Research On The Dispatching Center's Control Strategy Of VSC-MTDC Distribution Network

With the growing of electricity load,the greater difficulty of improving power quality,the penetration increase of renewable power and electric vehicles,a flexible DC distribution network is an important direction of distribution network reconstruction in modern urban areas.A flexible DC distribution network means applying flexible DC technology in the urban power distribution network.Distributed generation and power storage station can be switched into DC side directly.So is DC load.Voltage source converters of DC side can improve power quality remarkably in the same time.A flexible DC distribution network contains many power generation equipments with complex nature of renewable energy and a large number of power electronic converters.The traditional fine modeling will be a burden for simulation software.This paper will view all electrical nodes as black boxes and model them as simplified power sources based on the power-voltage curve,omitting the complex internal control model.Based on the simplified models of power generators,power loads and power converters,a simplified network model of a newly proposed multi-terminal flexible DC distribution network is derived.This paper introduces the three-layer control system of a flexible DC distribution network.They are the bottom converter control,the upper unified control and the optimal operation control.The droop control and the master-slave control are introduced as the main parts of the bottom converter control.The droop control with dead zone is also introduced.The meaning of translating the droop control curve for no-deviating regulation of power is firstly studied.The simulation of DC distribution power flow under a variety of bottom converter control strageties(the master-slave control and the droop control with different slopes)is done by the simplified network model.The influence of different converter control strageties on distribution power flow is summarized.This paper also studies the basic part of upper unified control —— dispatching control part.A new index called grid voltage error(GVE)is presented.It describes how much the actual state deviated from the ideal state in a flexible DC distribution network.Based on GVE index and the special dispatching characteristics of a DC distribution network,three specific dispatching schemes are proposed for DC distribution network's power dispatching centre.For an all-weather DC distribution network simulation model,the GVE values without and with a specific dispatching scheme are compared.It is considered that the proposed dispatching control schemes can keep a relatively stable state in a DC distribution network all the time.