The Research On Key Technologies Of Three-phase Four-wire Active Power Filter

With the rapid development of power electronic technology, more and more power electronic devices are widely applied in various fields. The nonlinear, impact and unbalanced electrical characteristics of the load, makes the transient impact, reactive power, harmonic and three-phase unbalanced problem in power grid become more and more serious, causing a serious impact on the quality of power supply. Therefore, eliminating the harmonic pollution had become an important task of quality of electric energy resarech. Active power filter(APF) which compensates the harmonic and reactive power, improves power quality, enhances the reliability and stability of power grid, is an effective device to solve power quality problem, and its good filtering performance has aroused extensive attention. In this paper, the key technologies on current harmonic detection, modulation algorithm in compensating current control, DC-link voltage control strategy design of APF is studied, and it has important theoretical and practical significance for improving APF performance.In order to fill the lack of traditional harmonic detection algorithm instantaneous reactive power theory and FBD algorithm, based on analyzing the traditional instantaneous reactive power theory and FBD algorithm, an improved harmonic detection method combining these two methods, was proposed. The proposed method reduces the computation caused by coordinate transformation in traditional instantaneous reactive power method, and improves real-time and dynamic performance. It also eliminates the unnecessary separation of the zero-sequence current, reducing the complexity of the algorithm. The proposed method compensates zero-sequence current separately, making the balance control of upper and lower capacitor voltage directly, to ensure the applicability in three-phase four-wire active power filter. For reducing the hysteresis error generated by the low-pass filter during the sampling and calculation and improving the harmonic detection accuracy, the low-pass filter was replaced by an improved moving average algorithm. The proposed method has theoretical and practical significance for improving the performance of harmonic detection and the compensation effect in active power filter.Based on the detailed analysis and deduction on the procedures of conventional 3D-SVPWM algorithm, some steps mentioned in conventional algorithm are proved to be unnecessary, and a novel 3D-SVPWM algorithm is proposed in this paper. By removing these unnecessary steps, the algorithm becomes extremely simplified and easily implemented. By applying the proposed algorithm, the complexity of modulation algorithm and the computational load can be drastically reduced, and the applicability and stability of modulation algorithm can be improved. In addition, it will provide more space for other control algorithm in system. On this basis, further application and improvement in the other three-phase four wire electric equipment can be explored, and it has the significance in theory and practical application.The mathematical relationship between the DC-link voltage and the system parameters for APF is analyzed. Based on the analysis, the required minimum DC-link voltage for APF is obtained by mathematical deduction. Then, a novel adaptive DC-link voltage controller for the three-phase four-wire shunt APF is proposed, in which the DC-link voltage reference value will be maintained in the required minimum voltage level. Therefore, the power consumption and switching loss will be reduced effectively. Furthermore, the DC-link voltage can be adaptively yielded different DC-link voltage levels according to different harmonic current and grid voltage levels, avoiding the harmonic current and grid voltage fluctuation impact on the compensation performance.The inherent influence of DC-link voltage controller on both DC-link voltage control and compensation performance of three-phase four-wire APF is investigated, and a variable parameter DC-link voltage controller is proposed. Compared with the conventional fixed parameter controller, the proportional and integral parameter in proposed adaptive controller will be varied according to the difference between the actual and the reference DC-link voltage, and the both optimal DC-link voltage control performance and compensation performance can be achieved. The design procedures of nonlinear PI algorithm for the adaptive DC-link voltage controller including stability and dynamic performance are considered and analyzed, so that the controller parameter can be designed accordingly.In summary, the key technologies of harmonic current detection, current control strategy and DC-link voltage control in three-phase four-wire shunt APF are studied in this paper. Representative simulation and experimental results in three-phase four-wire shunt APF prototype are presented to verify the analysis and also the feasibility and effectiveness of the proposed algorithm and controller. It has important theoretical and practical significance to improve the APF performance.