Probes for spot measurement of surface conductivity on polluted insulators K.L. Chrzan, C. Stec and M. Farzaneh Abstract: The surface conductivity of insulators in the ﬁeld is often non-uniformly distributed. In this case the integral surface conductivity is not an appropriate parameter of pollution severity. A better evaluation can be achieved on the basis of local conductivity measurements by means of special probes, for example, the rod probe or tissue strip probe. In this work the form factors of different probes are given and the form factor for the rod probe was calculated. The inﬂuence of wet-contamination area, the meniscus and the pollution layer thickness on the measured value of surface conductivity are shown. It was found that the surface conductivity on polluted sili- cone rubber measured by a tissue strip probe is proportional to the equivalent contamination deposit density (ECDD). 1 Introduction The problems arising from pollution of insulators have been recognised by utilities for nearly a century. With the aim of predicting ﬂashover and improving insulator design, con- siderable research work has been done towards understand- ing insulator surface contamination processes and ﬂashover mechanisms under pollution conditions [1]. In service, an insulator will carry a resident contamination layer, accumu- lated since its installation or the last cleaning operation. This layer, which may ﬂuctuate as a result of depositing and wetting events, is more or less stable. Whatever their nature, the pollutants, when dry, are rather inactive. However, when exposed to random occurrences like con- densation, frost and onshore gales, water and/or ionisable materials are added. This, depending on the design of the insulator, will increase its surface conductivity possibly leading to ﬂashover and consequent power outages. The surface conductivity is, together with equivalent salt deposit density (ESDD) and leakage current, a very import- ant parameter of contamination severity. For ceramic and glass insulators, specially designed probes appear to be accurate for surface conductivity assessment, because ﬁeld pollution accumulation is generally non-uniform. ESDD measurement is a reliable method for assessing contami- nation severity for porcelain, glass and polymer. It involves washing the contaminants off from the insulator surface, and measuring the conductivity imparted by the contami- nants. Polymer materials, on the contrary to porcelain, resist wetting because of hydrophobicity and their molecu- lar chains known to be non-stationary. For example, a thin ﬁlm of silicone ﬂuid literally engulfs contaminants thus pre- venting dissolution of the ionic species in the water and thereby providing a very low surface conductivity. Thus, for insulators made of silicone materials, ESDD rather than surface conductivity seemed to be a better parameter of pollution layer. However, during the washing of insulator all soluble components of the contaminant are dissolved. Therefore so measured ESDD value does not take into account the particular properties of silicone rubber. A new equivalent contamination deposit density (ECDD) parameter was introduced to replace ESDD for silicone insulators. The measurement procedure of ECDD is rather complicated. We show that surface conductivity, measured very simply by means of strip probe, is proportional to ECDD. The strip probe has an important advantage, it can be used both for porcelain and silicone insulators. 2 Insulator surface conductivity and ﬂashover prediction It is possible to evaluate the ﬂashover voltage U f of an insulator as a function of surface conductivity k s . The experimentally well conﬁrmed equation is [2] U f ¼ K f  k n k S (1) where K f and n k are constants inﬂuenced by the insulator proﬁle and other factors. The surface conductivity is related to the conductivity k and layer thickness h of the pollution layer by k s ¼ 1 R S ¼ k  h (2) If the surface conductivity is uniformly distributed on the insulator as in the case of artiﬁcial layers then its value can be calculated from the current measurement. Usually the current is measured at a voltage lower than the operating voltage (e.g. 700 V per 1 m of leakage distance) which causes no partial discharge or substantial temperature increase [3]. The so-called integral surface conductivity of a polluted insulator is then calculated from k si ¼ f  I U (3) # The Institution of Engineering and Technology 2007 doi:10.1049/iet-smt:20060044 Paper ﬁrst received 21st March and in revised form 28th November 2006 K.L. Chrzan and C. Stec are with the Institute I – 7, Wroclaw University of Technology, Wroclaw 50-370, Poland M. Farzaneh is with Universite ´ du Que ´bec a ` Chicoutimi, CIGELE, Icing Research Building, UQAC, Quebec, Canada, G7H 2B1 E-mail: krystian.chrzan@pwr.wroc.pl IET Sci. Meas. Technol., 2007, 1, (5), pp. 295–300 295