Why and Where Transformer Tap changer required??

Why Tap Changer required in Transformer??

Every equipment  is designed to operate at a particular voltage level. So it becomes extremely important to keep that voltage within limits. To keep terminal voltage within limits transformers are provided with tap changers. Tap changer can be provided either on primary or secondary depending upon requirement.

Principle behind the tap changer is same. For regulating secondary output voltage no. of turns in primary or secondary winding are changed.

e.g. let V(p),N(p) and V(s), N(s) are primary and secondary quantities. If N(p) decreased emf per turn on primary winding i.e. V(p)/ N(p) increases therefore it will leads to increase in O/P voltage as Secondary O/P voltage is {V(p)/ N(p)} XN(s).

Similarly if N(s) is increased and N (p) kept constant then by same formulae we find that same effect on primary and secondary voltages. So we can say that decreasing primary turns and increasing secondary turns have same effect on voltages.

Where Tap Changer to be Provided on Transformer??

Thus tap changers are provided on primary or secondary winding of transformer so as to alter turn ratio. But choice of fixing tap changer on primary or secondary winding is made by keeping in mind that voltage per turn will remains constant. If primary winding voltage per turn decreases the core flux will decreases which will leads to poor utilization of core. On the other hand if voltage per turn will be increased in primary winding then this will leads to magnetic saturation of core and more core losses.

As we know that At generating end Transformers  primary voltage will be kept almost constant so that is reason to provide tap changer on secondary winding not primary winding. If there are variable voltages as in case of transmission lines then it is advisable to keep tap changer on primary.

There are other factors which must need to be considered while deciding tap changer position on transformer winding:-

1.       Transformers with large turn ratio are tapped on HV side since it enables smoother control of O/P voltage as compared to LV side tap changer as LV side tap changer will leads to vary O/P voltage in large steps which is undesirable.

2.       Tap changer on HV side have to handle low currents although it has to handle high voltage or more insulation required.

3.       Also it is very difficult to put tap at LV winding as it is placed next to core due to insulation considerations. As HV winding is placed outside of LV winding so it is easily accessible can be easily tapped without any difficulty.

  Read about off-load tap changer Click here
http://electrialstandards.blogspot.in/2014/05/transformer-off-load-tap-changer.html