- 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
Some power transformers need continuous voltage regulation of transformer service. An on load tap changer adjusts the winding ratio without interrupting load. An OLTC transfers load current between adjacent taps through a controlled make-before-break sequence. A transition resistor or reactor limits current while both taps are bridged. Construction varies by rating and design. Some OLTCs use a separate tap selector and diverter switch in an oil compartment, while others combine those functions in a selector switch. Circular selector arrangements and separate diverter tanks are common examples rather than universal requirements.
The switching sequence below is for the illustrated resistor-type no load tap changer (NLTC), although a device that performs this sequence while carrying current is technically an OLTC. A true NLTC, also called a de-energised tap changer, must be operated only after the transformer is de-energised and isolated according to its instructions.
The illustrated OLTC has 16 steps and 17 positions; these numbers apply to this diagram, not every OLTC. It is shown moving from tap 4 to tap 5. First, M1 opens so the load current transfers to A1 through transition resistor R1. Next, A2 closes. Resistors R1 and R2 then bridge taps 4 and 5 in series and limit the circulating current caused by the step voltage. A1 then opens, transferring the load current to tap 5 through R2. Finally, M2 closes to carry the load current in the new steady position. The transition resistors are loaded only during the rapid diverter-switch interval. A reverse tap change uses the corresponding reverse contact sequence. For a wide regulating range, a reversing or coarse changeover selector can reduce the length of the tapped winding.
