ABB RELION 620 Series Technical Manual page 882

Section 12
General function block features
876
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
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 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.
1MAC504801-IB E
(Equation 116)
620 series ANSI
Technical Manual