Decentralized Control And Stability Analysis In DC Distribution Networks

Renewable Energy has attracting increasing attentions for its substitutability of fossil energy,and DC distribution networks are becoming more and more important for the efficiency of accessing renewable energy and DC electrical loads.To promote the further development of DC distribution networks,this paper detailedly studies several issues in DC distribution networks.The research route of this paper is: studying the DC power electronic transformer at first,which is the core equipment in a DC distribution network;then studying the primary control framework of a multi-voltage-level DC distribution network,where the sub-networks are interlinked by DC power electronic transformers;finally studying the simplified analysis method of the small-signal stability in an DC distribution network,which may be connected by large-scale power electronic converters.The researches can be depicted in detail as follows:Firstly,this paper points out that an ideal DC transformer possesses bidirectional voltage support capability due to its constant transformation ratio,which can play an important role in a DC distribution network.This paper further studies the method of using a constant-transformation-ratio-control dual-active-bridge converter(DABC)to build an ideal DC power electronic transformer.The built transformer can work in both of forward voltage support mode and backward voltage support mode,between which the transition is seamless,which means that it possesses bidirectional voltage support capability.In a DC distribution network,the built transformer can realize the mutual voltage support between the medium-voltage bus and the low-voltage bus,and realize the normalized decentralized power coordination between the energy storage systems at the medium-voltage side and the low-voltage side.Secondly,this paper studies the decentralized and per-unit primary control framework for a multi-voltage-level DC distribution network,where the sub-networks are interlinked by DC power electronic transformers.In the proposed framework,the power DC power electronic transformers adopt concise decentralized and per-unit difference-of- control,the energy storage systems(ESSs)adopt decentralized and per-unit - droop control,and the distributed generations(DGs)adopt decentralized and per-unit constant power control.The control framework make the ESSs at the different buses be regarded as connecting to a common bus with a per-unit voltage level,where the ESSs can be viewed as peers.The peer-to-peer relationships among the ESSs make the DC distribution network be compatible to the conditions of ESSs power sharing,part of the ESSs malfunctions,voltage level extension,and transmission grid connected.Finally,this paper studies the impedance-based decentralized small-signal stability analysis method for the DC distribution network,to solve the curse of dimension in small-signal stability analyses when large-scale power electronic converters are connected to the DC distribution networks.The proposed method defines decentralized,individual,and time-invariant port impedance specifications for the subsystems connected to the DC bus of the DC distribution network.Once each subsystem satisfies its own port impedance specification,the small-signal stability of the whole DC distribution network can be guaranteed.This method decouples the small-signal stability analyses of the subsystems in the DC distribution network,and can solve the curse of dimension.