ABB REF615 Applications Manual page 185

1MRS756378 D
REF615
Application Manual
The equivalent circuit is now used in the calculation. The U
voltage at a single phase-to-earth fault is U
component of the transient source voltage of a generator can be approximated by 1.1
p.u. The U voltage appearing at the measuring point of the relay is calculated with
1
the approximation formula:
X
U U U
1
( )
= + −
1 G 1 f 1 f
X
kB
GUID-6F5535FF-F9AE-41D2-A8BD-1FBE5E6C155C V1 EN
X
kG
X '
d
X
kB
X
y
If
x '
d
S
n
U
n
the setting is then calculated:
2
U
( 10000
' '
n
X = x = ×
0 2 .
d d
S ( 6000000
n
GUID-0BD1A603-E39D-4F41-96E0-7E25F8940FAE V1 EN
X = 1 Ω (reduced to the 10 kV voltage level)
kG
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
The value reduced to the 10 kV voltage level is X
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
= 0.67 p.u. The positive phase-sequence
1f
X
y
kG
U
(( . 1 1
+ −
'
X
X X
+
+
y
kG d
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
transient reactance of the generator of the power station
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
the measuring point of the relay
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
0.2
6 MVA
10 kV
2
V
)
=
3 33Ω .
VA )
Section 5
Protection functions
value of the fault-point
1
X
y
U
) ( 1 ) )
− −
1 f 1 f
X X X
+
y
kB
is reduced to the voltage level of
kB
= 1 Ω.
kB
' = 1 × (10/150)² Ω = 0.0044 Ω.
kB
= 0.3 Ω.
y
(Equation 9)
179