Volcanic Ash Contamination: Limitations of the Standard ... - IEEE Xplore

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J. Wardman et al.: Volcanic Ash Contamination: Limitations of the Standard ESDD Method for Classifying Pollution Severity

Volcanic Ash Contamination: Limitations of the Standard ESDD Method for Classifying Pollution Severity Johnny Wardman, Thomas Wilson Department of Geological Sciences University of Canterbury Private Bag 4800 Christchurch 8140 New Zealand and Pat Bodger Department of Electrical and Computer Engineering University of Canterbury Private Bag 4800 Christchurch 8140 New Zealand

ABSTRACT The pollution severity of airborne contamination on high voltage insulation has traditionally been quantified by calculating the contaminant’s equivalent salt deposit density (ESDD). Volcanic ash is a rare but severe form of airborne pollution, and the high conductivity of wet volcanic ash (often >1.3 x 10-4 S/cm) can cause pollution-induced insulator flashover. This paper presents the ESDD and non-soluble deposit density (NSDD) for four different fresh volcanic ash samples and two ash proxies measured at different thicknesses using a standardised plate test. Results show that there is a log-linear increase of ESDD with increasing NSDD. Tests indicate that a 3 mm thick deposit (NSDD between 158 and 231 mg/cm2) of fresh volcanic ash yields an ESDD between 0.02 and 0.7 mg/cm2, suggesting that ash can have high contamination severity and therefore potential to cause pollution-induced insulator flashover. Whilst the ESDD/NSDD method provides direct analysis of the ionic content of a contaminant, the procedure is time consuming, cannot accommodate the high NSDD of volcanic ash for site pollution severity classification and does not account for changes in the contaminant’s electrical conductivity under different environmental, chemical and physical conditions. Given these limitations, this study proposes an alternative, simple yet more comprehensive technique for investigating the electrical properties of volcanic ash by means of direct resistivity analysis. Index Terms — Volcanic ash, ESDD, NSDD, conductivity, contamination, electric breakdown

1 INTRODUCTION IT has long been established that electric power systems are vulnerable to pollution-induced insulator flashover and subsequent interruption of service [1–6]. Volcanic ash is a rare but severe form of airborne pollution. Worldwide accounts have reported adverse impacts to power systems from volcanic ash contamination, with ash-induced insulator flashover being the most common [7]. The product of explosive volcanic eruptions, volcanic ash consists of two primary components: (1) non-soluble, pulverised fragments (< 2 mm particle diameter) of rock, minerals and glass (SiO2); and (2) soluble Manuscript received on 12 November 2012.

salts which are adsorbed onto the surface of ash particles during ash–gas/aerosol interaction within the volcanic plume [8]. These salts supply ionic content to an otherwise electrically inert material. The fine-grained nature of volcanic ash makes it an excellent retainer of moisture and once the attached salts are dissolved into solution (e.g. by dew, fog or light rain) the ash becomes a conductive electrolyte [9]. Equivalent salt deposit density (ESDD) is a standard IEC parameter for Type A pollution (where solid pollution with a non-soluble component is deposited onto the surface of an insulator). An industry practise since the 1950s, the ESDD or “Solid Layer” method equates the amount of sodium chloride (NaCl) required to yield the same conductivity as the

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IEEE Transactions on Dielectrics and Electrical Insulation

Vol. 20, No. 2; April 2013

contaminant when dissolved in the same volume of water. Upon calculation, ESDD values are classified into insulatorspecific site pollution severity (SPS) indexes provided in IEC 60815-1. As a guide for this paper, however, an example of exposed ESDD levels for different pollution severities is shown in Table 1. Table 1. ESDD classifications for pollution severity [10]. ESDD (mg/cm2) 0-0.03 0.03-0.06 0.06-0.1 >0.1

Pollution Severity Very Light Light Moderate Heavy

The non-soluble deposit density (NSDD) is another standard pollution parameter prescribed by IEC 60815-1 that is used to quantify the amount of non-soluble residue on contaminated insulators (also expressed in mg/cm2). Previous work suggests that the non-soluble component of volcanic ash is a poor conductor [11] and the sensitivity of salt deposit density is higher on flashover voltage than that of NSDD [12]. However, other studies have shown that inert, non-soluble pollution can greatly reduce the flashover voltage of HV insulators [13, 14]. Insoluble deposits do not contribute directly to conductivity but instead convert the smooth surface of an insulator into a rough, irregular one which, in turn, can affect (1) the run-off rate of soluble material, (2) the hydrophobicity of the insulator surface, (3) the evaporation rate of the wetted layer, and (4) the local electric field strength [15]. Whilst most forms of pollution such as salt, combustion emissions, dust (e.g. earth, fertiliser, metallic, coal and feedlot), smog, and defecation (e.g. bird streamers) have been identified and studied in some detail [16-18], very little information exists on the ESDD of volcanic ash [e.g. 19-21]. This paper complements the existing knowledge of insulator contamination in different polluted environments by presenting the results from ESDD tests for four fresh volcanic ash and two comparative pseudo (artificial) ash samples. Based on the findings, an assessment of the overall suitability of the ESDD method for quantifying the pollution severity of volcanic ash is considered. It is intended this will aid power system operators to better understand the potentially wide-ranging physical, chemical and electrical characteristics of volcanic ash.

2 SAMPLES AND PROCEDURES 2.1 SAMPLES 2.1.1 FRESH ASH SAMPLES The low frequency of explosive eruptions and logistical difficulties in collecting pristine volcanic ash meant only four ash samples collected from four different eruptions were available for analysis. The volcano and eruption dates for each ash are listed in Table 2. All samples had been stored in dry conditions within sealed polyethylene bags since collection. Non-essential movement was minimized to reduce modification of ash properties. Freshly fallen volcanic ash loses its soluble content rapidly in the presence of moisture (such as rain or wet soil)

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so it is imperative that ash collection be carried out shortly after an eruption and the ash adequately stored to avoid leaching or erosion of the soluble components. Table 2. Fresh ash samples used in this study. Samples were collected by the authors or provided by research affiliates. Sample ID SDKE-10 SHIL-09 CHTN-08 RUAP-09

Volcano Shinmoe-dake Soufriere Hills Chaiten Ruapehu

Country Japan Montserrat (UK) Chile New Zealand

Date of collection 2-Feb-11 27-Nov-09 28-May-08 18-Jun-96

2.1.2 PSEUDO ASH SAMPLES To augment our limited fresh ash samples, a pseudo ash was developed to replicate the physical and electrical properties of freshly fallen volcanic ash to test against the fresh volcanic ash samples. Unweathered Stoddart olivine basalt (from Halswell Quarry, Lyttelton volcano, New Zealand)[22] was chosen as the non-soluble volcanic component for our ash proxy. It has been argued that fine-grained ash deposits (e.g. 1 mm particle diameter) [9, 23]. Thus, to investigate whether particle size has any effect on ESDD, two different pseudo ashes were created: (1) a predominantly fine-grained fraction (