Fault Current Limiter With New Topology And Its Application In DC Grid

At present,the DC grid technology based on MMC(Modular Multilevel Converter)is an effective technical scheme for the acceptance of new energy.The DC fault current rises rapidly due to the low damping performance of the VSC-H VDC(Voltage Sourced Converters,High Voltage Direct Current Transmission)system,which will seriously threaten the relevant electrical equipment in the system.So the use of the fault current limiter(FCL)has become an important technical means.In order to carry out an in-depth study in this area,the exemplary engineering project of Zhangbei DC grid is selected as the background project to be studied.The research on the topology of FCL,coordination strategy with protection and configuration scheme is carried out in this degree thesis,and the research work and the main achievements are as follows.1)A new topology of solid-state FCL is proposed.The theoretical analysis and simulation comparison of the resistance-type FCL and the inductance-type FCL are carried out,and the results show that the resistance-type FCL is better.In view of the problem that the power electronic devices need to withstand high voltage during the current limiting period of resistance-type FCL,FCL with new topology is proposed,which can reduce the voltage at both ends of the power electronic devices and save the cost,effectively.Then the topology,working principle,control mode and mathematical model of the FCL with new topology are described in detail,and the validity of the topology is verified in the Zhangbei HVDC transmission system.2)The coordination strategies of the FCL with overcurrent protection of the MMC sub module and the breaking of DC circuit breaker are given.Based on the working principle of over current protection and DC circuit breaker,the influence of FCL on the blocking of the converter valve and the breaking process of the circuit breaker is analyzed.Then According to whether or not the FCL is installed,the timing sequence of protection of the DC grid in two cases is analyzed.For the system with FCL,the coordination strategies between the FCL,the over-current protection and the DC circuit breaker in the converter station near fault and the converter station remote fault are given respectively.The advantages of the DC grid with the FCL are compared and analyzed by the simulation example,and the rationality of thestrategies are verified.3)The short circuit current calculation method of DC grid is derived,and the configuration scheme of FCL in the DC grid is proposed.Aiming at the Zhangbei bipolar DC grid,the transient characteristics of the bipolar short-circuit fault are analyzed,and the calculation method of the bipolar short-circuit transient current before the converter valve blocked is derived.Through the comprehensive analysis of current limiting effect and device cost,the installation location of the FCL is selected.Based on the transient current calculation method,the parameter configuration analysis of the resistance-type FCL is carried out.Through the PSCAD/EMTDC simulation platform,the validity and accuracy of the configuration scheme are verified.