A modified predictive control strategy of three-phase grid-connected converters with optimized action time sequence

Due to the excellent dynamic performance, the Finite Control Set Model Predictive Control has been widely used in various types of converters. However, when Finite Control Set Model Predictive Control is adopted, the switching frequency of converters varies significantly with system operating conditions. Consequently, constant-frequency predictive control strategy has been proposed. Two active voltage vectors and a zero voltage vector are selected within each sampling period. The action time sequence is then calculated. Due to the unsymmetrical distribution of current variation rates around zero, the calculated value of the voltage-vector action time will turn up negative. According to common sense, the voltage-vector action time is greater than or equal to zero. The action time is normally forced to zero whenever a negative value is predicted, resulting in the control failure and performance deterioration. To solve this problem, this paper proposes modified strategy. The modified strategy examines the action time calculated out. When negative action time comes out, the modified strategy reselects the active voltage vector accordingly, instead of forcing the action time to be zero. Optimized action time sequence is further determined by minimizing the cost function. The effectiveness of the modified strategy is clearly verified by experimental tests, and analytical remarks are all founded in practical results.