Study On Recognizing The Reliability Non-coherence Components Of Power Systems

Generally, adding an electrical power component to a power system, such as generating unit, transformer and transmission line, can improve the system reliability performance, which is the so-called reliability coherence phenomenon in power systems. Many research achievements on the power system reliability evaluation are based on this rule. However, the reliability non-coherence (RN) phenomenon exists in power systems due to the nonlinearity of power systems and their reliability evaluation models, which refers to the fact that if one or more components are added to a power system, the system reliability would not be improved, or even become worse. This component is designated as the reliability non-coherence component (RNC). The RNC not only increases the component capital investment and maintenance cost, but also reduces the system reliability performance.Supported in part by the National Natural Science Foundation of China ("Study on the RN mechanism and recognizing the RNCs of power systems", No. 50777067), this thesis studies the existence mechanism of the RN phenomenon, non-coherence indices, and methods for recognizing RNCs. It mainly focuses on the RN feature, influencing factors and RNC recognizing techniques of multiple parallel transmission lines (MPTL) with and without considering substation configurations, distribution substation configurations (DSC) and composite generation and transmission system (CGTS). This study can provide a useful decision information for power system reliability optimization and operations, and is of important significance to the reliability and economy of power system operations. This thesis mainly includes the following contents:â‘ The power system RN mechanism is analyzed, and the index system used to describe the degree of the system RN feature is proposed and applied in the RN analysis of MPTLs and DSCs. The RN mechanism of the simple series, parallel systems and complex systems is analyzed, and the impact of adding a component on the system reliability is divided into two parts: positive effect and negative effect. If the positive effect is larger than the negative effect, adding a component can improve the system reliability and the component is the reliability coherence component. Otherwise, the component is a RNC. The Expected Energy Not Supplied (EENS) index or other indices is used to act as the RN judgment standard for the complex power systems, and the RN index system is proposed to describe the degree of the system RN feature, which includes: RN probability, RN frequency, expected RN time and expected RN energy.â‘¡The RN features of MPTLs with and without considering substation configurations are analyzed, the RN identification models are built based on the analytical reliability evaluation model, and the analytical model of the RN threshold load (RNTL) is obtained. The reliability analytical models for double-circuit transmission line (DCTL) system, three-circuit transmission line (TCTL) system and four-circuit transmission line (FCTL) system are deduced. The RN identification models for MPTLs are also built based on the definition of RN, and the analytical model of the RNTL is obtained based on the transmission line rated capacity and component reliability parameters. Without evaluating the system reliability, the RN feature of a MPTL can be judged using the comparison between the system load and the RNTL. If the system load is larger than the RNTL, the system has the reliability coherence feature; otherwise, the system has the RN feature. In addition, the impacts of component reliability parameters and load levels on the RN feature, RNTL and RN indices are also analyzed.â‘¢The RN features, the existing conditions and influence factors of the RN phenomenon for DSC including the sectionalized single bus and bridge connections are analyzed, and the RNC identification model for DSC is built. The simulation results show that load level is the most important influence factor of RN feature. The component reliability parameters have a little impact on the system RN features. Only the failure rate of breakers can change the system RN feature; other parameters, such as the repair rate of breakers, the failure rate and the repair rate of transformers, only affect the system reliability and can not change the non-coherence feature.â‘£The reliability changing rule before and after removing a component from CGTS is analyzed. The identification indices of RNC in CGTS, which can be used to recognize the RNC, are discussed. In order to reduce the computation complexity of recognizing the RNCs, the thesis analyzes the RN phenomena of CGTS by using the experience of contingency ranking technique for power system, and proposes the indices, such as the transmission losses, system loadability, the minimum security margin of transmission lines and the weighted betweenness of transmission lines, or the comprehensive index which obtained by weighing the before-motioned indices, to recognize the RNCs of CGTS. The IEEE-RTS 79 is used as an example to illustrate the rationality of the proposed concepts. The analysis shows that there are significant correlation between the first three indices and the system reliability indices; however, the rank of the indices can not reflect perfectly the reliability changing rule, and the comprehensive index only recognizes some but not all RNCs in power systems. Therefore, although the algorithm can identify the RNCs preliminarily, the identification indices for CGTS still need more research.