Research On Control Strategy Of Power Unit Cascaded Static Var Generator

The constantly expanding of power system network and improving voltage level oftransmission lines promote the device compensating reactive and negative sequence current todevelop towards high-voltage and large-capacity, which puts forward higher requirements tothe capacity of reactive power compensation equipment. Power units cascaded SVG(StaticVar Generator)with the advantages that power units are mutually independent and easy tomodularization, without accessing to multiple transformers and requiring fewer switchingdevices in the same output level has become the mainly developing direction of high-voltageand large-capacity compensation devices. Therefore, it is important and meaningful to studythe control strategy of power units cascaded SVG.The working principle and topology of power units cascaded SVG are analyzed, andpower balance equation of cascaded SVG under general circumstances is deduced, whichobtains the sufficient and necessary conditions for realizing power balance between phasesand capacitor voltage balance of cascaded SVG. That is controlling active power of cascadedSVG per phase absorbed equal to zero. By analyzing the power balance equation, the methodof ensuring cascaded SVG power balance under general circumstances is proposed byintroducing a freedom of zero sequence voltage component which the cascaded SVG outputs.The current feed-forward decoupling control strategy and overall capacitor voltagecontrol method of cascaded SVG are designed. Using unipolar dual frequency modulation,the output waveform of the SVG single power unit pulses two times in one switching period,doubling the equivalent switching frequency. Combined with carrier phase shift modulationmethod to generate cascaded SVG power units trigger pulse, the cascaded SVG reaches tocoordinate and control the power units, and harmonic content of total output voltagewaveform is significantly reduced.The active power mathematical model of cascaded SVG exchanging with the grid isderived. By analyzing the impact of carrier phase shift modulation to capacitors voltage of thepower units within each phase, the reason of capacitors voltage unbalance within eachphase of cascaded SVG caused by pulse delay in the process of modulation is determined andthe same conclusion can be drown from the mathematical expression of energy absorption.The method to dynamically adjust power units active power within each phase is proposed,which is superimposed an active voltage in the same direction of cascaded SVG outputcurrent based on current feed-forward decoupling control strategy modulation signals,ensuring the balance of capacitors voltage within each phase.By analyzing the imbalance principle for capacitor voltage between phases and operationsituation of cascaded SVG under unbalanced condition, the method of satisfying capacitor voltage balancing between phases using cascaded SVG output fundamental zero sequencevoltage component is proposed, attached with zero sequence voltage solving process andevaluate expressions. The cascaded SVG control strategy under unbalanced condition isdesigned, which meets power units capacitor voltage balancing between phases and withinphases, and comprehensive compensation ability to reactive and negative sequence currents.The effectiveness and feasibility of designed control strategies is demonstrated by simulationanalysis.