Frequency protection 81U/81O: how it works

What protection 81 (frequency) is

System frequency reflects the balance between the power generated and the power consumed. If a large generator is lost, load exceeds generation, the machines slow down and frequency drops. If a large block of load is lost, or an island is left with excess generation, frequency rises. Nominal frequency is 60 Hz or 50 Hz depending on the system.

Function 81 measures the frequency of the voltage and issues a command when it leaves a band for a set time:

• 81U (under): underfrequency.
• 81O (over): overfrequency.

It is named this way:

• ANSI/IEEE C37.2: 81, frequency relay.
• IEC 61850: PTUF for underfrequency and PTOF for overfrequency.
• IEC 60617: represented as f< and f>.

The variant that measures the rate of frequency change, 81R or ROCOF, has its own article.

Where it is applied

• Underfrequency load shedding in distribution substations and industry. It is the most widespread application.
• Generators and turbines: steam and gas turbine blades have operating limits away from nominal frequency, with allowable accumulated times.
• Solar and wind plants: interconnection requirements set frequency bands within which the plant must stay connected and outside which it must disconnect.
• Industry with its own generation: separation from the public grid and control of the internal island.
• System separation schemes in transmission.

How it works

The relay calculates frequency from zero crossings or from the rotation of the voltage phasor, usually on a phase-to-phase or positive-sequence voltage so that a single-phase fault has less effect on the measurement. Each stage has:

• a frequency threshold, below (81U) or above (81O) nominal;
• an operating time, so that it does not act on transients;
• an undervoltage block: if the voltage is too low, the frequency measurement is not reliable and the stage is blocked.

Numerical relays offer several independent stages. IEC 60255-181 defines the functional requirements of frequency protection and how its accuracy and times are declared.

Load shedding

When frequency drops, the load shedding scheme disconnects load in steps: first a block of circuits with the highest threshold, then another with a lower threshold, and so on. Each step reduces the deficit until generation can hold the frequency. The order in which circuits are disconnected is chosen to protect critical loads, such as hospitals and essential services.

This scheme is known as UFLS (underfrequency load shedding). The load percentages, thresholds and times of each step are defined by the system operator with frequency studies and assigned to the utilities. Some schemes add steps supervised by the rate of change of frequency to react earlier in severe events.

Generators

On machines, 81U protects the turbine and the auxiliary systems; 81O protects against overspeed after a load rejection, as backup to the speed governor. IEEE C37.106 describes how to coordinate these stages with system load shedding: the generator must not disconnect before load shedding has had a chance to recover the frequency.

Setting criteria

• The system operator’s load shedding program: thresholds, times and load percentage per step are not chosen at the substation; they are taken from the official assignment.
• Turbine and generator manufacturer limits, coordinated according to IEEE C37.106.
• Interconnection requirements of the utility for generation and solar plants.
• Undervoltage block, set to avoid operations with de-energized buses and during faults.
• Operating time, long enough to filter transients without delaying recovery.
• VT ratio and measuring point.

Coordination between load shedding steps, generators and sensitive equipment is resolved in the protection coordination study and in the stability studies.

How each manufacturer implements it

• Siemens SIPROTEC groups frequency protection in stages with threshold, time and minimum voltage settings, and some models offer dedicated load shedding functions. It is configured in DIGSI.
• SEL offers several frequency elements, each settable above or below nominal, with voltage supervision and free use in its logic equations. It is set with AcSELerator QuickSet.
• Relion (Hitachi Energy 670/650; ABB 615/620) separates underfrequency, overfrequency and rate of change into independent functions, named after the IEC 61850 logical nodes, configured in PCM600.
• GE Vernova (Multilin) has underfrequency and overfrequency elements and, in some devices, load shedding and restoration schemes. It is configured in EnerVista.
• Schneider MiCOM / Easergy offers frequency stages and load shedding logic in feeder and generator relays, configured in MiCOM S1 or Easergy Studio.

What changes is the number of stages, the voltage used for the measurement, the calculation window and whether there is an automatic load restoration function once frequency recovers.

Why it trips or blocks

• Loss of supply to a bus with motors: the residual voltage generated by the motors decays in frequency and 81U reads it as underfrequency. It is corrected with an undervoltage block and suitable times.
• VT fuse failure or very low voltage during a fault: the measurement becomes erratic.
• Setting different from the assigned one from the system operator, or a wrong settings group.
• Generator stages faster than load shedding, which make the event worse instead of helping.
• Island with excess generation, which makes the plants’ 81O operate.
• Real trip from a system event: if it coincided with the loss of a large generator, the relay did what it should.

The first step is to compare the frequency recorded in the oscillography with that of other points in the system. If only the relay saw the drop, the problem is local: voltage, VT or motors.

How it is tested

With the Omicron CMC 356, voltage is injected and the frequency is varied in ramps or steps. The test verifies:

• the pickup of each stage;
• the operating time, measured from the moment frequency crosses the threshold;
• the undervoltage block;
• the reset;
• the outputs to each load shedding breaker.

The test is part of secondary injection. For load shedding it is also worth checking that each step trips its assigned circuits and no others.

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

References

• IEEE C37.2, device function numbers.
• IEEE C37.106-2022, Guide for Abnormal Frequency Protection for Power Generating Plants.
• IEC 60255-181, functional requirements for frequency protection.
• IEC 61850-7-4, logical node models.
• P. Kundur, Power System Stability and Control.
• J. L. Blackburn and T. J. Domin, Protective Relaying: Principles and Applications.

Frequently asked questions