ABB RELION REX640, Техническое руководство, Страница: 1154 / 1960

U k U U U k
A B C N
+ ⋅ +
( )
= ⋅ + ⋅
( )
1 2
Figure 651: Equation
If the system is symmetrical, that is, the system is healthy without any fault
U U U
A B C
+ + =
0
Figure 652: Equation
Combining
Figure 651
and
Figure 652
results in
U k U U k
A A N
+ ⋅ −
( )
= ⋅ + ⋅
( )
1 2
Figure 653: Equation
U U
k
k
N A
= ⋅
−
( )
+ ⋅
( )
1
1 2
Figure 654: Equation
Considering the neutral VT ratio, per unit voltage referred to the neutral side is
given by
U
U
k
k
U
N
A
N pr
=
⋅
−
( )
+ ⋅
( )
1
1 2
_
Figure 655: Equation
For example, if the impedance of the 20 kV system (U
A
= 11.547 kV) capacitance
drops by 5 percent, that is, k = 0.95, the neutral voltage is around 200 V according
to
Figure 654
. If the voltage rating of a neutral VT is 20 kV/100 V, then in per unit
terms refer to neutral side, the neutral voltage is around 0.01 · Un, that is,
Start value
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
20
3
100
3
100
3
kV V V
/ /
. Also, the
Primary voltage
and
Secondary voltage settings under Configuration > Analog inputs > Voltage
(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
20
3
100
3
kV V
/
, the
Primary
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
Protection functions 1MRS759142 F
1154 REX640
Technical Manual