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Frequency df/dt ramp generator for 81R tests

Set the initial frequency and the df/dt rate, and the duration or the final frequency: the tool draws the ramp f(t), checks the conditions of the standard and gives you the COMTRADE file to inject into the relay. It is a test signal: it does not calculate the setting or decide whether the relay passes.

IEC 60255-181§ Aequation of the signal with linear frequency variationIEEE C37.111 / IEC 60255-24COMTRADE file

Example cases
Frequency df/dt ramp generator for 81R tests

Only the basic data; the rest is taken with typical values.

E1Ramp
Hz

The standard takes it equal to the nominal.

Hz/s

Positive rises, negative falls.

s
Hz
E2Standard test
Hz/s

The df/dt you set in the relay; it is used to build the ramps from 1.2 to 10 times Gs.

E3COMTRADE file

Download the record

Not checked.

E4Signal
V
°
Hz
s
s
E5Ranges and times of the standard
± %
± %
± Hz/s
± Hz/s
ms
ms

Use a decimal point. The calculation runs in your browser and updates as you type.

Final frequency

— Hz

df/dt rate

— Hz/s

Ramp duration

— s

File

—

Injected frequency against time
  • Injected frequency
  • Rated
  • Effective range
  • Operating range
The frequency is at its initial value, changes with constant slope df/dt and stays at the final value. The lines show the nominal frequency and the limits of the ranges you set.
Voltage at the start of the ramp
  • VA
  • VB
  • VC
  • Constant initial frequency
Five cycles centred on the start of the ramp. The waveform is continuous: there is no step in phase or amplitude, only the frequency changes (6.1).
Voltage at the end of the ramp
  • VA
  • VB
  • VC
  • Constant initial frequency
Five cycles centred on the end of the ramp. The dashed line is the same signal without the ramp: the accumulated phase difference is the integral of the frequency.

Step-by-step calculation

Standard ramps for your setting Gs

What it means for your test

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    What it is for

    The rate-of-change-of-frequency function (81R, ROCOF) starts when the frequency changes faster than a value set in hertz per second. A frequency step is not enough to test it: you need a signal whose frequency changes linearly, at a known slope. This tool builds that signal, samples it and saves it as a COMTRADE record for your injection test set to play.

    The product standard tests the function with a sequence of ramps: it starts at the nominal frequency, raises the slope until the relay starts and compares with the setting, and repeats with slopes from 1.2 to 10 times the setting to measure the start time. The table «Standard ramps» builds that sequence for your Gs, with the duration and the ranges you set.

    How it is calculated

    IEC 60255-181§ Atest signal with constant frequency variation

    Frequency and phase (Annex A)

    f(t) = f0 + (df/dt)·(t − t0) · φ(t) = 2π·[ f0·t + ½·(df/dt)·(t − t0)² ] + φ0 · v(t) = Amp · sin φ(t)

    The phase is the integral of the frequency: differentiating φ(t)/2π returns f(t). Before t0 the signal is Amp·sin(2π·f0·t + φ0) and after the ramp the frequency stays at its final value with continuous phase. Amp is the peak value: √2 times the RMS you enter.

    Three ways to give the ramp

    ff = f0 + (df/dt)·T · T = (ff − f0) / (df/dt) · df/dt = (ff − f0) / T

    They are the same straight line given in three ways: rate and duration, rate and final frequency, or final frequency and duration. The tool calculates the missing one. The number of cycles of the ramp is f0·T + ½·(df/dt)·T².

    IEC 60255-181§ 6.1sampling and continuityIEC 60255-181Annex E

    Samples and phases

    vA = Amp·sin φ · vB = Amp·sin(φ − 2π/3) · vC = Amp·sin(φ + 2π/3) · n = ⌊(tpre + T + tpost)·fs⌉ + 1

    Sample k is at t = k / fs. The ramp starts at an exact sample, which is also the trigger instant of the file. The sampling must be at least 4,800 Hz when the signal is a COMTRADE (4,000 Hz is accepted for legacy equipment) and meet the Nyquist criterion.

    IEC 60255-181§ 6.2.1.2.1ramp duration and time beforeIEC 60255-181§ 5.2table 3, rangesIEC 60255-181§ 6.3.3table 16, slopes

    Standard conditions that are checked

    T ≥ MAX(1 s ; 1.5 × tstart) · tpre ≥ MAX(1 s ; 1.5 × tdisengaging) · slopes: ±1.2 · ±2 · ±5 · ±10 × Gs

    In the accuracy test the ramp starts at the nominal frequency and lasts the greater of 1 s and 1.5 times the typical start time; before each ramp the frequency stays at nominal for the greater of 1 s and 1.5 times the typical disengaging time. The injected frequency must stay inside the effective range (accuracy) or the operating range (start time) declared by the manufacturer; table 3 of the standard gives an example of ±5 % and ±10 %, and of ±5 and ±10 Hz/s. Those ranges and the typical times are editable in Advanced mode.

    The file and the test set

    • The record carries channels VA, VB and VC (or VA only) in secondary volts, with a 1:1 ratio, and a digital channel RAMPA that is 1 while the frequency changes. It is written with the same writer as the COMTRADE generator of the records tool, and read back with its reader before the download.
    • OMICRON TransPlay reads COMTRADE revision 1999 with a single sampling frequency; revision 2013, .cff and multiple rates require Advanced TransPlay. The other format presets are the same as in the fault generator.
    • Check in the test set program that the frequency played matches the chart before connecting the signal to the relay, and that the voltage is one the input accepts.

    Limits of the calculation

    • It is a test signal. It does not calculate the setting of the 81R function, does not decide whether a relay passes and does not replace the procedure or the data sheet of its manufacturer.
    • IEC 60255-181 describes type tests of the function and states that they are not intended for commissioning or routine tests, although their methods can be applied. The acceptance criteria of your project are those of the manufacturer and of your utility.
    • The ranges and typical times are the examples of the standard, not relay values: the manufacturer declares its own. The tool warns when the ramp leaves the ranges you set, but it does not set them for you.
    • It generates sinusoidal voltages without harmonics, amplitude variation or phase jumps. Tests with harmonics, frequency steps or phase jumps are not included.
    • The file is a simulation of the signal: it does not include the errors of your injection test set, whose own frequency accuracy you must verify.

    Frequently asked questions

    What is a df/dt frequency ramp and what is it used for?

    It is a voltage signal whose frequency changes linearly, at a fixed rate in hertz per second. It is injected into the relay to check the rate-of-change-of-frequency function (81R, ROCOF): the rate is raised until it starts and checked against the setting. IEC 60255-181 defines the equation of that signal in its Annex A.

    How is the COMTRADE file played in a test set?

    Open the .cfg and the .dat (or the .cff) in the program of your injection test set, assign channels VA, VB and VC to the voltage outputs and play it. The digital channel RAMPA marks the stretch where the frequency changes, to trigger the timing record. Check in the program that the frequency played matches the chart.

    What sampling rate should the ramp be generated with?

    IEC 60255-181 asks for at least 4,800 Hz when the test signal is a COMTRADE file, and accepts 4,000 Hz for legacy equipment. At 4,800 Hz there are 96 samples per cycle at 50 Hz and 80 at 60 Hz. The sampling must also meet the Nyquist criterion for the highest frequency of the ramp.

    We test your frequency protection in the field

    We inject the ramps, measure the pick-up and reset of the 81 and 81R and give you the report with the results and the verified settings.

    Contact us about on-site testingSee secondary injection

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