Research On Sliding Mode Vairable Structure Control Of Three-phase Four-wire Active Power Filter

Considering the main harmonic source of low voltage power distribution system aresingle-phase and three-phase load.This paper put the APF which can compensate forharmonics in three-phase four-wire system as the research object, on the basis of thefour-bridge three-phase four-wire main circuit structure, studies the the new method ofharmonic detection and compensation control strategy of active power filter.In the analysis of the traditional three-phase four-wire system zero sequence currentseparation，through mathematical derivation proves that three-phase current ia、iband ictoeliminate the zero-sequence component is unnecessary. Advancing an improved ip-iqharmonic method, the three-phase current operation of ip-iq algorithm is decoupled by thismethod, thus saves the three-phase to two-phase coordinate transformation and its inversetransform, It is capable of detecting single-phase and any one phase of three-phase four-wiresystem harmonic current, simulation results show that the improved algorithm effectivelyreduce the computation, improve the detection speed in the case of the same detectionaccuracy, meeting the requirements of real-time detection of power grid harmonic current.Analysising the space vector of four-bridge topology APF output voltage, and comparedwith three-phase APF space voltage vector, put forward to the new thinking of the outputvoltage of A, B, C phase bridge in four-bridge topology APF is decomposed into symmetriccomponent and zero sequence components. Symmetrical components is the same with thethree-phase APF space voltage vector, to further study the relationship between the outputcurrent of the A,B,C and the N-phase arm, come to the conclusion that A,B,C phasezero-sequence current component can be compensated by the N phase bridge, and ultimatelydetermine the A, B, C and N phases can be separated control. The control of the capacitorvoltage and compensating current are put into a unified mathematical model to consider, thusavoiding they are controlled separately by two closed loops. Taking into account thenonlinear nature of the active power filter, sliding mode variable structure control strategy isadopted because its smaller affecting by the system parameters and external disturbances withbetter static and dynamic performance, on the target of minimizing tracking error, define thesliding surface, Sliding mode controller is designed by equivalent control and reaching rate,analyze reachability conditions, determine the range of the sliding surface parameters andobtaine switching function. Comparied with hysteresis control, simulation results show thatthe proposed method has the advantages of good stability, good real-time, fast tracking speed.The research of this paper has some certain reference value for practical engineeringapplication of three-phase four-wire system active power filter.