Frequency protection (ANSI 81) and ROCOF — how a relay recognises loss of grid by the speed of frequency change

The guide on anti-islanding protection for PV inverters briefly mentions the rate of change of frequency (ROCOF) as one of the detection methods a PV inverter uses to recognise loss of the grid. This article goes into the underlying protection function itself in more depth: frequency protection, denoted by the ANSI codes 81O (overfrequency), 81U (underfrequency), and 81R (ROCOF, also called df/dt) — a function applied much more broadly than just for PV inverters, including for generators and for automatic load shedding.

Why frequency monitors something different from voltage or current

Unlike voltage at a specific point, grid frequency is (approximately) the same across the entire synchronously coupled grid: it reflects the balance between total generated power and total consumed power. If demand increases without generation growing along, frequency drops; if generation increases without demand growing along (or a large part of the load is lost), frequency rises. Frequency protection therefore indirectly monitors the balance of the entire grid, not just the local condition at one measuring point.

Over- and underfrequency protection (81O/81U)

Underfrequency protection (81U) is most commonly used for automatic load shedding: once the frequency drops below a set threshold — a sign the grid has a power deficit — the protection trips portions of the load in stages, to prevent the frequency from sagging further to a point where generators themselves start to trip (a cascade that can lead to a full blackout).
Overfrequency protection (81O) protects against a grid with too large a power surplus (for example after a large load is lost while generation doesn't ramp back fast enough), and is also used to disconnect generators or distributed generation when frequency exceeds a safe upper limit.

Both functions generally use several independently settable threshold-time pairs, so that a mild frequency deviation is handled with a longer time delay than an extreme deviation, which must be cleared faster.

ROCOF (81R, df/dt): reacting to the speed of change, not just the level

An ordinary over- or underfrequency protection only responds once frequency crosses an absolute threshold — depending on the threshold, that can take relatively long. ROCOF protection instead measures how fast the frequency changes (in Hz per second) and responds once that rate of change exceeds a set value, regardless of whether the absolute frequency has already crossed a normal 81O/81U threshold at that moment. This makes ROCOF suitable for two different applications:

Accelerated load shedding: a grid that suddenly loses a large part of its generation shows a rapid frequency drop well before the absolute 81U threshold is reached; ROCOF can respond to this earlier than ordinary underfrequency protection.
Islanding detection for distributed generation: when part of the grid with local generation is disconnected from the main grid, the remaining, much smaller "island" load is generally no longer exactly balanced with the local generation, causing the frequency in that island to start changing rapidly — a df/dt well above the normal, slow behaviour of a large, synchronous grid.

Why voltage supervision is needed to prevent false tripping

A brief harmonic distortion or measurement noise during a switching operation can produce an apparent, very fast frequency change without there actually being loss of grid. To prevent this, a ROCOF function is generally voltage-supervised: as long as the measured voltage is below a set lower limit (for example around 0.7 to 0.8 times nominal voltage), the ROCOF measurement is considered unreliable and the protection remains blocked, even if the calculated df/dt happens to show a high value at that moment.

Note: the exact 81O/81U thresholds, the ROCOF sensitivity (generally in the order of 0.1 to a few Hz/s, with a short set time delay), and the voltage-blocking limit follow from the grid operator's grid code and the system study of the specific installation; this article covers the principle, not a ready-made setting table for every grid.

Practical relevance

When connecting distributed generation (PV, CHP, battery storage) to an existing grid, it is important to verify that the frequency protection — including any ROCOF function — is aligned with the grid operator's requirements, rather than simply a generic factory default: an overly sensitive ROCOF setting can lead to nuisance tripping during normal, small frequency fluctuations of a large grid, while an overly insensitive setting misses the intended islanding-detection purpose.

Common mistakes

Applying ROCOF protection without voltage supervision — this makes the function susceptible to apparent frequency jumps caused by noise or harmonic distortion.
Applying the same 81O/81U/ROCOF settings to a small, isolated grid as to a large, strongly coupled grid — normal frequency dynamics differ significantly between the two situations.
Trying to combine load shedding and islanding detection with the same, undifferentiated setting — both applications generally require a different sensitivity and time delay.
Not reviewing frequency protection after a significant change in the generation mix or the grid load — the normal frequency dynamics of a grid change along with the ratio between slowly and quickly regulating generation.