Research On Reliability Evaluation Models And Methods Of Microgrid

Composing of large-scale, long-distance power transmission system and centralizedpower supply system is the typical characteristics of large power system. But, it appear somedisadvantages increasingly such as severe pollution, lack of flexibility, long constructionperiod, severe fault effect and etc. The shortage of fossil fuel propels people to develop thenew modes of power network. The microgrid, acting as the most effective method forexerting the efficiency of distributed generators(DG), has many advantages such asenvironmental friendliness, flexibility and rapid investment return. Microgrid and largepower network can support each other to reduce energy consumption, improve the powersupply reliability and electric power quality. Nevertheless, microgrid can switch seamlesslyand flexibly between grid-connected and islanded state, and that makes microgrid differentfrom traditional power distribution network. With unique running and controlling mechanism,Microgrid can receive different kinds of DG. Wide application of renewable energy DGmakes it significant in energy saving and environmental protection, but some characteristics,specially the intermittency, randomness and correlativity of DG output also create muchproblems for planning and quantitative evaluation of microgrid. Thus, it is urgent to study theprojects about microgrid reliability so that it can effectively instruct the microgrid planningand purchase the optimized comprehensive performance. Therefore, the dissertation studiesseveral imperative problems to be solved, including the reliability indices system ofmicrogrid, the model, analysis method and algorithm for microgrid reliability evaluation, andtheir application in microgrid planning. The main works are as follows:1. In order to analyze operation state of microgrid according to its network characteristicin microgrid reliability evaluation, by discussing the different characteristics between themicrogrid and the traditional distributed network, some notions are explored such as thecomponents, the network structure, the network-building and the operation mode of themicrogrid according to the definition of microgrid.2. Aiming at the relentless needs for establishing indices system for microgrid reliabilityevaluation, through analyzing the demands of reliability indices of microgrid according to theoperation and fault property of microgrid, the necessity and limitation of adopting the reliability indices used in distribution system is discussed. It shows that the new practicalindices for microgrid reliability evaluation need to be put forward. Then, this dissertationproposes microgrid reliability indices from four main aspects, those are, the equivalentfeatures of microgrid, the output characteristics of intermittent power source, energy storagedevice, the islanding state of microgrid, and the efficiency of microgrid. Meanwhile, a seriesof definitions and formulas for calculating those indices are given, and their meanings arediscussed. The numerical results of the case demonstrate that the proposed indices can reflectthe characteristics of reliability evaluation of microgrid. It provides the base for establishingthe microgrid reliability indices system.3. In order to evaluate the reliability of microgrid considering correlativity of DG output,the Nataf probability transforming theory and multi-Gauss numerical integration method areused to establish the mathematical models for correlated variables which obey non standardnormal distribution. Further more, on the base of considering the outage and the nonlinearoutput of DG, the power generation system reliability model is established for the microgrid,and the Monte-Carlo method is used to solve it. In the case, the factors including DG outputcharacteristics, total DG capacity vs. load capacity, and correlation of primary energy, whichinfluences microgrid generation reliability indices, is quantitatively analyzed. The resultindicates that the proposed model and method is valid and general.4. On the basis of analyzing the control strategies of the point of common coupling(PCC)and microsource(MS), aiming at the characteristics that islanded microgrid need to reduceloads because of insufficient generation capacity and the microgrid has bidirectional powerflow, the tactics and optimization algorithm for load shedding of islanded microgrid ispresented, considering the power flow and operation safety constraints. The numerical resultsshow that the proposed model and method, which considering the active and reactiveconstraints, can provide the load state concerning loads shedding for evaluation of microgridreliability.5. On the basis of analyzing the operation and control characteristics of the microgrid,the method for solving reliability indices of loads is proposed. The hybrid method based onMonte-Carlo and fault mode and effect analysis (FMEA) is applied to evaluate the microgridreliability, under a series of influence factors of utility’s outage, MS failure, line’s fault, the generation characteristic of MS, loads shedding of islanded microgrid. The numerical casesevaluate the adequacy of microgrid with different kind of MS. Through analyzing, it provedthat the proposed model and method can thoroughly assessing the microgrid reliability.6. The microgrid power source planning model for the aim of optimized comprehensiveperformance is built, under certain restrictions including reliability indices. The dynamicprogramming method is used to convert the planning model into a two-stages-decisionprocess. The GAOT tool based on Matlab are combined to solve a numerical case. The resultshows that the reliability evaluation model proposed by this dissertation can be applied inpower source planning of microgrid effectively.