Research On Reliability Evaluation Methods Of Microgrid And Active Distribution System

Microgrid(MG)and active distribution system(ADS)are two emerging paradigms for effective integration of renewable distributed generations(DGs).MG integrates renewable energy with loads to achieve regional autonomous operation,while ADS can integrates large-scale renewable energy and MGs to achieve global optimal operation.These new distribution systems show many different features from the traditional distribution network,such as the power fluctuation of intermittent renewable DGs,the temporal correlation of energy storage devices,the interaction of loads,the flexibility of islanded operation mode and operation strategy,as well as the complexity of protection and control systems.These features aggravate the complexity of fault effect analysis and make it difficult to evaluate the reliability of the distribution system.Based on the reliability evaluation method of traditional distribution system and the analysis of the characteriscs of MG and ADS,three evaluation methods are proposed for different application scenarios in the reliability studies of MG and ADS.The system reliability can be quantitatively evaluated and studied from different perspectives by the proposed methods.The main works in this dissertation are summarized as follows:1)An improved network equivalent method and a reliability evaluation method based on fault impact matrix are both proposed for the traditional complex distribution system.First the the concept of zone node is defined and a complex distribution system is simplified at two levels on the premise of ensuring its accuracy.Subsequently,zone node adjacency matrix,fault influence mode matrix and fault influence parameter matrix are defined,and load zone node reliability indexes can be obtained based on matrix operations.Based on the reliability indexes of load zone nodes the feeder and system reliability indexes are calculated easily.The effects of failure and refusing action of various switching devices are all taken into account in this evaluation method,and the practical engineering problems are modeled in a succinct mathematical form,which is easy to be programmed.2)Aiming at power fluctuation of intermittent renewable DGs such as wind and solar based generating systems,a fast probabilistic evaluation method based on hybrid computation combining analytical method and Monte Carlo simulation method is proposed for fast pobabilistic evaluation of MG island operation reliability.A probability correlation model of DG power output and load demand is established based on Latin Hypercube Sampling(LHS)technology combined Choleskey Decomposition method,so as to calculate the reliability index of loss of load probability(LOLP)in island.With a practical load shedding strategy that the curtailed load would not be restored in short-term island until reconnected to main grid,the probability distributions of load point interruption duration can be calculated using a set of analytical formulas derived from a proposed geometric distribution model.On this basis,reliability indexes of load point and microgrid can be obtained comprehensively.Compared with Monte Carlo simulation(MCS)method,it is faster in calculation with the same accuracy in this method in which source-load fluctuation is modeled accurately,and over-optimistic estimation of load point interruption frequency index can be avoided effectively.The practical load shedding strategy adopted in this method avoids the adverse effects of load frequent interruptions,and it is more suitable for engineering application.3)An improved sampling technique and a practical island scheduling strategy are proposed to evaluate the reliability of microgrid containing large capacity energy storage with island balancing capability.A practical island global optimal scheduling model with island load shedding frequency constraint and load continuous operation duration constraint is established,which eliminates the adverse effects of load frequent interruptions and short-time intermittent operation in many existing strategies.Sequential Latin Hypercube Sampling(SLHS)technique is utilized to select system failure state of distributed network including MG,and the established island scheduling strategy is integrated into the Monte Carlo simulation of island state assessment so as to calculate the load point and system reliability indexes.Evaluation results of the example system show that the proposed scheduling strategy improves the reliability of MG on the premise of ensuring economy,and it is helpful to ensuring the safety of electric equipment and improving customer satisfaction.Compared with the simulation evaluation method using random sampling,the SLHS technique applied in this method can greatly reduce the simulation calculation time.4)For the reliability evaluation of ADS with MGs,a Monte Carlo simulation method considering practical island coordination optimal scheduling strategies is proposed.Two configuration schemes of sectionalized circuit breaker and interconnection tie line between MGs installation in appropriate locations are proposed,and island operation modes of ADS after fault isolation are divided into simple island and joint island.Subsequently,two improved optimal scheduling strategies,centralized coordination strategy and hierarchical coordination strategy,are proposed for joint island.The sampled system failure state is preliminarily evaluated by using the proposed reliability evaluation method of traditional complex distribution system based on the fault impact matrix,and operation mode of each zone together with its mode duration are determined quickly.Practical island coordination scheduling model is integrated into Monte Carlo simulation of joint island state assessment in this method.Evaluation results of the example system show that the configurations of sectionalized circuit breaker and interconnection tie line are of greatly benefit to the reliability of local region.Centralized coordination scheduling strategy adopted in ADS can effectively improve the overall reliability of joint island,and its comprehensive promotion effect is better than hierarchical coordination scheduling strategy,while hierarchical coordination scheduling strategy can better deal with diversified reliability goals of different MGs.The research works and results in this dissertation are helpful to correctly evaluate the reliability of MG and ADS,and can be used as the references for the planning and operation of MG and ADS,which will promote the practical application of DG in distribution network.