- RMS
- DFT which is a numerically calculated fundamental component of the signal
- Peak-to-peak
- Peak-to-peak with peak backup
- Wide peak-to-peak
In extreme cases, for example with high overcurrent or harmonic content, the measurement modes function in a slightly different way. The operation accuracy is defined with the frequency range of f/fn=0.95...1.05. In peak-to-peak and RMS measurement modes, the harmonics of the phase currents are not suppressed, whereas in the fundamental frequency measurement the suppression of harmonics is at least -50 dB at the frequency range of f= n x fn, where n = 2, 3, 4, 5,...
RMS
The RMS measurement principle is selected with the Measurement mode setting using the value "RMS". RMS consists of both AC and DC components. The AC component is the effective mean value of the positive and negative peak values. RMS is used in applications where the effect of the DC component must be taken into account.
RMS is calculated according to the formula:
| n | Number of samples in a calculation cycle |
| I i | Current sample value |
DFT
The DFT measurement principle is selected with the Measurement mode setting using the value "DFT". In the DFT mode, the fundamental frequency component of the measured signal is numerically calculated from the samples. In some applications, for example, it can be difficult to accomplish sufficiently sensitive settings and accurate operation of the low stage, which may be due to a considerable amount of harmonics on the primary side currents. In such a case, the operation can be based solely on the fundamental frequency component of the current. In addition, the DFT mode has slightly higher CT requirements than the peak-to-peak mode, if used with high and instantaneous stages.
Peak-to-peak
The peak-to-peak measurement principle is selected with the Measurement mode setting using the value "Peak-to-Peak". It is the fastest measurement mode, in which the measurement quantity is made by calculating the average from the positive and negative peak values. The DC component is not included. The retardation time is short. The damping of the harmonics is quite low and practically determined by the characteristics of the anti-aliasing filter of the protection relay inputs. Consequently, this mode is usually used in conjunction with high and instantaneous stages, where the suppression of harmonics is not so important. In addition, the peak-to-peak mode allows considerable CT saturation without impairing the performance of the operation.
Peak-to-peak with peak backup
The peak-to-peak with peak backup measurement principle is selected with the Measurement mode setting using the value "P-to-P+backup". It is similar to the peak-to-peak mode, with the exception that it has been enhanced with the peak backup. In the peak-to-peak with peak backup mode, the function starts with two conditions: the peak-to-peak value is above the set start current or the peak value is above two times the set Start value. The peak backup is enabled only when the function is used in the DT mode in high and instantaneous stages for faster operation.
Wide peak-to-peak
The wide peak-to-peak measurement principle is available in products where it is necessary for overcurrent protection to operate already starting from as low frequency as 2 Hz during the generator start-up or shutdown phase. The wide peakto-peak measurement principle is selected with the Measurement mode setting "Wide P-to-P".
The measurement mode calculates the average from the positive and negative peak values over the 500 ms wide measurement window. Retardation and reset times are longer due to the length of the measurement window. The frequency of the fault current affects the operate time. The damping of the harmonics is quite low and practically determined by the characteristics of the anti-aliasing filter of the protection relay current inputs.
Operation accuracy in the frequency range 2...85 Hz is ±1.5% or ±0.003 xIn. Operate time accuracy in definite time mode is ±1.0% of the set value or ±60 ms when IFault = 2 · set Start value and the fault current frequency is 10...85 Hz.