Frequency Adaptive Feedforward Dual-Mode Repetitive Control and Proportional Control for Grid-Connected Inverters in Microgrid

Repetitive control (RC) based on the internal model principle can achieve zero tracking error for periodic reference and disturbance in a steady state. It has been widely used in grid-connected inverters to improve the current control performance. Owing to a recursive form and an inherent delay of one fundamental period in the internal model, the RC has a relatively slow transient response. At the same time, grid frequency fluctuations in the microgrid lead to the higher harmonic content of the inverter grid current. Therefore, this paper proposes the frequency adaptive feedforward dual-mode repetitive control and proportional control (FA-FDMRC-PC) scheme for grid-connected inverters in the microgrid to track the fundamental reference signal and suppress the harmonic signals simultaneously. First, the principle of feedforward dual-mode repetitive control (FDMRC) is analyzed. The feedforward links are added to the internal models of the dual-mode repetitive control (DMRC) to improve the dynamic performance of the system. From the open-loop bode plots of FDMRC, DMRC, and RC, it can be seen that the FDMRC has much larger open-loop gains at the frequencies of interest with better harmonic suppression capability. Then, the FA-FDMRC-PC scheme is proposed, which connects the FDMRC and the proportional control in parallel to form the feedforward dual-mode repetitive control and proportional control (FDMRC-PC), further improving the dynamic performance of the system and realizing the fractional delay through the infinite impulse response (IIR) filter. The stability analysis provides the relevant stability conditions, which are then utilized in designing the proportional gain, internal mode filter, compensator, phase lead compensator, and repetitive control gain. Subsequently, a single-phase LCL-type grid-connected inverter experimental platform is set up. The controller based on repetitive control and proportional control (RC-PC) and FA-FDMRC-PC are compared at the grid frequency of 50 Hz to verify the harmonics suppression capability and dynamic performance of the proposed control scheme under the ideal grid. The total harmonic distortion (THD) of the grid current of the proposed FA-FDMRC-PC is 1.0%. However, for the RC-PC, the THD value is 1.5%, higher than the THD of the FA-FDMRC-PC. To verify the dynamic performance of the FA-FDMRC-PC, output current waveforms are recorded and analyzed when the peak value of the reference current is changed from 5 A to 10 A. The waveform of the grid current tends to be stable after about two cycles for the FA-FDMRC-PC when the reference current changes abruptly, which shows that the proposed FA-FDMRC-PC has a faster transient response than the RC-PC. When the grid frequency is 49.6 Hz, the THD of the FA-FDMRC-PC is only 0.8%, but the THD of the FDMRC-PC rises to 1.6%. When the grid frequency is 50.4 Hz, the THD of the FA-FDMRC-PC is only 0.8%, but the THD of the FDMRC-PC rises to 1.9%. When the grid frequencies are 49.6 Hz and 50.4 Hz, the waveforms of the grid current tend to be stable after about two cycles for the FA-FDMRC-PC when the peak value of the reference current is changed from 5 A to 10 A. Experimental results verify that the proposed FA-FDMRC-PC scheme has a better immunity to grid frequency fluctuation and a faster dynamic response when the grid frequency fluctuates. © 2024 China Machine Press. All rights reserved.