| The voltage source converter based high voltage direct current(VSC-HVDC)transmission technology is important in promoting energy restructuring and enabling wide-range energy interconnection.The true bipolar system comprised of combination VSCs has enhanced the flexibility and reliability of VSC-HVDC transmission system and is more suitable for the high-voltage,large-capacity DC grid with overhead lines.The positive and negative converter valves can be separately controlled in the true bipolar VSC-HVDC system,which means coordination between converter stations is more flexible.Therefore,an efficient and reasonable coordinated control strategy of the VSC-HVDC grid is the key to the safe and reliable operation of the system as well as the effective and full utilization of renewable energy on a large scale.The efficient electromagnetic transient modeling method of modular multilevel converter(MMC)was studied.The method for converting all the cascaded submodules in an arm into a single time-variant capacitor was analyzed.Based on trapezoidal integration,the transient companion model of the equivalent time-variant capacitor was proposed.Then the efficient model supporting steady and transient simulation of VSCHVDC grid was established.The slicing power flow calculation method for the true bipolar VSC-HVDC grid was studied.Based on the analysis of the construction and the power distribution characteristics,the solving methods of the node admittance matrix and the power loss of a converter station were studied.Considering the various station control strategies of VSCs,the power flow calculation method applicable for the true bipolar system in symmetric mode and asymmetrical mode was proposed.The coordinated control strategy of the true bipolar VSC-HVDC grid was studied.The characteristics of the traditional droop control strategy and the control of the bipolar system were analyzed,based on which,the coordinated control structure was studied at system level.The method of operating point optimization seeking to achieve both safe and cost-effective operation of the true bipolar system was studied.The efficient modeling method of MMC,the structure of the true bipolar VSCHVDC grid and the power flow calculation method were analyzed in this paper.And considering the safety,reliability and economy of the system,the coordinated control technology of the true bipolar VSC-HVDC grid was mainly studied. |
