ABB RER620 Technical Manual page 552

Section 10
General function block features
546
Consequently, the measurement mode can be selected according to the application.
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
1
2
I ∑
=
I
RMS i
n
i = 1
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 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.
1MAC309294-MB F
(Equation 68)
RER620
Technical Manual