Research On Coordinated Control Strategy Of DC Distribution Network

Compared with AC distribution network,DC distribution network has more advantages such as better power quality,large capacity of power supply,no need of reactive power compensation,suitable for new energy access and so on,thus providing a bright prospect of development.DC distribution network,which is based on voltage source converter,has been a research hotspot and also has a wide application prospect with the extensive use of HVDC-Flexible technology based on voltage source converter.DC voltage is a significant parameter of the power balance of DC distribution network,and a reasonable control strategy is the key to the safe and continuous operation of DC distribution network.As a result,it is necessary to do intense research into the coordinated control strategy between the control units and to design reliable local controllers to guarantee the stability of the DC distribution network's voltage.It is also needed to design a matched upper controller and to provide reference values of the control signal for the local controller in order to optimize the operating mode of the system.This paper does research into the local coordinated control strategy of DC distribution network and the parameter adjustment to the corresponding upper control,which mainly includes the following parts:(1)The topology and voltage level of DC distribution network are induced and summed up on the basis of existing research.The basic control model of every control unit in DC distribution network is analyzed and the applicability of margin control,droop control and DC voltage drop control in DC distribution network is discussed.It focuses on the analysis of the realization method of mode switching control,As the basis for the coordinated control strategy.(2)In order to reduce the dependence on the upper control and communication system,this paper puts forward a coordinated control strategy which can adjust the slope of droop curve automatically for hand-in-hand type DC distribution network.The voltage is divided into several intervals according to the variation of the voltage,and the control unit has different coordinated control strategy in different intervals.Control units will switch to proper control models automatically by monitoring the local DC voltage.(3)The droop control section of DC voltage drop control has been improved.By monitoring local active power,the control units switched into droop control method are able to adjust the slope of droop curve automatically on the basis of its real-time power margin,thus making the best of the power margin of every control unit and realizing the optimum allocation of power by avoiding the overload of a certain control unit due to the mismatch of the fixed droop slope and the real power margin.Scattered self-discipline control is achieved without long-distance communication or real time monitoring of upper controller,only monitoring the local information is needed.Moreover,a model of DC distribution network is established based on PSCAD/EMTDC software platform to verify the availability of the control strategy in the paper in a variety of working conditions.(4)The specific functions of the upper level control in the coordinated control strategy are analyzed.Based on the power flow calculation of DC distribution network,a method of parameter adjustment for control mode switching point is proposed.Finally,an optimal power flow model of DC distribution network is built,and the optimization method of the reference values for the active power of each converter station is proposed.