| The increase in the capacity of high voltage direct current(HVDC)system based on modular multilevel converter(MMC,MMC-HVDC)causes the increase in fault current.which exceeds the breaking range of the circuit breaker and makes the system unsafe.It is higher requirements for relay protection.Reactance fault current limiter can effectively suppress the fault current.But the fault current suppression capability of the fault current-limiting reactance is not proportional to fault current-limiting reactance.Unreasonable configuration will cause waste of resources and reduce system economy.So it is necessary to optimize the selection of fault current-limiting reactance parameters.The fault current analytical formula is the basis for parameter design of fault protection devices such as circuit breakers.However,because of the high order of Multi-terminal MMC-HVDC(MMC-MTDC)system,there is no accurate analytical solution for the fault current,and the time domain simulation solution takes a long time.Which is not conducive to the design of fault current-limiting reactance and relay protection parameters.Therefore,the study of fault current limiting and the method of calculation is significant for MMC-MTDC system.The main contents are as follows.(1)There is no accurate analytical solution for the fault current on the DC side of MMC-MTDC system,which makes it impossible to determine the affected of various lines’ current by fault.In order to solve this problem,a method to calculate the fault current by the curve fitting of approximations is proposed.Approximate analysis is used to qualitatively and quantitatively analyze the impact of the fault,and then a method to determine the severely affected area is proposed.The results show that the calculation error of the method proposed in this paper is within the acceptable range of actual engineering calculations.It is used to determine the area seriously affected by the fault and can determine the area seriously affected by the fault.(2)The time-domain simulation of high-order nonlinear systems takes a long time to solve the fault current,which is not conducive to the design of relay protection parameters.In order to solve this problem,a decoupling calculation method for the DC side fault current of the MMC-MTDC system is proposed.The failure energy flow direction of the four-terminal MMC-HVDC system is analyzed,and the equivalent model of each mesh is established according to the energy equivalence principle to reduce the order of the system,and solve the fault current.Then the decoupling dimensionality reduction method is extended to the fault current calculation of the N-terminal MMC-HVDC system.The calculation example shows that the method proposed in this paper calculates the fault current.Within the acceptable range of current calculation error,the short-circuit current calculation time is reduced by this method.(3)In order to solve the problem that unreasonable configuration will cause waste of resources and reduce system economy.This paper proposes a method for optimal selection of current-limiting reactance parameters.Clarify the fault current suppression capability of each line fault current limiter.Combining the fault influence characteristics and the performance requirements of the cir cuit breaker,select the objective function and constraint conditions,and calculate the fault current-limiting reactance parameters.The calculation example shows the fault current of each line is suppressed to the breaking range of the circuit breaker by the optimized selection method of fault current-limiting reactance parameters proposed in this paper,and the sum of fault current-limiting reactance parameters is reduced.In summary,a method for the fault current calculation and current-limiting reactance parameter optimization selection of the MMC-MTDC system is proposed.The efficiency of fault current calculation and the economy of parameter selection is verified,and it will provide a theoretical reference for MMC-MTDC system relay protection. |
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