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Distribution transformers are manufactured exclusively of grain oriented steel assembled with interleaved joints and fastened, with particular care, by bout steel reinforcement in order to reduce to a minimum the hum and additional losses.
This type of power transformer contain windings of soft electrolytic copper, insulated by pure cellulose paper of concentric moored type so that it may withstand the stresses coming from short circuit. An oil filled transformer will generally be constructed with large cooling ducts which allow an easy oil circulation.
The tanks made of stout sheet – steel are provided with tubes or radiators in order to completely discharge the produced heat. Power transformers rated at 100 kVA and above are cooled by means of radiators as explained above. Up to 10000 kVA rating natural cooling (ONAN type) is applied as standard. Off load (off circuit) voltage regulation is effected within the typical range of 10pct (+/- 5 pct) whilst On-load voltage regulation for transformers rated above 2000 kVA is provided within ranges of 20 pct (+/- 10 pct) or 32 pct (+/- 16pct) which are also typical for distribution transformers. Both types of tap changers are mounted together with core and windings inside of transformer tank.
It is very common that power transformers are internally painted with a rust preventer, which is oil-heat proof and externally painted with antirust and two coats of anticorrosive enamel. These types of protection are suitable for installation in a tropical climate.
Mineral oil of type in accordance with the last standards, it is rightly filtrated and dried under vacuum. A pole mounted transformer and pad mounted transformer if requested can also be manufactured with an uninflammable synthetic fluid of a silicon type.
Various fittings and accessories can be included with any transformer. A Buchholz relay and thermometer with two electrical contacts can be supplied, which are used for signalizing and releasing the transformer in case of damage. Other fittings which are seen on pole mounted transformers and pad mounted transformers, both step up transformers and step down transformers include: oil level indicator, dehydrating breather, oil sampling plugs, oil drain valve, earthing terminals, jacking lugs, traction rollers or wheels and connecting box or control cubicle.
A rigid inspection/test procedure is carried out on all power transformers including routine tests that meet all the requirements of BS/IEC standards. Type and special tests are carried out when requested.
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