The Phase Four Wire System Three Level Detection Method Research Of Active Power Filter

Three phase four wire power supply system is mainly used in China now.When thethree-phase system is balanced, the current flowing through the zero line is almostzero.When the three-phase system is unbalance, the current in the zero line is large, andmay be even1.5times large as the phase current. the grid.The single-phase load is nowused as residential load, which will cause the zero line over current by producing a largenumber of zero sequence harmonics superimposed on the zero line.Too large zero linecurrent causes very serious consequences. On the one hand, the cross-sectional area of zeroline is less than the phase conductors, once the current flowing through the neutral wire istoo large, the zero line inevitably will be severely heat. The other hand, automaticprotection device does not be instaled on the zero line, when a current flowing through theneutral wire is too large, the fault line can not be automatically removed.In the three-phase four-wire system, the active power filter is not only necessary tocompensate for the phase sequence current, but also necessary to compensate for thezero-sequence current to avoid excessive current which may cause a fire hazard.Three-phase four-wire active filter is mainly used to compensate the harmonic currents,which is consisted by two parts: Testing part and control links. Harmonic current detectionpart refers to the detection of the harmonic current scilicet that inside the load current.Control link refers to the harmonic current compensation, in other words, the detectedharmonic current value is changed into a control pulse signal by some algorithm, and theinverter is controlled to produce the compensation current corresponding to the currentgeneration, which will be transfered back to the load in the grid.The detection of three-phase four-wire three-level active filter section is mainlystudied. A mathematical model of the three-phase four-wire three-level active filter isbuilded in the matlab/simulink environment. The detection method of harmonic currentbased on Fast Fourier Transform (FFT) and harmonic current detection based oninstantaneous reactive power (ip-iq) is used. While a DC voltage control (PI) links isintroduced and SVPWM control method is also adopted to achieve the overall function ofthe simulation; the software design of the active filter is also completed in the CCSenvironment, the hardware circuit design of the active filter is also completed by adoptingthe DSP F28335as the Core board.