Research On Additional Reactive Power Control Of Hybrid Multi-Infeed HVDC System

Flexible HVDC technology has the advantages of self-commutation,independent regulation of active and reactive power,and power supply to passive system.It can make up for the shortcomings of traditional HVDC and complement each other's advantages,thus becoming an important research direction of HVDC technology.Therefore,with the development of power grid,the traditional DC and flexible DC are fed into the same AC system or the AC system with relatively close electrical distance,forming a hybrid multi-Infeed Direct Current(HMIDC).For HMIDC,because VSC-HVDC has the ability of reactive power control and its inverting station is close to LCC-HVDC inverting station,it can provide reactive power support for LCC-HVDC through reactive power regulation of VSC-HVDC,and stabilize the bus voltage of LCC-HVDC inverting side converter.This regulation mode of VSC-HVDC is equivalent to static var compensator(STATCOM),which avoids frequent switching of LCC-HVDC inverter side reactive power compensator.Firstly,this paper introduces the development process of HVDC,including several forms of HVDC,and briefly summarizes LCC-HVDC technology and VSC-HVDC technology.The basic principle of VSC-HVDC system,the functions of each part,the topological structure of converter and the connection mode of the system are introduced in detail.On this basis,the hybrid multi-infeed HMIDC model and its conventional control method are established.The control method of hybrid multi-infeed HMIDC system is studied and the control system model is established.The causes of commutation failure of HMIDC are analyzed,and the measures to restrain commutation failure are introduced.Secondly,from the aspect of reactive power balance optimization,VSC-HVDC additional reactive power control strategy is designed to improve the stability of HMIDC receiving system.Because of the difference of reactive power control between the two systems,two additional reactive power controllers are built without changing the existing control structure.The reactive power control strategy based on transient voltage and the transient reactive power control strategy based on gamma angle are designed.Through the simulation results,the advantages and disadvantages of the two additional control structures are compared and analyzed,which can guarantee the safe and stable operation of HMIDC.Finally,in view of the maximum limit of reactive power support in VSC-HVDC,when the voltage drop of HMIDC receiving bus is very serious,the demand for reactive power compensation exceeds the support capability of VSC-HVDC reactive power control,which requires the use of other reactive power compensation devices,such as STATCOM.By analyzing the operation principle and control strategy of STATCOM,a coordinated control strategy between STATCOM and hybrid multi-infeed HVDC transmission system is established.When the reactive power of VSC-HVDC reaches its maximum,the reactive power of the system is compensated to stabilize the voltage of the receiving AC bus.