Research On Compensation Characteristics Of Cascaded Static Var Generator

With increasingly complex grid structure of power systems, Static Var Generator SVG is an important reactive power compensation device which plays a role in maintaining the safe and stable operation of the power system. SVG of cascaded H-bridge structure has advantages of using no transformer, lower loss, lesser occupied area. And harmonic characteristics is improved by using PWM modulation technique. The advantage of using the main circuit of modular H-bridge is that we can get the output level of the same number by less needed switching devices. Cascade SVG,as a representative and an ideal solution, has attracted wide attention and study. Aiming Single-phase modeling and control methods for cascade H-bridge multi-level SVG, and the command current and control methods for compensating unbalanced load for three-phase SVG connects in two connection are studied.In order to maintain stable operation of cascade SVG and accurate compensation of reactive power, firstly, the mathematical model of single-phase cascade SVG is built, and the control strategy of single-phase chain SVG are studied. In this paper, the idea is hierarchical control, that is, under the premise of achieving a stable DC voltage control and current tracking control, to acquire the DC voltage balance control. Upper control using DC voltage PI regulator and current deadbeat control to generate a modulated signal. Lower control is designed to ensure the inverter output voltage to meet the prerequisite for upper control, fine-tune modulated signal power of each unit to guarantee a reasonable allocation of power input active power, thus securing DC voltage balance between all power units. In this paper, the control strategy has been verified by simulation.Secondly,in this paper,compensation for unbalanced load conditions are studied and compared in two connecting structure of SVG. The focus of unbalanced load compensation is to keep active power balance of the DC capacitor, which is a prerequisite for the normal operation of the grid converter. Core work is to find the current command and voltage directive in line with control targets. In this paper, the zero-sequence circulating current method for angular Cascade SVG and zero-sequence voltage injection method for star Cascade SVG are conducted in-depth research and analysis under the same power grid conditions for compensating the same unbalanced load. In order to compare the structure and compensation characteristics of both systems,the simulation results are analysis and compared. Finally, the paper describes the design cascade SVG device, the experimental results are analyzed, and some key technologies are summarized. Experimental results show that the system can achieve high equivalent switching frequency at a lower switching frequency, the system has output harmonic characteristics, and a good ability to regulate reactive power.Aiming at the reactive power compensation devices applied in the high voltage occasion of distribution grid as well as unbalanced load occasions, the topology and control strategy of cascade SVG, the command current and control methods compensating load unbalance when three-phase SVG connects in two way are studied in this paper. Multi-level and high-voltage chain SVG device is developed. Combined with the application, a specific program of some key technologies are provided. Contents of this paper can provide reference basis for the development of high-voltage SVG.