Abdul Syakur1*, Tumiran1, Hamzah Berahim1, Rochmadi2
1Department of Electrical Engineering, Gadjah Mada University
2Department of Chemical Engineering, Gadjah Mada University
Jln. Grafika 2, Yogyakarta, Indonesia
ISBN : 978-7-900780-15-7
Abstract — currently the use of polymer materials as electrical insulators on transmission and distribution lines has been increasing such as epoxy resin. Several advantages of using epoxy resin as an insulating material are its low density, better dielectric properties, and epoxy resin has higher volume resistivity than that of the glass and porcelain. However, epoxy resin has some disadvantages when it is used in tropical areas concerning with the humidity factor, high ultraviolet radiation, acid rain and effects of contaminants. Consequently, insulator surface will be easily damaged due to electrical tracking, which is indicated by the surface tracking.
In this paper, the surface tracking on epoxy resin samples has been investigated. The test was based on the method of Inclined-Planed Tracking (IPT) IEC 587:1984 with NH4Cl as contaminants. The flow rate of contaminant was 0.3 mL/min. AC high voltage 50 Hz was applied to the high voltage electrodes.
The experimental results showed that the surface tracking was affected by voltage applied, contamination and contact angle. By using micro-cameras, the surface damage was detected. The severest damaged sample surface on a sample had small contact angle. On the other hand, samples with the greatest contact angle needed longer time to have surface damage in the surface discharge. This shows that it is more difficult for large contact angle samples to have surface discharge.
Keywords: surface tracking, contact angle, discharge current, contamination.
Polymer materials have been widely used in the distribution and transmission line for their good dielectric properties, light weight and compact, when compared to the porcelain or glass insulators (X. Wang et al 2000). However, polymer outdoor insulator showed degradation due to climate stresses such as ultraviolet in sunlight, moisture, temperature, humidity and the other contaminants so that the surface discharge, tracking, and erosion can occur, and degradation may reduce the performance. This reduction is actually the result of chemical and physical changes taking place on the surface of polymer (Berahim et al 2003).
Epoxy resin is an important electrical insulating material. It is a thermoplastic polymer in which two components are mixed to eventually form a glassy product at room temperature.
Epoxy resins are used in a large number of fields including surface coatings, adhesives, in potting and encapsulation of electronic components, in tooling, for laminates in flooring and to a small extent in molding powders and in road surfacing.
Compared with the polyesters, epoxy resins generally have better mechanical properties and, using appropriate hardeners, better heat resistance and chemical resistance, in particular, resistance to alkali. The electrical properties of epoxy resins have a dielectric constant about 3.4 – 5.7, and a dielectric strength about 100 – 220 kV/cm. Power factor of resin epoxy resins are about 0.008 – 0.04 (Brydson, 1982).
When insulators made of epoxy resin are used outdoor, the presence of contaminants on the surface of the insulator becomes a serious problem. Different materials have different contamination performance. Generally non-ceramic insulators perform better than ceramics when new. However, due to aging of polymer housing, this relative difference can change with time at a rate depending on the environment (Gorur, 1999). According Berahim (2005), epoxy resin is a hydrophilic material, therefore, in particular, in the tropical area; humidity and rainfall play an important role in accelerating of degradation process on the surface of the insulator. Contamination layer will formed on the surface of the insulator and it would be spread on the surface. Leakage current will increase, especially when the insulator surface is wet caused by fog, dew or light rain. Leakage current will initiate a process of heat conduction which occurs on the surface of an insulator and finally flashover or insulation breakdown would occur. (Berahim, 2005).
This paper presents investigation steps on surface tracking of epoxy resin using Inclined-plane tracking method based on IEC 587:1984 with contaminants NH4Cl and 3.5 kV being applied. The procedure of test will be explained in this paper and conditions of material surface will be observed by using micro photo. The influences of contact angle and stochiometric value were studied.
II. THE INCLINED PLANE TRACKING SET UP
The inclined plane tracking (IPT) test is used as specified in IEC 587:1984. Preparation was conducted as follow:
The test materials used were epoxy resins with the dimensions 50 mm x 120 mm with a thickness of 6 mm. Test materials must be drilled to place electrodes.
Electrodes in this experimental were made from stainless steel material.
Contaminant and Filter paper
Contaminant used had concentration of 0.1 ± 0.002 % by mass of NH4Cl (ammonium chloride). These contaminants were flowed on the surface of materials using a peristaltic pump. There were eight layers of filter-papers as a reservoir for the contaminant, which were clamped between the top electrode and the specimen.
