Protection
A volts-per-hertz test checks that a device 24 relay detects excessive flux in a generator or transformer core, picks up where it is set and times out along its curve, so the unit is tripped before overexcitation heats it.
Device 24 is the volts-per-hertz relay, which guards generators and transformers against overexcitation. It responds to the ratio of voltage to frequency, which tracks the magnetic flux in the core. The test applies a controlled voltage and frequency from a relay test set and checks where the relay picks up, how it times out and where it resets. Many relays have an alarm stage with a definite delay and a trip stage that follows an inverse-time curve, or two definite-time stages, so each stage and curve is checked. The relay may be an electromechanical unit, a solid-state unit or a function inside a microprocessor-based generator or transformer protection relay. The test is performed on the relay itself, along with the voltage input and the output path to the trip circuit.
When voltage rises or frequency falls, the core flux climbs, the iron saturates and flux spills into the surrounding structure. The result is rapid heating of core laminations, bolts and nearby metal that ordinary overcurrent protection does not see, and the damage builds with time at the elevated level. Overexcitation arises during a startup or shutdown of a generator with the regulator in service, after the loss of load, with a faulty regulator, or when a system disturbance depresses frequency. A relay that picks up late, times out too slowly or fails to trip can leave an expensive machine to cook, while one set too tightly trips a healthy unit during normal transients. Testing proves that the in-service setting and curve are the intended ones.
Induced voltage in a winding is proportional to flux and frequency, so flux is proportional to the ratio of voltage to frequency. The relay computes that ratio from a measured voltage and frequency, usually referred to the rated values of the protected unit and expressed per unit, and compares it with its settings. The alarm stage warns the operator, and the trip stage follows a time characteristic in which a larger excess trips sooner, matching how the thermal capability of the core falls as excitation rises. A test set can raise the voltage at constant frequency, lower the frequency at constant voltage, or do both, and each approach checks the same calculation from a different direction. Because the relay uses a ratio, the voltage and frequency references must be known and correct, and the inverse characteristic must be tested at several points.
The relay is isolated from its trip and alarm circuits, or those circuits are blocked, following the owner's lockout and work-permit procedures, so the test cannot trip a generator or transformer. The voltage input is disconnected from the voltage transformer secondary and connected to the test set, and the secondary is never back-fed. The relay's settings, including the rated voltage and frequency references, are read and recorded. The test set raises voltage at constant frequency until the alarm and trip stages pick up, then the pickup is repeated by moving frequency. Timing is measured at several excitation levels along the inverse curve or at each definite-time stage, with the test set starting its timer when the level is applied and stopping when the output contact closes. The relay is allowed to reset between runs, because many designs integrate the excess over time and carry a memory of earlier runs. Reset behavior is also observed. Afterward the connections are restored and the settings are compared with the protection study.
The record contains the as-found pickup of each stage, the timing measured at the points along the curve, the reset behavior, the voltage and frequency references used for the calculation, the relay identification and settings, the test set used with its calibration status, and any adjustment made with the as-left result. The protected unit and the voltage transformer ratio are noted so that the result can be tied to the application.
Pickup and timing are judged against the settings in the protection study, within the accuracy the manufacturer states for the relay, and against the capability curve the protected equipment's maker publishes. The inverse curve should follow its expected shape across the points tested, and the alarm stage should operate before the trip stage. Comparison with earlier tests of the same relay shows drift. ANSI/NETA ATS-2025 describes acceptance testing of protective relays, and maintenance testing follows ANSI/NETA MTS-2023. Refer to your own licensed copies for the values and tolerances.
The equipment classes and standard sections that call for this test, and the UltraDb forms that carry it.
Core flux depends on the ratio of voltage to frequency, not voltage alone. A machine running slowly with normal voltage can be overexcited just as one running at normal speed with high voltage, so the relay watches both together.
The core saturates and flux leaks into structural parts, producing heat that normal overcurrent protection does not detect. Damage builds with time at the elevated level, so the relay trips along a time curve in which larger excursions trip sooner.
Many relays integrate the excess over time to follow the core's heating, so a recent run leaves a memory that shortens the next one. Allowing the relay to reset gives each timing run a fair starting point.
UltraDb references ASTM, ANSI, IEEE, NETA, NFPA, NEMA, FAA, TIA, and IEC standards descriptively, to identify the published methods its forms are built from. UltraDb and Verlecta are not affiliated with, endorsed by, certified by, or licensed by NETA, ASTM, ANSI, IEEE, NFPA, NEMA, FAA, TIA, IEC, or Megger. Standard names and section numbers are the property of their respective organizations. Always test to the edition your contract specifies. Where a standard is sold through the ANSI webstore, Verlecta may earn a commission on a purchase made through our link, at no extra cost to you. A commission never decides which standards we cite or how a form is built.