Research On Forward Failure Diffusion Algorithm For Reliability Evaluation Of Distribution Networks Considering Capacity And Voltage Constraints

With the growing impact of distribution system failures on users, the distributionsystem reliability has been increasingly concerned and emphasized. Therefore，it isessential to research on an efficient and practical distribution network reliabilityassessment algorithm.In this paper, a full access to domestic and foreign reliability assessment research ismade, the features of reliability assessment for a medium-voltage distribution networkare pointed out, the definition of complex distribution networks is given, and theexisting research status on reliability assessment of distribution networks is presented aswell. In the aspect of network connectivity analysis, several commonly used analyticalmethods are analyzed and compared, and their scope of application is shown. To obtainthe outage and isolated ranges, existing methods have to carry out the network topologyanalysis once for each failure event or load point.In order to solve the executing time problems of network topology searching andpower flow calculation, an efficient forward failure diffusion algorithm of distributionnetwork reliability assessment is proposed with the capacity and voltage constraintsbeing considered, Firstly, an efficient node optimization ordering method is used fordistribution network nodes so that the network topology searching can be easy andquick. Secondly, network elements are traversed using father searching method in adescending numbering order: when a node is reached, it‘s determined whether itsdownstream nodes may be incorporated into its father branch and father node’s outagerange or isolated range according to the switch setting situation of its father branch.Then, all load points are classified into four categories according to their differentoutage time (no outage, isolating and locating time and switching time of the upstreamswitch, isolating and locating and switching time of interconnection switch, outageduration) resulting from an element. Also, A quick searching and calculation method isproposed with the element current-carrying capacity, node voltage constraint and a dailyload curve being considered：only one time of father searching is needed from the endof each lateral branch to calculate and record the maximum voltage drop of the lateralbranch; the power change and node voltage change of any main trunk can be identifiedand estimated easily and quickly from both ends of the corresponding tie line usingfather searching, and the voltage constraint of each lateral branch’s nodes can be considered automatically according to the lateral maximum voltage drop which isrecorded in advance. At last, an annual outage time of each load point is calculated byweights using the node classification and the load transfer proportions resulting fromcapacity and voltage constraints. Each period’s power can be estimated by global loadfactor with a daily load curve considered.With the proposed algorithm being employed, all the elements’ outage ranges,isolated ranges and alternative supply ranges can be found with only some times ofelement traversing and the traversal times of distribution network reliability assessmentare weakly correlated with the network size, resulting in many less traversal times for alarge scale distribution network and much less searching and calculation time with theelement current-carrying capacity, node voltage constraint and a daily load curve beingconsidered.The reliability assessment is conducted for two typical distribution networks in twocases with capacity and voltage constraints being ignored and considered using theproposed algorithm. The results is analyzed and compared to other literatures’ results.Examples’ results show that the proposed algorithm has a relatively small executingtime with the requirement of engineering accuracy being met.