The study on the technology and the prospect of flexible DC distribution network

摘要

As the high speed growing of city electrical load, the improving demand of the user power quality, andrnthe requirement of the new energy grid-connect, the traditional AC distribution network can’t adaptrnthese new demands. Using ±7.5kV flexible DC distribution line to replace part of the 10kV ACrndistribution line and constituting the compound distribution line is proposed in this paper.rnThis flexible DC distribution line based on Voltage Source Converter is composed with 3 DC cablernconsisting dipolar line. As the calculating result, the transmission capacity of one ±7.5kV flexible DCrndistribution line almost equal to three 10kV AC distribution line. And when the supplies powerrndistance longer than 10kV, the economic advantage is obvious. The DC distribution line getting thernpower from more than two transformer substation’s 10kV side, converting it into DC, transmittingrnthough DC cable, then converting to AC in the distribution box consists the flexible DC distributionrnpower net. When the fault happened in AC system, it can be isolated by the phase locked loop to keeprnthe voltage stable of DC net.rnThe new city distribution network can increase the transmission capacity of lines and improve the userrnpower quality, provide DC accessing network condition and raise the distribution line efficiency.rnThe digital simulation, designing plan, control strategy, and the protective relaying of flexible DCrndistribution network are discussed in this paper. The digital simulation researches on the model ofrnequipment, load, and the whole system to build the multi-terminal flexible DC distribution simulatingrnsystem with impact load, electro-mobile, accumulation system, renewable energy sources in PSCAD.rnThe steady-state load flow analysis, transient load flow analysis are based on this model. Therndesigning plan studies on topological structure of the flexible DC distribution network and analysesrnthe reliability, stability and economics of different structure and the most proper topological structurernis given based on the study. The control strategy studies on multi-terminal flexible DC distributionrncoordination control method which using an up level strategy to improve the safe stability. Thernprotective relaying studies on the fault diagnosing and protective relaying to find the best protect way.rnThe demonstration project of flexible DC distribution network can be built according to the researchrnof these four fields, and it also provides the foundation to the large-scale construction which is therndirection of the future city distribution network.