Research On The AC-DC Hybrid Distribution System For Energy Internet

Due to the rapid expansion of modern cities,rapid economic development,and increasingly serious environmental,gradually increasing the share of electricity in energy use is an important way to solve power problems.In order to solve the problems existing in existed power grids,such as weak grid structure,large short-circuit current,lack of dynamic reactive power and failure to meet the interconnection and supply of energy internet.A more flexible,economical,and environmentally friendly power distribution method is proposed.That is named AC-DC Hybrid Distribution System(AC-DC HDS).The proposed AC-DC Hybrid Distribution System adds a new DC part consisting of DC power distribution parts to the original power grid,which is more convenient and flexible,and lays a solid foundation for the future development of comprehensive energy interconnection and interoperability.According to the existing distribution system structure,the topology of AC-DC HDS is divided into three types: radiation type,two-end type and ring type.Based on the analysis of the topologies of different between AC-DC HDS and traditional topologies,the topology and the voltage level which are suitable for AC-DC HDS has been selected.AC power distribution systems,DC power distribution systems,and AC-DC HDS are also analyzed using the radiation type structural distribution network as an example.The transmission loss and efficiency in the system are separately calculated,and the efficiency of the three are compared.Through this theoretical calculation and analysis,the development prospects of AC-DC HDS in the future transmission efficiency are obtained.Taking a demonstration project as a case to analyze the economic benefits of AC-DC HDS.According to the proposed topology of AC-DC HDS,the distributed power supply has systematically analyzed and studied.For different energy sources,different grid connection interfaces are set according to their characteristics.At the same time,the steady-state simulation of the AC-DC HDS with multiple distributed power sources is performed,and the simulation results under various operating conditions are obtained.The types of possible faults have been analyzed,and the fault characteristics of the AC-DC HDS are described in three different situations.The simulations of the AC-DC HDS under different fault conditions are performed.In view of the two typical DC fault conditions,the effects of the parameter changes on the current and voltage at the fault are compared between the three groups(electrode capacitance,current limiting reactance,and load capacity)which are under different system parameters.According to the simulation results,two effective fault current limit protection methods are proposed.In order to suppress the active power fluctuation of the AC-DC HDS and ensure the safe and stable operation of the system,the method for suppressing the DC bus voltage fluctuation is described.The coordinated operation control strategy for the active power fluctuation of the distribution system is analyzed and proposed.A simulation model is established in the PSCAD/EMTDC software,and the steady-state model including the coordinated control of the energy storage system is established.According to the proposed coordinated operation control strategy between the energy storage system and the converter station,the short circuit faults in the AC-DC HDS are compared and simulated.The simulation results show that the coordinated operation control strategy between the energy storage system and the converter station can effectively control the active power fluctuations generated under the system fault condition.When the active power of the DC bus is frequently switched,the control strategy can respond quickly,reducing the requirements on the protection equipment and ensuring the stable operation of the system.The converter station energy storage coordinated operation control strategy can be used as a method to suppress the power grid fluctuation of the AC-DC HDS.Finally,in order to improve the acceptance of new energy in AC-DC HDS,a full-model predictive control AC-DC HDS is proposed.And based on the system,a three-tier energy management and control system(EMCS)has been proposed.In the EMCS,the first layer is the Maximim Power Point Tracking(MPPT)control algorithm and the battery safety control strategy for maximum power point tracking.When the State of Charge(SOC)is lower than the set point,Photo Voltaic(PV)and wind energy are MPPT control states.The second layer is the power converter control algorithm based on Model Predictive Control(MPC)to provide power compatibility between different power supplies of the power distribution system.The third layer is the coordination control layer,which can automatically achieve power balance and keep the system stable.In order to verify the effectiveness of the proposed energy management system,a lab-scale HIL test platform for AC-DC HDS has been built based on MATLAB&RT-Lab.It consists of a mixture of wind energy,solar energy,and energy storage systems.The HIL test results show that the EMCS based on AC-DC HDS can quickly realize power balance and stable operation.And it also can achieve energy exchange in AC or DC state.The system responds quickly and has positive significance for the stability of the Energy Internet.