Research On Key Technology Of Optimized Operation And Simulation In AC/DC Hybrid Distribution Network

With the successful application of HVDC technology and the development of power electronic technology,domestic and foreign scholars pay more and more attention to the technology of low voltage DC power distribution.Compared to AC distribution networks,DC power distribution can transmit higher capacity of electricity and ensure the power transmission distance and power quality.With the abundant of load diversity,the increased n on-linear load results in more line loss of AC distribution network.In addition,the application of distributed generations which need to be converted into direct current and then converted into alternating current introduces a large number of power electronic devices.The use of DC power distribution can reduce the converter devices and the network losses,so as to improve the operation of the power grid.The key technology of optimized operation and simulation in ac/dc hybrid distribution network is studied in this thesis.Firstly,an AC/DC hybrid distribution network model is established.According to the characteristics of AC/DC distribution network,the models of AC distribution network,DC distribution network,converters and distributed generations are established.In the AC distribution network,considering the three-phase asymmetry characteristic,the models of line,transformer and load are established under abc coordinates,and the corresponding admittance matrix form is given.In the DC distribution network,the models of converters are considered.The DC-DC converter is equivalent to a Buck converter or a Boost converter according to the requirements,and the AC-DC converter uses constant AC power control or constant DC voltage control according to the target.Distributed generations use PQ control based on its grid connection and the network characteristics.Secondly,the power flow method of AC/DC hybrid distribution network with alternating iteration method is studied.The alternate iteration obtains the entire network state by continuously solving the AC network and the DC network.Therefore,the power flow method of AC distribution network and DC distribution network is introduced respectively.The power flow method of AC distribution network considers the three-phase asymmetric models and uses Newton-Raphson method.Besides,the new node types introduced by distributed generations are analyzed and dealt with accordingly.The power flow method of DC distribution network uses Newton-Raphson method and can handle multiple V-node networks.The distributed generation is equivalent to the corresponding node type according to the output characteristics.The power flow method of AC/DC hybrid distribution network which uses alternating iteration is given based on these two kinds of power flow methods and the handling method of converters.The calculation is completed only if the AC part result and the DC part result converge at the same time.The DC lines are added to the original IEEE 3 3-node case to constitute the case of AC/DC hybrid distribution network so as to do simulation calculation.Finally,a method of regional load transfer using DC line is proposed.The advantages that DC line load transfer method is not affected by AC node voltage or frequency,and the output power is controllable are analyzed.Based on the topology,specific method of regional load transfer using DC line is expounded,including the corresponding constraint.On this basis,considering the regional distribution capacity,the input power of the upper power grid is distributed according to the regional distribution capacity to ensure stable operation of regional power grid.In addition,the load transfer path which causes the minimum network loss is selected to improve the economy of the grid operation.To complete load transfer simulation calculation,IEEE 14-node case which is divided into three distribution areas is used.