On Load Tap Changer or OLTC
In larger electrical power transformer, for proper voltage regulation of transformer, on load tap changer is required. As there is no permission of switching off the transformer during tap changing. The tapping arrangement, is placed in separate divertor tank attached to electrical power transformer main tank. Inside this tank, the tap selectors are generally arranged in a circular form. The divertor switches have contacts operating in rapid sequence with usually four separate make and break units.
Figure shows a typical selector and switching arrangements with no load tap changer or NLTC on neutral and of the star connected winding of a electrical power transformer.
The OLTC selector arrangement for 16 steps and 17 positions. In the diagram, the on load tap changer is connected to tapping 4 while changing from tap position 4 to tap position 5, M1 is opened first and this transfers the load current via A1 with resistor R1 in series. Then A2 closed and two resistors R1 and R2 are in series across tapping 4 and 5. A circulating current will flow through these resistors because of step up voltage between tapping 4 and 5. After that A1 opens and interrupts the circulating current and the load current is transferred to tapping 5 passing through resistor R2. Finally M2 closes and takes load current, completing the tap changing. For a tap changing in opposite direction, the sequence is reverse. When the tap changer has several tappings it is advantageous to halve the length of tapping winding and to introduce a reversing or changeover selector.