Control Strategy Research And Equipment Development Of High Voltage Cascaded Static Var Generator

In recent decades, with the continuous development of power electronic technology, a cascade multilevel power quality compensator based on the H-bridge structure has become the research focus in high voltage reactive power compensator which has strong modular design, easy extension and excellent output performance. In this paper, cascade-connected SVG based on the H-bridge structure is treated as the research object. The main work of this paper are as follows:First, the background, aim and significance of reactive power compensation in power system is presented. A comparative analysis of several common passive compensation devices and active compensation devices is discussed for many types of high-voltage reactive power compensation devices. The conclusion show that the cascade-connected SVG based on H-bridge topology is promising development in the distribution network field of 35kV and below voltage classes.Second, this paper explains the basic principle of cascade-connected SVG. Mathematical model of it is successively established in the three-phase stationary coordinate system and the two-phase orthogonal stationary coordinate system. The unipolar dual-frequency CPS-SPWM is selected as modulation strategy in cascade-connected SVG after analyzing and simulating bipolar CPS-SPWM and unipolar dual-frequency CPS-SPWM, which can get a higher equivalent switching frequency and output voltage waveform closer to a sine wave.Third, an overall control strategy is proposed for the three-phase star SVG based on its main circuit topology. And then the principle and realization method of cascade-connected SVG to common connection point voltage control are analyzed. Associate PR control strategy in two orthogonal αβ stationary coordinates is selected as the inner current tracking control without current component coupling. What is more, the unbalance of sub-module capacitor voltage is analyzed to design a hierarchical voltage balance control. This balance control adopted traditional PI control in the upper and superimposed on an active voltage vector on the modulation signal of inner current control output to achieve sub-module voltage balance control in the lower. Finally the simulation results show the effectiveness and feasibility of the control method.Fourth, the reasons and influencing factors of DC side voltage fluctuation of cascade-connected SVG sub-module are analyzed, and the source of the two harmonic wave of DC side voltage is pointed out. Because of the existence of the two harmonic wave, the sampling circuit of the DC side voltage of the control circuit is sure to introduce the filter. Second order low-pass filter and second order band-stop filter are used as DC side voltage sampling feedback link. By using the Routh Hurwitz stability criterion under the continuous system, and combined with the zero pole distribution diagram and the root locus diagram of the voltage closed-loop system, the conditions of voltage stability when using different filters are analyzed. The effectiveness and correctness of the stability conditions are verified by simulation analysis.Five, design a three-phase star SVG that has a rated voltage of lOkV, capacity of ± 2Mvar. Hardware design of main circuit, control circuit of the device and control system software design are included in it. Through the hardware debugging of the main circuit and the control circuit, the basic debugging of the single-phase cascade-connected SVG device is finished. A description is made of the low voltage and high voltage single-phase cascade-connected SVG, which has been completed, which shows the correctness of the design of the hardware circuit in this chapter.In this paper, based on the research of the power quality compensator of the medium and high voltage distribution network, the cascade-connected SVG of the H bridge topology is designed, and the overall control strategy of the three-phase star SVG is proposed. What is more, the voltage stability of the sub-module is analyzed. The hardware platform and simulation platform of cascade-connected SVG main circuit and control circuit are built, and the control strategy is verified.