The Performance of High Voltage Insulator Based on Epoxy-Polysiloxane and Rice Husk Ash Compound in Tropical Climate Area

This paper presents the effect of natural aging upon the performance of the polymeric insulat ion materials made from Epoxy resins, Polydimethylsiloxane (PDMS), and Rice husk ash compound. The sa mples of epoxy resin insulation material consist of Dig lycidyl Ether of Bisphenol A (DGEBA), Meta Phenylene Diamine (MPDA) as curing agent, and 325 mesh Rice Husk Ash (RHA) as filler, treated with variat ion of PDMS content. The research aims are to observe the Equivalent Salt Deposit Density (ESDD), crit ical leakage current, and flashover voltage on the insulation material surface that has undergone a natural aging. Experimental method was carried out through the following procedure. The samples were placed outdoor for natural aging test, outside the building of electrical engineering and informat ion technology of Gadjah Mada University in Indonesia. Then the ESDD, crit ical leakage current, flashover voltage on the sample surface was measured every 2 weeks. The experiment results show that the performance of insulator material fluctuates during 52 weeks. The higher PDMS with RHA filler content the lower ESDD and surface leakage current. Furthermore the flashover voltage increases.


Introduction
The use of epoxy resins as high voltage insulator material is appropriate, because of its high dielectric strength (25-45 kV/ mm) [1]. But epo xy resin insulating materials is very sensitive when used in areas with h igh temperature, high humid ity, the existence of pollutants and ultraviolet radiation [2]. The main advantages of epoxy resin over porcelain material as insulator material are that it is much lighter (because of low density), easy handling, easy blending with additives, and having hydrophobicity.
Many researchers revealed that the first generation polymer insulator material of epo xy resin based on bisphenol A with silica sand filler for outdoor insulators had satisfactory performance in normal air conditions . But, for long term outdoor service, their performance was unsatisfactory. There was a crack on the surface because it was n ot res is t ant to u lt rav io let rad iat ion [3]. Th e next generation is the cast epoxy resin cycloaliphatic. It shows good performance under normal at mospheric condition, but the performance will be decreased under polluted atmospheric condition [4,5]. The weakness of the cast epoxy resin cycloaliphatic is the impurity of its filler, Alu mina Trihydrate (ATH), which may consist of Natriu m Oxide (Na 2 O) and Kaliu m Oxide (K 2 O). Those two alkali o xide along with water will form Alkali Hydro xy l (NaOH and KOH), a strong electrolyte, which can change dielectric property of cast epoxy resin cycloaliphatic [6].
An aging test to analyze Silicon Rubber (SiR) type polymeric polysilo xane insulator and Ethylene Propylene Diene Monomer (EPDM) insulator perfo rmance was held outside Anneberg Bulk Station, West Coast Sweden by observing the relationship between surface condition and performance of poly meric insulator. Six SiR and three EPDM samples were analy zed by using Electron Spectroscopy for Chemical Analysis (ESCA), Attenuated Total Reflection Fourier Transform Infrared (ATR-FTIR), and Scanning Electron Microscopy (SEM ). The results conclude the relationship between insulation material surface condition and hydrophobic property, leakage current and voltage resistance. In EPDM insulator, a h igh magnitude of leakage current and intensity of arc occurs. But, in insulator of SiR only a very low leakage current occurs.
Polysilo xane (i.e. silicon rubber) is able to create hydrophobic properties on insulator surface in which prevents water layer formation and leakage current occurrence on its surfaces. Low Molecule Weight (LMW) co mponent is diffused fro m the bulk of SiR to the outer surface layer. LMW will then film the pollutant and maintain the hydrophobic properties (i.e. prevent water layer format ion on insulator surface) [8].
Adding ATH to SiR g ives better resistance of erosion and tracking. Surface degradation of elastomeric silicon with lower filler (0-25%) will make tracking phenomenon occurs faster, while higher filler (50%-70%) will delay surface degradation due to dry band arc phenomenon and reduce water absorption [9].
The application of polysilo xane on the mixture of epoxy and rice husk ash as the filler affects the hydrophobic properties. The h igher filler content in insulator the bigger contact angle of water droplets on the insulator surface [10].
Insulator contaminations become a troublesome in electrical power system operation. Wet atmospheric condition will form thin water layer on the insulator surface. Then, the present of the contaminant inside the layer will create leakage current on the insulator surface [11]. The contaminant layer accu mulat ion at the surface of the insulator will increase the leakage current in wh ich causes flashover voltage and give damage to insulator [12].
Considering that electrical properties of poly meric insulators are influenced by the type of material and the environmental conditions, therefore this paper will discuss performance of epoxy-polysilo xane poly meric insulator materials with rice husk ash as the filler (EP-RHA) under natural tropical climate. The performance is measured as equivalent salt deposit density, surface leakage cu rrent, and the flashover voltage.

