Abstract: The tracking of the distorted current signal without static error cannot be completed by a single PI

control. In order to improve the quality of the grid current，a harmonic compensation method for parallel active filters under extreme loads was proposed. Create the main circuit topology of the parallel active filter in the three-phase fourwire system under extreme load to obtain the distortion current to be compensated. Establish a composite control system based on SAPF control and PI control. Based on the fundamental period characteristics of the distorted current signal，the error signal was repeatedly controlled to improve the stability and accuracy of the tracking signal. On this basis，a fixed-order no static error control technology was incorporated to realize harmonic compensation of parallel active filters. A three-phase four-wire system platform was built for experiments. The results show that the method can reduce the current distortion rate，improve the sine of the current waveform，and has superior dynamic performance and steady-state performance.

      Key words: extreme load；shunt active power filter（SAPF）；harmonic compensation；distortion current；

compound control