- In transformer in generating stations, the primary voltage can be kept almost constant , so the tappings should be provided on the secondary side while in transformer installed at the end of a transmission line i.e. where primary voltage varies, the tappings should be provided on the primary side.
Tapping are usually provided on the high voltage winding due to the following reasons:
- The number of turns on the hv winding is large, and therefore, a fine and smoother control of the output voltage is obtained.
- It is not possible to tap other than an integral number of turns, and this may cause difficulty with lv windings. For example 260V phase winding with 13V per turn can not be tapped closer than 5 %.
- It is difficult to tap lv winding, since it is placed next to the core, due to insulation consideration. on other hand the hv winding, which is placed outside the lv winding, is easily accessible and can, therefore be tapped easily.
- Tap Changers on hv side will have to handle low currents though it will need more insulation. there is an additional advantage of providing tapping on hv winding in case of stepdown transformer transformers.
- On light load, for decreasing the voltage on secondary side , the number of turns included on hv side is increased resulting in reduced emf per turn and so in the flux density and therefore core loss.
- Another important point about the tapping is that whether these should be provided at the phase ends, at the neutral point, or in the middle of the windings.
- The choice is largely a constructional question. The advantage of having the tapping at the phase ends is that the number of bushing insulators required is reduced, which may be useful where cover space is limited.
- Some transformers have reinforced insulation at the phase ends, it is essential that in such cases either the tapping should not be provided or the reinforcement should be carried beyond the lowest tap.
- The insulation between various phase parts, when the tapping are made at the neutral ends, is small.
- These arrangement is very economical specially in the case of high voltage electrical transformers.
- For achieving large voltage variations, tapping should be placed near the center of the phase windings in order to reduce magnetic asymmetry.
- However, this arrangement not possible with Lv windings placed next to the core( as in core type transformer) because of accessibility and insulation considerations.