Simplified approach for the reliability estimation of large transmission and sub-transmission systems J. Van der Merwe Dissertation Presented for the Degree of MSc(Eng) in the Department of Electrical Engineering Faculty of the Built Environment UNIVERSITY OF CAPE TOWN February 2014 Eskom The copyright of this thesis vests in the author. No quotation from it or information derived from it is to be published without full acknowledgement of the source. The thesis is to be used for private study or non- commercial research purposes only. Published by the University of Cape Town (UCT) in terms of the non-exclusive license granted to UCT by the author. University of Cape Town I know the meaning of plagiarism and declare that all the work in the document, save for that which is properly acknowledged, is my own. i Acknowledgements I would like to acknowledge my co-supervisor, Prof Trevor Gaunt for his invaluable guidance and support with this dissertation. I would also like to thank my colleagues at Eon Consulting for their knowledge, ideas and guidance through this project. ii Abstract Various specialised power system reliability modelling software are commercially available to analyse the expected performance of a utility’s transmission and sub-transmission network. The software requires a physical network model to be constructed, representing all network components. A high level of accuracy is obtained using such software, but significant effort is required to create these models, especially when large utility-scale networks are modelled. Another limitation of the software is that specific design strategies can only be modelled by physically changing the network model, which again requires significant effort. A simplified approach is therefore required to enable utility engineers to analyse the reliability of different network configurations, reliability improvement strategies and planning criteria. The aim of this research is to provide a simplified reliability approach that will assist engineers in managing the reliability of their transmission and sub-transmission networks. The approach should be simplified to require minimum user inputs and it should be capable of quantifying the impact of different substation and line configurations on a system level. It is not expected that this approach will have the same level of accuracy as the detailed software models, but it should enable engineers to calculate system indices with much less effort, while still maintaining an acceptable level of accuracy. The scope of this research is limited to the transmission and sub-transmission networks (lines and substations). Power stations and MV distribution feeders are excluded from the analysis. Only technical, customer-based performance indicators are modelled, no load-based or economic performance indicators are calculated. An analytical approach is considered for the simplified reliability modelling, starting with a failure mode and effect analysis. The contribution of substation and sub-transmission events is decoupled and a detailed model of the substation is created, including all internal components. A reliability analysis is performed for each substation, to determine the unavailability experienced by customers connected to each busbar. An equivalent system model is then generated by replacing all substations with busbars, of which the outage frequency and outage duration are equal to that of the substation equivalent. The simplified substation reliability estimation is compared with detailed substation modelling using specialised software. The results obtained with the simplified reliability estimation show a good correlation with the detailed software models. The simplified reliability methodology was programmed into MS Excel and used to model the expected availability of the Ghana transmission network. Different scenarios were then modelled, analysing the impact of design and operational changes on the expected reliability of the network. The simplified reliability model developed through this research is capable of calculating system level technical performance indices for utility-scale networks, requiring much less effort than detailed software models, but still providing an acceptable level of accuracy. The technical system indices (SAIDI and SAIFI), calculated by means of the simplified reliability approach, provide an indication of the technical performance of the network, but they do not provide information on the economic impact of network outages. These technical indices have the potential to result in funding decisions that are not closely linked to economic interest. For this purpose economic indices are required, and it is recommended that the approach be extended to include the calculation of economic indices. iii Keywords Reliability, availability, sub-transmission, transmission, performance evaluation, SAIDI, SAIFI, network planning, substation design. iv Table of Contents Acknowledgements ....................................................................................................................................... ii Abstract......................................................................................................................................................... iii Keywords ...................................................................................................................................................... iv Table of Contents .......................................................................................................................................... v List of Figures ............................................................................................................................................. viii List of Tables ................................................................................................................................................ xi Definitions ................................................................................................................................................... xiii Abbreviations .............................................................................................................................................. xvi 1. Introduction ............................................................................................................................................1 1.1. Problem statement ........................................................................................................................1 1.2. Research objective ........................................................................................................................2 1.3. Scope .............................................................................................................................................2 1.4. Outline of dissertation ....................................................................................................................3 2. Literature review ....................................................................................................................................4 2.1. Evaluation techniques ...................................................................................................................4 2.1.1. Failure mode and effect analysis (FMEA) .............................................................................4 2.1.2. Markov models ......................................................................................................................4 2.1.3. Network reduction ..................................................................................................................6 2.1.4. Minimal cut-set method .........................................................................................................8 2.1.5. Monte Carlo simulation ..........................................................................................................8 2.1.6. Specialised reliability simulation software .............................................................................8 2.1.7. Decoupled composite models ...............................................................................................9 2.1.8. Conclusions from evaluation techniques ............................................................................ 10 2.2. Network outages ..........................................................................................................................10 2.3. Reliability indices .........................................................................................................................11 2.4. Reliability modelling test networks ..............................................................................................11 2.5. Component failure rates, maintenance frequency and repair times............................................12 2.5.1. Component failure rates and duration ................................................................................ 12 2.5.2. Maintenance frequency and duration ................................................................................. 15 2.6. Conclusions from literature review ..............................................................................................16 3. Reliability evaluation approach ........................................................................................................... 17 3.1. System description ......................................................................................................................17