Underfrequency load shedding (UFLS): how the scheme works

What underfrequency load shedding is

When a large generating unit trips, or an interconnection that was importing power opens, load exceeds generation. The kinetic energy of the machines covers the deficit for a few moments and frequency falls. Primary frequency response from the generators reacts, but if the deficit is large it is not enough. If frequency keeps falling, the frequency protection 81 of the units operates, more generation trips and the system collapses.

Underfrequency load shedding (UFLS) sheds blocks of load in stages to restore the balance before that point. It is a system scheme: it does not protect one piece of equipment, it protects the operation of the grid. See the map of protection schemes.

Which protections make it up and how they are coordinated

• Frequency relays 81 in distribution substations, fed from the bus VT, with several threshold and delay stages.
• Rate of change of frequency (df/dt, 81R) in some stages.
• Tripping of the feeder breakers assigned to each stage.
• Supervision by undervoltage or by measurement quality, so the scheme does not act with the VT disconnected.
• Generator frequency protections, which must be set below the load shedding and coordinated with it. IEEE C37.106 covers frequency protection of generating plants.

Who defines the program

The scheme belongs to the power system, not to one substation. The system operator, or the planning or reliability coordinator, studies it and sets the design criteria. Typical criteria:

• The trip of the largest unit in the system must not activate the first stage.
• Frequency must never fall below a minimum value set for the system.
• Some seconds after the event, frequency must be back above the first-stage threshold.

The program fixes the share of demand to be shed, the number of stages, the load percentage per stage, the thresholds and delays, and whether some stages use df/dt. The utilities that own the feeders implement it, test it and report it. In North America, NERC PRC-006 sets the requirements for UFLS programs. Programs are reviewed periodically, so thresholds and delays can change from one review to the next: always take them from the program in force for your system, not from this guide. Our team adapts the settings to the local code and to the requirements of your utility or system operator.

Where the system-wide scheme is not enough, for example in radial areas, supplementary schemes are studied between the system operator and the utilities. See area separation and islanding and remedial action schemes, which act on the cause of the deficit and not on frequency.

Logic and sequence

1. A large unit trips. Frequency falls at a rate that depends on the deficit and on the system inertia.
2. If it crosses the first-stage threshold and stays there for the delay, the relays of that stage trip their feeders.
3. If the deficit continues, frequency reaches the next stage and another block is tripped.
4. When the shed load equals the deficit, the decline stops and frequency response recovers it.
5. Load is restored in a controlled way, by operator instruction or with automatic restoration.

Variants

• Interconnected system: stages by frequency threshold, sometimes with df/dt, distributed among the utilities.
• Radial or weak areas: supplementary schemes with their own stages.
• Industry with on-site generation: when the plant islands, internal shedding by frequency or by power balance protects the plant. Power-balance schemes, which measure the lost power and shed an equivalent load, act faster than frequency-based ones.
• Undervoltage (UVLS): a similar scheme on low voltage, for voltage stability problems.

Design criteria

• Load percentage, number of stages, thresholds and delays according to the system operator’s studies.
• Feeder selection: leave out essential loads, consider the connected distributed generation and spread the stages geographically.
• Frequency measurement that is robust against transients and voltage sags.
• Coordination with the generator protections and with equipment limits.

How each manufacturer implements it

Almost all feeder and bus relays from Siemens SIPROTEC (DIGSI), SEL (AcSELerator QuickSet), Hitachi Energy and ABB Relion (PCM600), GE Vernova Multilin (EnerVista) and Schneider Easergy / MiCOM (Easergy Studio) have several frequency and df/dt stages. What changes is how frequency is measured, the number of stages, the df/dt measurement window and its voltage supervision. Some utilities use dedicated load shedding relays that distribute the stages among many feeders, by hardwiring or by GOOSE.

Why it fails or misoperates

• Trip on a voltage sag without undervoltage supervision.
• df/dt tripping on oscillations or switching, because of a short measurement window.
• Stage assigned to a feeder that no longer carries the expected load or that has generation.
• Settings from an earlier program that were not updated after the last review.
• Generators tripping before the load shedding because of poorly coordinated frequency protections.

How it is tested

Each relay is tested by secondary injection with frequency ramps: thresholds, delays, df/dt and voltage supervision, up to the trip of the assigned breaker. Testing and reporting follow the procedure set by the system operator or the applicable reliability standard.

Need to update the UFLS settings in your relays or review an operation? Our team configures the frequency stages remotely. See remote relay configuration or contact us.

References

• NERC PRC-006, Automatic Underfrequency Load Shedding.
• IEEE C37.117-2007, Guide for the Application of Protective Relays Used for Abnormal Frequency Load Shedding and Restoration (inactive-reserved since 2019; cited as a technical reference).
• IEEE C37.106-2022, Guide for Abnormal Frequency Protection for Power Generating Plants.
• P. Kundur, Power System Stability and Control.

Frequently asked questions