Epoxy Resin Polymeric Insulator Material
Epo xy is a thermosetting chemical co mpound. It consists of o xygen and carbon atomic bond which is generated by chemical reaction between epichlorohydrin and bisphenol A. A complex structure of epoxy resin has molecu le bond as shown in Fig.1. Epo xy resin will harden when mixed with hardener, catalyst and filler. It is widely used as insulator, house hold equipment, machinery co mponent, automotive, tank, p ipe, aeroplane body part, bridge construction, etc. An epoxy resin formation react ion with MPDA hardener is shown in Fig.2.

Pol ysiloxane Polymeric Insulator Material
Polysilo xane is a polymeric material that consists of silicon and oxygen atom on its main chain. Po lysilo xane main chain is more flexib le than vinyl poly meric and polyolefin. The chemical structure of polysilo xane is shown in Fig. 3. Several applications of polysilo xane are statue and frame mould ing material, mixture co mpound, filler liquid and insulation coating of electrical and e lectronics equip ment, filler co mpound in polyester, electric insulation material, chemical and ultravio let radiation proof material for hu man body treatment product.
Polysilo xane can be formed trough addition and condensation polymerizat ion. Polysilo xane, in the addition polymerization is formed at silo xane bonds (Si-O) which have double bond as vinyl (--CC--) at the edge. While in the condensation polymerization is formed in silo xane bond which has hydroxy l (OH) group at the edge. Thermal stability of Si-O bond is very good, but has a relatively high ionic property. This property makes the bond easily broken on a high concentration of acid or alkali [13]. Strong bond of Si-O g ives very high resistance properties of SiR against damage fro m environ ment condition or corona phenomenon. This Si-O also has similar property to glass or quartz wh ich will not leave a conductive layer when burnt, e.g. burning caused by arching. Instead of the good thermal properties, SiR has stable elasticity within temperature range of -50℃ up to +230℃ (long thermal stability). This latest property is the most important property criteria of electrical insulator [14].

Rice Husk Ash
In Indonesia, rice husk is usually used for main firing material (fuel) in brick industry or as plant seed medium, while its ash is usually used for cleaning-rub ashes or even wasted. Rice husk burning will produce RHA with very high SiO 2 content (>80% fro m total weight). When continuous burning at 500 to 700℃ for 1-2 hours applied, RHA will contain large amount of amorphous silica which can increase mechanical property of cement-RHA mixture at certain dosage [15,16].

Equi valent Salt Deposit Density (ES DD)
The outdoor insulator surface will be contaminated by pollutant which is carried by the wind. Pollutant materials basically div ided as 2 major co mponents: conductive components and inert components. The conductive components mostly consist of ion ic salt such as sodium chloride (NaCl), sodium sulphate (Na 2 SO 4 ), magnesium chloride (MgCl 2 ), etc. Inert components are a solid materials or cations which can not being unravelled to ions in liquid (e.g. kaolin, silica, bentonite, cement, etc). These inert co mponents can create mechanical bond with conductive component particles. Kaolin, cement dust and silica make the insulator surface become hydrophilic , while oil creates hydrophobicity. Contamination level wh ich is caused by a certain salt can be measured by using Equivalent Salt Deposit Density (ESDD) method which is expressed in mg/cm 2 . The ESDD procedure includes measuring conductivity of cleaning materials i.e. water and cotton, before and after cleaning. This is done at certain roo m temperature and then converted to standard 20℃ conductivity by using correction factor as shown in Table 1 [17].

