There are two types of tap changers:
1. On Load Tap Changer OLTC
Can change tap position carrying a full load.
2. De Energized Tap Changer DETC
Can be changed only when the transformer is de energized completely.
Principle of operation of OLTC was patented in 1927 by Dr Jansen, and today this holds. "Make before break" is the definition. Next tap should be connected before the previous one is disconnected. During a short period while both are connected, there is a short circuit of the portion of the regulating winding. To minimize this current, different philosophies are applied.
There are two philosophies of operation for OLTC:
1. Resistor type
2. Reactor type
It is very common to find OLTC in the primary (HV winding) of a transformer in Europe, and they are resistor type. On the contrary OLTC is usually in the secondary (LV winding) of transformers in the USA and it is mostly reactor type.