An Improved Grid-Voltage Feedforward Strategy for High-Power Three-Phase Grid-Connected Inverters Based on the Simplified Repetitive Predictor

Abstract

When faced with distorted grid voltage, more harmonics will appear in the output currents of the grid-connected 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 (<5 kHz), and the grid-interface inductance is small (<0.5 mH). 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. Furthermore, 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 250-kVA solar power generation system, where the current harmonics are effectively attenuated. In addition, the inverter starting current is suppressed.

A DSOGI-FLL-Based Dead-Time Elimination PWM for Three-Phase Power Converters

“…However, in the digital control system, the current measurement delay Tmd caused by the conditioning circuit is inevitable, and the alternation command of drive pulses will take effect with a control delay Tcd [31], which can together affect the accuracy of the obtained current polarity. Therefore, the conventional SOGI-FLL will be modified with delay compensation in next section.…”

“…Thus the output of SOGI-FLL can achieve a lead phase compared to the input signal, effectively compensating control and measurement delays. Similarly, the closed-loop structure for implementing the delay compensation is also adopted in the repetitive predictor [26], [31], which includes a delay term in the feedback path and has an infinite-gain controller.…”

“…control and measurement delays) in the digital-control system can seriously deteriorate the performance of converters, numerous researches have been reported in the compensation of delays. Various delay-compensation methods have been proposed for enhancing the damping ability of LCL filters [26], [27], improving the stability and dynamic performance of converters [28]- [30], suppressing output current harmonics [31]- [33], etc. The linear prediction is implemented by the iteration algorithm using consecutive history samples [28].…”

“…However, the phases of other-frequency components will also be influenced to different degrees, presenting a nonlinear-compensation characteristic for signals containing components of different frequencies. The repetitive predictor is effective for compensating the delay in periodic signals [26], [31] while has drawbacks of high frequency-variation sensitivity and low dynamic response. The DSOGI-FLL adopted in this paper has noise-attenuation and frequency-adaptability characteristics, as well as the ability of extracting both the positive-and negativesequence components, which can work well for detecting current polarities even in harmonic and unbalanced conditions.…”

Comparison of two PwWM schemes for SiC-device-based split output converters in high-switching-frequency applications

“…However, a significant delay may be introduced to the sensed current, e.g. the delay caused by the conditioning circuit with the second order Butterworth low-pass filter can reach up to 100µs [11], which equals to the time of 10 switching cycles at the switching frequency of 100kHz. 2) Using the auxiliary circuit to detect the current direction, the delay in the sensed current direction can be effectively reduced, which is expected in high-switching frequency applications [8].…”