ABB RELION REX640, Техническое руководство, Страница: 1153 / 1960

side. The terminal side's residual voltage component is obtained directly by VTs
connected in open delta or by deriving open delta voltage using all three terminal
side phase voltages. The neutral side's residual voltage is measured directly by
connecting a VT between the neutral point and earth. It is advisable to block the
function with the fuse failure function.
Example 1, ungrounded SCB
Figure 646
illustrates how much the neutral voltage changes in case of failure of
some phase elements of the ungrounded SCB.
U
A
U
B
U
C
U
N
Z Z Z
I
A
I
B
I
C
I
A
+I
B
+I
C
=0
Figure 646: Ungrounded SCB
In a healthy ungrounded capacitor bank, the sum of the phase currents is
I I I
A B C
+ + =
0
Figure 647: Equation
If the capacitor bank is symmetrical, that is, there is equal impedance in each phase,
the equation can be
U U
Z
U U
Z
U U
Z
A N B N C N
−





 +
−





 +
−





 =
0
Figure 648: Equation
If the fault in the capacitor bank changes the phase A impedance to kZ
U U
kZ
U U
Z
U U
Z
A N B N C N
−





 +
−





 +
−





 =
0
Figure 649: Equation
U
kZ
U
Z
U
Z
U
kZ Z
A B C
N
+ + = ⋅ +






1 2
Figure 650: Equation
1MRS759142 F Protection functions
REX640
Technical Manual
1153