We shall be discussing the distribution transformers in detail in the next unit.Here, we briefly describe the remaining components.
•Transformer Mounting Structure
Transformers can be mounted outdoors (Figs. ) in one of the following ways: Plinth mounting, H-pole mounting and direct mounting. We describe these mountings, in brief.
|Transformer Mountings: a) Plinth Mounting; and b) H-Pole Mounting|
Plinth mounting: The transformer is mounted on a plinth made of concrete. The plinth has to be higher than the surroundings. The
method can be used for all sizes of transformers. Where the distribution substations are plinth mounted, they are efficiently
protected by fencing so as to prevent access to the apparatus by unauthorized persons.
H-pole mounting: The transformer can be mounted on cross- arms, fixed between two poles, which are rigidly fastened to the
poles. The transformer has two base channels, which rest on the transformer mounting structure.
Direct mounting: The transformer is clamped directly to the pole by suitable clamps and bolts. This method is used for transformers up
to 25 kVA only.
The HT side of all transformers is normally protected by drop out expulsion type fuse. Three 11 kV drop out fuse units comprising a set are installed on mounting cross-arm. The fuse element is soldered on both ends between woven wires, which are sufficiently strong to withstand tension when fixed to the terminals on both ends.The element is housed in an insulated tube of paper or insulating material. Horn gap fuses are also used on distribution transformers on HT side. The fuse wire is fixed between arcing horns. The advantage is that ordinary fuse wire of rated capacity can be used for replacement while for drop out fuses, fuse elements are required to be stocked for replacement.
Pipe earthing or rod earthing is provided for the distribution substation.Three electrodes forming an equilateral triangle are provided so that adequate earth buffer is available.
11 kV lightning arrestors 9 (kV) of outdoor type are used for diverting the lightning surges to earth resistance of earth. The lightning arrestor should be installed on the HT side and its lead should be kept at a minimum.
•LT Distribution Box
For transformers of 100 kVA and above, sheet metal LT distribution box consisting of LT breaker and fuse cut-outs is provided from where distribution feeders are to be taken out. The size of the box has to be suitable for accommodating MCCB, fuse cut-outs, cable connectors, bus-bars, etc.
The load incident on the distribution system is mostly inductive, requiring large reactive power. The best method is to compensate the reactive power at the load end itself but it is difficult to implement in practice. Hence, providing compensation on the distribution system is essential. So wherever the power factor is low, reactive compensation may be provided on the distribution transformers.
The shunt capacitor supplies constant reactive power at its location,independent of the load. Fixed or automatic switched type capacitors of adequate rating are to be provided on the LT bus of the distribution transformers. In the switched capacitor system, the capacitors are switched on and off along with the load to avoid over-voltage during low load operation.