ABB Relion 620 Series - Calculated Measurements

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.
11.6 Calculated measurements
Calculated residual current and voltage
The residual current is calculated from the phase currents according to equation:
Io I I I
A B C
= − + +( )
(Equation 206)
The residual voltage is calculated from the phase-to-earth voltages when the VT
connection is selected as “Wye” with the equation:
Uo U U U
A B C
= + +( ) / 3
(Equation 207)
General function block features
1MRS757644 H
1166 620 series
Technical Manual