The cost of these substations is less as they require less space and no building. as such to function.
The layout diagram of a pole-mounted transformer substation is shown in Fig.
Such substations are erected for mounting distribution transformers of capacity up to 250 KVA.
Such substations are the cheapest, simple and smallest of substations. Triple pole mechanically operated (TPMD) switch is used for switching on and off of HT transmission line.
HT fuse unit is installed for the protection of the HT side.
To control LT side Iron Clad Low Tension Switch of suitable capacity with fuses is installed.
Lightning arresters are installed over the HT line to protect the transformer from surges. The substation is earthed at two or more places.
Generally, transformers of capacity up to 125 kVA are mounted on a double-pole structure, and for transformers of capacity above 125 kVA but not exceeding 250 KVA, a 4-pole structure with a suitable platform is used.
This type of pole-mounted substation is erected in a very thickly populated location.
The maintenance cost is low and by using a large number of such substations in a town, it is possible to lay the distributors at a lower cost.
But owing to the increase in the number of transformers, total kVA is increased, no-load losses increase and cost per kVA is thus more.
The economy is the main consideration when a choice is made for such substations. HT side of a transformer is protected by a horn-gap fuse.
Beyond 250 kVA capacity, transformers become heavy and hence arc mounted on plinth-mounted platforms.
They are also called foundation-mounted substations. Since equipment required for such substations is heavy, therefore, the site selected for these substations must have good access for heavy transport.
The switchgear consists of circuit breakers of suitable type on both sides but with the increased reliability of the modern transformers, the practice is to dispense with the circuit breaker on the incoming side from the economic consideration. The isolating switches thus serve the purpose.
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