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| Injection Moulding of Silicone Polymer - Housings on Metal Oxide Element Stacks |
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Polymeric Housed Surge Arresters :
The Metal oxide element stack with the terminals is wrapped to sufficient thickness by resin bonded ECR grade fiber glass tape and cured to hard set thus making void free module which gives the desired mechanical strength and provides an uniform dielectric at the surface of the Metal Oxide Elements. The modules are moulded with service proven silicone polymer housing in an Injection Moulding Machine. The void free construction eliminates the void discharge under normal and polluted conditions.
All the Polymer Arresters undergo Seal Leak test followed by Partial Discharge test, leakage current test at MCOV, Reference Voltage test and the Residual discharge voltage test before dispatch. |
| Main Features of OBLUM Polymer Arrester |
Main Features of OBLUM Silicone Polymer Housing
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- No internal space & so no moisture ingress
- Improved thermal conductivity as metal oxide elements can dissipate heat developed during surge discharge into surrounding dielectric to polymer housing and then
to atmosphere.
- High cantilever and torsional strength.
- Suitable for installation in polluted environments.
- Non-explosive failure mode.
- Low weight & small size.
- Exceptional tolerance to seismic disturbance.
- Vandal proof.
- Resistant to transport damage and careless handling.
- Easy to install.
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- Hydrophobic nature.
- Insensitive to UV Radiation.
- High resistance to Ozone and Corona.
- Excellent tracking resistance.
- Excellent mechanical impact and
abrasive resistance.
- Excellent resistance to ageing under
climatic and electrical stress.
- Non-combustibility & self extingui-
shing property.
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Polymer for Outdoor Use:
Stability under exposure to discharges and arcing, differentiates outdoor insulation from indoor insulation. Corona discharges and dry band arcing occur when the surface of an energized insulator is covered by an electrolytic film, formed by the presence of moisture and contamination. The intense localized energy of the dry band arcs can cause material degradation in the form of tracking and / or erosion. In addition, environmental factors such as ultraviolet (UV) from sunlight, moisture, temperature, etc., can contribute to the degradation of the polymer which could lead to premature failure. In contrast, porcelain and glass insulators are inert materials and are not degraded by corona and dry band arcing.
Many polymers have been tried for outdoor insulators. These include silicone rubbers (both room temperature vulcanized-RTVs, and high temperature vulcanized (HTVs), ethylene propylene rubbers (EPR) Ethelene Propylene Diene Monomer (EPDM), epoxies (both bisphenol and cycloaliphatic), Polyolefins, polyurethane, polyethylene, and Teflon (PTFE). For silicone rubber housings, using the RTV weather sheds, the tear resistance of the RTV was inadequate. Earlier generation epoxy insulators were deficient in the resistance to moisture, tracking and Ultra- Violet radiation from sunlight. Butyl rubber and EPR compounds had poor UV resistance. Teflon has a high arc resistance & it poses manufacturing problems. Polyethylene is flammable and insulators using this material have sometimes resulted in pole fires.
The Arrester Housing should withstand Power Frequency and Impulse voltages in dry and wet conditions, should have proper Creepage distance for pollution performance as well as to withstand power arcs and environmental abuses. The resistance to power arc depends on the tracking property of the material. The polymer container has to withstand physical, chemical, mechanical & thermal requirements in addition to good ageing property for outdoor use. The polymer should not get carbonised for power arcs and arcs during pollution conditions.
In the evaluation of tracking resistance, it was discovered early that the use of an inorganic filler like Alumina Trihydrate (ATH) provides a means to increase the arc tracking resistance. No specific base polymer material alone has the necessary properties and characteristics for an outdoor weathering material. It was realized that the long term electrical performance is not only dependent on the polymer's ability to withstand high energy arcing, which is mainly dependent on the filler, but also on its ability to control leakage current and thereby the probability of flashover. The relationship between the polymer surface state and its electrical performance has been the subject of a number of recent studies.
Detailed studies have shown that the surfaces of polymers are relatively dynamic in comparison to porcelain and glass. Polymer molecules have much greater freedom for rearrangement in the bulk or at the surface. The surface discharge activity developed differently on different types of polymers, the effects of discharge are directly related to the chemical nature of the polymers. The recovery of hydrophobicity of silicone coatings after exposure to discharges was directly related to the molecular structure. The hydrophobicity or water repellency of the surface of polymers plays an important role in the electrical performance of polymeric housings.
The use of Surge Arresters in abnormal service conditions require enhanced external insulation for altitudes over 1000M, hostile atmospheres with excessive contamination and high humidity. Such conditions require high Creepage, high dry arcing distance and resistance to surface contamination. The construction of Polymer Surge Arrester affords a much improved thermal conductivity and after a surge discharge the metal oxide elements can rapidly dissipate the heat developed, into surrounding dielectric and then via the polymeric housing to the surrounding atmosphere.
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| Accessories: |
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OBLUM Surge Monitor consists of discharge counter to count the surge discharges through the arrester and a milli-ammeter to monitor the leakage current. Regular Monitoring of the Surges through this device helps to understand the frequency of surges that occur in the system. The leakage current drawn is a measure of healthiness of the Surge Arrester. |
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OBLUM Ground Lead Disconnectors : For distribution class arresters, disconnectors are used for disconnecting the ground lead from the arrester to avoid supply interruption, violent shattering of arresters, and also to give visible indication of the failed arrester. In the remote event of Arrester failure, the conductive rubber housed inside gets heated and the heat produced is sufficient to ignite the pyrotechnic producing profuse gas enabling the Disconnector getting separated thus disconnecting the arrester from the earth connection. |
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Aluminium alloy is processed in an Induction Melting Furnace. Components are manufactured in 400 Tonnes Pressure Die Casting Machine to achieve high strength components highly resistant to corrosion. |
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ZnO Arrester selection steps are:
- Determine the continuous operating voltage of the system (COV). Select the Arrester with MCOV higher than the system C.O.V.
- Determine the rated voltage of Arrester with respect to coefficient of earthing.
- Voltage Vs Time Characteristics of the Arrester should be higher than the Temporary over voltages of the System.
- Pollution levels also to be considered, while selecting the rated voltage of arrester.
- Select the energy class of the Arrester based on the transmission line discharge requirements and energy due to transient over voltage in the system caused by closing or re-closing operations, and disconnection of capacitor banks or cables with restriking circuit breakers etc.
- Select the pressure relief class of the Arrester based on the maximum short circuit level at the location of Arrester installation.
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OBLUM Metal Oxide Surge Arresters have many outstanding features that make them perform better:
- Operates cooler and can absorb and dissipate high energy surges in rapid succession due to high specific heat and thermal conductivity.
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Has excellent thermal stability for high energy surges, external pollution and temporary over voltages due to liberal design and very low losses at operating voltages.
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Draws very little current at the system voltage.
- Holds the voltage with little change during conduction period of surge current and ceases to conduct at very nearly the same voltage at which conduction has started.
- Has quick response of the order of Nano seconds.
- Has excellent contamination performance.
- Nearly constant residual voltage within a large range of current values.
- Has a high volume energy capability.
- Discharges any transient energy from the power system to earth while limiting the voltage not exceeding the protection levels.
- Provides greater reliability along the sea coast and in areas of heavy industrial pollution.
- Superior performance on low impedance cables and capacitor circuits.
- Is lighter, simpler, rugged and reliable in operation.
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