ABB RELION REX640 Technical Manual page 1154

Protection functions
U
+ ⋅
A
Figure 651: Equation
If the system is symmetrical, that is, the system is healthy without any fault
U
+
A
Figure 652: Equation
Combining
U
+ ⋅ −
A
Figure 653: Equation
U
=
N
Figure 654: Equation
Considering the neutral VT ratio, per unit voltage referred to the neutral side is
given by
U
=
N
Figure 655: Equation
For example, if the impedance of the 20 kV system (U
drops by 5 percent, that is, k = 0.95, the neutral voltage is around 200 V according
Figure 654
to
terms refer to neutral side, the neutral voltage is around 0.01 · Un, that is,
should be set around 0.01.
In addition, assume that residual voltage (terminal side) is measured directly using
an open-delta VT having ratio of
and
(UoB,VT) in a grounded system are set as 11.547 kV and 100 V, respectively. Similarly,
if the VT connected at the neutral point has a ratio of
voltage and Secondary voltage settings under Configuration > Analog inputs >
Voltage (UoB,VT) must be set as 20 kV and 100 V, respectively.
4.12.5.8 Signals
1154
( )
(
k U U U
+ = ⋅ 1 2 + ⋅
B C N
U U
= 0
+
B C
Figure 651
and
( )
( )
k U U k
= ⋅ 1 2 + ⋅
A N
( )
1 k
−
U
⋅
A
( )
1 2 k
+ ⋅
( )
1 k
−
U
⋅
A
( )
k
1 2 + ⋅
U
N _ pr
. If the voltage rating of a neutral VT is 20 kV/100 V, then in per unit
Secondary voltage settings under Configuration > Analog inputs > Voltage
)
k
Figure 652
results in
20 kV 100 V 100
/ /
3 3
= 11.547 kV) capacitance
A
V
3
Primary voltage
. Also, the
20 kV 100 V
/
3 3
, the
Technical Manual
1MRS759142 F
Start value
Primary
REX640