Calculated Measurements - ABB RELION SMU615 Technical Manual

General function block features
n
I
i
DFT
The DFT measurement principle is selected with the
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.
9.3

Calculated measurements

Calculated residual current and voltage
The residual current is calculated from the phase currents according to equation:
Io
= −
The residual voltage is calculated from the phase-to-earth voltages when the VT
connection is selected as "Wye" with the equation:
Uo
=
Sequence components
The phase-sequence current components are calculated from the phase currents
according to:
I
=
0
I
=
1
I
=
2
The phase-sequence voltage components are calculated from the phase-to-earth
voltages when
248
The number of samples in a calculation cycle
The current sample value
( I I I )
+ +
A B C
( U U U ) / 3
+ +
A B C
( I I I ) / 3
+ +
A B C
2
( I a I a I ) / 3
+ ⋅ + ⋅
A B C
2
( I a I a I ) / 3
+ ⋅ + ⋅
A B C
VT connection is selected as "Wye" with the equations:
1MRS758407 C
Measurement mode setting
(Equation 16)
(Equation 17)
(Equation 18)
(Equation 19)
(Equation 20)
Technical Manual
SMU615