| The unified power flow controller(UPFC)is a flexible ac transmission systems(FACTS)that enables parallel compensation,series compensation,phase shifting,and terminal voltage regulation.In order to solve the problem of uneven distribution of power flow and insufficient voltage support capability in the load center of China's power grid,China is developing the world's largest 500 kV class unified power flow controller with the largest transmission capacity,the highest voltage level and based on modular multilevel converter(MMC),which represents the highest level of the world's flexible transmission technology.In order to optimize the insulation coordination of the 500 kV MMC-UPFC project and ensure the safe and stable operation of the project,it is urgent to study the electromagnetic transient over-voltage characteristics and suppression methods of the MMC-UPFC system.Compared with the traditional two-level UPFC,the topology of MMC-UPFC system is more complicated,and its control strategy and electromagnetic transient modeling are the hotspots of current domestic and foreign scholars.Since the MMC-UPFC system is divided into series and parallel sides,the control model and electromagnetic transient characteristic law established based on the existing flexible direct current transmission system(MMC-HVDC)cannot be directly extrapolated to the MMC-UPFC system.In view of the above problems,this paper first focuses on the MMC-UPFC control strategy and electromagnetic transient modeling problem,studies and analyzes the basic structure and operation principle of MMC-UPFC,and establishes the mathematical models of the parallel side and series side converters respectively,designs the double-loop control strategy of parallel converter based on PI control and series converter dual-loop control strategy based on cross-decoupling control,and the designed control system can control line flow stably.Based on this,the slow-wave-front electromagnetic transient model of the 500 kV MMC-UPFC project in Sunan was established.The simulation of the steady-state operating conditions and power step conditions of the project proved that the established control model has good steady state and dynamic performance.Secondly,based on the slow-wave front process voltage characteristics and suppression methods of the 500 kV MMC-UPFC project in Sunan,based on the established electromagnetic transient model,consider sixteen typical faults appearing on the AC side,the converter valve and the DC side of the project,the slow-wave front over-voltage characteristics and the protection level of the arrester of the key nodes of the equipment under different fault conditions are obtained,and the influence of the fault protection action on the over-voltage level is obtained.The calculation results show that the slow-wave front insulation level of main equipment of Sunan 500 kV MMC-UPFC project in southern Jiangsu can meet the requirements.Finally,aiming at the lightning overvoltage protection problem in the Sunan 500 kV MMC-UPFC system station,this paper studies and establishes the calculation model of lightning intrusion overvoltage in 500 kV AC substation and MMC-UPFC station by analyzing the propagation path of lightning intrusion wave in MMC-UPFC station.The lightning overvoltage level of MMC-UPFC equipment under three operation modes of single-line,double-line and STATCOM is calculated.The influence of the grounding resistance of the incoming line and the phase angle of power frequency voltage on the lightning overvoltage level of equipment is analyzed.The checking results show that the insulation level of equipment in Sunan 500 kV MMC-UPFC station meets the requirements of lightning overvoltage protection.The research work in this paper can provide support for the safe and stable operation of 500 kV UPFC and the design,overvoltage and insulation coordination of MMC-UPFC engineering,and has important theoretical and engineering application value for the development of flexible transmission technology. |
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