ABB Relion 611 Series, Technical Manual

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I
n
I
RMS
i
i
n
=
=
∑
1
2
1
A070883 V3 EN
(Equation 50)
n The number of samples in a calculation cycle
I
i
The 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.
Section 9
1MRS757454 D
General function block features
488 611 series
Technical Manual