Research On Security And Power Supply Reliability Assessment For Hybrid AC/DC Microgrid

With the rapid development of power electronics technology,more and more DC distributed generator(DG)and DC loads are connected to the microgrid,and the consumption of DC and AC will coexist for a long time in the future.It is difficult to meet the actual needs of microgrid simply using a certain form of power supply,which promotes the application and development of hybrid AC/DC microgrid.The hybrid AC/DC microgrid includes an AC subsystem,DC subsystem and an interlinking converter(ILC)that connects AC and DC buses.It can supply power directly to AC load and DC load.The hybrid AC/DC microgrid will be the preferred microgrid model for the efficient development and utilization of distributed renewable energy,and it will also be an important form of power distribution system in the future.The random failure of intermittent DG,the uncertainty of output and the two-way flow of AC/DC power in hybrid AC/DC microgrid are not conducive to the security and reliability operation of hybrid AC/DC microgrid.In this paper,the static security assessment,long-term power supply reliability assessment and short-term power supply reliability assessment are carried out for the hybrid AC/DC microgrid.The main research work is as follows:1)The conventional power flow and random power flow calculation of hybrid AC/DC microgrid with islanded operation are carried out.Based on the conventional power flow calculation mechanism,the conventional power flow calculation of power system based on node power equation is analyzed.The conventional power flow model of hybrid AC/DC microgrid is established,and the conventional power flow of AC/DC microgrid with / without balance node is solved based on Levenberg-Marquart with a nonmontone line search(LMNL)power flow algorithm.On this basis,the alternating iterative method is used to solve the conventional power flow of hybrid AC/DC microgrid,and the random power flow calculation of hybrid AC/DC microgrid based on Monte Carlo sampling is proposed.The example verification and analysis are carried out.2)A method for static security assessment of hybrid AC/DC microgrid is proposed.Based on the analysis of the basic mechanism of power system static security assessment,the static security of hybrid AC/DC microgrid is analyzed,and the static security risk assessment index of hybrid AC/DC microgrid is established.Inthis paper,a static security analysis criterion based on N-1 accident and contingency scheme is proposed,and a new partition load shedding strategy is proposed to solve the problem of islanded system after fault resolution,on the basis of which the static security risk index value of the system is obtained.The effectiveness and efficiency of the method are verified by the example.3)A method for long-term power supply reliability assessment of hybrid AC/DC microgrid is proposed.In view of the flexible topology of the islanded hybrid AC/DC microgrid,the system faults are classified,and the AC subsystem,DC subsystem and ILC are transformed into equivalent node models to simplify the process of fault analysis.The reliability model of components is established,and the two-stage sampling method is used to sample the components of the system.On this basis,the long-term power supply reliability assessment of hybrid AC/DC microgrid is carried out.The correctness and effectiveness of the method are verified by the example4)A method for short-term power supply reliability assessment of hybrid AC/DC microgrid is proposed.A time-varying probability tree contingency screening technique(TVPTCST)is proposed based on time-varying probability tree.Establish a time-varying model of the component and obtain a time-varying probability model of the load point that is interrupted by two different causes.The sequence operation theory adopted is to obtain the probability distribution of random variables through probability sequences.Define two short-term power supply reliability assessment indicators.On this basis,the short-term power supply reliability assessment of hybrid AC/DC microgrid is carried out.The correctness and effectiveness of the method are verified by the example.