| At present,with the rapid development of renewable energy and full utilization of wind energy,offshore wind power is applied widely in China.Due to the advantages of long transmission distance,flexible control,continuous bidirectional independent adjustment of power,etc,the flexible DC transmission technology has been widely used in offshore wind power delivery.However,when a quantity of electronic devices are connected to AC grid,the mutual influence between control strategies of converters during faults causes the current to have a variety of characteristics,such as affected by various control strategies and harmonic enrichment.At this time,the performance of traditional AC protection will be affected.Therefore,analyzing fault characteristics of MMC-HVDC connected wind integration system,studying protection's performance,proposing a protection configuration scheme,is of great significance for ensuring safe and efficient grid-connected power generation of renewable energy.This thesis starts with the analysis of the fault characteristics of the outgoing line,and builds a model of MMC-HVDC conmected wind farm integration system,which equips with LVRT capability,realizing the coordinated control of each converter during the fault.Using the established model,faults are set on the AC side of GMMC,and the fault characteristics under different fault loacations and different fault types are analyzed from the perspective of sequence component and harmonic.Combined with the mathematical model and LVRT coordinated control strategy,the analytical expression of short-circuit current is proposed and the change of current is revealed.Comparing calculated values with simulated values,the correctness of the expression is verified.Using the analytical formula,the thesis comprehensively analyzes the performances of zero-sequence current protection,longitudinal distance protection and longitudinal differential protection under the fault of the outgoing line.For the problem that the distance relay of MMC-HVDC side of the transmission line cannot operate correctly,a correct action boundary is proposed for the occasion that multiple faults occur in the midpoint of the outgoing line through different transition resistors,which provides a basis for analyzing the performance of relays under different faults.Through simulation in PSCAD,the correctness of the adaptability analysis is verified.Combined with the performance of the protections,a protection configuration scheme suitable for system is proposed. |
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