Control Strategy Of The Cascaded H-bridge SVG And Its Link Unit Test System

ABSTRACT:With the development of power electronic technology, power electronic switching devices are widely used in industry field. A large number of nonlinear loads emerge in power grid, and the demand to reactive power is becoming more and more. Static var generator (SVG) as a new generation of reactive power compensation device with fast response speed, low loss, small energy storage device and low harmonic content of output current, can effectively solve this problem. In recent years, reactive power compensation device is in the direction of integrated function high voltage and large capacity. Adding a variety of work modes can also make SVG access a variety of functions. Cascade is an important method to design the reactive power compensation device of high voltage and large capacity.Based on the project of "10kv/7.2Mvar cascaded H-bridge SVG" of CEPR, the control strategies of cascaded H-bridge in SVG,optimization of the link unit test in the process of production are studied in this thesis.The control strategy of cascaded H-bridge, mainly includes the current work mode (realizes reactive power compensation) and voltage working mode (realizes prevention of voltage drop). The important points in the current work mode are decoupling control of reactive current and active current, three-level balance control of DC capacitor voltage.Common cascaded H-bridge SVG only has the function of reactive power compensation. But the voltage sags problem also exists in the actual operation process. If we design another device to prevent voltage drop, the cost will increase. The cascaded H-bridge SVG in this thesis adds a voltage mode, which makes the SVG possesses two functions, reactive power compensation and voltage drop prevention..In common SVG mathematical model, active current and reactive current interact with each other, which increases the difficulty of control. Aiming to solve this problem, the active current and reactive current are decoupled in this thesis, and the control strategy is simplified.In Cascaded H-bridge SVG, DC capacitors are independent with each other. This simplifies the main circuit, which leaves out the phase-shifting transformer and rectifier circuit, but also has brought balance control problem of the DC capacitor voltage. In order to solve this problem, a method of three-level balance control of the DC capacitor voltage is proposed. This method can make the DC capacitor voltage reach expectation.Based on the MATLAB/SIMULINK simulation model and experimental platform, the simulation and experimental validation are carried on in this thesis. The results show that the system has a good compensation effect and dynamic response.Finally, in view of complex flows of test experiment in the production of SVG link unit, a test system is devoloped, which can simplify experiment operation and improve the work efficiency.