Reliability Study Of Protection System Based On Smart Process Bus Technology

Substations are key nodes of smart grid, deliver energy from remote generation to load centers. Protection relays play an important role in substation to ensure safety and stability of the power system. The IEC61850standard is applied in smart substation network communication, and the switched Ethernet based process bus is introduced to replace the miles of copper cables between the primary apparatus in the switchyard and secondary system in the control building. Compared to the traditional relay protection, the first generation of process bus based digital protection system differs in many ways such as the physical layouts and operating mechanism, so its reliability needs further study. Especially the inherent defect of Ethernet network (such as real-time performance and security problem) may decrease the reliability of protection function. Hence, the performance evaluation of process bus has very important significance for the reliability analysis and network optimal design of protection system. To maintain the reliability of protection system, the preventative maintenance is required. How to utilize limited resources to keep the best condition of protection system is a topic which is worth studying.Based on above discussion, this dissertation has made a deeply study of the reliability of process bus based digital protection system. The main works and achievements are as follows:1) According to the working principle of switched Ethernet and the publisher/subscriber mechanism specified in IEC61850, the minimal space model of network is proposed to construct the optimal switch space of process bus. The multicast configuration method with respect to the switch space is presented which provides a theoretical basis of optimal design of process bus network. 2) The block models of two types of process bus data (synchronous and non-synchronous data) are deeply studied. The transmission delay calculation formula for the certain priority message is derived for the QoS based communication. A message capture time based evaluation method of network latency which can work without the help of synchronous clock source and provide very high measurement precision is proposed. It can be used for the real-time monitoring and diagnosis of process bus network performance.3) A process bus simulation system is developed, including network traffic generator and capturer. The network performance of two process bus solutions for a typical substation is tested using the digital protection simulation system, which validates the availability of proposed models and methods.4) The communication failure of process bus is investigated. The influence of communication failure on protection function is analyzed from the aspects of safety and reliance. To assess the power supply reliability, the miss trip model of relay protection caused by communication failure is built. According to the transmission characteristic of sampled value, a method for detecting the fault of process bus is proposed.5) Several typical process bus network solutions are introduced considering the Chinese relay protection design criteria together with the redundancy protocols recommended by IEC61850-90-4Network Engineering Guidelines. For the reliability analysis of complicated network architecture, a reduced cut set method is presented and used to analyze the reliability of proposed process bus solutions.6) An optimal principle based reliability allocation method is presented for distributed protection system. The optimization model of reliability allocation is constructed to pursue two goals:the lowest totalcosts of system design and the highest reliability of protection system. The multi-objective problem is transformed into a single objective problem by means of the evaluation function method to solve Pareto optimal solutions.7) Considering the condition-based and periodic maintenance, the maintenance model of protection system is constructed by means of the state space method. A novel maintainability allocation method of protection system is proposed to plan the operation and maintenance of digital protection system.