Analysis Of Relay Protection Applicability Of AC-DC Hybrid Distribution Network

The construction of flexible AC/DC distribution network is one of the important directions for the development and reform of distribution network in the future.Flexible ac/dc power distribution network is composed of converter station,distributed energy,ac/dc load,the inverter adopts modular multilevel converter(MMC),is the realization of the core equipment of interconnected ac/dc system,MMC commutation failure does not exist,can supply power to passive network,adjust the active and reactive power independently,has the incomparable advantage over other inverter.The flexible DC power supply network supplying power to the passive network is suitable for wind farm grid-connected,isolated island,city center and other application scenarios.Considering that there are few literatures on the flexible DC power supply network supplying power to the passive network,this paper carries out research on the flexible DC power supply network supplying power to the passive network.The fault characteristics are closely related to the control strategy of the flexible system.The control strategy of the flexible system supplying power to the passive network is different from the control strategy of the flexible system connected to the active network at both ends,and its fault characteristics are different from those of the active network connected to both ends.Because the flexible direct current system contains large-scale power electronic devices,its ability to withstand overcurrent is weak,and the fault current may reach the self-protection threshold of power electronic devices in a very short time after the fault occurs.Therefore,it is necessary to use the improved control strategy to actively control the fault current,reduce the fault current amplitude,and achieve fault traversing.Firstly,according to the working principle and control strategy of MMC,a flexible DC power supply network for passive network is built on the real-time simulation platform of RTDS.Then,on the basis of the MMC inner loop current controller,the positive-negative sequence active controller and positive-negative sequence passive controller are designed respectively based on the principle of the positive-negative sequence decoupling technology of the dual synchronous rotating coordinate transformation phase locked loop(DDSRF-PLL)as the MMC AC transient control strategy.Then the AC/DC fault characteristics and the output short-circuit current characteristics of the flexible system are analyzed from the theoretical and simulation aspects,which can provide some reference for the control and protection research of the flexible system in the future.Supply power to passive network protection technology is the one of the important content of flexible dc power supply network research,based on pure ac system fault feature configuration and setting of traditional communication distribution network line protection,dc system without considering the flexible access for ac electrical effects of the characteristics of distribution network protection,may result in relay protection action reliability is reduced.However,there are few studies on the protection technology of the flexible DC power supply network supplying the passive network.In view of this,this paper studies the applicability of the relay protection of the AC lines of MMC inverter station in the flexible DC power supply network supplying power to the passive network.In the flexible DC power supply network system supplying passive network,the MMC short-circuit current amplitude is greatly affected by the control strategy when the AC line of the modular multi-level converter(MMC)inverter station fails,which may affect the performance of relay protection.Therefore,based on the idea of coordination of control and protection,the influence of control strategy on relay protection is mainly studied.On the basis of the above research,the principle of distance protection of high-frequency fault components based on S-transform is introduced.This protection principle requires the use of high-frequency impedance equivalent model of MMC AC side and the extraction of high-frequency fault components of 5ms before and after the fault to avoid the influence of MMC control strategy on relay protection.Finally,the effectiveness of the protection principle under typical fault condition is verified by simulation.