All tap changers perform the same function — changing the active number of turns — but the fundamental difference is: can the change be made while the transformer is supplying its loads? This question separates two completely different families in design and operation.
The fundamental comparison
| Aspect | OLTC (on-load) | OCTC / DETC (off-circuit) |
|---|---|---|
| Changing while in service | Yes, without interrupting supply | Prohibited — the transformer must be de-energized and isolated |
| Complexity | A transition mechanism with resistors/reactors and a separate arcing chamber | A simple selector switch |
| Number of operations | Thousands of operations per year (often automatic) | A few times: seasonal adjustment or during installation |
| Maintenance | Periodic and intensive (contact wear, tap changer compartment oil) | Very limited |
| Typical use | Power transformers in transformation substations | Small distribution transformers |
How does an OLTC change taps without interruption?
The secret lies in the transition mechanism: before leaving the current tap, the tap changer grips the next tap through a transitional resistor or reactor that limits the circulating current between the two taps, then completes the transition and releases the first one — all within fractions of a second and inside an oil compartment separate from the main transformer oil to contain the arc.
Practical points
- The oil in the OLTC compartment becomes contaminated faster than the main transformer oil due to repeated arcing — it has its own independent inspection and replacement schedule.
- The OLTC operation counter is a key reference for scheduling its maintenance.
- With an OCTC, make sure the switch is firmly seated on a tap position (not between two positions!) before re-energizing.
Sample answer: OLTC changes taps while the transformer is under load without interrupting supply and is used in power transformers, while OCTC can only be changed after the transformer is fully de-energized and is found in small distribution transformers. OLTC needs a transitional resistor (or reactor) because, during the transition, it momentarily grips two adjacent taps together, and the resistor limits the circulating current between the turns of the two taps until the transition is completed safely.
Assuming any tap changer can be operated while the transformer is in service. Operating an OCTC under load generates a damaging arc. The rule: never touch a tap changer before confirming its type from the nameplate and catalog.
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