a stand-alone inverter, whereby the comparison between the power quality of stand-alone inverter with battery storage (off-grid) and power quality of the utility network is presented. Multi-loop control techniques for a single phase stand-alone inverter are used. A capacitor current control is used to provide active damping and improve transient and steady state inverter performance. Capacitor current control is cheaper than inductor current control, where a small current sensing resistor is used. The output voltage control is used to improve the system performance and also control the output voltage. The inner control loop uses a proportional gain current controller and the outer loop is implemented using Internal Model Control Proportional-Integral-Derivative (IMC-PID) to ensure stability. The optimal controls are achieved by using the Sisotool in MATLAB/Simulink. The outcome of the control scheme of the numerical model of stand-alone inverter has nice dynamic performance but also strong robustness under load variations. The numerical model of the stand-alone inverter and its power quality are presented, and power quality is shown to meet IEEE 519-2014. Furthermore, the power quality of the off-grid system is measured experimentally and compared with the grid power, showing power quality of off-grid system to be better than that of the utility network.