Research On Relative Techniques Of Three-Phase Four-Wire Shunt Active Power Filter

With the development of the modern power electronic technology and the application of the power electronic devices, the power electronic devices are main harmonic source in power grid, seriously threatening the safety of power grid and electrical equipment. Three-phase four-wire distribution power system has been widely used in manufacturing facilities and residential buildings, etc. to supply low-level voltage. The typical loads connected to the three-phase four-wire power system may be balanced 3-phase loads or single-phase loads. Most of these loads have a nonlinear input or unbalanced characteristic, which may cause two problems of high input current harmonics and excessive neutral current. The harmonic currents produced by balanced 3-phase non-linear loads are positive-sequence harmonics and negative-sequence harmonics. However, harmonic currents produced by single-phase non-linear loads that are connected phase to neutral in a 3-phase 4-wire system are zero-sequence harmonics. Although each device power is small, the harmonic pollution is extremely serious for its large numbers. This can result in neutral current that can reach more high magnitudes than the phase current, which may potentially damage the neutral conductor and distribution transformer while affecting the safety of the consumers. The existence of current harmonics in power systems increases losses in the lines, decreases the power factor. Therefore, the compensation of harmonic and unbalance of three-phase current in three-phase four-wire system are important significance and have become an important research field of power quality control in power system.Aiming to the problem of harmonic current suppression and neutral current cancellation in three-phase four-wire power system with four-leg active power filter (4-leg APF), the aspects are discussed and studied in this paper as follow:Firstly, the harmonic detection method for three-phase four-wire system based on instantaneous reactive power theory is discussed the principle and characteristics of different methods, including p-q method and p-q-r method, are analyzed and compared. Harmonic detection method based on instantaneous reactive power theory is a coordinate transformation method. After coordinate transforming is applied, the fundamental positive-sequence component becomes the dc component and the harmonics and negative-sequence components become the ac components. The division of the dc and ac can be obtained across a low-pass filter. The reference current signal can be achieved by the ac component through a counter-transformation. However, mains voltage may be unbalanced and/or distorted in industrial systems. Under such unbalanced and/or distorted conditions, control of the 4-leg APF using the p-q theory does not provide good performance. The phase-locked loop (PLL) can be used to eliminate harmonic detection error. The principle and structure of the conventional phase-locked loop are analyzed in the paper, a new method of harmonic detection based on the nonlinear PI controller three-phase-locked loop is proposed. The simulation and experiment results show that the proposed method has better detection precision and dynamic performance.Secondly, the IRP theory was widely applied to calculate the compensating currents assuming ideal mains voltages. However, the coordinate transformation of complex calculation, filter and phase-locked loop are needed. Therefore, a novel harmonics detecting algorithm under unbalanced and/or distorted mains voltage case in 3-phase 4-wire system based on linear adaptive notch filter without PLL and low-pass filter is proposed. The proposed harmonic detection method based on structure characteristics of three-phase four-wire system decouples three independent single-phase systems from three-phase four-wire system. The harmonic detection algorithm proposed employs a linear adaptive notch filter to extract asymmetrical fundamental component for each phase respectively. The fundamental positive sequence component in the asymmetrical fundamental component can be obtained with instantaneous symmetrical component algorithm. Because of two-dimensional linear filter is constructed by adaptive notch filter and the system parameters and the voltage supply no relationship, therefore, has good robustness and accuracy. The simulation and experiment results under balanced main voltage case, unbalanced main voltage case, balanced-distorted mains voltage case and unbalanced distorted mains voltage case show that the effectiveness and the viability of the proposed method.Thirdly, because of its very simple and easy to implement, hysteresis current control strategy is widely used in active power filter for its high precision, good stability and dynamic performance, but has several known disadvantages such as uncontrollable high switching frequency and induced important switching losses. With theoretical analysis and system simulation, the main factors affecting the switching frequency is determined. A novel fuzzy adaptive hysteresis band current control strategy is developed, where the band is modulated with the system parameters to maintain the modulation frequency nearly constant. The proposed control strategy significantly reduces the number of switching and the variation range of switching, compared with a conventional hysteresis control. Based on the conventional hysteresis control, adaptive hysteresis control and fuzzy adaptive hysteresis control simulation and experiment, the results show that switch frequency decreased by nearly 60%, the frequency range of only about 30% by the fuzzy adaptive hysteresis control mode.Fourthly, a three level four-leg asymmetry inverter structure is proposed by analyzing the structure and principle of the single-phase asymmetric three level inverter in detail. The structural features of the four-leg three-level asymmetry inverter are proposed. The SVPWM control strategy based on the abc coordinate system for the four-leg three level asymmetry inverter is proposed. The SVPWM control strategy for single phase three level asymmetry inverter is discussed and applied to active filter, harmonic suppression is realized. The reasons of capacitor voltage imbalance in DC side are analyzed. The method of control of neutral point potential is proposed and good performance of control of neutral point potential can be realized. Simulation and experiment results are presented to validate the proposed modulation and control method.At last, the three-phase four-leg shunt active power filter prototype is developed based on the TMS320F2812 DSP. The extensive simulation and experiment are carried out for validate theories and methods of the proposed harmonic detection method for 3-phase 4-wire APF, fuzzy adaptive hysteresis control strategy and The four-leg APF with asymmetry inverter. Its can be used to development and production of APF.