Tap changer (ON LOAD type) can be used to increase or decrease transformer output voltage without break in the voltage to the load.
The tap changer consists of a diverter switch. The odd taps are taken on one side and even on another side as shown in figure.
The diverter switch is provided so that there will be no break in the supply to the load and also no cut of transition resistance when the tap changing is achieved. Transition resistors are provided to limit the current when the windings are shortcircuited by the diverter switch
Operation: In the above figure, the voltage at tap 2 is 406 V. the position of diverter switch is shown. It short-circuited with transition resistance.
We want to increase voltage to 420 V at tap 5. When we begin to change the tap the diverter switch connects 2 transition resistance and when the tap changeover is achieved that is when the tap changer reaches tap 5, the diverter switch short circuits transition resistance and thus the resistance is eliminated. The diverter switch switching time is very high. This is to reduce arcing, which can decompose the oil. The whole assembly is immersed in oil. OLTC is connected to HV side, because in LV side current handled will be more. But in HT side current to be handled is lesser than LV side.
