An Improved Grid Voltage Feedforward Strategy for High-Power Three-Phase Grid-Connected Inverters based on the Simplified Repetitive Predictor
When faced with distorted grid voltage, more harmonics will appear in the output currents of the gridconnected inverters. The grid voltage feedforward strategy, as the most direct solution to compensate the harmonics, however, is seriously affected by the errors in the grid voltage feedforward loop, such as delays. This issue is more significant for high power inverters where the switching frequency is relatively low (<5kHz), and the grid-interface inductance is small (<0.5mH). The errors mainly include the signal distortion caused by the conditioning circuits, the control delay of the digital controller, and the zero-order hold (ZOH) characteristic of pulse width modulation (PWM). In this paper, several improvements have been made to reduce the signal distortion and compensate the delays.
A second-order Butterworth low-pass filter in the conditioning circuit is carefully designed with the maximum flat magnitude response and the almost linear phase response to avoid distorting the measured grid voltage. Further, based on the conventional repetitive predictor, an open-loop simplified repetitive predictor is proposed to compensate the delays in the grid voltage feedforward loop. Three predictive steps are achieved by the open-loop simplified repetitive predictor to compensate the delays: one step for the delay caused by the conditioning circuit, the second step for the control delay of the digital controller, and the third step for the ZOH characteristic of PWM. The effectiveness of the improved grid voltage feedforward strategy are experimentally validated on a 250kVA solar power generation system, where the current harmonics are effectively attenuated. In addition, the inverter starting current is suppressed.