Fault Transient Analysis of Flexible Medium Voltage DC Distribution

Abstract

The analysis of fault transient characteristics of flexible dc distribution is the basis of dc distribution protection technology. Based on the construction of the two-terminal power supply code flexible dc distribution system, the problems of different system grounding methods are analyzed in detail, and the connection of the converter transformer using Yn/△ and the dc capacitance using split midpoint high-resistance grounding method are proposed. Under this grounding method, transient models of transient grounding faults of ac and dc lines are established and their transient characteristics are studied. Using PSCAD/EMTDC software to build the simulation model, through the analysis and simulation proved that the system grounding method can effectively limit the instantaneity ground fault transient fault current during the period of over current levels, to maintain the dc voltage stability, improve power supply reliability of the system, has the strong ability of fault crossing ability and fault recovery. Introduction With the gradual change of China s industrial structure, load types are diversified. Meanwhile, the development and application of distributed power supply, energy storage and other equipment have posed an impact on the existing ac distribution network. In recent years, the flexible dc transmission technology based on voltage source converters has been extended to the distribution network, which makes the flexible dc distribution network become a hot research topic. The research of flexible dc distribution mainly focuses on control strategy, network topology, fault analysis and protection technology. The paper [1] discusses in detail the fault characteristics under the double-pole short circuit of the dc line, and divides the fault into the dc capacitance discharge stage, the diode natural commutation conduction stage, and the diode simultaneous conduction stage, and establishes the fault transient model. In reference [2], the fault model of dc micro grid is established, and the transient characteristics of different feeders under different location faults are analyzed, but the grounding mode of the system is not considered. Literature [3] established a simulation model for single-pole grounding fault of converter dc line and asymmetric fault of ac outlet, and the study showed that the fault characteristics of voltage between dc poles were affected by the grounding mode of converter transformer. The fault characteristics of the flexible dc system are affected by the system grounding mode. DC power distribution system is proposed in this paper a kind of flexible topology structure, the failure characteristics of different grounding modes are analyzed in theory, puts forward the converter transformer adopts Yn/delta connection, dc side with the method of dividing capacitance midpoint high resistance grounding, effectively limiting the fault current flow level, improve the power supply reliability of the system. Modeling of Flexible Medium Voltage DC Distribution The topology structure of the flexible dc power distribution system proposed in this paper is shown in figure 1. Double-ended ac power supply is adopted for power supply. The 110kV ac system at both ends is reduced through 110kV/10kV converter transformer respectively. 2nd International Conference on Electrical and Electronic Engineering (EEE 2019) Copyright © 2019, the Authors. Published by Atlantis Press. This is an open access article under the CC BY-NC license (http://creativecommons.org/licenses/by-nc/4.0/). Advances in Engineering Research, volume 185