Calculated Measurements - ABB Relion SMU615 Technical Manual

Section 9
General function block features
9.3
230
n
1
2
I I
∑
=
i
RMS
n
i 1
=
A070883 V3 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.

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
GUID-B9280304-8AC0-40A5-8140-2F00C1F36A9E V1 EN
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 ) / 3
A B C
= + +
GUID-03909E83-8AA3-42FF-B088-F216BBB16839 V1 EN
Sequence components
The phase-sequence current components are calculated from the phase currents
according to:
( ) / 3
I I I I
0
= A + B + C
GUID-2319C34C-8CC3-400C-8409-7E68ACA4F435 V2 EN
2
I ( I a I a I ) /
= + ⋅ + ⋅
1 A B C
GUID-02E717A9-A58F-41B3-8813-EB8CDB78CBF1 V2 EN
3
1MRS758407 B
(Equation 14)
(Equation 15)
(Equation 16)
(Equation 17)
(Equation 18)
SMU615
Technical Manual