What is the main difference between an OLTC and a DETC?

An OLTC changes ratio while the transformer carries load. A DETC may be operated only after the transformer has been de-energized and safely isolated.

Does every power transformer require an OLTC?

No. An OLTC is justified when the network requires voltage regulation without an interruption. A stable system with infrequent adjustment may be adequately served by a DETC.

What tap-changer data should be included in an enquiry?

State the regulating winding, range, step size, number of positions, normal position, maximum current, control method, expected operating duty and applicable standard.

OLTC vs DETC: How to Select a Transformer Tap Changer

An on-load tap changer (OLTC) changes transformer ratio while the transformer carries load. A de-energized tap changer (DETC, also called an off-circuit tap changer) may be operated only after the transformer is de-energized and safely isolated. The choice affects voltage regulation, transformer cost, control equipment and maintenance.

Parallel OLTC operation: tap schedules and regulator control must be coordinated before units share a bus. Review the required data in Transformer Parallel Operation.

Selection rule: Use an OLTC when the system must regulate voltage without interrupting supply. Use a DETC when taps are intended mainly for commissioning or infrequent network adjustment during a planned outage.

OLTC and DETC compared

Comparison of transformer OLTC and DETC tap changer operating conditions — Both devices alter the effective winding turns, but their permitted operating conditions and system roles are different.

Decision point	OLTC	DETC
Operation	With the transformer energized and carrying load	Only when de-energized and safely isolated
Typical purpose	Automatic or commanded voltage regulation	Commissioning adjustment or infrequent ratio correction
Main equipment	Diverter/selector arrangement, transition impedance, drive and controls	Simpler tap selector and operating mechanism
Maintenance	More components and operating-duty records to manage	Lower mechanical duty, but position and contacts still require inspection
Commercial effect	Higher initial and lifecycle complexity	Lower complexity where continuous regulation is unnecessary

Why an OLTC needs a transition circuit

During a tap change, load current must continue while the connection moves from one tap to the next. The switching sequence therefore uses a make-before-break principle with transition resistance or reactance to prevent an open circuit and limit circulating current between adjacent taps. This is a controlled switching operation, not a simple selector movement.

Simplified make before break sequence in an on-load tap changer — Simplified OLTC transition sequence. The actual arrangement, timing and protective system depend on the selected tap-changer design.

Data needed to specify the tap range

A statement such as “OLTC ±10%” is incomplete. The supplier also needs the regulating winding location, number and size of tap steps, normal tap position, constant-flux or variable-flux requirement, maximum through-current, insulation level, short-circuit duty and the control philosophy. The system study should show the voltage range that the transformer must correct under realistic load and source conditions.

Nominal HV and LV voltages and allowable bus-voltage range
Total regulating range, step percentage and number of operating positions
Manual, remote or automatic voltage control requirements
Line-drop compensation, paralleling and master-follower requirements
Expected operations per day and maintenance constraints
Alarm, blocking and protective-device interfaces

Transformer tap changer selection questions for regulation range duty and maintenance — A tap-changer enquiry should define operating need, range, duty and maintenance—not just the letters OLTC or DETC.

When a DETC is the better choice

A DETC is often sufficient where the source voltage is stable, the transformer is not expected to regulate a live bus, and planned outages are available for occasional adjustment. It can avoid unnecessary equipment and maintenance. However, operators must never treat a DETC as an OLTC: the transformer must be de-energized, isolated, proved dead and handled under the site’s safety procedure before its position is changed.

Procurement checks before approval

Confirm the tap table on the nameplate drawing, the voltage associated with each position, the normal position, the direction of voltage change and the motor-drive supply. For an OLTC, also review its type-test evidence, protective device, monitoring contacts, control cabinet, operation counter and maintenance instructions under the applicable IEC 60214 requirements.

Practical conclusion: Do not pay for an OLTC simply because a transformer is large. Select it when the network study demonstrates a need for live voltage regulation and define the complete operating duty in the enquiry.