Control Strategy Research And Equipment Development Of Cascade-connected SVG

In recent decades, power electronics technology has made rapid development. Power quality control devices have been widely applied for low voltage distribution network with proven technique. While in the high voltage distribution network a H-bridge cascade-connected topology is becoming a hot research subject for its modularization characteristic, easy to expand, excellent output performance etc. In this paper, control strategy research and equipment development are discussed and analyzed based on the H-bridge topology cascade-connected SVG, the main work of this paper are as follows:First, the background and significance of reactive power compensation in power system is presented. A comparative analysis of several common reactive compensation devices topologies in topology characteristic and development situation is discussed for high-voltage and high-capacity applications. The conclusion show that the cascade-connected SVG based on H-bridge topology is promising development in the distribution network field of 35 kV and below voltage classes.Second, the working principle of the cascade-connected SVG is explained and mathematical model of it is successively established in the three-phase stationary coordinate system, the two-phase orthogonal stationary coordinate system and the two-phase rotating coordinate system. The different current coupling characteristics are analyzed for each coordinate system. Multi-level modulation technique is a key technique of the cascade-connected SVG research, comparing the feature both multi-carrier modulation and multi-level SVM modulation, at last choose the unipolar dual-frequency CPS-SPWM as the selected modulation technique.Third, an overall control strategy is proposed for the single-phase cascade-connected SVG which can provide a control strategy reference both in electrified railway, solar power and other renewable energy application situation specialized. The voltage stability control, the current tracking control and DC voltage balance control are respectively designed. The control strategy improves the traditional single-phase inverter output performance, obtaining a better current tracking effect, fast dynamic response and zero steady-state error. The proposed method can be directly applied in single-phase power network applications and also be used in individual phase control of a three-phase SVG with triangle configuration. Simulation analysis and experiment verified the feasibility and effectiveness.Fourth, the cascade-connected SVG mainly are applied in three-phase circuit, the cell number of three-phase SVG with star configuration is less than the triangle configuration under the same voltage level. Besides, the proposed single phase control strategy can be used in individual phase control of a three-phase SVG with triangle configuration so that we focus the research on the control strategy of three-phase SVG with star configuration. An overall control strategy for three-phase SVG is introduced in this part. The DC voltage balance between phase circuit under the asymmetry power system voltage is analyzed specially, adopting a zero-sequence voltage injection method to rebalance the phase power and expounding the detailed mathematical derivation. In the two-phase rotating coordinate system, without current component coupled, using the quasi-PR controller obtained good tracking performance. Build a three-phase SVG with star configuration simulation platform for the proposed control strategy simulation.Five, design the main circuit hardware and the DSP+FPGA dual-core control system of a three-phase cascade-connected SVG with star configuration topology. The designed SVG with rated voltage of 10 kV and capacity of ± 2MVar. After assembling and debugging the main hardware circuit, control system, accomplished the basic testing of single-phase cascade-connected SVG, the experiment of the single-phase cascade-connected SVG verified the proposed single-phase control strategy in this paper.In this paper, research have been made of the power quality control equipment especially in high-voltage and high-capacity, the control strategy of the single-phase and three-phase cascade-connected SVG are proposed respectively. Establish the simulation platform for both application situation in PSIM software and designed hardware prototype of a three-phase cascade-connected SVG with star configuration, the work of this paper may provide a reference of control strategy and prototype design for cascade multilevel converters.