Study On Control Strategy Of Static Var Generator

With the increasing size of power system,various power consumption situations are becoming more and more complex.Especially with the access of large-capacity impact asymmetric loads and the power electronic devices,three-phase unbalance,reactive power and harmonic problems have become the hot issues.These problems not only seriously reduce the power quality,but also threatens the safe and stable operation of power grid seriously.In many static var compensation devices composed of power electronic devices,SVG has the advantages of fast response,flexible control,high compensation accuracy and low harmonic content of compensation current.It can realize continuous dynamic,smooth and fast reactive power compensation and become an important part of modern flexible AC transmission system.In this paper,three-phase three-wire system SVG is studied through principle explanation,model establishment,system simulation and system implementation.Firstly,the necessity of reactive power compensation,improvement of three-phase unbalance and harmonic suppression,and the development status of reactive power compensation devices are described.Then,the working principle of SVG is elaborated in detail and its mathematical model is established.Besides,the instantaneous reactive power theory and common compensation current instruction detection methods are compared and analyzed.In order to compensate for the shortcomings of traditional PLL,this paper uses an improved second-order generalized integrator PLL and verify that the PLL can accurately track the voltage phase of power grid under non-ideal power grid conditions.Secondly,in order to realize optional and flexible compensation,under the condition of three-phase asymmetric load,the equivalent mathematical models of the system under each sequence are established through the double-sequence control strategy.On the basis of this mathem,atical model,the closed-loop control block diagram of SVG based on current double-sequence indirect control is deduced by feed forward decoupling control.Because traditional SVPWM uses a definite combination of switches to control the power switches,so some high-order harmonics with large amplitude will be generated in the output current and voltage,and these harmonics mainly concentrate near the carrier frequency and its integer multiple frequency.In order to further improve the harmonic characteristics of the output current of SVG and reduce the THD of the grid current,the working principle of random SVPWM is analyzed in detail in this paper.The control strategy of combining the idea of double random SVPWM with the idea of double sequence synchronization control is adopted and the simulation model is built by MATLAB/SIMULINK.The simulation results prove the effectiveness and feasibility of the control strategy in theory.Finally,a three-phase three-wire SVG prototype is built with DSP chip TMS320F28335 as the core.Reactive power and negative sequence current compensation experiments are carried out under two sets of three-phase asymmetric inductive load.The experimental results verify the effectiveness of the adopted control algorithm.