Research On Power Flow Calculation And Reliability Evaluation Of Microgrid

In the background of energy shortage and environmental degradation nowadays,microgrids have gained more and more reception in the world.Microgrid is a small power system consisting of distributed generations,energy storages,energy conversion devices,loads,monitoring and protection equipments,and it is also an autonomous system with self-control,self-protection and self-management.The flexible and efficient application of distributed generation can be realized when distributed generations and loads operated together in the form of microgrid,so the operation difficulty of large-scale distributed powers connecting to the grid is also resolved.Microgrid is also an important way to achieve self-healing,user-side interaction and demand response of the future smart distribution network.With the development of the power system,there will be more and more microgrids accessed to the distribution network.And microgrid has become a hot research field in the power system.The power supply and load are in close proximity in microgrid,so long-distance transmission is not needed.The transmission line impedance characteristics of microgrid are similar to that of the traditional distribution network.But the access of distributed power,the diversity of distributed power control method and the flexibility of microgrid operation mode make the power flow distribution and calculation of microgrid different from that of the traditional distribution network.In addition,uncertain factors in microgrid,such as intermittent power output of distributed generation,random faults of line,distributed generation,and the fluctuations of the system load,are harmful to the safe,stable and reliable operation of the system.This dissertation,which has been sponsored by the National High Technology Research and Development Program of China project(863 Program)“Demonstrations and key technologies of efficient and reliable operation of the microgrid cluster”(Grant No.2012AA050217)and the National Natural Science Foundation of China project “Power Flow Model and Efficient Algorithm for Microgrids Under Different Operating Modes”(Grant No.51507149),researches on unified conventional power flow algorithm of microgrid which can be used for the conventional power flow calculation of microgrid under different operation modes,and it further studies the continuous power flow calculation,probabilistic load flow calculation,short-term reliability assessment and static security evaluation of microgrid.The research results have a certain guiding significance for the planning design,optimizing operation and the protection control of microgrid.Main researches can be summarized as follows:1.A unified conventional power flow calculation method for microgrid based on Levenberg Marquardt method combined with trust region technique(LMTR)power flow algorithm is proposed.According to the diversity of microgrid load flow calculation under different operation modes,a unified simplified microgrid power flow model is established,and its characteristics and the solving requirements are analyzed.LMTR power flow algorithm is proposed to slove the unified microgrid power flow model.This algorithm can resolve the difficulties brought by Jacobi matrix singular and narrow solution scope of power flow equations.Meanwhile,it has a global convergence,and it has little dependence on the initial value.The results and analysis of examples verify the correctness and validity of the proposed LMTR power flow algorithm,and it can be applied to conventional power flow calculation of microgrid under different operation modes.2.A load type continuous power flow calculation method of microgrid without balance node is presented.The continuous flow model of microgrid without balance node containing load power parameter is established,and a continuous power flow calculation method is proposed.In this method,the initial points are obtained by LMTR power flow algorithm.At the prediction step,the tangent prediction method combining local parameterization technique is used.Then at correction step,a new hyper-spheres parameterization method is proposed,and a combination Newton method that combines the traditional Newton algorithm and Newton algorithm with Armijo-type line search is used for correction.The results and analysis of examples show that the proposed method can ensure the success of continuous power flow calculation,and can achieve a greater fixed-step prediction at a higher calculation precision during the whole continuous power flow calculation.3.A microgrid probabilistic load flow calculation method based on Monte Carlo simulation method with combination sampling is put forward.According to different characteristics of input random variables in the probabilistic load flow,a Monte Carlo simulation method with combination sampling for probabilistic load flow calculation is proposed.In the combination sampling process,the Latin Hypercube sampling method is used to sample the random variables of continuous probability distribution,and the improved importance sampling method is applied to sample the random variables of small probability event and discrete probability distribution.Firstly,the main random factors that needed to be considered in microgrid probabilistic load flow are analyzed and they are regards as input random variables.Then,the probability distribution models of input random variables are established,and the combination sampling models are built.Lastly,the system input data obtained by each sampling are used to calculate conventional power flow by LMTR power flow algorithm,so the probability distributions of the system power flow are obtained.The simulation results show that the proposed combination sampling method can improve the convergence speed of Monte Carlo simulation significantly,and can reduce the calculating time.4.A short-term reliability evaluation method of microgrid in islanded operation mode based on time-varying universal generating function(TVUGF)is proposed.After TVUGF is defined,the improved TVUGF calculation methods are proposed in order to decreases the number of states evaluated and improve speed of reliability evaluation,which including merging similar items method and states re-division method.The state probabilities of forecasting wind speed are obtained based on TVUGF model of continuous-time homogeneous Markov chain.According to time-varying state probability of two-state component,the time-varying state probability of microgrid system is established.Further,considering microgrid splitting planed and island system load shedding strategy,the fault impact for each system state is analyzed by classification,so the microgrid short-term reliability model based on TVUGF is established.The correctness and validity of the proposed short-term reliability evaluation method are verified by the simulation results and analysis.5.The static security risk assessment of microgrid without balance node is studied.In accordance with characteristics of microgrid without balance node,the static security analysis criteria combining N-1 contingency with splitting scheme are proposed.Considering the characteristic of islanded system caused by outage that it has a large number of uncertain factors and has not a balance node,a probabilistic power flow simulation calculation method based on Levenberg Marquardt method with new inexact line-search(LMNIL)algorithm is proposed to calculate probabilistic power flow of the islanded system.Then,the probability distributions of static security risk indices considering frequency limit violation risk index are obtained.Assessment results on the microgrid system verify the correctness and effectiveness of the proposed static security risk assessment method,and this method can be more effective to assess the static security risk of microgrid without balance node.