IEC 60255-181 requires the manufacturer to declare two ranges (the effective range, where accuracy is met, and the operating range, where the function acts), the accuracy of the frequency or df/dt threshold, and the accuracy of the operate delay. Without those three data and their validity range, a catalog figure cannot be compared.
Why a single number is not enough
A frequency relay advertises, for example, an «accuracy of ±10 mHz». That statement means nothing unless you know at what voltage, between which frequencies, for which settings and with what delay. IEC 60255-181 solves this with one rule: the manufacturer declares ranges, accuracy and conditions in a common format, so the data of two devices can be compared.
Clause 5.1 clarifies the scope: since the standard sets minimum requirements, it only specifies the performance needed to meet them and defines how the manufacturer must document it.
Effective range and operating range
The standard defines two ranges for each input quantity:
- Effective range (clause 3.15): the part of the operating range in which the accuracy requirements are met.
- Operating range (clause 3.14): the range in which the relay can perform its intended function according to the specified requirements.
Time accuracy is guaranteed within the effective range. The function must operate over the whole operating range (note to clauses 4.4.1.2 to 4.4.1.4).
Clause 5.2 requires both ranges to be declared, for voltage and for system frequency. For the 81R the rate-of-change range is declared as well. Tables 2 and 3 of the standard show the format with example values, which depend on each device’s technology:
| Quantity | Effective range (standard example) | Operating range (standard example) |
|---|---|---|
| Voltage | 5% to 150% of rated | 2% to 200% of rated |
| Frequency | 0.95 to 1.05 of rated (47.5 to 52.5 Hz at 50 Hz) | 0.9 to 1.1 of rated (45 to 55 Hz at 50 Hz) |
| ROCOF (81R only) | −5 to +5 Hz/s | −10 to +10 Hz/s |
These are format examples, not required limits. Other details of clause 5.2:
- The voltage range can be expressed as a percentage of rated voltage or in volts at the voltage transformer (VT) secondary.
- If the 81R algorithm does not depend on the frequency value, the manufacturer writes «not applicable» on that row and declares its behavior outside the range.
- If the device uses currents as input, it also declares its current ranges.
- If the voltage inputs have a flux limit (V/Hz), the manufacturer declares the maximum voltage versus time curve they withstand.
Accuracy of the characteristic quantity
Clause 5.3 asks for the accuracy to be declared together with the setting range where it applies, and the behavior of the function over the whole operating range.
| Function | How it is expressed | Standard example |
|---|---|---|
| 81U and 81O | Absolute frequency value; a value relative to the setting (Gs) may be added and the larger of the two is taken | ±0.01% or ±10 mHz, whichever is greater |
| 81R | Value relative to the setting, absolute df/dt value, or both; it may vary across the setting range | ±5% or ±15 mHz/s, whichever is greater |
The verification method (clause 6.2.1) in short: a continuous signal is applied to the relay input. Its frequency (or its slope, for the 81R) is brought toward the setting in small steps until the start signal is activated. The measured value must fall inside the band formed by the setting plus and minus the declared accuracy. The band is the acceptance limit.
Three rules of that method are worth knowing:
- The start must be clean. An output contact may bounce for up to 4 ms. On a solid-state output or a GOOSE message no oscillation is allowed.
- The test is repeated several times per setting point and is done at rated voltage and also at the minimum voltage and maximum of the effective range.
- It is repeated for every rated frequency the device supports, 50 Hz and 60 Hz.
Setting points cover from the minimum to the maximum of the range, with emphasis on values close to rated frequency. The standard does not ask the user to repeat this in the field (clause 6.1).
Operate delay accuracy
The operate delay is the time between the start signal and the operate signal (clause 5.6). Its maximum error is declared in one of two ways:
- as a percentage of the setting plus a fixed error, taking the larger (standard example: ±5% or ±20 ms);
- as a fixed error (example: ±20 ms).
It is declared with the delay range where it applies. The standard also requires the manufacturer to express delay accuracy in the same way as the start time. The reason is that the total time from the event to operation adds both errors (clause 6.4.3).
How to read an accuracy figure
- For the 81U and 81O the standard always asks for an absolute value in hertz or millihertz. The value relative to the setting is an optional complement.
- For the 81R accuracy can be a percentage of the setting, an absolute df/dt value or both, and it can change across the setting range. Ask for which part of the range the figure holds.
- The effective range tells you at which voltages and frequencies the accuracy holds. Compare it with the real conditions of your bus, for example a frequency that departs far from rated in an electrical island.
At Tiza Group we verify that the start and delay of your frequency functions stay within what is declared, using secondary injection in Colombia, or elsewhere on request. If you also need your relay settings reviewed, see our remote relay configuration, available worldwide. To choose the test equipment, see the secondary injection test set.
Related guides
- IEC 60255-181: what it requires of frequency protection
- Operating and reset characteristic of the 81
- Start and disengaging time of the 81 protection
- Hysteresis and reset ratio of the 81 protection
- Functional tests of frequency protection
References
- IEC 60255-181:2019, clauses 3.14, 3.15, 4.4.1, 5.1 to 5.3, 5.6, 6.1, 6.2.1 and 6.4.3, Tables 2, 3, 5, 6 and 8.
- IEC 60255-1, common requirements for measuring relays and protection equipment.
Frequently asked questions
What is the difference between effective range and operating range?
In the effective range the function meets the declared accuracy. The operating range is wider: the function must keep operating, but without an accuracy guarantee. This is how IEC 60255-181 defines them in clauses 3.14 and 3.15.
How is the accuracy of a frequency protection expressed?
In hertz or millihertz (mHz) for 81U and 81O, optionally with a value relative to the setting, taking the larger of the two. For the 81R it is expressed as a percentage of the setting, in Hz/s, or both. The standard defines it in clause 5.3.
What accuracy does the standard require from a frequency relay?
No minimum figure. IEC 60255-181 does not set an accuracy value: it requires the manufacturer to declare it, over which setting range it holds, and to verify it with the method of clause 6. The numbers it shows are format examples.