Classification of Sub States

Classification of Sub States Points : classification of sub states, indoor type substations, outdoor substations, foundation mounted substations, indoor substations, substations of the integrally build up type, substations of the composite built up type, outdoor substations, pole-mounted substations, foundation mounted substations, (i) Indoor Type Substations In such substations the apparatus is installed within the substation building. Such substations are usually for a voltage up to 11 kv but can be erected for the 33 kv and 66 kv when the surrounding atmosphere is contaminated with impurities such as metal corroding gasses and fumes, conductive dust etc. (ii) Outdoor Substations These substations are further subdivided into: (a) Pole Mounted Substations Such substations are erected for distribution of power in localities. Single stout pole or H-pole and 4-pole structures with suitable platforms are employed for transformers of capacity upto 25 kva, 100 kva and above 100 kva respectively (b) Foundation Mounted Substations For transformers of capacity above 250 kva the transformers are too heavy for pole mounting. Such substations are usually for voltage of 33,000 volts and above. INDOOR SUBSTATIONS Indoor ‘distribution and transformer substations as well as high voltage switch boards consist of a series of open and enclosed chambers or compartments. The main equipment of the given installation is arranged in these compartments. The chamber space within which the equipment of any one main bus bar connection is mounted, as a whole, is called as a cell, cubicle or compartment. 1. Substations of the Integrally Build Up Type In such substations the cell structures are constructed of concrete or brick. 2. Substations of the Composite Built up Type In which the assemblies and parts and factory or workshop prefabricated, but are assembled on site within a substation switchgear room. The compartments of such substations take form of metal cabinets or enclosures, each of which contains the equipment of one main connection cell. Outdoor Substations As already said outdoor substations are of two types namely (i) pole-mounted substations and (ii) foundation mounted substations. (i) Pole-mounted Substations Such substations are erected for mounted distribution transformers of capacity upto 250 kva. Such substations are cheapest, simple and smallest of substations. All the equipment is of outdoor type and mounted on the supporting structure of h.t. distribution line. Triple Pole Mechanically Operated (T.P.M.O) switch is used for switching. ‘on’ and ‘off’ h.t. transmission line. H.T. fuse unit is installed for protection of h,t. side. To control l.t. side iron clad low tension switch of suitable capacity with fuses is installed. Lightning arrestors are installed over the h.t. line to protect the transformer from the surges Substations are earthed at two or. more places. Generally transformers up to 100 kva are mounted on double hole structure and for transformers of capacity above 100 kv but not exceeding 250 kva 4-pole structure with suitable platform is used. This type of pole mounted substation is erected up very thickly populate location.

The maintenance cost of such substations is low and by using a large number of such substations is a town it is possible to lay the distributors, at a lower costs. But owing to increase in number of transformers, total kva is increased, no load losses increase and the cost per kva is thus more. Economy is the main consideration when a choice is made for such substations. Typical 200 kva pole mounted substation. (iii) Foundation Mounted Substations These substations are built entirely in the open and in such substations all the equipment is assembled into one unit usually enclosed by a fence from the point of view of safety. Substations for primary and secondary transmission and for secondary distribution (above 250 kva) are foundation mounted outdoor type.

Since equipment required for such substations is heavy, therefore, site selected for these substations must have a good access for heavy transport. .The switch gear consists of circuit breakers of suitable type of both the sides but with the increased reliability of the modern transformers, the practice i to dispense with the circuit breaker on the incoming side from the economics consideration. The isolating ‘switches thus serve the purpose.