- OLTC Definition: An On Load Tap Changer (OLTC) is a device in power transformers that regulates voltage without switching off the transformer.
- No Load Tap Changing Transformer: No Load Tap Changers (NLTCs) are used for changing transformer taps without load.
- Tap Changing Process: The tap changing process involves a sequence of operations using resistors to manage current during tap switching.
- Voltage Regulation: OLTCs are crucial for maintaining proper voltage regulation in large power transformers.
- Components and Arrangement: The tap selectors and divertor switches in OLTCs are arranged in a circular form within a separate divertor tank.
On Load Tap Changer or OLTC
In largerpower transformers, for proper voltage regulation of transformer, an 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.
The figure shows a typical selector and switching arrangement with a no load tap changer (NLTC) on the neutral end of the star-connected winding of an electrical power transformer.
The OLTC selector arrangement has 16 steps and 17 positions. In the diagram, the OLTC is connected to tap 4 while changing from tap position 4 to tap position 5. First, M1 opens, transferring the load current via A1 with resistor R1 in series. Then A2 closes, putting resistors R1 and R2 in series across taps 4 and 5. A circulating current flows through these resistors due to the voltage step between taps 4 and 5. After that, A1 opens and interrupts the circulating current, transferring the load current to tap 5 through resistor R2. Finally, M2 closes and takes the load current, completing the tap change. For tap changing in the opposite direction, the sequence is reversed. When the tap changer has many tappings, it is beneficial to halve the length of the tapping winding and use a reversing or changeover selector.
