| Power system failure,large load mutation or large quantity of reactive power would result in power quality issues including voltage imbalance,voltage sags,as well as voltage deviation.In order to ensure the quality of power supply,the series and shunt power quality controllers based on voltage source converter(VSC)have achieved rapid development.Shunt power quality controller such as static synchronous compensator(STATCOM),can dynamically compensate reactive power and online adjust the distribution bus voltage;and series power quality controller such as dynamic voltage restorer(DVR),can solve the problems of voltage sags,swells and other voltage quality disturbances.The modeling,the design of main circuit component parameters and the control strategy are thoroughly analyzed and studied in this thesis,and its results could provide theoretical support and technical reference for the design of voltage quality controller.It is well-known that distribution bus voltage suffers from negative sequence or harmonics.These non-fundamental positive sequence components could degrade the performance of STATCDM,and cause resonation when its parameters are wrongly selected.In this thesis,the resonance mechanism of STATCOM,under the sine pulse width modulation,is analyzed,and its mathematical model is built and the resonance condition is pointed out.Additionally,it has been found that there is a group of LC parameters which could eliminate the negative component or harmonics in compensation current,and an optimized design method targeting to minimize the dc voltage fluctuation or non-fundamental positive sequence components in compensation current is presented.Using the conventional switching function model,it is inevitable that negative or harmonics currents are injected into the power grid when STATCOM is adopted to compensate the unbalanced voltage or current.In this thesis,the relation between dc voltage fluctuation and the degree of unbalanced current or voltage is analyzed,and the rules of harmonic current spectrum are revealed.Based on above results,an improved VSC switching function for voltage imhalancecontrol strategy of STATCCOM is proposed.which could deduce the degree of voltage unbalance and improve the quality of the compensation current.A new topology of STATCOM,whose filter reactor is in series with capacitor,has the characteristic of low dc voltage.Compared with traditional STATCOM,it could reduce the cost of device under some voltage class.In this thesis,the working principle of the new topology of STATCOM and the regulation of VSC are analyzed,and the design method of capacitor is also proposed.Based on above results,the reverse phase modulation method of switching function and the strategy of feedforward decoupling voltage control are presented.Finally,the switching losses,the startup performance and current total harmonic distortion(THD)of the new topology of STATCOM are much better than the traditional.Capacitor coupled DVR is a kind of power quality controller for resolving the voltage quality problem.However,the parameter design of coupled capacitor lacks theoretical basis.By analyzing its working principles,the parameter design method of coupled capacitor and filter reactor,based on the equivalent circuit and vector diagram,are presented in accordance with the minimum output current principle of VSC.At the same time,the advantages and the disadvantages of several typical injection ways of series compensation voltage are carefully analyzed.In addition,a double loop mutual compensated strategy to compensate load voltage is designed.The proposed method and the control strategy can not only meet the needs of the different amplitude of voltage sag compensation,but also able to adapt the requirements of a wide range of load current changes.Based on above results,simulation models of STATCOM and DVR are established,and the prototypes are also developed.Simulation and experimental results show that the main circuit parameter design methods and the control strategies are feasible. |
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