We acknowledge Transpower New Zealand, Ltd. (Wardman, Wilson) and the Ministry of Science and Innovation Grant C05X0804 (Wilson) for funding support. We thank Keith Comer, Erik Brogt for their help in refining this manuscript and two anonymous reviewers for their insightful and constructive comments.
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Chapter 5
Influence of volcanic ash contamination on
the flashover voltage of outdoor HVAC
suspension insulators
John Wardman1,Stewart Hardie2, Thomas Wilson1, Pat Bodger2
1 Natural Hazards Research Centre, Department of Geological Sciences, University
of Canterbury, Private Bag 4800, Christchurch
2 Department of Electrical and Computer Engineering, University of Canterbury,
Private Bag 4800, Christchurch
Intended for submission to: IEEE Transactions on Power Systems
OVERVIEW
Volcanic ash-induced insulator flashover is the most likely impact to disrupt HV power systems during and/or following an ashfall (Chapter 2). Until now, our understanding has been based on the Mount St Helens study and anecdotal field observations (Chapter 2). A lack of empirical data on the external factors influencing the flashover mechanism has prompted the need for a systematic analysis of the subject. Chapter 5 presents the results from electrical tests carried out to assess the vulnerability of a range of different HV insulators commonly used in New Zealand’s transmission system to volcanic ash-induced insulator flashover.
5.1 ABSTRACT
High voltage (HV) station and line insulators used on alternating current (AC) systems are vulnerable to volcanic ash-induced flashover, yet little quantitative data exists on the environmental, volcanological and electrical parameters most influential in reducing their flashover voltage. This chapter presents results from clean-fog rapid flashover tests for 5 different suspension insulators of either ceramic or non-ceramic construction under different environmental and volcanic ash contamination scenarios. Results suggest composite polymer insulators have higher dielectric strength (pollution performance) than ceramic equivalents under light to heavy pollution severities due to their hydrophobic properties. However, all insulators tested here perform comparably when critically contaminated (i.e. both top and bottom surfaces coated in ash). Based on these and other findings, recommendations for best insulator selection and maintenance are provided.
5.2 INTRODUCTION
The process of insulator contamination, associated flashover and subsequent loss of service has been a major problem on electric power systems since their inception in the early 1900s (Baker et al., 2009). Volcanic ash is an infrequent, but potentially highly disruptive form of contamination capable of causing insulator flashover across station and line insulators (porcelain, glass or polymeric) (Chapter 2). Of all eruptive hazards, ashfall can affect the most people because of the wide areas that can be covered by fallout (Blong, 1996). Considering 9% of the world’s population lives within 100 km of a historically active volcano (Horwell and Baxter 2006), and the increasing reliance of society on electricity to maintain normal operations, there is a desire to increase power system resilience to volcanic ashfall hazards.
Most efficiently produced by explosive volcanic eruptions, volcanic ash consists of two chief components: (1) non-soluble, pulverised fragments (<2 mm particle diameter) of rock, minerals and glass (SiO2); and (2) soluble
salts which form on the surface of ash particles during ash–gas/aerosol interaction within the volcanic plume (Delmelle et al., 2007). These attached surface salts supply ionic content to an otherwise electrically inert material. Once the attached salts are dissolved into solution (e.g. by dew, fog or light rain) the ash becomes a conductive electrolyte and poses a flashover hazard to the power system (Chapter 3).
While the pollution flashover phenomenon has been studied in great detail (e.g. Adler et al., 1948; Lambeth, 1971; Jolly, 1972; CIGRE Taskforce 33.04.01, 2000; Gencoglu and Cebeci, 2008), little knowledge exists on the pollution performance of HV insulators subjected to volcanic ash contamination. Although anecdotal evidence (e.g. Wilson et al., 2009; Chapter 2) and limited experiments (e.g. Nellis and Hendrix, 1980; Matsuoka et al., 1995) provide some understanding of ash-induced flashover processes, a systematic examination of this problem has not been conducted to-date. This study presents the results from clean-fog rapid flashover tests for 5 different suspension insulators subjected to a range of contamination scenarios. The minimum flashover voltage and the dielectric strength for each insulator is measured and compared against each other. Based on the findings, considerations for appropriate insulator selection for ashy environments are discussed.