Frequency protection needs a minimum voltage to measure well. That is why it is usually combined with elements that block or restrain it: undervoltage, phase unbalance, current, power or df/dt supervision. IEC 60255-181 does not define those elements, but it obliges the manufacturer to declare which ones it uses and with what accuracy.
Why the 81 needs blocking
Frequency is calculated from a voltage. If that voltage is very low, distorted or gone, the value the relay calculates no longer represents the system. IEC 60255-181 acknowledges this in two ways. In the definition of operating range it states that a minimum voltage level is required to calculate frequency correctly (clause 3.14). In clause 4.5 it gathers the additional influencing functions or conditions, which can change the behavior of the function.
The standard describes them as separate elements that act on the frequency protection through binary inputs in a predefined way. Its main example is blocking the protection when the voltage is below prescribed limits.
Two typical situations illustrate the need:
- a nearby fault that pulls down the bus voltage;
- a bus with motors that lost its supply: the residual voltage of the motors, which decays while they slow down, drops in frequency and a relay without blocking reads it as real underfrequency.
The second case is described in the guide on frequency protection 81.
Which elements the standard cites
| Function | Influencing elements cited in IEC 60255-181 |
|---|---|
| Under and overfrequency (81U and 81O), clause 4.5.2 | Undervoltage block; three-phase unbalance supervision; df/dt supervision; current supervision; power supervision (f/P function) |
| ROCOF (81R), clause 4.5.3 | Undervoltage block; three-phase unbalance supervision; under and overfrequency supervision; current supervision |
These are “examples” in the standard, not a closed list. The standard names them but does not describe their logic:
- Undervoltage: prevents the function from operating if the measured voltage falls below a threshold.
- Three-phase unbalance: detects that the three voltages are no longer symmetrical, which can falsify a measurement made with a single phase or a combination of phases.
- df/dt, frequency, current and power (f/P) supervision: additional conditions combined with the measured frequency. Their logic and settings are defined by each manufacturer.
These associated functions are outside the scope of the standard (clause 1): “only their influence on the frequency protection is covered.” That is, IEC 60255-181 does not explain how each one is built.
What the standard requires of the manufacturer
The standard imposes three obligations.
- Describe their behavior. The performance of these features must be described in the documentation (4.5.1). The binary inputs that connect them to the 81 must appear in the logic diagram (4.3).
- Declare which ones it uses. The manufacturer must state which restraint or blocking elements it employs (5.9).
- Declare the accuracy of those elements when they are threshold-based, for example the accuracy with which undervoltage is detected (5.9).
The functional tests of these elements are not described in the standard. Instead, the methods with which the manufacturer checked them must be described in the type test report (4.5.1).
How it relates to the voltage range
The manufacturer declares two voltage ranges (clause 5.2), as a percentage of rated voltage or in volts at the secondary of the voltage transformer:
- the effective range, where the declared accuracy is met;
- the operating range, where the function must act according to the requirements.
The undervoltage block must sit below the zone where the measurement is reliable. If the voltage falls below the blocking threshold, some devices switch to measuring frequency with current. The standard (4.2) reminds us that this serves for frequency tracking, but is not recommended for protecting.
An example of ranges in the standard’s format
As an example of format, Table 2 of the standard presents an effective voltage range of 5 % to 150 % of rated. The operating range goes from 2 % to 200 %. The first figure indicates from which voltage the measurement is expected to meet its accuracy. An undervoltage blocking threshold must be chosen with that data in view: if the block acts above the lower limit of the effective range, the function is out of service where it would still measure well; if it acts far below, it can allow measurements outside the declared accuracy.
Those values are only illustrative. Each manufacturer declares its own, and the study of the installation decides the blocking threshold.
What you should check
- Which blocks are active and whether the 81U, 81O and 81R settings use them equally.
- What happens to the protection during the block. A long block in an island or with on-site generation can leave the plant without frequency protection exactly when it is needed most.
- What signal the block produces: an alarm, a command to another relay or just the loss of the function.
- What the protection does when voltage returns, with the block released and the frequency still out of band.
- The release time of the block relative to the delay set in the protection.
A badly coordinated block produces the opposite of the intended effect: trips with a healthy system or no trip on a real event. For on-site generation, see frequency trip 81 in plants with on-site generation. For the voltage signal that feeds the measurement, see loss-of-potential alarm from VT fuse failure.
At Tiza Group we review blocking schemes using the relay records and settings, remotely and worldwide. If a frequency protection tripped or failed to trip when it should have, see the relay trip analysis service and we will quote the analysis. Blocks are validated with secondary injection, varying voltage and frequency at the same time, in Colombia or elsewhere on request.
Related guides
- IEC 60255-181: frequency protection requirements
- Inputs and characteristic quantity of the 81 protection
- Operating and reset characteristic of the 81
- Phase jump and sudden voltage changes in the 81
- Undervoltage protection 27 (PTUV): what it is and how it works
References
- IEC 60255-181:2019, clauses 1, 3.14, 4.2, 4.3, 4.5.1 to 4.5.3, 5.2 and 5.9.
- IEC 60255-1, general requirements for measuring relays and protection equipment.
Frequently asked questions
Why does an 81U trip when the system frequency is normal?
Often it is a false measurement caused by very low or distorted voltage, for example during a nearby fault or with the decaying residual voltage of motors on a bus that lost its supply. An undervoltage block avoids a good share of those cases.
Does IEC 60255-181 say what value the undervoltage block is set to?
No. The standard gives no setting values. It requires the manufacturer to declare which blocking elements it uses and, when they are threshold-based, the accuracy of those thresholds (clause 5.9). The value is defined by the study of the installation.
Are the blocking elements tested with the standard's methods?
No. Clause 4.5.1 states that the tests of those elements are not described in the standard, but the manufacturer must describe in the type test report the methods it used to check them.