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This chapter describes the "active filters for power conditioning" or simply uses the term "active filters" as long as no confusion occurs. Much research has been performed on active filters for power conditioning and their practical applications since their basic principles of compensation were proposed around 1970. In order to distinguish active filters in power processing from active filters in signal processing, the term "active power filters" often appears in many technical papers or literature. Active filters are divided into alternating current and Direct Current (DC) filters. The DC terminals of the diode rectifier and the active filter form a common DC bus equipped with an electrolytic capacitor. The active filter is controlled in such a way as to present zero impedance for the fundamental frequency and to act as a resistor with high resistance of K for harmonic frequencies. A hybrid active filter for harmonic damping has been installed at the Yamanashi test line for high-speed magnetically-levitated trains.
The instantaneous active and reactive power theory or the p-q Theory is widely used to design controllers for active filters. This paper deals with some problems due to the misinterpretations of this theory. A historical background of the theory is presented and the problem caused by distorted voltages at the point of common connection (PCC) is analyzed. In addition, the appearance of source current harmonic component not present in the load current (hidden current) caused by different filtering characteristics for the calculation of the oscillating real and imaginary power components is discussed. The problem caused by the voltage distortion can be solved using a phase locked loop (PLL) circuit. For the hidden current, filters with similar characteristics can avoid them. These analysis and solutions are presented to clarify some aspects of the p-q Theory not clear in the original approach of the theory.
