Simulation Model Of Multi-terminal DC Distributed Network Based On MMC And Fault Isolation And Location

MMC-MTDC(Multi-terminal DC distributed network based on MMC),which has low loss, large power transmission capacity, a flexible connection to distributed source and energy storage system, is considered an important direction for future development of the distribution network. Nowadays, with the development of DC breaker technology, it has basically become possible to isolate dc fault rapidly in DC distribution network. In this paper, operation issues relevant to MMC-MTDC are investigated by means of theoretical analyses and simulation validations. The research on short-circuit and ground fault analyses, fault isolation and location are the key work.Firstly, MMC-MTDC in parallel and MMC-MTDC in series are compared in operating characteristics. Besides, a method of adding a fault current limiter module in MMC has been proposed to protect electronic equipment. Based on hierarchical control and protection theory, partition protection of MMC and coordinated control between MMCs are designed.Secondly, Fault feature on dc lines and ac lines are summarized in order to give an effective dc fault isolation strategy. Moreover, a four-terminal MTDC distributed network with parallel-radial topology simulation model is built in RTDS / RSCAD, so short-circuit and ground fault on dc lines and ac lines can be further analyzed.Finally, When short-circuit fault occurs in dc line, protection criterion will lose selectivity, which gives all CB signal to break. Therefore, this paper presents a methods to identify the fault dc line, which uses two-terminal direction of current. Furthermore, in order to give complex systems a better solution, another method, which uses only single terminal direction to isolate fault, is proposed. The method can rapidly clear the fault and take full advantage of the first stage to the second stage of the locked state of MMC to charge the capacitor in sub-module, which is conducive to the rapid recovery in the latter part of the non-defective part. Afterwards, a fault location module is designed. From its discharge current, parameter of attenuation coefficient and the oscillation frequency can be calculated by the fast Fourier transform and numerical fitting method. Double-end measurement is carried out to eliminate the inductance, avoiding the errors caused by the non-uniform distribution of line inductance. The influence of transition resistance, fault location and other variables to measurement is analyzed.