What is the difference between an on-load tap changer (OLTC) and an off-load tap changer?

• A. OLTC changes taps while energized; off-load requires removing power to change taps.
• B. OLTC changes taps only when the transformer is de-energized.
• C. OLTC and off-load both require removing power to change taps.
• D. Off-load tap changers change taps automatically in service.

Answer: (C)

The key idea is whether you can change the transformer taps while it is energized and delivering power. An on-load tap changer is designed to shift taps without disconnecting service, so the transformer can regulate its output voltage as load and system conditions change. The switching is coordinated through a control system and contacts arranged to handle the arc and transient behavior while the windings stay energized, keeping customers on the line with minimal interruption.

In contrast, an off-load tap changer is used only when the transformer is de-energized. You must remove power and isolate the transformer before changing taps, so there is no load current to contend with. This means voltage adjustment cannot respond to real-time changes in load and is typically done during maintenance or scheduled steps.

So, the difference is about live operation: on-load changes taps under load, off-load changes taps only after de-energizing the equipment.