Hierarchical Islanding, Protection And Control Of Distribution Network With Multi-MicroGrids

The needs to improve energy efficiency reduce greenhouse gas emission,and restructure of energy markets have favored the development of distributed generation(DG)technology.As the penetration of distributed generation increases,current recommendations for DG integration are no longer valid.The anti-islanding techniques are susceptible to common mode tripping,which can decrease the stability of total system and increase the requirement for spinning reserve.The intentional islanding of DGs,such as the microgrid and self-start island,can support local supplies for critical customers in the event of network failure,and thus increase the system reliability and maximize the use of DGs.That is,it plays a key role in active networks and becomes a valuable option.When more microgrids or DGs be integrated into electrical networks, not only single-user microgrid,but also a large number of muti-users microgrids can be formed.The new concept of multi-microgrids is related to a higher level structure,consisting of several microgrids and DG units connected to adjacent feeders.The efficiencies and advantages of microgrid offerings to customers and utilities,i.e.improved energy efficiency,minimisation of overall energy consumption,reduced environmental impact,improvement of reliability and resilience,network operational benefits and more cost efficient electricity infrastructure replacement,can be exploited and extended further through multi-microgrids concept.In this complex system,the scope and operating modes of multi-microgrids,the interaction and cooperation of multi-microgrids are critical to be investigated.To take full advantages of DG contribution and to improve the reliability and availability of active system,the existing control schemes and protection arrangements of the distribution networks should be regulated to accommodate more microgrids.The objectives of this paper are to investigate the operation,protection and control strategies of distribution networks with multi-microgrids.The operating modes of active system were evaluated by reliability analysis.The reliability indices of a typical network adopting reclosing, automatic backup switching,anti-islanding and intentional islanding were calculated respectively.The evaluation model and SAIFI-Island curve for evaluating the reliability were also derived.The results show that distributed generators often have a negative impact on reliability indices through re-closing failure and other improper operation strategies.The anti-islanding relay can rule out these negative impacts,and proper intentional islanding coordinated with auto reclosing can make an improvement on system reliability. According to the principle of power balance,proprietorship and network topology,the operation concept of hierarchical islanding of distribution network with multi-microgrids including Cell Islanding and Aggregation Islanding was presented.The main purpose of hierarchical islanding is to achieve stable islanding and make the best use of DG to improve system availability.An active system with more microgrids can be split hierarchically into Aggregation Islands and Cell Microgrids according to load matching when inter-ties faults or major system disturbances occur. Aggregation Islanding is always the first choice.When the inter-ties faults occur,the Aggregation Island is split from the grid and achieves stable islanding,the loads besides the Aggregation Island is reclosed by the grid. If the stable islanding can not be obtained,the Aggregation Island would be split into several Cell Islands,and the loads besides Cell Islands would be reclosed by the grid.When the grid supply is recovered,re-synchronize the Aggregation Island at first,if it fails to reconnect,the Aggregation Island should be splitting to Cell Islands.These Cell Islands will be reconnected subsequently.The benefits of hierarchical islanding include:(1)the Aggregation Islands and Cell Islands are formed by splitting,and the loads besides islands will be reclosed by the grid,so the seamless of seamless transition between interconnected mode and islanding mod is achieved;(2) the islanding and re-closing are combined to improve the reliability,and then the operation is not limited to the success probability of islanding;(3) the Aggregation Island is the maximum island for maximum exploitation of intentional islanding.To achieve seamless transition,the proper splitting strategy of DG islands is necessary.The splitting strategy of Aggregation Island is more complicated than Cell Islands because a combinatorial explosion of strategy space happens in a large-scale power system.This paper proposed rooted tree to model DG island separation according to the radial structure and service restoration process of distribution system.The level and directed properties of rooted tree were adopted to reduce the solution space from exponential to linear.Depth first search of node-weight rooted tree and edge-weight rooted tree were employed to determine island splitting strategies in the phase of planning and online-decision respectively.Simulations showed that by this method,proper splitting strategies can be found quickly.This method is effective for preplanned creation and dynamic creation of DG intentional islanding.The advantages of new method include:(1)The islands can be formed at the end of the feeders but not in the middle of the radial network,so the re-closing can be combined easily.The seamless transition,reliability improvement and smoothly service restoration can be also achieved.(2)The maximum island can be formed to support the most loads and reduce blackout area. (3)The algorithm has linear complexity to be effective for real time decision. The protection strategies of multi-microgrids system must be able to deal with both system and microgrid faults,which occur in the interconnected mode,Aggregation Islanding mode and Cell Islanding mode.The multi-agent systems and the wide-area protection based on longitudinal comparison were introduced into multi-microgrids system,and a three-phase protection strategy was presented.The first phase is local protection which the protection agents take action based on full local information.The second phase is area protection including primary protection zone and backup protection zone,which the agents in local area exchange knowledge by fully decentralized mode.In primary protection zone,the agent just coordinates with adjacent agents to protect the element it located.The protection of backup zone covers the adjacent elements and is just activated when the primary protection is disabled.The selectivity of wide area relaying protection system was achieved by wide area fault information instead of different operation time delay.The third phase is centralized backup protection,which executed by central protection agent based on wide area information of all agents.The proposed three-phase protection strategy has the advantages include:(1) the settings and performance of protection relays are independent of the fault location and operating modes,so the seamless transition between interconnected and islanding can be realized.(2)the protection performance in weak feed backside can be improved.(3)the degradation function,local backup protection and centralized backup protection can solve the problems of information deficiency,IED faults and mal-operation.The possibility of having a controllable Aggregation Island requires the use of hierarchical control scheme,which corresponds to the hierarchical islanding.The first layer is cell layer,including DG control agents(DGCA), load control agents(LCA)and Cell MicroGrid agents(CMGA).The second layer is Aggregation Island control agent(AICA),and the third layer is distribution network operator(DNO).CMGA manages the Cell MicroGrid; the AICA manages the operation of Aggregation Island consisting of several microgrrids,DG units and loads;and the DNO manages the whole local distribution network.The CMGA and AICA exchange information via high-speed communication network and realize functions as following:(1) Splitting into islands hierarchically.(2)Real-time load shedding.The power imbalance within an island is calculated with the measured power flow change of tieline,and the real-time load shedding is executed with the consideration of island regulation capability.(3)Re-closing and re-synchronization.To realize the smoothly service restore,the reclosing and resync are combined and self-adaptive to the operating modes.To improve the reliability of the management system,a two-phase control strategy was adopted.One is centralized decision,which the central control agent DNO collects all the information to make a decision. The other is local decision,which the CMGA and AICA make the decision according to local information and report these decision messages to DNO. If there is no feedback from DNO by a pre-set delay,the CMGA and AICA will take action by local information.$A decision support system for dispersed plant paralleling(DSSDP) based on expert system was programmed.The effects of distributed generation on power system operation were quantified by simulation results.The functions of coupling points decision,inter-connection devices checking,and protection & control system configuration were achieved on the platform of simulation database and rule base.The relay setting module was programmed based on rule-based reasoning,while the protection & control strategies configuration module and device selection module were designed based on mixed reasoning mechanism of rule-based reasoning and case-based reasoning.The hierarchical islanding,splitting strategies of Aggregation Island, protection and control system of the distribution networks with multi-microgrids were investigated to extend the microgrids concept and its efficiencies.As a result,the penetration of DG in electrical networks can increase without compromising the quality and roughness of the system which has to be safe,and the contribution and benefits of DG and microgrids can be fully exploited.