Study On The Lightning Intruding Wave In ±500 KV Two-Terminal MMC-HVDC Transmission System And Its Influence On DC Circuit Breakers

Zhangjiakou-Beijing MMC-HVDC project has the highest voltage and the largest capacity in China,which is a milestone in the development of MMC-HVDC transmission system.This project has lots of new features,but also faces many challenges,for example,overhead transmission lines will be used in DC side,the probability of lightning and short-circuit faults increase sharply,the DC circuit breakers will be installed in the DC side of converter station to clear the faults.The steepness and frequency of lightning intruding wave are high,which would threaten the insulation safety of converter station equipment(especially the power electronic devices like IGBTs in hybrid DCCBs)and cause the operation failure of converter station or even the power cut accident.However,the propagation characteristics of the lightning intruding wave in DC side of the MMC-HVDC transmission system are rare.Based on Zhangjiakou-Beijing project,a ±500 kV two-terminal MMC-HVDC transmission system with hybrid DCCBs was established to study the influence of lightning intruding wave overvoltage and the lightning intruding wave performance on the DCCBs.The main works of this thesis are as follows:First,a ±500 kV two-terminal MMC-HVDC transmission system is built in PSCAD/EMTDC simulation software.The equipment model and electrical insulation in converter station are analyzed.At the same time,the branch parameters of the Hybrid DCCBs are studied in detail and the parameters of lightning current are ensured by simulation.Then,intruding wave overvoltage caused by back-flashover/shielding failure in converter station is analyzed,and the MMC-HVDC transmission system is in the monopole running state in the simulation to strictly consider the lightning overvoltage.This paper analyzes the influence of lightning location,pole line voltage,ground resistance and insulation level of transmission line on lightning overvoltage in the converter station invasion.Simulation results indicate that the ground resistance of tower should not exceed 10 ? to limit the equipment overvoltage and insulation level.Besides,the double DC pole line arrester of insulation coordination solution should be adopted when the insulator string length of DC side transmission line is 8.65 m.Finally,based on the above work,the most serious condition of the lightning invasion wave of the converter station is obtained,the impedance frequency characteristics and the influence of the lightning intruding wave on the performance of DCCBs are further studied.Meanwhile,the influence of the DC circuit breaker parameters and stray inductances on the shunt characteristics and the overvoltage distribution are discussed.A series of consistent simulation conclusions are obtained: The RCD branch can shunt the intruded lightning wave in an obvious "window period";and the RCD branch capacitance and stray inductances can suppress the impulse current level of the RCD branch.Besides,the impulse current steepness decreases and its amplitude increases with the increase of the capacitance.Therefore,RCD branch capacitance need to consider the influence of circuit breaker breaking performance,mitigate the turn-off voltage of the DCCB and the lightning impulse stresses across the RCD branch.The achievements of this paper would be able to support the reliability check of the key equipment' insulation coordination in the converter station.Besides,the regular conclusions could be the boundary conditions for insulation design and device selection for hybrid DC circuit breakers in ± 500 kV MMC-HVDC transmission systems.This paper can be read for reference in the actual project.