Reliability Assessment Of Micro-grid Based On Markov Model

With the global energy crisis and environmental issues becoming increasingly prominent,coupled with the increasing demand for power supply reliability,utilization of renewable energy has attracted more and more attention from all over the world.As a single controllable unit,micro-grid can absorb distributed generation(DG)supply effectively,reduce loss,improve power supply reliability and relieve the energy crisis and environmental pollution pressure.However,micro-grid access changes the single source radiating structure of traditional distribution network fundamentally.In addition,DGs in micro-grid are random and fluctuating,such as wind power,photovoltaic power generation and other renewable energy generations.The reliability evaluation model,method and reliability indexes of traditional distribution network are difficult to adapt to the new situation.In view of the above problems,this paper studies the reliability evaluation of distribution network with micro-grid.In this paper,the major research focus on the research status of micro-grid at home and abroad and the problems in existing research of micro-grid reliability.Firstly,the definition and structure of micro-grid are summarized,and its operating characteristics are analyzed.In order to evaluate the reliability of distribution network with micro-grid comprehensively,this paper compares and analyzes the common reliability evaluation methods,and finally selects the minimal path method.The article is divided into two parts,one is the distributed generation modeling,and the other is the micro-grid reliability analysis.Summarized as follows:(1)This paper proposes a method of DG modeling based on improved Markov chain.Wind power(WT),photovoltaic(PV)and energy storage systems(ESS)are considered in this paper.ESS just meets the charge and discharge constraints and energy constraints.WT and PV modeling ideas are as follows:based on the original first order Markov chain,considering the diurnal,seasonal and weather characteristics of WT and PV,this method directly use WT and PV output history data to generate multiple state transition matrix.The target states are generated through state transition according to climate characteristics and time attribute discrimination selection.The modeling method proposed in this paper,eliminate wind speed and solar irradiance modeling and energy conversion process,without two or multiple modeling,which making WT and PV model more accurate and concise.(2)Analytical method is applied to small power system with simple structure.In this paper,the structure of the micro-grid is simple,and the minimal path method is selected to evaluate the reliability.In addition,traditional reliability indexes cannot evaluate the contribution of micro-grid to the reliability of the whole distribution network comprehensively.New assessment indexes containing system average interruption frequency reduction coefficient MSAIFI,average service availability enhancement coefficient MASAI and expected energy not supplied reduction coefficient MEENS are proposed in this paper.And the influence of different DG access capacity and access position on micro-grid load and the distribution network reliability are analyzed.The improved IEEE RBTS Bus6.testing system is analyzed,and the conclusion can be obtained as follows:?micro-grid can improve micro-grid load and distribution system reliability effectively,?with the increase of micro-grid capacity,micro-grid load and distribution system reliability are constantly increasing,but the increasing trend is more and more gentle,? micro-grid position influence its load reliability obviously,the reliability of load close to DG is significantly higher than the same level load far away from DG.Finally,the new assessment indexes proposed in this paper are proved to evaluate the contribution of micro-grid to distribution system reliability more directly and accurately through a numerical example,which can guide the planning and construction of micro-grid more scientifically.