In this paper, a novel transformer structure is proposed in order to manufacture a high-power active filtering device. The novel transformer structure contains one primary and multiple secondary windings. The system configuration and compensation principle are addressed in detail. The fundamental magnetic flux compensation in the case of the transformer with multiple secondary windings is fulfilled. In addition, a three-phase active power filter topology consisting of three independent transformers with multiple secondary windings is presented so that the filter can run under unbalanced loads. The parameters of hysteresis current control are also analyzed in detail. A simple and practical fundamental detecting approach is discussed. Transient response and fault protection schemes are explained. A set of automatic three-phase active power filtering devices has been constructed and operated in the field. The field test results verify the validity of the novel transformer structure and the active power filtering system.

In this paper, a novel practical principle of series hybrid active power filter based on the fundamental magnetic flux compensation (FMFC) of a series transformer is proposed. In the active power filter, the fundamental current component of power utility is detected and tracked to produce a fundamental compensation current, which is injected into the secondary winding. According to the law of superposition, when the injected fundamental current and the fundamental current component of the power utility satisfy the FMFC, the series transformer can really exhibit very low impedance to fundamental and naturally magnetizing impedance to harmonics. As a result, the series hybrid active power filter with FMFC really acts as a "harmonic isolator". The current control scheme and fundamental detecting method are analyzed. The validity of the novel principle and excellent filtering characteristics are verified by the experimental results of the two kinds of typical harmonic-producing loads.

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In the series hybrid active power filter (SHAPF) based on fundamental magnetic flux compensation (FMFC), the harmonic voltage across the magnetising inductance of the series transformer will disturb the inverter output current and worsen the filtering effect of the SHAPF. However, this influence caused by harmonic voltage was ignored in the previous research. Therefore there is blindness and casualness in the parameter selection of the SHAPF based on FMFC. Taking a 10 kV, 1 MVA industrial application of the SHAPF in a manufacture factory in Guangdong Province as an example, this study establishes the accurate global mathematical model of the SHAPF. Furthermore, based on the proposed global mathematical model, the series transformer parameters are analysed in detail to show its influence on the filtering performance of the SHAPF. This mathematical model can be used as foundations of the parameter selection of the SHAPF. Finally, an engineering prototype is manufactured and applied. The practical experimental results verify the theoretical analysis.