Research On Reactive Power Compensator Using AC Chopper

The reactive power compensation is very important to the safe operation of modern power system. High power induction motors, transformers and power-electronic loads consume a large amount of reactive power. They reduce the power factor of the grid and affect the efficiency of power transmission. A large number of nonlinear loads are connected to the power grid, which can inject harmonic current into the power network and seriously deteriorate the power quality. Therefore, reactive power compensation and harmonic suppression are imperative. However, the cost of the existing reactive power and harmonic compensation methods is too high or they can't meet dynamic compensation requirements of the future power networks. In this paper, the reactive power and harmonic compensation scheme based on the two-level and three-level single phase Buck-type AC chopper topology is studied. The main research work of this paper is as follows:The working principle of two-level system is studied. The small signal model of the system is derived to analyze system variables of AC chopper on the response of different disturbances. The transfer function formula is also derived. On this basis, the compensation network is designed. Static and dynamic reactive power compensation control scheme is designed, and verified by simulation. To scale the compensator to realistic grid voltage level, three-level topology of AC choppers is studied. The three-level flying-capacitor-type and cascading two level module implementation scheme are focused on, and the former one is analyzed in detail. The flying-capacitor charging balance and 2:1 ratio transformer scheme are presented to control flying capacitor's voltage. Comparative analysis results are verified by simulation.The formula of different harmonic current components is obtained by analyzing the two-level system's even harmonic modulation strategy. Meanwhile the frequency domain model of the strategy is established. The sensitivity analysis is proposed to get the relationship between current harmonics and even harmonic modulation. The reactive and harmonic current compensation strategy are designed and verified by simulation model.Finally, the experiment platforms are set up. Meanwhile the theoretical analysis and simulation analysis are verified by experimental results.