ABB REF615 - Page 185

The equivalent circuit is now used in the calculation. The U
1
value of the fault-point
voltage at a single phase-to-earth fault is U
1f
= 0.67 p.u. The positive phase-sequence
component of the transient source voltage of a generator can be approximated by 1.1
p.u. The U
1
voltage appearing at the measuring point of the relay is calculated with
the approximation formula:
U U U
X
X X
X
X X
U U
X
X
G f f
y
kB
kG
kG d
y
kB
y
f f
1 1 1
1 1 1 1
1 1
= + −
+
+
+
+ − − −( )
’
(( . ) ( )
XX
y
)
GUID-6F5535FF-F9AE-41D2-A8BD-1FBE5E6C155C V1 EN (Equation 9)
X
kG
short circuit reactance between the farthest point of the created island, the
substation B in this case, and the measuring point of the relay reduced to
the voltage level of the latter
X
d
’ transient reactance of the generator of the power station
X
kB
short circuit reactance of the network as seen from the substation B towards
either the substation A or the substation C. The smaller of these values is
used in the calculation. The value of X
kB
is reduced to the voltage level of
the measuring point of the relay
X
y
the higher value of the overreaching of the distance relays of the
substations A and C. The value used in the calculation is reduced to the
voltage level of the measuring point of the relay
If
x
d
'
0.2
S
n
6 MVA
U
n
10 kV
the setting is then calculated:
X x
U
S
V
VA
d d
n
n
’ ’
.
( )
( )
.= = × =
2
2
0 2
10000
6000000
3 33Ω
GUID-0BD1A603-E39D-4F41-96E0-7E25F8940FAE V1 EN
X
kG
= 1 Ω (reduced to the 10 kV voltage level)
The short circuit reactance of the network seen from the substation B towards the
substation A is 1 Ω and towards the substation C 1.2 Ω => X
kB
= 1 Ω.
The value reduced to the 10 kV voltage level is X
kB
’ = 1 × (10/150)² Ω = 0.0044 Ω.
The overreaching first zone of the distance relay of the substation A covers a length
of 0.3 Ω of the line BC. The overreaching first zone of the distance relay of the
substation C covers a length of 0.25 Ω of the line BA => X
y
= 0.3 Ω.
1MRS756378 D Section 5
Protection functions
REF615 179
Application Manual