ABB RELION REX640 Technical Manual page 1062

Section 4
Protection functions
1056
U U U 1
A B C

U
+ + = N ⋅

kZ Z Z kZ

GUID-D6E94D4E-1C60-4CBB-848A-79BA3EEE6FD1 V1 EN
U k U U U
1 2
( )
(
A + ⋅ B + C = N ⋅
GUID-2EAB2220-7360-438A-BF59-A126B9E33264 V1 EN
If the system is symmetrical, that is, the system is healthy without any fault
U U U = 0
A B C
+ +
GUID-6519AFB6-21E0-4312-99E6-DD5BC18327C0 V1 EN
Combining Equation 252
U k U U
1 2
( )
(
A + ⋅ − A = N ⋅ + ⋅
GUID-7C251C96-3B10-46FD-99DE-F00DD2877FCD V1 EN
1 k
( )
−
U U
N A
= ⋅
k
1 2
( )
+ ⋅
GUID-CC5C8341-5EF1-4D79-A6B3-F1CFAE973928 V1 EN
Considering the neutral VT ratio, per unit voltage referred to the neutral side is given
by
k
1
( )
−
U
A
⋅
1 2 k
( )
+ ⋅
U
N
=
U
N _ pr
GUID-F837E8C7-EADA-496C-9873-7F589699AF9B V1 EN
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 to
Equation 255. 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
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
Secondary voltage settings under Configuration/Analog inputs/Voltage (Uo,VT)
must be set as 20 kV and 100 V, respectively.
2

+

Z

k
)
+ ⋅
and Equation 253 results in
k
)
20 kV 100 V 100
/ /
3
3 3
20
1MRS759142 C
= 11.547 kV) capacitance
A
V
. Also, the Primary voltage and
kV 100 V
/
, the Primary voltage and
3 3
Technical Manual
(Equation 251)
(Equation 252)
(Equation 253)
(Equation 254)
(Equation 255)
(Equation 256)
REX640