Research On A Zero Sequence Filter Based On Magnetic Flux Compensation

The harmonic problem is one of the key factors that affect power quality in the power system. Due to the extensive use of power electronic devices, harmonic problem becomes particularly serious in the low voltage distribution power system nowadays. One of them is called zero-sequence harmonic which has the same size and the same phase and can be added up in the neutral line. Which cause the neutral line to be overheated and brings the power system some instability. At present there is no filter which specifically designed to filter out harmonics in the neutral line. It is not probable to use the LC filter or the APF to solvel this problem considering the economic factor and filtering effect.Flux compensation principle can be simply interpreted that by controlling the frequency, magnitude and direction of current that flow through the secondary side of the transformer according to some rules set in advance, the equivalent impedance in the primary side can be regulated. Then the impedance has different values in different frequencies in the primary side. According to this principle, this paper presents a novel zero sequence current filter in order to eliminate the harmonic in the neutral line. Because of zero sequence flux compensation, the zero-sequence filter exhibits small leakage impedance to the zero-sequence current while exhibits the great value of excitation impedance to the fundamental current. The filter connects in parallel with the harmonic source. Then the filter can reduce the harmonic current flow through the main circuit. This paper analyzes the magnetic circuit characteristics of the filter. Then a three-dimensional core structure is proposed to apply to this novel filter. Theoretical analysis and calculation comparison shows the sectional area of the yoke is 1/√3 that of the leg in the new filter which has the advantage of reducing the iron loss and the no-load loss. At the same time completely symmetrical three-phase magnetic circuit also helps to improve the zero-sequence current filtering effect.In order to analyze the effect of the filter, a 10kVA test prototype is made. In this paper, the detailed design process is given. The leakage magnetic distribution of the zero sequence filter is analyzed based on the theory of leakage flux linkage and deduced the calculation method of the leakage reactance. The magnetic circuit is analyzed using the ANSYS software which has the powerful electromagnetic computing ability. Then the main parameters are calculated. Finally, the experiment measuring circuit is designed. The test results show that the zero-sequence filter can well play a role in the filtering of zero sequence current, harmonic current neutral line fell by more than 90%.