Research On Power Flow Algorithms For Multi-terminal Flexible DC Distribution System

Power industry is the vitals of the national economy.Power flow calculation and optimal power flow calculation are widely used in power system planning and operation.With the continuous development of distribution system,new energy generation technology and DC distribution technology have gradually reached a mature state.Distributed generation is widely connected to distribution network.The traditional power flow calculation and the optimal power flow calculation methods can no longer meet the operation requirements of the contemporary power system.There is only active power flow in the DC distribution network,so when AC distribution system and DC distribution system are interconnected,the traditional power flow calculation and optimal power flow calculation methods can no longer meet the operation requirements of the modern power system.When interconnected,the traditional power flow can not meet the calculation requirements of AC-DC hybrid system.Therefore,this paper focuses on the power flow calculation method.The main work of this paper is as follows:(1)The access of DC distribution network can improve the ability of distribution network to accept distributed generations.Because of the less using of DC/AC inverters,the efficiency of power transmission is improved.In view of the complementary advantages of AC-DC distribution network,several typical topological structures of DC distribution network are studied firstly.Then,according to the different purposes of grid-connected converters,several typical outer-loop control modes of converters are studied.At the same time,several typical power flow calculation methods of distribution network are analyzed.(2)The droop control can be used to coordinate the parallel converters.Owing to the droop character of each converter,the DC bus voltage can be controlled at the same value.However,in the analysis of steady-state or optimal scheduling,each converter needs to participate in power flow calculation under the droop curve equation's conditions.In other words there are multiple voltage control sources on the slack bus,but the voltage value needs to converge to the same value.So an algorithm based on virtual master-slave strategy is proposed Firstly,the network structure and the control strategy of AC/DC distribution network are discussed,and then an algorithm of power flow calculation is designed for multi-converter parallel connection.The parallel DC bus of multi-converter can be regarded as a slack bus.The relationship between the active power of the first number converter and the droop character of other converters can be deduced by using the power balance equation and the droop characteristic equation on this node.According to the droop characteristic curve,the power of the first number converter and the DC voltage value can be solved.The other number converters' DC voltage value can be obtained by the droop characteristic curve.At the same time the active power can be updated.Finally,the problem of power flow calculation for multi-converter simultaneous control of slack bus voltage is solved.(3)There is only active power flow in DC distribution system,which is very different from AC distribution system.Therefore,more direct theories and methods for voltage stability analysis can be provided by studying the existence and uniqueness of power flow solution.Banach fixed point theorem is an important theorem which is about the existence and uniqueness solutions of algebraic or differential equations' solutions.Based on Banach fixed point theorem,the existence and uniqueness of power flow solutions for DC distribution systems are proved,and a voltage stability analysis method is proposed based on the feasible power flow solutions.