Research On Reliability And Risk Assessment Of Protection Systems

As the first defense line of the power system, protective relays should be selective, speedy, sensitive and reliable. The reliability of protection systems is one of the most important factors that effect the stable operation of power systems, as well as an important reference for protection design and operation. Based on investigation of the reliability requirement of protection systems, this thesis focuses on the research of objective and accurate reliability model and reliability assessment method according with the characteristics of protection systems.Based on the concept and features of protection system reliability,'3RF'technique, i.e. Relay Failure Mode Effect Analysis (RFMEA), Relay Fault Tree Analysis (RFTA), Relay Failure Reporting, Analysis and Corrective Active System (RFRACAS) is firstly proposed mainly for reliability assessment and improvement of protection hardware system. It is a set of cooperative procedures related to failure prevention, failure analysis, failure elimination and basic data collection, which could be used for design, development and maintenance of protection systems. The RFMEA supplies modeling basis for the RFTA by analyzing failure modes and their effect. The RFTA with stronger adaptability includes reliability model of protection system and its analysis methods, the model integrating Markov state space and Dynamic Fault Tree(DFT) takes into account dynamic characteristics of protection system such as fault and repair, topology and logic, spare and block, each part of which has a definite physical signification, so it is easy to implement modular modeling; the indices calculation method based on the structure function of DFT and the sequential Monte Carlo simulation takes advantage of Markov chain and Monte Carlo simulation while reduces the state-division difficulty of Markov method and solution complexity of system-level fault tree model. By quantitative calculation of reliability indices, they could supply reference for weakness identification, design improvement and maintenance cycle establishment. The RFRACAS is an information system which supplies data basis for protection reliability assessment by collecting timely and exact failure information and defect information. As a whole, the3RF technique helps to save time and cost under the same reliability requirement, while to improve protection system reliability under the same investment.Secondly, a two-level calculation model on failure probability of protection principle is built, which is based on Programmable Time Petri Net (PTPN) finishing dynamic simulation of protection logic and sequential characteristics, and Fuzzy Petri Net(FPN) finishing probability calculation of protection elements and their combination. It could be used to simulate protection logic as well as to calculate instantaneous failure probability of protection principle according to system operation mode and protection settings. It supplies reference to research on matching degree between protection characteristics and specific faults or operation modes, and overcomes the shortage of routine method which ignores the effect of real-time operating conditions and protection settings on protection systems reliability.Thirdly, on the basis of recent development on communication-based protection, the reliability assessment of the support technique in this type of protection systems, i.e. the reliability of communication systems is presented. The major focus is put on the hierarchical analysis and synthenic assessment of connection reliability of the communicateon system in limited wide area protection which could comprehensively reflect related communication systems.Finally, a protection system reliability assessment method considering competition of failure modes is presented based on protection failure machanism analysis, which gives full consideration of combined effects of different failure modes on protection reliability in an unified model. A new way of integrated protection risk assessment method is proposed in succession which takes into account the combination of reliability analysis and vulnerability assessment as well as the combination of risk assessment, protection action sequence and protection coordination, it strictly follows the sequential characteristics of associated relay actions and therefore improves the authenticity and accuracy of protection risk assessment. The study cases show that the risk assessment contributes to the online monitoring and analysis of protection reliability level, the protection setting verification and the awareness of potential problems of protection systems and power systems.