Leakage Current on Insulator Surface
Most of high voltage insulator applied outdoor in electrical power trans mission systems. Polluted environ ment will create a long term pollution coating on insulator surface. This pollution coating does not have a detrimental effect when the insulator is dry. The electrostatic field determines the voltage distribution of such a dry insulator and a very small capacit ive leakage current flo w across the entire insulator. However in the at mospheric condition is wet, the contamination particles on the insulator surface will dissolve into the water and provide a continuous path of conduction between the high voltage electrode and ground.
When an insulator is wet, a resistive leakage current flows in a conducting path on the insulator surface wh ich has greater value than capacitive current (in case the insulator is in dry condition). At this point, the equivalent circu it can be represented by resistance (R w ). This leakage current creates in non-uniform heating of the contamination layer that eventually causes dry bands to be formed at the narrow sections where the surface leakage current density is highest.
The voltage distribution along the surface of wet polluted insulators is very non-uniform when a dry band is formed in series with the conductive film. Since the resistance of the dry band is very high, the whole applied voltage across the insulator appears across the dry band. As a result, the breakdown occurs across the dry band when it reaches the air crit ical flashover voltage and generates small sparks between the separating moisture films .
The heat resulting fro m the small sparks creates carbonization and volatilizat ion on the insulator surface which left permanent carbon tracks on the surface. This process will continue until the breakdown happen due to the formation of carbon bridge between electrodes [18]. Th is phenomenon is usually occurs in wet contaminated insulator surface.
The equivalent circuit after the appearance of dry band is represented by parallel resistance (R d ) and a capacitor (C d ).
The leakage current phenomenon on the insulator surface and its electrical model is shown in Fig. 5.

Fl ashover Voltage on Insulator Surface
Flashover voltage is an external disturbance on insulator surface or fire arcing process on insulator surface. It may occur in solid surface or gas. Flashover voltage value is smaller than puncture voltage on an insulator. Several factor affect flashover voltage, such as : surface resistance of a material, surface condition and electric field shape between electrodes and insulator.
Flashover voltage on polluted condition follows this following sequence [19,20]: 1) Insulator with dry band and conductive pollution layers are represented as a series of arc path along x (air gap) and resistance of polluted layer at every length unit R'=R'(I), as in Figure 6.
2) The surface of insulator between two electrodes is an electric arc which is in series with conductive polluted layer.
3) The arc ing can extend or shut-off if the electric field strength reaches certain value in dry band, then the electric discharge occurs and initiates flashover. 4) Flashover voltage occurs when the arc covers entire dry band and further the entire insulator surface between two electrodes.

Fi gure 6. Model of insulator with dry band and contaminated layer to determine critical leakage current (I) and voltage (U)
The relat ionship between surface conductivity and flas hover voltage can be shown in the fo llo wing equations (eq. 7eq. 9): where: V fo = flashover voltage (kV); E = electric field (kV/cm);  = surface conductivity due to pollutant (S/cm); L = length of arc ing when the flashover occur (cm). During the flashover test, the voltage is increased step by step until the critical voltage for every insulator samp le reached. The measurement value is then calibrated by using standard value in standard air condition [21] as the following equations (eq. 10, eq. 11):
All tools are available in the High Vo ltage Laboratory, Electrical Engineering and Informat ion Technology, Gadjah Mada University, Yogyakarta, Indonesia.

Testing Procedure
The test samples are installed outdoor, outside Electrical Engineering build ing, Gadjah Mada Un iversity. Samp les are placed in support bracket with 45 tilt, flipped every week and taken for performance test every 2 weeks. Th is procedure applies for 52 weeks experiment period.
The ESDD procedure will follow these steps: 1) Preparing a beaker, measuring cup, and cleaning cotton; 2) Filling a beaker with 200 ml d istilled water; 3) putting the cotton inside the beaker, measuring the temperature and conductivity; 4) Separating the pollutant layers from the samples by rubbing the soaked cotton, then putting the pollutant and the cotton into the beaker, and finally stirring them; 5) Measuring the temperature and conductivity of water and cotton mixtu re. The conductivity with and without pollutant are conversed into standard conductivity at 20℃ by using b factor based on IEC 507 standard (1991), then calculating salt concentration (in %) by using equation (2). The ESDD can be calculated the by using equation (3).
Leakage current measurement procedure can be explained as following steps: 1) Putting test sample into fog chamber with 70% humidity (after fogging); 2) Applying voltage. Every samp le is tested by applying increase voltages with a step of 1.5 kV/sec, fro m 11.5 kV up to 50% flashover voltage; 3) Recording. Every surface leakage current d isplayed and recorded by an oscilloscope; 4) Multiplying the voltage value fro m the oscilloscope with mu ltiply ing factor of voltage divider equivalent for obtaining leakage current value; 5) Calculating critical surface leakage current.
Flashover measurement procedure can be exp lained as following steps: 1) Putting test samples into fog chamber with 70% hu midity (after fogging); 2) Applying voltage. Every samp le is tested by applying increase voltages with a step of 1.5 kV/sec until flashover occurs; 3) Recording temperature, hu midity and air pressure then converting flashover voltage value by using standard value correction factor (as eq. 10 and eq. 11).
The measurement tools arrangement is shown in Fig. 7.

