Research On The Failure Analysis And Protection Strategy Of Grid Connected Large-scale Wind Farm

In recent years，large-scale wind turbine offline accidents occurred frequently，thisphenomenon caused much attention on the security and stability of the Grid-connectedwind farm from the Government and research institutions。The accident investigationresults show that the main reason for large-scale wind turbine offline was its weeksurvivability during the time of fault exists and voltage recovery。In addition to this, it isalso very difficult for protection strategies and programs of wind farm at the present stageto cope with the challenge from the Grid-connected large-scale wind farms。Currently, in connection with the phenomenon of large-scale wind turbine offline, Inorder to maximize ability to realize fault ride-through, we not only to improve its transientcharacteristics from the view of its control strategy, but also to coordinate protection andcontrol strategies of grid with the low voltage ride through characteristics (LVRT)of windturbine。Therefore，this paper firstly introduced the unit structure、mathematical model andshort-circuit current calculation of the asynchronous wind generator and doubly-fed windturbine, then simulated and analysised the transient characteristics and short-circuit currentcharacteristics of wind turbine under different fault type by using PSCAD/EMTDCsimulation software。Simulation results show that there will be continued short circuitcurrent output during the fault period for wind turbine with LVRT capability, whichmakes great influence on the grid protection selectivity and sensitivity。Moreover, becauseof its own control strategy to limit fault current and regulate voltage, the transientcharacteristics and LVRT characteristics of the double-fed wind turbine is superior to theinduction wind turbine。Secondly, because the action delay of current protection for wind farm collector linesis too long, Adaptive inverse time low-voltage direction protection strategy was proposedin this paper for wind farm collector lines。Its start criterion is low voltage, action delay isthe inverse time based on bus voltage of collector lines and current in fault line,Protectionvalue is set by Electrical quantities under the condition of three-phase short circuit faultwith the system under maximum operating mode. The protection strategy not only tocoordinate with fuse, but also to reduce the requirements for the LVRT capability of windturbine on non-fault lines, it can remove fault fast, accurately and effectively. Inconnection with the grid-connected characteristics of offshore wind farms based onVSC-HVDC, distance protection strategy for wind farms DC transmission line wasproposed in this paper, the action criterion is fault distance calculated by the measurementvalue of voltage and current at protection installation, action delay between the protection is set by the overcurrent characteristics of anti-biased diode in VSI. this method not onlyto ensure the security of power electronic devices during fault period, but also to realizethe reliability and selectivity for metal faults and avoid the drawbacks of wind turbinestotally removal in the DC cable fault.Finally, in order to verify the reliability and correctness of the protection methods, amodel of a typical wind farm structure was built based on PSCAD/EMTDC simulationsoftware in this paper, and took simulation experiment in each type of fault, theverification results show that the adaptive inverse time low-voltage protection can achieveselective, liquid and reliability in many kinds of fault conditions, but, distance protectionstrategies for wind farms DC transmission, such as theoretical analysis, is sensitivity andhigh sensitivity under metal fault condition, but grounding resistance makes greatinfluence on it.