Research On The High-voltage Direct Current Transmission Control Strategy And The Reactive Power Compensation Technology

In the present world, with the rapid development of power electronicdevices, VSC-HVDC using IGBT as its converter is widely discussed andstudied by academia and industry.Because it overcomes the shortcomings ofLCC-HVDC and controls the active and reactive power independently. Amongseveral transmission topologies of VSC-HVDC, MMC-HVDC has a betterprospect due to its advantages including low switching frequency, low loss,strong scalability, working without alternating current filtering devices and so on.This paper makes a systematic research of the operating principle and controlmode in different situations of LCC-HVDC, VSC-HVDC and MMC-HVDC,and the main contents are follows:The structure and working principle of LCC-HVDC is analyzed, itsmathematical model is built and the control strategies including rectifier sideusing current control and inverter side using direct current voltage control orconstant extinction angle control are studied. Meanwhile the study simulatemodel in the MATLAB platform is finished, the results show that the inverterside using constant extinction angle control compared with constant voltagecontrol has high power factor and less consumption of reactive power, but the stability of direct current voltage is weak when the whole system work stably.The running mechanism of VSC-HVDC is studied and the mathematicalmodel is built. This paper builds the VSC control system and makes simulationin MATLAB including inner current control and outer loop power control. Theresults shows that the VSC controller built in this article realizes the independentcontrol of active and reactive power, and has fast response and high stablility.The structure and working principle of MMC-HVDC are analyzed, thesystem mathematical model is built. This paper builds the simulation models andcontrol system of MMC-HVDC in MATLAB. The simulation results shows thatthe controller in this article can restore stability quickly, and has a good controleffect, and realizes the independent control of active and reactive power aftersystem power flow reversing.The new control strategies based on symmetrical component analysis areproposed relying on the mathematical model of VSC under the unbalancedthree-phase system and simulated in MATLAB platform, in order to suppress thenegative sequence current and active power frequency-doubled fluctuationcomponent generated by VSC-HVDC when the system is unbalance.Thesimulation results shows that the control model built in this paper can effectivelysuppress the negative sequence current and active power frequency-doubledfluctuation component according to the requirement of the system and assure theVSC-HVDC system can still run stably when unbalanced fault occurs. Then thispaper sets up a three-terminal hybrid transmission model made up by LCC-HVDC、VSC-HVDC and MMC-HVDC, and studies the correspondingcoordinate control strategy so that the system can also stably and safely runwhen the system fault occurs in alternating current side. The system controlstrategy and fault simulat are finished in MATLAB platform, the result showsthat the control strategy proposed in article is correct and effective, and it canrun stably and reliably according to the specified operating mode when thesystem fault occurs.Three kinds of compensator for HVDC including shunt capacitor (FC),static compensation device (SVC) and static var compensation generator(STATCOM) are studied deeply, and corresponding simulation model andcontrol system are built in MATLAB platform. The simulation results follow:SVC and STATCOM compared to FC can achieve continuous smoothadjustment from capacitive reactive to inductive reactive, play the role ofdynamic reactive power support; In term of response time, STATCOM is betterthan SVC and FC, which can track the reactive power changing faster and better,so STATCOM is better than SVC and FC in the effect of reactive compensation.