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Electrical Porcelain:
Electrical porcelain with outstanding features like white colour, non-porous, high insulating properties, high mechanical strength and chemically inert material is manufactured by OBLUM Insulators which is a division of OBLUM. These Raw materials are weighed, ball milled with treated water by reverse osmosis process to get a defined particle size distribution, dewatered in filter presses and the Cake is kneaded and extruded through vacuum pug mills, turned in vertical lathes, and the articles are dried, glazed and sintered in high temperature shuttle kilns, and finally cut and ground at the ends for proper sealing.
Each and every Insulator undergoes visual inspection, dimensional check and electrical tests. Mechanical cantilever test, porosity test, thermal shock tests and bursting pressure tests are conducted as acceptance tests. The tubular porcelain is available for In-house consumption and also for outside market for Arresters, Instrument Transformers and Circuit Breakers. |
| Metal Oxide Gapless Surge Arresters |
Porcelain Housed Surge Arresters: |
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Station class porcelain housed Arresters consist of a stack of Metal Oxide elements
held rigidly by a tie rod with Pressure release arrangements at both ends consisting
of reverse acting rupture diaphragm which gets ruptured against a knife edged pressure
release plate, in the remote event of arrester failure, to release the pressure built up due to short circuit and the gases are expelled through directional venting ports avoiding catastrophic failures. |
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The assembled station class arresters are evacuated and filled with dry air of 30oC due point and sealed. Each and every arrester is routine tested for seal leakage test by vacuum over water for medium voltage distribution arresters and 1500mm water dip test followed by testing for leakage current, reference voltage and partial discharge tests.
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Arrester rated upto 132KV are tested for protection level at 10KA in OBLUM High Voltage Lab using 480KV 86.4KJ Impulse Current Generator to give 8x20m sec. wave. Each and every Arrester of all ratings are routine tested for Partial Discharge and Reference Voltage and Leakage Current at MCOV |
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Installation and Maintenance of Surge Arresters:
For effective protection, the following factors need to be considered during the installation of surge arresters:
The Surge Arrester is mounted on an Insulating Base where Surge Monitor is to be connected in series with the Arrester. The Surge Monitor is to be fixed at a height of 1.5 meters from the ground level for better viewing of the meter. Arrester should be properly connected to effectively earthed system of 0.8 coefficient of earthing for effective discharge of the surge current and also to limit the voltage to rated voltage of the Arrester.
Arrester should be mounted close to the equipment to be protected and connected with shortest possible lead on both the line and ground side to reduce the inductive effects while discharging large surge currents.
For transformer winding protection, it is desirable to interconnect the ground lead of the arrester with the tank and also the neutral of the secondary. The interconnection reduces the stress imposed on the transformer winding by the surge currents to the extent of the drop across the earth resistance and the inductive drop across the ground lead.
Periodic cleaning of the Porcelain Arresters is necessary in polluted areas and particularly in the highly polluted areas such as cement plant location, coastal areas, areas of chemical pollution, desert areas, areas of industrial smoke and areas exposed to strong winds carrying sand, salt smoke and chemicals. The deterioration of the Arrester under heavy pollution conditions is not ruled out. This is because of the fact that arrester would experience severe stresses due to the presence of thick polluted layer on the housing which could endanger the life of the arrester.
The clearance between the arrester and the other equipments should be met as per the Insulation coordination practices. Note that if the clearance is less than the suggested values, the voltage distribution of the arrester would be disturbed.
Besides, it is important to note that if the distance between the arrester and the equipment to be protected is high the later will be subjected to a higher over voltage. The voltage stress on the equipment to be protected (U2) can be estimated using the following emperical relationship : U2=U1+(2 x s x I)/V. {Where U1 – residual voltage of arrester, s-steepness of the incoming wave (KV/micro seconds), l-distance between the arrester & the equipment to be protected, V-Velocity of wave propagation (m/micro seconds)}.
Maintenance:
No maintenance is necessary except for cleaning of the Porcelain Housing (if the surface of the arrester housing is contaminated) before taking the reading as the ammeter reads the leakage current of the arrester housing and the metal oxide element as well.
Connecting lead between surge arrester and surge monitor should be insulated. Insulated aluminium cable of 35 sq.mm. cross section is adequate as connecting lead. The connecting lead can get raised to high potential during Arrester operation.
The earth terminal of the Arrester should never be isolated. In the remote event of Surge Monitor disconnection, it should be by-passed by an earthing shunt. Surge monitor being sensitive instrument proper handling of the instrument is suggested.
Indication of failure of Porcelain Arrestes:
A red diaphragm with the inscription “THIS RED DIAPHRAGM BLOWS OUT IN THE REMOTE EVENT OF ARRESTER FAILURE”, closing the Pressure Relief vent blows out in the remote event of the arrester failure. This helps in locating the failed arrester and eases in its replacement.
Periodic cleaning of the Porcelain Housed Arresters is necessary in polluted areas and particularly in the highly polluted areas such as cement plant location, coastal areas, areas of chemical pollution, desert areas, areas of industrial smoke and areas exposed to strong winds carrying sand, salt smoke and chemicals. The deterioration of the Arrester under heavy pollution conditions is not ruled out. This is because of the fact that arrester would experience severe stresses due to the presence of thick polluted layer on the housing which could endanger the life of the arrester. |
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