Transformer protection — overexcitation / V/Hz protection (ANSI 24)

The guide on transformer differential protection (87T) covers protection against internal short circuits inside a power transformer. This article covers a fundamentally different failure mechanism, which 87T does not detect because there is no current imbalance between the primary and secondary side: overexcitation caused by an excessive voltage-to-frequency ratio (V/Hz), and the ANSI 24 protection that monitors for it.

Why the V/Hz ratio matters, not voltage alone

The magnetic flux density in a transformer core (and in a synchronous generator) is proportional to the applied voltage and inversely proportional to frequency: a voltage rise at constant frequency, or a frequency drop at constant voltage, raises the flux in the same way. Once the flux exceeds the saturation limit of the core material, core saturation occurs: the magnetising current rises sharply and non-linearly, and significant, unwanted eddy currents develop in structural parts that are not designed to carry them — core clamping plates, the tank, and other metal components outside the laminated core stack. This causes localised overheating that, unlike a short circuit, cannot be detected by differential protection because no current imbalance arises between the windings.

Typical situations in which overexcitation occurs

Load rejection: when a large load suddenly drops off, the voltage on the generator side can temporarily rise sharply before the voltage regulator has corrected it. Rotating generators and connected transformers are then briefly exposed to an elevated V/Hz ratio. Related: the [guide on transformer inrush current](/guides/nen-1010/transformator-inschakelstroom-magnetiseringsstroom-selectiviteit) covers a different, brief saturation phenomenon that occurs at energisation, with its own characteristic.
Island operation with falling frequency: on an island running with insufficient generation capacity, frequency drops while the voltage regulator tries to maintain voltage — this raises the V/Hz ratio even without the voltage itself exceeding the rated value.
Manual or incorrect voltage-regulator setting: a voltage reference set too high on a generator's excitation controller, combined with a light load, can cause a prolonged, mild overexcitation that is not acutely dangerous but accelerates insulation ageing over time.

Operating principle of the ANSI 24 relay

The ANSI 24 protection continuously calculates the V/Hz ratio (typically expressed in per-unit, relative to the rated voltage-to-frequency ratio) and compares it against a set pickup value. A common rule of thumb is that transformers should not continuously withstand more than roughly 1.05 pu V/Hz at full load and 1.10 pu V/Hz at no load, but the actual, time-dependent thermal withstand of each specific machine is defined by a manufacturer capability curve.

Note: the ANSI 24 relay operates with an inverse-time characteristic: the further the measured V/Hz ratio rises above the pickup value, the faster the protection trips. This characteristic is designed to track the manufacturer's thermal capability curve, typically with a safety margin of 80–90% relative to that curve, so the protection intervenes before the actual thermal damage limit is reached.

In addition to the inverse-time characteristic, a modern ANSI 24 implementation typically also includes one or more fixed alarm and trip thresholds as an additional, unambiguous limit above the inverse-time curve.

Practical relevance

When setting or reviewing the protection settings of a power transformer or generator, it is important to check the ANSI 24 setting against the actual, manufacturer-specified V/Hz capability curve of that specific machine, rather than applying a generic rule of thumb — saturation sensitivity differs by core design and core material.

Common mistakes

Assuming differential protection (87T) also protects against overexcitation, when it responds only to a current imbalance between the windings and does not detect core saturation without an internal fault.
Applying a generic V/Hz threshold without checking it against the specific thermal capability curve provided by the manufacturer of the transformer or generator in question.
Overlooking overexcitation during island operation with falling frequency, because attention often focuses exclusively on voltage rise, while a frequency drop at constant voltage causes the same increase in the V/Hz ratio.
Not maintaining a safety margin between the ANSI 24 relay's inverse-time curve and the manufacturer's thermal capability curve, so the protection only intervenes after irreversible thermal damage has already occurred.