Research On Communication-Network-Based Resilient Protection Strategy For Distribution Systems

In the context of the development of renewable energy-dominated novel power systems,distribution systems are now transitioning from the conventional role of unidirectionally transferring electricity into a new one of integration and distribution of multi-energy,due to which the technical structure of distribution system protection is now confronted with emerging challenges.For a long time,because of the phasic and regional development gaps of distribution systems,insufficient configuration or unbalanced deployment issues of protective resources,such as potential transformers,circuit breakers,etc.,linger in the field of distribution system protection,compromising the capability of fault sensing and handling.Besides,influenced by the complicated operational environment of distribution systems,some abnormal operational issues,such as sampling failures,breaker failures,unexpected operations of protection relays,etc.,still occasionally occur,which bring about operational risks to distribution system protection.In today’s new situation,these issues,now gaining more concerns,would accent the presence of protection configuration differences and protection operational failures.To cater to the new security and operation requirements of distribution systems in the new era,two feasible solutions have been introduced to distribution system protection.The first one is the “rigid upgrade” of protection system hardware configurations,which follows the configuration modes of transmission systems to enable a holistic equipment upgrade for distribution systems.The second one is to establish a new solution combining with digital development process,which integrates the independent configuration association from a“resilience promotion” perspective and facilitates system protective resource interoperability and protective function complementariness.The solution of “rigid upgrade” is limited by investment factors and environmental conditions and thus,the protection configuration issues still remain unsolved,and more difficulties and risks in the coordination of protection system are inevitably introduced.Meanwhile,with the tremendous driving forces of artificial intelligence,cloud computing,internet of things and big data,communication-network-based protection,a new form of distribution system protection,has now derived.By taking advantage of the information sharing attribute of communication-network-based protection,distribution system protection can integrate protective resources in a much larger range and coordinate global resources and functions of the whole system to counter operational disturbances faced by protection system.The new implementation of communication-network-based protection is now stimulating the considerable potential of “resilience promotion” of distribution system protection.It is becoming a vital development trend for future distribution systems to build a resilient protection system besides upgrading the protection configuration.Accordingly,to cope with the aforementioned protection configuration difference issues and protection operational failure issues,this paper takes into account the resilience promotion requirements of distribution protection system and conducts a series of research on communication-network-based resilience promotion methods as well as applications.Centering on two resilience promotion perspective of flexibility enhancement and robustness enhancement and covering the full-chain of protection functions,i.e.,sampling process,action process,and coordination process,we proposed the resilience promotion targets of distribution system protection to maximize the exploitation of protection configuration resources and minimize the impacts of protection operational failures.On this basis,communication-networkbased enhancement strategies and optimization methods for resilient distribution system protection are presented.Based on the strategies and methods proposed in this paper,a communication-network-based resilient protection system as well as its devices have been developed in cooperation with relay manufacturers and were successfully implemented in the smart grid demonstration projects of China Southern Power Grid.The main works of this paper includes:(1)A communication-network-based resilient protection architecture is proposed.According to the two perspective of flexibility enhancement and robustness enhancement for resilient protection,and centering on the full-chain resilience promotion targets of the distribution system protection sampling process,action process and coordination process,communication-network-based optimal measuring models,action models and coordination models are established.Virtual measuring equations are formulated and the state estimation theory is deployed in this paper to estimate the values and calculate the errors of virtual measurements,through which the quantitative analysis and advantage characterization of observability of communication-network-based protection in the sampling process are conducted.Full-response action equations and dynamic coordination equations are formed,combined with the topologies and configuration relation deduction,to provide the quantitative analysis and advantage characterization of action performance and coordination complexity of communication-network-based protection in the action process and the coordination process.On these bases,the implementation model and technical architecture of communicationnetwork-based resilient protection for distribution systems are constructed.(2)A virtual-measurement-based resilient protection method with “redundant” samplings against sampling anomalies is proposed.By calculating virtual measurements,the redundancy of samplings can be obtained.The sampling features of protection systems can be depicted using numerical analysis of the error distribution between virtual measurements and actual measurements.The proposed method can identify the sampling feature differences between the faulty status and the sampling abnormality status of distribution systems and thus,provide the reference to screening abnormal samplings and limiting fault ranges.Accordingly,a communication-network-based auxiliary protection block criterion is formulated.The resilience of protection system is improved by fully integrating sampling resources and effectively coping with sampling anomaly issues.(3)A virtual-measurement-based resilient protection method with “supplementary”samplings against sampling missing is proposed.By calculating virtual measurements,the missing samplings can be supplemented.Furthermore,the protection features of protection systems generated by protective criteria with the supplementary sampling inputs are matched with their expected values of each fault scenario.On this basis,the proposed method can screen out the corresponding fault scenarios and accordingly,formulate the communication-networkbased complementary protection action criterion.The resilience of protection system is improved by fully and flexibly supplementing sampling resources and effectively tackling sampling missing issues.(4)An “equivalent” full-response-action-based resilient tripping strategy for distribution system protection is proposed.With respect to the differentiated switch configurations,the proposed strategy provides a communication-network-based full-response tripping mechanism considering the sequential coordination of multi-switches,which can replace the conventional tripping strategy with complete configuration of circuit breakers.Switch resource sets considering full-response actions are defined and the state transition theory is applied to track the variations of the switch resource sets in the process of fault handling.Based on the dynamic optimization of switch resource sets,the resilience of protection system is improved with optimal operation combination and optimal operation coordination.(5)A dynamic-coordination-based resilient timing optimization strategy for distribution system protection operation “acceleration” is proposed.With respect to diversified protection principle application and differentiated protection relay configurations,the proposed strategy presents a two-stage timing optimization method and network-based communication implementation mechanism,which can strike the balance between initial timing setting from the configuration perspective and dynamic timing adjusting from operation perspective.On this basis,the proposed method can dynamically extract the signals of protection response to fault conditions and thus,the resilience of protection system is improved with automatic cooperation adjustment and protection operation acceleration.(6)An engineering application of communication-network-based resilient protection for distribution systems is implemented.Based on the resilience promotion methods proposed in this paper,a communication-network-based resilient protection system and its devices are developed in cooperation with relay manufacturers,which have passed the inspection by thirdparty authority institutes and the RTDS closed-loop test.Besides,a pilot application have been successfully implemented in the smart grid demonstration project of China Southern Power Grid.This research is sponsored by the National Natural Science Foundation of China and several science and technology projects of China Southern Power Grid.The research achievement has won the first prize of science and technology progress of China Southern Power Grid and the first prize of China Electrical Technology.