Research On Optimal Control Of Dc Distribution Network With Distributed Power Supply

Compared with ac distribution networks,the dc distribution network based on power electronic inverter interface not only has flexible control method,but also has high energy conversion efficiency.In view of this,it becomes a more ideal way for the access of distributed power supply network.However,the influence of distributed power access on dc distribution network steady state operation and transient fault isolation is increasingly significant.Not only does it increases the fluctuation of power supply and voltage in a distribution system,but also causes millisecond over-current shock and voltage fluctuation to the system during the dc short circuit fault,which threatens the quality and safety of dc distribution system.As the problems above are closely related to operation and control,safety and protection of the dc distribution network,therefore,we start with the study of the influence of the distributed power supply access on dc distribution network power flow,network loss in operation and system operation characteristics when faults occur.Taking voltage distribution equilibrium,minimum network loss,safe and stable operation after short-circuit fault as the study objectives,the optimal control method for dc distribution network with distributed power supply was proposed to improve the system operation efficiency,power supply quality,transient operation stability and fault riding-through ability.The main contents of the paper are as follows:(1)Under the conditions of different distributed energy permeability and dc power grid topology,the change rules of the network loss and voltage distribution of dc distribution network were studied.A mathematical model of optimization power flow for dc distribution network was established with network loss and voltage unbalance taken as objective functions.According to the change of system power flow,an optimized instruction value was provided for each AC/DC and DC/DC converter,and the power flow of DC distribution network was optimized by controlling the converter control instruction so as to reduce the network loss and voltage imbalance.Two cases of the distributed energy access to radial distribution network and meshed distribution network were selected to verify our method.Indexes of the network loss and voltage imbalance degrees of DC distribution network were compared before and after the optimization by PSCAD/EMTDC simulation,and the feasibility of the proposed method was then verified.(2)On the basis of analyzing the loss reduction method of dc distribution network,a method was proposed to reduce the loss of dc distribution network with distributed energy by optimizing the upper control instruction.It does not need to change the existing network topology,and the cumbersome power flow algorithm and the additional equipment are not needed.Firstly,the formula of network loss was derived based on power flow calculation,and the variation of network loss is studied.After that,a mathematical model of optimal power flow(OPF)for dc distribution network was established with the lowest network loss taken as the objective function.The Artificial Bees Colony algorithm(ABC)was used to solve the power flow optimization model.On this basis,for dc distribution network with distributed power supply which adopts master-slave control,a method was proposed to control power flow accurately and reduce network loss by optimizing control instruction.Finally,a typical IEEE16 node case was selected to verify the feasibility of the proposed method using software MATLAB/SIMULINK.(3)To enhance the fault detection and protection performance against the millisecond shock current and voltage transient during short circuit fault of the dc distribution network,the short circuit fault characteristic expression of dc distribution network with distributed power was derived,and the fault response of different fault stages was studied.The influence of each distributed power supply access on its fault current,voltage amplitude and peak time was analyzed.According to the mathematical relationship between circuit parameters in fault characteristics of dc distribution network,an optimal control method for suppressing impulse current by series fault current limiter was proposed.Based on the structure of the bridge-type AC current limiter,to improve the continuous and stable operation of the system and the safety of the device under the condition of large current failure,the structure of the dc FCL was designed and the parameters were determined.By using the designed dc FCL,small current faults can be detected at the same time,providing sufficient running time for fault detection and protection unit according to the fault ride-through target.When a bipolar short circuit fault occurs in the low-voltage dc distribution network,a low voltage riding-through strategy based on DC FCL was proposed for the bipolar short-circuit fault of LVDC distribution network.Finally,MATLAB/Simulink simulation was used to verify the rationality of the proposed FCL and its ride-through strategy.