Results and Discussion
The following sections below show the performance results of insulator materials of epo xy-polysilo xane with rice husk ash filler for each composition during 52 weeks experimentation. Fig. 8 shows the results of ESDD of EP-RHA insulator in each composition against natural aging. The experiment period is 52 weeks. It is calculated by using equation (3).

ESDD Performance of Epoxy-polysiloxane Pol ymeric Insulator with Rice Husk Ash Filler
ESDD value of EP-RHA insulator material with various compositions is fluctuating during the aging process. This phenomenon happened due to variation of composition and local climate condition during the test. The highest the filler composition the lo wer the ESDD value is . The increase of filler material then makes methyl group (CH 3 ) on insulator surface also increases. This makes the insulator material more hydrophobicity [10]. When there is a rain, the EP-RHA insulator surface will be wiped up by the water. This hydrophobic property will decrease the ESDD and surface leakage current, so the power losses will be decrease as well [20]. Fig. 8 shows ESDD value fro m in itial aging until week 12 th wh ich fluctuates and tends to increase (Nov 2010 to Jan 2011). It is because of the tropical climate with low rain intensity. From week 14 th to 28 th (Feb -May 2011), the ESDD value of EP -RHA insulator surface tends to decrease due to high rain intensity (rainy season). Fro m week 30 th to 44 th (Jun -Sep 2011), the ESDD tends to increase due to entering summer season. Fro m week 46 th to the end of week 52 nd (Sep-Nov 2011), the value of ESDD decreases due to beginning of rainy season.  The results of measured ESDD for every samp le are shown in Table 2.

Epoxy-polysiloxane Pol ymeric Insulator with Rice Husk Ash Filler
The Fig. 9 shows the two dimensional results of crit ical leakage current of EP -RHA insulator in each co mposition against natural aging. The experiment period is 52 weeks. The critical leakage current values of EP-RHA insulator material with various compositions are fluctuating during the aging process. These values have the same trend with fluctuating value of ESDD. The higher ESDD the higher critical leakage current is , as shown in Fig. 10.
The natural atmospheric condition covers those EP-RHA insulators with pollutants. The pollution layer increases the conductivity on the insulator surfaces. In wet atmospheric condition, the salt component in the pollution layer will dissolve in the water which is a form of electrolyte. As the result, the EP-RHA insulator surfaces become conductive, and therefore the leakage current flo ws on its surface. The higher ESDD the higher the conductivity is on the EP-RHA insulator. The higher conductivity, the higher surface critical leakage current is .
The filler material is measured in percentage of overall composition. The higher filler co mposition the lower the ESDD is. For examp le in case of 10% co mposition (RTV EP 1 ) the average ESDD is 0.0226 mg/cm 2 . In the other hand in case of 50% co mposition the average ESDD is 0.0171 mg/cm 2 . Finally, a lower ESDD makes the critical leakage current lower o f EP-RHA insulator surface. The results of critical leakage current on insulator surface for every samp le are shown in Table 3.

The Flashover Voltage Performance of Epoxy-polysiloxane Pol ymeric Insulator with Rice Husk Ash Filler
The flashover voltages of EP-RHA insulator material with various compositions are fluctuating during the aging process (52 weeks) as shown in Fig.11.
The standard flashover voltage for EP -RHA insulator with various compositions tends to decrease until week 52 nd . The standard flashover voltage is affected by the ESDD value and surface leakage current. The ability of an insulator to withstand surface voltage is affected by pollution layer and surface wetting phenomenon [22]. In wet condition, a contaminant layer is formed on the surface, so that the insulator surface is conductive properties. The surface conductivity will increase if the surface pollution layer is thicker. This make the leakage current increase, so that the leakage current creates in non-uniform heating of the contamination layer that eventually causes dry bands .
The insulator surface between both electrodes is a form of dry band arcing that series connected with a conductive pollutant layer [19,20]. The dry band arcing can extend or shrink/vanish. When the electric field strength reaches certain value in dry band, then the surface electric d ischarge occurs, initiates a flashover voltage on the EP-RHA insulator surface.
The higher filler content in EP -RHA insulator, the lower the ESDD and the critical leakage current. If the ESDD and the critical leakage current are decreased, then the flashover voltage is increased. The results of standard flashover voltages of EP-RHA insulator for every samp le are shown in Table 4.

Conclusions
The performances of ESDD, critical leakage current and standard flashover of epoxy-polysilo xane poly meric insulator material with rice husk ash filler (EP-RHA) are fluctuating depend on local climate where the experiment is held.
During natural aging, the higher filler content in EP-RHA insulators results the lower value of ESDD and critical leakage current. If the ESDD and the crit ical leakage current are decreased, then standard flashover voltage is increased.