Research On Compensation Equipment Of Three Phase Four Wire Distribution Grid With Unbalanced Load

With the rapid growth of newfashioned power load, requirements for power quality are increasingly more critical. Meanwhile, nonlinear and unbalanced loads are still increasing in the low voltage distribution network, which has growing influence on grid power quality. Thus it is of great significance to implement fast compensation device on the distribution system to improve power quality of the grid. For three-phase unbalanced load management, the traditional way is through the early stage of reasonable planning, to averagely distribute unbalanced load to three phase. The second method is to install different kinds of passive and active compensation device. Based on this background, this article focuses on the research of unbalanced load current compensation of three-phase-four-wire system in low voltage distribution grid technology.Firstly, issues of three phase load imbalance in low-voltage distribution grid and its conventional treatment methods are introduced and the current application status of unbalanced current compensation device and solutions are discussed. Secondly, based on symmetrical component method, the components of general three-phase current are analyzed and reference current signals for imbalance compensation are determined by calculating the fundamental negative and zero sequence current component. After an overview of the traditional current detection method, the detection method of double synchronous coordinate rotation transformation using LPF+FIR two-level filtering is proposed. The improved detection method can realize zero-error detection of the three-phase current components of positive, negative and zero sequence in fundamental frequency. Then several common compensating topology structures are introduced, and the four-leg VSC with four-branch LCL connecting interface is finally adopted, followed by the modeling and analysis of this topology. For the interface coupling phenomenom caused by unbalanced components, a decoupling analysis of zero and non-zero sequence equivalent circuits is given, followed by the designing method of LCL filtering circuit parameters.The study of controlling strategy of the compensation device is a core issue of this paper. At the beginning of this chapter, mathematical modeling of the proposed four-leg VSC with four-branch LCL connecting interface in three different coordinates is conducted. By comparison, the ??0 stationany coordinate is taken to be optimal. To take a universal control of the four-leg VSC, modulation algorithm of 3D-SVPWM is adopted so that Phase N does not need to be controlled independently, which improves the utilization rate of DC side voltage and output waveform. Since the reference signals under ??0 stationany coordinate are sinusoidal, the proportional resonant regulators are adopted in place of PI regulators. A modified proportional quasi-resonant regulator is proposed, which performs better ability to extract reference signals of fundamental frequency from hybrid frequency components. Simulation results have verified the validity and rationality of the proposed control stratery.Finally, based on theoretical research and simulation verification, an experimental prototype is set up to execute compensation experiments with different unbalance loads. It is proved that compensation equipment using 3P4L-VSC with 4-B LCL connecting interface demonstrates sufficient feasibility and superiority in unbalanced load current compensation of Three-Phase-Four-Wire low voltage distribution power grid.