柔直送出下光伏送端电网交流送出线路的不对称故障距离保护方案OA

The fault characteristics of large-scale photovoltaic(PV)sending-end grid under VSC-HVDC are significantly affected by the control strategy and system topology form of double-end power electronic equipment,and the adaptability of traditional relay protection is challenged.Considering the fault control strategy of the converter,the topology of the sending-end grid,the transformer wiring mode and the fault location,a fault-equivalent model of the AC transmission line of the PV sending-end grid in terms of both equipment modeling and system modeling is established.By analyzing the composite sequential network of the system,the dif-ferential current characteristics of asymmetrical faults and fault distance impedance characteristics are extracted.Based on the fault characteristics of the VSC-HVDC side,asymmetric fault distance protection schemes for PV sending-end grid are proposed.For the distance protection scheme of grounded short-circuit fault,the fault distance impedance is calculated by using the sequential component characteristics,which is not affected by the transition resistance in principle.For the distance protection scheme of the ungrounded short-circuit fault,the fault distance is calculated based on the impedance complex-plane method by utilizing the fault-phase differential-current characteristics.Based on the simulation results of PSCAD/EMTDC,the proposed schemes can accurately calculate the fault distance impedance by utilizing only the single-ended quantity information of the VSC-HVDC side,reliably distinguish between internal and external faults,and improve the ability of the distance protection to tolerate the transition resistance in the double-ended weak-feeding scenario.