Research On Suppression Strategy Of Continuous Commutation Failure In Hybrid Multi-Infeed HVDC System

Line commutated converters based high voltage direct current(LCC-HVDC)technology has the advantages of large transmission power,long transmission distance and low line loss,etc.It is the key to realize the optimal allocation of resources and solve the reverse distribution of energy center and consumption in China.However,the failure of the receiving end of the grid is prone to cause commutation failure,which restricts the development of DC transmission technology.Voltage source converter based high voltage direct current(VSC-HVDC)technology does not suffer from commutation failure and can achieve fast and independent control of active and reactive power,but there are disadvantages such as high construction cost and small transmission capacity.In order to give full play to the advantages of both technologies,combining them to form hybrid multi-infeed HVDC system(HMIDC)has become a hot research topic.In fact,HMIDC system consisting of LCC-HVDC and VSC-HVDC have been formed in some regions of China.However,with the increase of DC transmission projects,the phenomenon of "strong DC and weak AC" in AC/DC hybrid power grid will become more prominent,which brings new challenges to the safe and stable operation of the power grid in the future.Therefore,how to combine the advantages of the two transmission technologies,make full use of the unique advantages of VSCHVDC to enhance the reliability of LCC-HVDC operation and prevent the occurrence of continuous commutation failure of LCC-HVDC,to ensure the safe and stable operation of the HMIDC system is of great significance.The main research content of this paper is as follows:1)The working principle and control strategy of HMIDC system are studied.The basic structure of HMIDC system is summarized;The operation principle of LCC-HVDC is analyzed,and the structure and control strategy of LCC-HVDC control system are introduced;The working principle and control method of VSC-HVDC are analyzed,and the mathematical model of internal and external loop control strategy in VSC converter station level control is obtained,which provides the basis for the subsequent HMIDC system continuous commutation failure suppression strategy.2)A fast reactive power control strategy of VSC considering reactive capacity constraint is proposed.The mechanism of commutation failure of LCC-HVDC and the main factors affecting commutation failure are analyzed;On the basis of considering the operating characteristics of VSC converter,the mathematical relationship between VSC reactive power output limit and active power and AC bus voltage is obtained;The mapping relationship between LCC DC current and VSC reactive power output is established,and a VSC fast reactive power control strategy considering reactive capacity constraint is designed.The corresponding simulation model is built in PSCAD/EMTDC,and the effectiveness of the proposed method in this chapter is verified by comparative analysis.3)A collaborative control strategy to suppress the continuous commutation failure of HMIDC system is proposed.The working principle and inherent defects of the voltage dependent current order limiter(VDCOL)in the LCC-HVDC control system are analyzed;A dynamic nonlinear VDCOL controller is designed by using variable slope(tanhx)function,which can flexibly adjust the slope of VDCOL curve to limit the growth rate of DC current;Combined with the control strategy proposed in Chapter 3,a collaborative control strategy for suppressing continuous commutation failure of HMIDC system is constructed.A simulation model is built in PSCAD/EMTDC for comparison and analysis,and the results show that the proposed method improves the recovery characteristics of the HMIDC system while improving the ability to resist continuous commutation failure.