Overexcitation 24 V/Hz (PVPH): How It Works

Short answer

Overexcitation protection (ANSI 24) monitors the ratio between voltage and frequency (V/Hz), which is proportional to the magnetic flux in the core of transformers and generators. When that ratio exceeds what the core can withstand, the iron saturates, stray flux and localized heating appear, and the function alarms or trips with a time that depends on how far the limit is exceeded.

What 24 protection is (overexcitation or V/Hz)

In a transformer or a generator, the magnetic flux in the core is proportional to the applied voltage and inversely proportional to frequency. So the quantity that matters is not voltage alone but the V/Hz ratio. If the voltage rises at rated frequency, or the frequency falls at rated voltage, the flux increases. Beyond a certain point the core saturates, the exciting current grows sharply, the flux escapes into structural parts that are not laminated, and the resulting heating damages the insulation within minutes.

Function 24 calculates V/Hz in per unit of the rated values and operates when the threshold is exceeded.

ANSI/IEEE C37.2: device 24, volts per hertz relay.
IEC 61850: logical node PVPH (volts per hertz protection).
IEC 60617: symbol U/f>.

Where it is applied

Generators and generator step-up transformers, where it is practically mandatory. Generator protection guides treat it as basic.
Substation power transformers, especially those connected to networks that may be left with overvoltage after a load rejection or in an island.
Solar and wind plants with a step-up transformer, when the grid or the plant control can drive the voltage up.
Industry with on-site generation that starts and synchronizes frequently.

How it works

The relay measures a voltage (phase or line, depending on the connection), calculates the frequency and obtains the ratio:

V/Hz (pu) = (V measured / V rated) / (f measured / f rated)

With that value it applies two kinds of element:

Definite time: one or two stages, normally one for alarm and one for trip.
Inverse time: a curve in which the trip time decreases as overexcitation increases. Some relays come with predefined curves and others allow a point-by-point curve that reproduces the capability curve of the equipment manufacturer.

Many relays include a thermal model with memory: if the condition repeats, the accumulated time counts and the relay trips sooner. The reset time of that memory is also set.

Overexcitation is related to transformer differential protection 87T: the exciting current of an overexcited core has a high fifth-harmonic content, and the 87T usually uses it to block so it does not mistake that condition for an internal fault.

Setting criteria

The values come from the capability declared by the equipment manufacturer, not from a generic number:

Manufacturer’s V/Hz capability curve for the transformer and the generator. When a generator feeds its step-up transformer, the setting follows the more restrictive of the two curves, referred to the same base.
Voltage base: it must match the rated voltage of the protected equipment, which is not always the nominal system voltage. If the transformer has taps, decide which tap is the reference.
Alarm below the trip, so the operator or the regulator can correct before the unit is lost.
Coordination with the automatic voltage regulator (AVR) and its V/Hz limiter: the limiter must act first, and the 24 is the backup.
Voltage transformer feeding the function, and behaviour on loss of potential.

The transformer and generator standards set the minimum capability to operate above rated voltage under defined conditions; the applicable values depend on the manufacturing standard and the nameplate data. Final settings are confirmed in the coordination study.

How each manufacturer implements it

Siemens SIPROTEC (7UT for transformers, 7UM for generators) with DIGSI: definite-time elements and a configurable thermal characteristic.
SEL (for example SEL-300G, SEL-700G, SEL-487E) with AcSELerator QuickSet: definite-time stages and inverse curves, including a user-defined curve in some models.
Relion: Hitachi Energy (RET670, REG670) and ABB (RET615), with PCM600: the function is added as a block and its curve is chosen from predefined ones or a table of points.
GE Vernova (Multilin T60, G60, 845) with EnerVista: V/Hz elements with their own curves and reset logic.
MiCOM P34x and P64x (Schneider Electric Easergy MiCOM and GE Vernova MiCOM Agile) with Easergy Studio or MiCOM S1 Agile: stages with selectable characteristics.

What changes between brands is how the base is defined, whether the voltage is phase or line, the shape of the curves and how the thermal memory behaves. The same per-unit setting can give different times in two brands.

Why it trips or blocks

Causes of operation we see often:

Generator start-up or shutdown with the regulator in automatic and the machine below rated speed.
Load rejection that raises the voltage before the regulator responds.
Voltage regulator failure or loss of its feedback signal, which drives it to maximum excitation.
Operation on the wrong transformer tap, or high system voltage under light load.
Electrical islands where the frequency falls and the voltage holds.

Causes of unwanted trips or odd measurements:

Voltage base entered incorrectly, or confusion between phase and line voltage.
Wrong voltage transformer ratio.
Thermal memory that did not reset between two consecutive events.

The first step is to check the voltage and frequency in the record at the moment of the trip and compare them with the configured base.

How it is tested

With secondary injection and a test set such as the Omicron CMC 356, the voltage, the frequency or both are varied to cover the characteristic. In general the test checks:

The V/Hz pickup, varying voltage at rated frequency and frequency at rated voltage.
The operating times of the definite-time stages and several points of the inverse curve.
The thermal memory and its reset time.
The alarm, the trip and their assignment to the correct outputs.

On generators, confirm that the regulator’s V/Hz limiter acts before the relay.

Did your relay trip or block on this function and the reason is not clear? At Tiza Group we review the events and settings remotely. See remote relay configuration or write to us.

References

IEEE C37.2, Standard for Electrical Power System Device Function Numbers, Acronyms, and Contact Designations.
IEEE C37.102, Guide for AC Generator Protection.
IEEE C37.91, Guide for Protecting Power Transformers.
IEEE C57.12.00, General Requirements for Liquid-Immersed Distribution, Power, and Regulating Transformers.
IEC 61850-7-4, Compatible logical node classes and data object classes.
J. L. Blackburn and T. J. Domin, Protective Relaying: Principles and Applications.

Frequently asked questions

Why did the 24 trip during generator start-up?

If the voltage regulator brings the machine to rated voltage while the speed is still low, the V/Hz ratio rises above the limit even though the voltage looks normal. Check the excitation ramp, the regulator's V/Hz limiter and the frequency recorded in the event.

Are the 24 and the fifth-harmonic blocking of the 87T the same thing?

No. Fifth-harmonic blocking prevents the differential from tripping on the exciting current of an overexcited transformer. The 24 is the function that protects the core and decides whether that condition lasts too long.

Why does the 24 measure a different V/Hz value from the one I calculated?

Most often the relay base value does not match the rated voltage of the protected equipment, or the relay uses phase-to-phase voltage while you calculated with phase-to-neutral, or the other way round. Confirm the base, the type of voltage measured and the voltage transformer ratio.