ROCOF - Device 7F (... not 81R)

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The IEEE / ANSI Device Numbers described in IEEE C37.2 (latest version 2022) have been a fundamental basis of describing functions in the electricity industry.

Of course the generic purpose of such "nomenclature/semantic" standards is to eliminate ambiguity and arbitrary definitions that are not necessarily understood in the same way by others.

However there are many in the industry who have not delved into a full understanding of the entire C37.2 Standard and so can fall into "common errors" simply because they have seen someone-else's proliferation of incorrect terms.
One such misunderstanding is about Restricted Earth Fault Device 64 as I have written about here: Restricted Earth Fault: 64, 64R, 87N ?

Sadly over the last 6-9 months I have seen an apparently growing trend in several references to this wrong use of a particular ANSI/IEEE C37.2 Device Number “81R” … probably increasing due to the stronger focus on renewables and Rate‑Of‑Change‑Of‑Frequency (ROCOF) issues.

If we are to operate as a "professional industry" with respect for our Standards and clarity of meaning in our terminology, I would call on you all to stop using device “81R”.

“81R” DOES NOT EXIST in any meaningful way!

The correct, dare I say “professional”, device number would be “7F”.

For those in the electrical power industry, you will probably have realised that Rate‑Of‑Change‑Of‑Frequency (ROCOF) has become an increasing issue.

In the “good old days” of large synchronous machines it was a somewhat lesser importance (although by no means to be ignored) as function simply because the inertia in the system of the large synchronous machines tended to limit ROCOF .. it was simply a "slower issue".

However, with the HUGE increase in renewables and inverter based generation, frequency is less “stable” and can change VERY rapidly …refer South Australia Blackout 2016 caused by seven tornadoes taking out 27 towers on three critical power lines! I hasten to add ROCOF was in place but far too slow to operate!


So-called “ANSI Device Numbers” are published as IEEE C37.2 (2008) “IEEE Standard for Electrical Power System Device Function Numbers, Acronyms, and Contact Designations”
(or the public listing ... interestingly with references indicating these Device Numbers are amongst the oldest Standards used in our industry .. so you'd think we'd all know how to use them correctly by now)

Device 81 describes Under Frequency, Over Frequency as well as Rate of Change of Frequency applications, ... so I can see the potential basis of confusion.

 “81R” has appeared in several specifications and even vendor literature ... presumably referring to ROCOF.
Even the Standard IEC 60255-181 (2019) "Measuring relays and protection equipment - Part 181: Functional requirements for frequency protection", Table 1 also refers to device 81R
It has also been referenced in the June 2022 issue of PAW World where the opening paragraph of the article about ROCOF refers to “ANSI 81R” relays. (Just USD 25/year subscription - I am an Honorary Advisory Board Member)

However C37.2 Section 3.5 specifically describes the various suffixes in different “use-cases”.
Section 3.5.5 does effectively identify “81U” and “81O” would logically apply for Under and Over Frequency respectively.

However "81R" has no context under the defines suffixes.

  • 3.5.1 Auxiliary Devices: Raising relay
  • 3.5.2 Actuating quantities:  no reference for R
    this is where it should logically be found if it meant Rate of Change
    Note suffix F is used for Frequency, but clearly 81F is basically a tautology and makes no sense, and is certainly of no help between Under/Over/ROC
  • 3.5.3 Main Device: Reactor / rectifier / room / rotor
  • 3.5.4 Main device parts: no reference for R
  • 3.5.5 Other suffix letters: Right / raise / reclosing / receiving / remote / reverse

However 3.1.7 "Device number 7—rate-of-change relay" is CLEARLY specifically a device operating on Rate‑Of‑Change.

In the IEC 61850 world we can see corrections have already been incorporated:

  • IEC 61850-5:
    • 2003 Section 11.1.1 only identifies 81
    • 2013 Section 8.5.3 identifies 81 and 7 
  • IEC 61850-7-4:
    • 2010 Table 7: Frequency : IEEE C37.2 code 81 – Nodes PTOF, PTUF & PFRC
    • AMD1:2020 Table 4: Frequency Prot : IEEE C37.2 code 81 – LN PTOF, PTUF, IEEE C37.2 code 7 – LN PFRC

Conclusion: “7F” is the correct ROCOF Device Number.

Watch this space ...

I have been unofficially informed that C37.2 is under revision and that the new edition will identify ROCOF as code 7, and not 81.

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