Research On Control Strategy Based On Power Unit Of Parallel Static Var Generator

As single-phase traction load in the electrified railway, electric locomotives can beequivalent to phased rectifier load, and large amounts of reactive power and harmoniccomponents which are produced by running electric locomotives pollute the grid, resulting inlower power quality of power system. Therefore, it is necessary to solve the problem byresearching compensation device. Static var generator (SVG) equipment as an importantcompensation device is becoming a hot spot of research. Because of high voltage level andlarge compensation capacity in the electrified railway traction power supply system, a singleSVG compensation device is difficult to meet the requirements. The thesis mainly studies thecompensation strategy for multiple modules of parallel SVG.In the thesis, three commonly used single-phase current detection methods are studied,including the single-phase instantaneous reactive power method, phase demodulation methodas well as synchronous detection method. The test results of three methods are analyzed indetail by modeling and simulation, and the characteristics of the three detection methods aresummarized.In the thesis, the control strategy of DC voltage and AC current is studied. Thecommonly-used control methods of DC voltage include PI method and active power balancemethod, which are introduced in detail. AC current control methods mainly includeinstantaneous value comparison method and triangle wave comparison method. Instantaneousvalue comparison method is mainly introduced by two states hysteresis control and threestates hysteresis control principle. And the control effect is compared between the twomethods through simulation, proving that the maximum switching frequency of threehysteresis current control is only about a quarter of the two state hysteresis current control,and the pulse is well-distributed. Triangle wave comparison method focuses on PI controlmode and proportion resonance (PR) control mode, and the PI control mode is analyzedthrough the simulation.The thesis studies the principle of the back-to-back structure of a single SVG powermodule, introducing the exchange of active power, reactive power compensation andharmonic suppression principle. A single SVG power module is simulated by adopting twokinds of control method. The first method is that current detection method adopts single-phasesynchronous current detection method, and voltage active power balance method is used tothe DC side, and three states hysteresis current control. the other method is that currentdetection adopts balance compensation method, and PI control method is used to the DC sidevoltage, moreover, AC side current is controlled by the triangle wave comparison of PRcontroller. The thesis analyzes the principle of SVG power modules in parallel, and sets simulationwith two methods which are used to the single SVG power module to test, and it proves thefeasibility of two methods. Due to the problem of supply current harmonic content in SVGpower modules in parallel, optimizing control strategy of power modules in parallel SVG isput forward, which uses carrier phase shift technology. Building a simulation model, andcomparing with non-optimized control strategy, the simulation shows that the optimal controlstrategy reduces the high harmonic content of the supply current, and proves the superiorityand feasibility of the method.