Research On The Power Flow Calculation And Emergency Control Method Of Medium Voltage Hybrid AC/DC Power Systems

With the development of various new energy sources such as photovoltaic and wind power,and the diversification of load types in power systems,hybrid AC/DC power systems have found a wide application in different areas,such as multi-terminal direct current(MTDC)transmission technology,hybrid AC/DC microgrids and ship power systems(SPS).Among them,the medium voltage hybrid AC/DC power system has attracted many scholars’ attention because of its flexible topology and wide application.Typical applications include hybrid AC/DC microgrids and medium voltage hybrid AC/DC ship power systems.These systems share some similarities but have their own characteristics,thus,it is important and meaningful to research the system modeling,power flow calculation and emergency control for medium voltage hybrid AC/DC power systems.Firstly,several typical topology of medium voltage hybrid AC/DC power system are described in this paper,for example,the modern smart building power system and the next generation medium voltage ship power system,the structure and operation characteristics of these typical topologies are analyzed,and a new modular multilevel type solid-state transformer based topology of shipboard power system is proposed.The widely used voltage source converter(VSC)in hybrid AC/DC power system is modeled in detail,including the equivalent model at AC side,the accurate loss model,the control strategy and the operation constraint.Then,the power flow calculation methods of hybrid AC/DC power systems are elaborated in detail which can divide into two types: the sequential methods that solve AC and DC networks separately in an iteration manner using an interaction procedure and the unified methods that solve AC and DC power flow simultaneously.A sequential power flow method is proposed for the DC network embedded hybrid AC/DC power system.The power flow models of the AC and DC side are established,and the additional iterative computations of DC slack bus is designed.The proposed algorithm is implemented in the three terminal hybrid AC/DC power system and the results verified the feasibility of the proposed method.But it should be mentioned that the sequential methods have convergence problem when deal with some weak connected AC/DC system,amid at this problem,a modified power flow calculation method of hybrid AC/DC power system based on unified method is proposed.By adding the new state variables related to the power of DC slack converter,the power flow calculation equation is simplified,and the proposed method is taken into consideration for each generator and load node from AC/DC networks.The case studies were performed using the modified DC zonal electrical distribution systems(DC-ZEDS)model based new ship power system,and the results verify the feasibility and effectiveness of the proposed algorithm.Finally,the emergency control strategy of the hybrid AC/DC power system is expounded,which include the local control based on the control capability of the converter and the centralized control aiming at the optimization of the system operation.For the medium voltage hybrid AC/DC ship power system,the necessity of emergency control of the system under disturbance is described.Aim at the deficiency of the existing analysis methods,a new reconfiguration theory based emergency control strategy of hybrid AC/DC ship power system is proposed,which is taken the maximum load power consumption and the minimum switching actions as the emergency control targets,meanwhile,the system power flow and the operation boundary constraints are taken into consideration.The genetic algorithm(GA)is used to solve the optimization model.The case studies are given to prove the effectiveness and superiority of the proposed method,and the results also demonstrate that the proposed unified method of power flow calculation is applicable to the splitting DC networks.