Research On Several Important Issues In Power System Reliability Assessment

With the surging demand to transfer electricity from the western grid to the eastern grid as well as the interconnection of the southern grid and the northern grid, the electrical power grid of China has developed into a long-distance and huge capacity interconnected system with AC and DC transmission lines in parallel. However, potential system contingencies may lead to even greater losses due to this highly interconnected system structure. In recent years, blackouts in interconnected power system frequently happened at home and abroad. Thus, research on power grid reliability and safety is of great engineering value and practical significance. Several major issues in power system reliability analysis are presented in this dissertation. The main contents and the related conclusions of this dissertation are outlined as follows:(1) Statistical analysis of power system basic reliability data is focused on. For the first time, a customer interruption costs investigation method that is fit for the characteristics of customers in China is proposed. This investigation method is discussed in detail in Chapter 3. Three new concepts, PAC (Preparatory-Action-Cost), WTP (Willing-to-Pay) and WTA (Willing-to-Accept) are proposed for the first time. The average reliability indices for power generation and transmission units in China during the past five years (2002-2006), as well as indices for six HVDC systems in China during the time frame, are calculated. Analysis on the power system operational reliability status is carried out based on the calculation results.(2) Reliability assessment of the relay protection system is studied. In Chapter 4, a general Markov model for reliability analysis of ultra high voltage (UHV) power grid relay protection system is presented. This model takes into account the characteristics of self-checking and online monitoring of the protective relays as well as their impacts on its reliability. This model also account for the redundant configuration characteristic of the current UHV power grid line protection. Based on this model, the reliability of the main protection operation and the availability of the protection system are analyzed, and the impacts of element failure rate on the system availability are presented. The principles to determine the optimal interval for routine test of protective relays are also discussed.(3) A reliability assessment method, called the GO methodology, is introduced to the field of protective relay system reliability assessment. The research result shows that we can analyze the system quantitatively and accurately based on the derivation of status probability formula. However, when the system contains shared signals, this method may become very complicated. A new quantitative calculation method is discussed to circumvent the complicated procedures in the conventional GO method. This new algorithm has been tested on the GO program. In addition, the fault tree analysis method is also used to analyze the same system, and the analysis results are completely in accordance with the new algorithm based on traditional GO method. In the end, the two methods are compared and the comparison shows that the new algorithm based on traditional GO method has superior performance over the fault tree analysis method.(4) The theory and algorithm of the GO methodology is further discussed in the reliability analysis of a typical repairable system - the UHVDC system. Considering the actual operating mode of UHVDC system, the equivalent model and algorithm of serial structures with correlations are discussed. Based on the above discussion, the corresponding GO program is developed. Based on the developed program, the reliability parameters of all the signal flows in the system can be calculated exactly considering the components' correlation. The analysis process shows that, application of GO methodology for reliability analysis of repairable system is much more complicated than un-repairable system. The necessity and advantage of the GO methodology in repairable system are proved through comparison between results with and without consideration of correlation of system components.(5) Configuration of large-scale power system out-of-step separation, which is the third defense-line of power grid in China, is discussed. Through the simulation studies of a typical large-scale power system, the China Northwest Power Grid, a new finding on large-scale power system ouf-of-step configuration as well as two new concepts - the "main out-of-step mode" and the "minor out-of-step mode" are proposed. The detailed study and analysis in Chapter 7 may inspire further improvement to the method of selecting separation points that is widely used in engineering applications nowadays.