Frequency Relay (Device 81) Testing: Underfrequency and Overfrequency Elements

What it is

A frequency relay, ANSI device 81, watches the frequency of a voltage signal and operates when it falls below or rises above a set value. Underfrequency elements guard against a shortage of generation and are the basis of load shedding schemes. Overfrequency elements protect generators and turbines from overspeed after a loss of load. Some relays add a rate-of-change element that responds to how quickly frequency is moving, and a function that acts on a frequency band for island detection or generator separation. The test confirms that each element picks up at its set frequency, operates after its set delay, is blocked when the voltage is too low to measure frequency reliably, and drives the correct output. It applies to electromechanical, solid-state and numerical relays on generators, substations, industrial plants with on-site generation and utility interconnections.

Why it is done

Frequency tracks the balance between generation and load. When a large generator trips or a system separates, frequency drops, and the only practical defense is to shed enough load quickly and in stages. A relay that picks up late, early or not at all breaks the staging and risks a wider collapse. At generators, a frequency element that fails to act can leave a unit running outside the range its turbine and generator can tolerate, with damage that builds up quietly. At interconnections, a slow or missing element can keep a local source energizing an island. Because frequency relays rarely operate, errors in settings or wiring may sit unseen for years, and a periodic test is often the only evidence the scheme still works.

The principle

The relay measures the period of the incoming voltage waveform, usually between zero crossings or through a digital filter, and converts it to frequency. It compares that value with its setting and, once the condition has lasted for the set time delay, closes its output. Measurement is only reliable when the voltage is large enough, so a frequency element is paired with an undervoltage inhibit that blocks operation when the voltage collapses, since a dying voltage gives an erratic reading and a false trip would be harmful. Rate-of-change elements compare the frequency over a short window and respond to a steep slope before the absolute value has dropped far. Some relays use several stages, each with its own setting and delay, so the response is graduated. Harmonics or a poor test source can distort the reading, so the source must be accurate and stable in frequency.

How it is performed

With the relay isolated from the voltage transformers and trip circuits under the owner's safety plan, the technician records the relay identification and reads each stage setting, delay and inhibit level. The insulation between the relay circuits and the frame is measured first at the level the standard assigns. A relay test set then supplies a voltage of the correct magnitude to the voltage inputs, starting at normal frequency. For an underfrequency stage the frequency is lowered in small steps, or ramped slowly, until the relay picks up, and the value is noted; the process is repeated for the dropout. Overfrequency stages are tested the same way in the opposite direction. The undervoltage inhibit is checked by reducing the voltage and confirming that the element is blocked below its inhibit level. Timing is measured by stepping the frequency from normal to a value beyond the setting and recording the time to output, and each stage is timed. Where a rate-of-change element is present, the test set ramps frequency at a chosen slope. Outputs are monitored for correct contact operation and the correct target, and any wiring to load shedding or generator trip circuits is checked in a controlled way. Settings are restored afterward.

What is recorded

The record covers the relay identification and settings of each stage, the voltage applied during the test, the pickup and dropout frequencies found, the operating times, the behavior of the undervoltage inhibit and any rate-of-change element, the insulation reading, the test source and its accuracy, and the as-found and as-left condition.

How results are judged

Pickup and dropout are compared with the configured setting and the manufacturer's stated accuracy, with each stage judged on its own. Operating times are compared with the configured delay plus the relay's fixed processing time. The inhibit should block at the intended voltage level and release when voltage returns. Results across stages are checked for consistency, and any change from earlier tests of the same relay is investigated, even if it remains within tolerance, because drift can indicate a failing reference or input. The insulation reading is read against the standard's table for the circuit voltage. The frequency function is among the protective relay tests described in ANSI/NETA ATS-2025 for acceptance and ANSI/NETA MTS-2023 for maintenance. For the values and criteria, refer to your licensed copies of both and to the utility or interconnection requirements that fix the settings.