A fully digital hysteresis current controller for current regulation of grid connected PV inverters

Grid connected voltage source inverters require both current regulation and synchronisation to control the power flow and to maintain stability during grid disturbances. For this type of application, non-linear current control of VSIs using techniques such as hysteresis current regulation offer advantages compared to linear current regulation, such as inherent over-current protection, robustness to load/filter parameter variation and very rapid dynamic response. However these techniques suffer from variable switching frequency and their implementation can require significant analog circuitry. Furthermore, sensorless grid synchronisation can improve the inverter´s robustness to disturbances such as electrical line interference, sensor failure and rectifier commutation voltage notches. This paper presents a fully digital constant frequency three-level hysteresis current regulator for single phase PV inverters that addresses these issues. The proposed control strategy also uses the average inverter voltage to adjust the hysteresis band to maintain a constant frequency, detect the output voltage polarity and estimate the grid voltage. The result is a robust current regulator with increased flexibility and easy adaptability for grid connected inverters.