The tests were carried out using a high voltage AC 50 Hz. The test voltage 3.5 kV was applied to the top electrode while an electrolyte flowed along the underside of the sample. In this test, the constant voltage method was used, and the time to start tracking was also determined.
III. THE PROCEDURE OF TEST
Before AC high voltage was applied on top electrode, the test sample was prepared by cleaning with soft paper and was then soaked overnight in the ammonium chloride (NH4Cl) solution.
The test sample was mounted with eight layers of filter paper underneath the electrode to act as a reservoir for the electrolyte. The electrolyte was obtained using 0.1% ammonium chloride solution and its measured conductivity was 2.7 mS. The electrolyte was delivered by a peristaltic pump with flow rate 0.3 mL/minute.
IV. RESULTS AND DISCUSSION
4.1 Surface Tracking
Electrical tracking on the surface of the material sample is due to heating at the surface of the material at the time of leakage current to flow and once the process of erosion. Leakage current on the surface of the material occurs due to the contaminants that flow on the surface material.
When the voltage of air critical flashover was achieved, carbonization process took place and water vaporization happened. Permanent carbonized path were formed. These process were continuously and cumulative and finally insulation breakdown was happen. Surface discharge also was happen. Erosion at samples surface was followed by formed of pattern filament in which produce electrical treeing. Surface condition for each stoichiometric varied is shown in Table 1.
Table 1 shows that the severest damage took place at epoxy resin RTV70. Higher epoxy resin stoichiometric value produces severer damage at the sample surface.
A method to characterize the level of surface damage was needed to determine the surface degradation of material. One of the methods was micro-photo method. The following picture shows the surface damage in various stoichiometric values.
The experimental results shows that there was structural change at the epoxy resin surface, and hence it can be concluded that 3.5 kV AC high voltage application and NH4Cl contaminant flow caused surface degradation of the insulator. Tracking occurred from the bottom electrode to the top electrode.
4.2 Mathematical equation for surface tracking
Methods to assess the degradation of polymer insulating epoxy resin due to the process of tracking will be examined using a mathematical model that states the relationship between the stoichiometric value and the contact angle versus time tracking.
In this experimental test, the applied voltage was 3.5 kV. The insulator would end when the flashover voltage was equal to the applied voltage.
The mathematical model of insulation degradation can be solved by statistical regression model with the data of research conducted on samples RTV30, RTV40, RTV50, RTV60 and RTV70 n = 5. The mathematical model of insulation degradation can be described by a statistical model of multiple regressions with the assumption that the performance data of each sample was independent.
Based on the experimental results, it can be concluded as follows.
1. The increase of stoichiometric value of epoxy resin caused the decrease of contact angle. Highest contact angel was obtained by RTV30.
2. The increase of stoichiometric value of epoxy resin accelerates the insulation breakdown, or facilitates the ncrease of leakage current at the surface. It also accelerates the the path of carbonization process at the surface.
3. The rise of stoichiometric value will reduce contact angle and tracking time at the surface.
. X. Wang, L.Chen and N. Yoshimura, “Erosion by acid rain, accelerating the tracking of polystyrene insulating material” J.Phys.D: Appl. Phys. 33 (2000) p. 1117-1127 Printed in the UK.
. H. Berahim, K.T. Sirait, F. Soesianto, Tumiran, “ A new performance of RTV Epoxy Resin Insulation material in tropical climate ” Proceedings of the 7th International Conference on Properties and Applications of Dielectric Materials, June 1-5, 2003, Nagoya .p. 607
. J.A. Brydson, “Plastic Materials” 4th edition, Butterworth Scientific, 1982 p.693-695
. Gorur, “Outdoor Insulators” Ravi S. Gorur, Inc., Phoenix, Arizona 85044 USA, 1999.
. H.Berahim, “Methodology to assess the performance of silane epoxy resin insulating polymer as high voltage insulator materials in the tropical areas ” Dissertation doctorate degree at Department of Electrical Engineering, Gadjah Mada University, 2005, Indonesia
. IEC 587, BS Test Method for Evaluating Resistance to Tracking and Erosion of Electrical Insulating Materials used under severe Ambient Conditions, British Standards Institution, 1984
. Abdul Syakur, Rochmadi, Tumiran, Hamzah Berahim, “Study on electrical tracking of epoxy resin insulating material using Inclined-Plane Tracking Method ” Proceedings of Conference on Information Technology and Electrical Engineering, July 20, 2010 Gadjah Mada University, Indonesia