ABB RELION REX640 Technical Manual page 1152

Protection functions
U
A
U
B
U
C
Figure 644: Ungrounded SCB
The operating signal for the compensated neutral voltage unbalance in per unit with
reference to the neutral side of an ungrounded SCB is as indicated in
Operating signal =
Figure 645: Equation
U
Neutral
U
o
U
unb
n
Uo
U
N_sr
U
N_pr
The rated
U
Neutral
Analog inputs > Voltage (Uo,VT).
In monitored data, xUn refers to neutral VT rated voltage.
4.12.5.7 Application
Shunt capacitor banks are provided with an unbalance protection function which
is mainly used to detect problems within capacitor units using the presumed
symmetry in the impedances of the protected bank. The unbalance protection
responds to unbalances in the measured signals caused by changes in the
impedance of the capacitor bank.
In compensating unbalance protection, the operating signal compensates both the
inherent capacitor bank unbalance and the system unbalance. The method involves
measuring the residual voltage at the capacitor terminal side and at the neutral
1152
N n
U
Neutral
G g
PT
U
Neutral
−
U
_
N sr
Measured shunt capacitor neutral voltage
Measured open delta voltage
Natural unbalance compensation component
Voltage ratio of open delta VT
Rated secondary voltage of neutral connected VT
Rated primary voltage of neutral connected VT
Primary voltage and Secondary voltage for the neutral voltage channel
, and for U and U
N_pr
The operating signal setting is given as multiple of the neutral VT rated
voltage.
U
0_T
Measured (by open
delta) or calculated
residual voltage
n
Uo
U U
o unb
⋅
−
U
_
N pr
are set in the global setting under Configuration >
N_sr
1MRS759142 F
Figure 645
.
REX640
Technical Manual