Coordination And Parameter Optimization Of Grid-side Current-limiting Equipment In DC Grid

With the full development of renewable energy in the world,DC power grid based on Modular Multilevel Converter(MMC)has become an important development direction of DC transmission in the future.However,the converter station,which is the core equipment of DC power network,has the characteristics of low damping,and the fault current develops rapidly when the DC side short circuit fault occurs,which endangers the safe operation of DC power network.Drawing lessons from AC power grid fault clearing methods,DC Circuit Breaker(DCCB)can selectively clear faults,which is considered as an effective means to realize fault clearing.However,with the development of DC power grid,DC circuit breaker has the problem that it can’t cut off the big fault current quickly,so it needs additional fault current limiting device to cooperate with DC circuit breaker to clear the fault.At present,a variety of fault current limiting methods have been proposed,but there are still no current limiting devices other than flat wave reactance put into practical engineering.Therefore,it is necessary to summarize the action mechanism of various current limiting devices in theory,sum up the coordination and current limiting methods of current limiting devices,and optimize the parameters of these current limiting devices,so as to give full play to the current limiting ability of each current limiting device on the premise of a certain economic cost.The main work of this paper is as follows:First of all,for the current current limiting devices,the topology and working principle of various devices are briefly introduced and from the location of the current-limiting device,it is divided into two types:the source side and the network side,from the current-limiting mechanism,the various current-limiting devices are divided into two types:voltage reduce and resistance increase.Based on the above classification method,a theoretical analysis of the current limiting mechanism of various devices is carried out.It is pointed out that the source-side device mainly limits the current by changing the input duty cycle of the converter sub-module capacitor,and the network-side device mainly changes the discharge characteristics of fault current.Through the constraints of various equipment in terms of input time and limit parameters,the principle of cooperating with grid-side current-limiting equipment to suppress fault current is given.Secondly,based on the coordination principle above,the fault current when the reactance-type fault current limiter and the DC circuit breaker cooperate with each other to limit the current is calculated.The proposed fault current calculation method takes into account the overvoltage problem in the actual current-limiting reactor input process and the energy absorption of the parallel MOA branch.The fault current is decomposed into multiple stages according to the switching of the current limiter and the circuit breaker.The energy absorbed by the MOA at each stage is accurately calculated.A four-terminal DC power grid model was built on the simulation platform for verification,and the maximum relative error between the calculated results and the simulation results is only 0.6%,indicating the validity and accuracy of the calculation method.Finally,the properties of the possible current-limiting components of the fault current limiter are studied.Through theoretical analysis,it is pointed out that:limited by the voltage stress,when the reactive fault current limiter meets the voltage stress requirements,the current-limiting effect is almost zero,so reactance cannot be used alone,which also proves the rationality of considering the current-limiting reactance in parallel with MOA energy absorption when calculating the fault current.Based on this calculation method,with the minimum breaking current of the circuit breaker and the minimum energy absorption of various energy absorption equipment as the optimization goals,the smoothing reactor and current-limiting reactor in the DC grid are optimized.