An Instantaneous Reactive Current Individual-phase Detection Method and Digital Realization Based on Fourier Transform

Currently, the frequency-domain detection methods based on Fourier Transform theory are widely used for reactive current detection, but they usually need the voltage phase information by means of phase locked loop (PLL), which will increase the algorithm complexity and may cause the reactive current detection errors. Therefore, this paper introduces an instantaneous reactive current detection method based on Fourier transform, which abandons the PLL to improve the comprehensive performance of the reactive current detection. Moreover, a synchronous tracking method for system frequency and a compensation strategy for the time delays derived from signal sampling, analog-digital conversion and detection algorithms are embedded in the proposed method, for the purpose of further reduce the reactive current detection errors. In addition, a digital realization means for this method based on recursive Discrete Fourier Transform (DFT) is introduced. The simulation and experimental results show that, with this novel method, the reactive currents could be individual-phase detected from the three-phase distorted unbalanced load currents accurately and quickly.

I. Introduction

Fast and accurate reactive current detection is crucial to some power quality devices for their reactive compensations, such as active power filter (APF) [1]–[3] and static var generator (SVG) [9]–[11]. Currently, there are two classes of typical methods widely in application. The first class of methods are based on the instantaneous reactive power theory [1]–[6]. By these time domain methods, the fundamental positive sequence reactive currents can be accurately detected from arbitrary three-phase systems. However, in an unbalance system, due to the existence of negative sequence and zero sequence currents, the positive sequence reactive currents are surely not equal the load reactive currents. In general, it needs to reconstruct a three-phase symmetrical systems for each phase [5]. The other class of methods are the frequency-domain detection methods based on Fourier transform theory [7]–[11]. They can accomplish the reactive current individual-phase detection, but they usually need the voltage phase information by means of hardware or software phase locked loop (PLL), which will increase the algorithm complexity and may cause the reactive current detection errors. Moreover, when the voltage is distorted, it is essential to filter the voltage harmonic components firstly, which will not only increase the amount of calculation, but also cause the phase lag of the fundamental voltage.