(Equation 403)
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, independently of the
Frequency
adaptivity
setting value. Retardation and reset times are longer due to the length
1MRS759142 F
General function block features
REX640
Technical Manual
1909
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