Research On AC Fault Ride-through At The Receiving End And DC Fault Clearing Technology For The Flexible High Voltage Direct Current System Integrating Wind Farms

With the large-scale development and utilization of wind power resources in China,wind power transmission based on the overhead line transmission method through the flexible high voltage direct current(HVDC)network outgoing solution because of its high reliability,low unit transmission cost gradually become the focus.The high failure rate of overhead lines and the low inertia and damping of the DC network can easily lead to "local failure and network shutdown".In order to ensure the uninterrupted operation of the system during AC and DC faults,two key issues of AC fault ride-through at the receiving end and DC fault clearing in the flexible HVDC system integrating wind farms are investigated in this paper.In order to ensure the uninterrupted operation of the flexible HVDC system integrating wind farms when the near-area AC short-circuit fault occurs at the receiving-end converter(Hereinafter called receiving-end AC fault),this paper proposes a fault ride-through strategy where the adaptive nearest level modulation(ANLM)strategy and the chopper resistances in wind turbines work together.After the fault occurs,the sending-end converter cooperates with the chopper resistances to quickly realize the aim of wind farm load-shedding.In order to gain enough time for the wind farm load-shedding,ANLM strategy enlarges the equivalent capacitance of the converter by decreasing input numbers of sub-module to absorb unbalanced energy of the HVDC and to limit DC overvoltage before the great reduction of the wind farm power.The unbalanced energy stored in the converter is gradually released after the wind farm completes load-shedding.The proposed strategy can reduce the cost only by using the regulation potential of the system.Finally,based on the simulation model of PSCAD/EMTDC software,the effectiveness of the proposed strategy is verified by taking the most serious receiving-end AC fault scenario as an example.Fault clearing technology based on DC circuit breaker(DCCB)is the key technology to guarantee the uninterrupted operation of the system during DC faults.The existing two-port DCCB scheme requires a large number of IGBT devices,which is costly and each line needs to be configured independently,and the cost will further increase with the development of DC network.In order to reduce its cost and to solve the problems of fast evolution of existing DCCB fault currents,large amplitude and energy consumption of lightning arrester,a fault clearing scheme based on multiport current-limiting DCCB(MP-CLDCCB)is proposed.MP-CLDCCB topology of multiple steady-state low-loss branches share a current-limiting branch and isolation branch,with fault current suppression and reliable fault line removal capability,in addition,increase the energy-consuming branch so that the non-fault side has the ability to flat wave quenching,to relieve the arrester energy consumption pressure.And a detailed comparison between the MPCLDCCB scheme and the commonly used ABB type hybrid circuit breaker(ABB-DCCB)scheme in terms of performance and economy based on the simulation model constructed by PSCAD/EMTDC.Fault clearance scheme based on MP-CLDCCB is proved to be superior in terms of performance and economy.In terms of performance,the MP-CLDCCB scheme can significantly reduce the peak fault current and shorten the time required for fault isolation.In terms of economics,the MP-CLDCCB scheme can not only significantly reduce the number of power electronics used,but also reduce the energy consumption requirements,extend the life,and reduce the investment cost of the arrester.