Research On Control System Of Cascade SVG And Its Performance Testing

The reactive power is a great threat to the safe and stable operation of power system,which not only increases the burden of power system,but also seriously affects the power quality of power system.With the improvement of people's living standard,modern industrial,commercial and residential users have higher requirements on power quality.So more reactive power compensation equipment need to be put into power system.With the large number of the equipments are put into operation,one of the biggest problems we have to face is to ensure the reliability,stability and accuracy of reactive power compensation and voltage regulation.Because the control performance of reactive power compensation equipment directly reflects the characteristics of reactive power compensation,it is necessary to investigate the control system of reactive power compensation equipment and strictly require the control process to achieve accurate,flexible and effective control result.As a new type of reactive power compensation device,cascade static var generator(SVG)is widely used for its superior performance.In this paper,cascade SVG is taken as the research object,and its control system is studied deeply and its performance testing is carried out.Firstly,this paper introduces the basic working principle of SVG and investigates the basic structure of cascade SVG and cascade H-bridge SVG mathematical models which are established in the static coordinate system and the rotating coordinate system,respectively.Secondly,the three control strategies of cascaded SVG are analyzed based on the idea of hierarchical control,such as constant bus voltage control strategy,constant reactive power control strategy and current DQ decoupling control strategy.Then,the modulation technology of cascaded SVG is studied,in which the bipolar and unipolar CPS-SPWM modulation method are analyzed and compared in detail by the method of spectrum analysis.Because the unipolar multi-frequency CPS-SPWM modulation method has better harmonic elimination characteristics and makes the wave form more close to the sine wave,the unipolar multi-frequency CPS-SPWM modulation method is employed in this paper.And then,for the problem of unbalanced DC capacitor voltage,the causes of the imbalance are analyzed.On the basis of the three-level balance control method,an improved balance control method is proposed.The simulation model of cascade SVG is established to analyze control performance of the cascade SVG and verify the validity of the control strategy based on the MATLAB/SIMULINK.Finally,combining with the field test,the performance testing of control system is carried out based on reactive power compensation device SVG in wind farm.As a conclusion,this paper puts forward a set of engineering testing methods and establishes test platform for dynamic reactive power compensation device controller.