ABB RED615 Technical Manual page 846

Section 11
General function block features
840
n
1
2
I
∑
=
I
i
RMS
n
=
i 1
A070883 V2 EN
n the number of samples in a calculation cycle
I the current sample value
i
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 IED 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.
1YHT530004D05 D
(Equation 109)
615 series
Technical Manual