Study Of Wide-area Backup Protection Principle And Its Communication Technology

With the fast development of power systems and the continuous advancement ofelectricity market operation, the structure and operation environment of power gird areincreasing complicated. The relay protection is a critical component to ensure the operationsafety of power equipments, which is also the “first defense line” of the safety guaranteesystem of power grid. The performance of relay protection has significant influence on thesafe and stable operation of the whole power systems. Traditional backup protectionsmainly employ partial information, like local information or port information of theprotected components, for the detection and judgment of fault. The coordination betweendifferent protection devices is realized by fixed settings, which causes poor adaptability ofprotection devices and complicated coordination of settings. In recent years, with the fastdevelopment and gradually maturity of wide area synchronous measurement technique, thestudy of novel wide area backup protection based on wide area measuring information hasattracted extensive attention. The application of wide area measuring information providesa new technical measure to improve the performance of relay protection substantially. It ishelpful to solve the problem of poor adaptability and complicated coordination of settingsof traditional relay protection. It can also improve the safety and reliability of protectiondevices in complicated fault and abnormal operation conditions. It is of very importanttheoretical and practical significance to ensure the operation safety of power equipmentsand power grid and prevent the occurrence of large-scale blackoutsSupported by the National Natural Science Foundation of China (No.50837002), thefundamental problems such as the principle of novel wide-area backup protection system,network communication and reliability are analyzed and studied in depth.So far, one or more of the following problems exist in the proposed wide area back upprotection algorithm: high requirement for sampling synchronism, large amount of widearea communication traffic and low fault tolerance of fault identification. Aimed at these, a novel wide area backup protection algorithm which has low requirement for samplingsynchronism, small amount of wide area communication traffic and high fault tolerance isproposed in this paper. Centralized wide area protection system structure is adopted. Whenfault happens, slave stations in the area identify the transmission line which is nearest tofault element with amplitude comparison of group branch currents. And then the slavestations transmit the fault current of these transmission lines to master station. Then masterstation will choose a few suspected fault lines as candidate lines in accordance withreceived current fault components and their distribution characteristic. After that, masterstation will acquire the operation situations of traditional protections and directioncomponents of these candidate lines from slave stations. In the end, the information fusionof these multi-source evidences is carried out with modified D-S evidence theory toidentify the practical fault line. Simulation results show that the proposed algorithm iscorrect and has good fault tolerance of information.With the development of the smart grid, information modeling and informationexchange of intelligent electronic device in the smart substation will be unified under theframework of IEC61850. IEC61850standards will surely become one of the main criteriain the future smart grid field. But for now, the communication model for wide area backupprotection is not described in IEC61850standards, which has been an important problemneeded to be solved in practical application of wide area backup protection. For this reason,a generalized IED modeling procedures in wide area backup protection is presented firstlyaccording to the layer-on-layer modeling method of IEC61850standards. Then taking the220kV digital substation and the proposed algorithm of wide area backup protection forexamples, tree structure of master station and slave stations of wide area back up protectionin accordance with the principles of IEC61850standards are given. And the interactiveprocess model of message flow between master station and slave stations are given. Finally,a communication service model is established, including the transmission model betweenclients and server, the transmission model for analog value, and the transmission model forswitching values. The proposed communication modeling method for backup protectionhas characteristics of good adaptability, small communication traffic of information servicemodel, high transmission reliability. It can satisfy the application requirements of wide area protection with different structures and principles.As wide area communication system is the support for information interaction of widearea backup protection, excellent communication system plays a critical role to wide areabackup protection correctly and rapidly removing the failure of power grid. But the existingwide-area communication system still cannot meet the requirements of wide area backupprotection. Therefore, according to the communication features of wide area backupprotection, the communication system for wide area backup protection including wide-areacommunication network and service model is designed. In the communication networkaspect, the Ethernet over SDH (EOS) transmission mode by Ethernet access way isproposed, and the network structure of wide area communication system based on EVPL isestablished. The parallel redundancy protocol (PRP) is adopted for station and wide areacommunication network to realize zero recovery time in communication failure condition.In order to estimate the property of wide area communication system, evaluation indexbased on network time delay, time delay variation, throughput, utilization factor ofbandwidth and packet loss ration is proposed. Taking network system of IEEE14nodes forexample, the overall performance of proposed wide area communication system isanalyzed. The feasibility is verified.Wide area backup protection is much more complicated than traditional backupprotection in aspects of construction mode, decision-making principle and implementationtechnique. Specially, it involves wide area communication that makes the physicalsimulation and performance evaluation very difficult. According to the structure andoperation characteristics of wide area backup protection, a synchronous co-simulationplatform called Wide Area Protection Synchronizing Simulator (WAPSS) associated withOPNET and PSCAD/EMTDC based on HLA (High-Level Architecture) and Agent isdeveloped, which provides a effective tool for overall performance analysis and evaluationof wide area backup protection under the environment of network communication. Thestructure of WAPSS platform, the function of important components, interface model andthe Multi-Agent design are discussed in detail. A simulation example is implemented toverify the correctness and availability of the platform.Compared with traditional protection, the wide area backup protection which is relatedto numerous components has big change in protection principle, implementation and action scope. Hence, the number of the influence factor of reliability increases. The study on thereliability of wide area backup protection is of important significance. The wide areabackup protection system is divided into several subsystems to study respectively. Withcomprehensive consideration of the failure and repair process of various components, thereliability models of various subsystems and the whole system are built up with utilizationof Markov state-space method and fault tree analysis method. In the end, the reliabilityevaluation of wide area backup protection is carried out by combining fault tree structurefunction and Monte Carlo simulation.Finally, the achieved research results are concluded and prospective about the futurestudy is highlighted.