IEC 60255-181 defines the functional type tests of a frequency protection: start value accuracy, hysteresis, start and disengaging times, delay accuracy, effect of harmonics and stability against voltage changes. They are manufacturer tests with a continuous signal; in the field the device instructions are followed.
What the standard’s functional tests are
Clause 6 of IEC 60255-181 describes how to check that a frequency protection meets what the manufacturer declares in clause 5. They are a mandatory part of the type tests: the manufacturer runs them once per design and publishes the results in a common format. This lets the buyer compare the manufacturer’s documentation with what is needed.
Clause 6.1 clarifies a point that is often misunderstood: these tests are not intended for commissioning or for routine tests of an installed relay. In the field the manufacturer’s instructions are the reference, although several methods and requirements of the standard can serve in commissioning and maintenance.
Common conditions
Before each test, the standard sets setup rules:
- The signal is injected at the device interface. That is, at the voltage terminals or as an equivalent digital signal with the right protocol. This exercises the complete hardware and firmware.
- Operation is taken from the outputs (contacts) whenever possible. The manufacturer must declare which output the times were measured with: trip contact, solid-state output or GOOSE message. If the device has several, it declares how the times change.
- The signal is a rated-voltage sine wave whose frequency is varied according to the test. In transitions it is continuous, with no angle or magnitude jump other than in frequency. The voltage may be adjusted to respect the flux limit (V/Hz) of the inputs.
- If some settings are in primary values, a transformation ratio is chosen for the tests.
- Tests are expressed as a percentage of the setting range and repeated for every rated frequency the device supports.
Constant-slope frequency ramps are generated with the equation of Annex A, and abrupt changes without discontinuity with the procedure of Annex E.
What each test checks
| Test (clause) | What it verifies | What is reported |
|---|---|---|
| Start value accuracy (6.2.1) | That the function starts within the declared error band around the threshold | Measurement error per setting point and voltage (Tables 5, 6 and 8) |
| Hysteresis or reset ratio (6.2.2) | That the reset value is within the declared one and bounce-free | Hysteresis in mHz or reset ratio (Tables 10 and 12) |
| Start time (6.3) | How fast it starts on a frequency step and on ramps | Minimum, mean and maximum, with chart or histogram (Tables 15 and 17) |
| Operate delay accuracy (6.4) | That the time between start and operate matches the setting | Absolute and relative error (Table 20) |
| Disengaging time (6.5) | How fast the signal resets when frequency normalizes | Minimum, mean and maximum (Tables 23 and 25) |
| Performance with harmonics (6.6) | How much accuracy changes with a distorted signal | Error with and without harmonics (Tables 27 and 28) |
| Stability against voltage changes (6.7) | That a phase jump, magnitude change or voltage dip does not cause a false start | Stable, or the minimum delay needed |
Each one has its own guide in this series, and all follow the same pattern: a defined injection method, a fixed number of repetitions, an acceptance criterion and a report format.
Start accuracy. A frequency ramp in small steps is applied until the start signal activates. The start must be clean, with no bounce beyond the normal time of a contact (clause 6.2.1).
Hysteresis. Two ramps in opposite directions identify the start value and the reset value. The difference is the hysteresis.
Times. Start time is measured with a frequency step and with ramps of different slopes. The operate delay is measured between the start signal and the operate signal, at several settings across the timer range (clause 6.4.2). If the start and operate signals cannot be measured at the same time, the manufacturer must indicate another method.
Harmonics and stability. Tests are repeated with a distorted signal or with phase and magnitude jumps.
What changes in the field
Commissioning a frequency protection does not repeat the type test. With the device and the project setting, you verify that:
- the relay measures the correct frequency with the actual voltage transformer (VT) voltage;
- each stage starts and operates at the set value and time, within the tolerance declared by the manufacturer;
- reset does not cause oscillation;
- blockings, for example by minimum voltage, work;
- each output reaches the circuit breaker or the intended logic, including load shedding.
This is done with a secondary injection test set that generates frequency ramps and steps with phase continuity. Results must go into a report that allows each test to be audited.
Common mistakes when testing frequency
- Injecting a frequency step without phase continuity, so the relay also receives an angle jump.
- Measuring with a window different from the one declared by the manufacturer.
- Measuring time on a different output than the one the manufacturer used.
- Testing only the threshold and not the reset, the blocking or the output to the circuit breaker.
- Accepting a time as good without knowing whether it includes the set delay.
When to call us
At Tiza Group we perform secondary injection on frequency and ROCOF protections, with frequency ramps and steps, in Colombia, or elsewhere on request, and deliver the results report. To choose the equipment, see the secondary injection test set. If you also need your frequency stages adjusted, see our remote relay configuration, available worldwide, or contact us.
Related guides
- IEC 60255-181: what it requires of frequency protection
- Frequency protection ranges and accuracy
- Hysteresis and reset ratio of the 81 protection
- Start and disengaging time of the 81 protection
- Type test report of a frequency protection
References
- IEC 60255-181:2019, clauses 6.1 to 6.7, Annexes A, B and E.
- IEC 61869-9, instrument transformers with digital interface (sampling rate).
- IEC 60255-1, common requirements for measuring relays and protection equipment.
Frequently asked questions
Can I use the IEC 60255-181 methods for commissioning?
The standard clarifies that its tests are not intended for commissioning or routine testing. In the field the manufacturer's instructions are the reference, although several methods and requirements can be applied in commissioning or maintenance.
How should the test signal be injected?
Directly at the device interface: at the voltage terminals, or as an equivalent digital signal over the corresponding protocol. Operation is taken from the output contacts whenever possible. During transitions the signal must be continuous, with no angle or magnitude jumps other than in frequency.
Can I test with a COMTRADE file?
Yes. When the test signal is played back from a COMTRADE file (IEEE C37.111) or another binary format, the sampling rate of the signal must be at least 4,800 Hz. The standard allows 4,000 Hz for older devices.