ABB RELION REX640 Technical Manual page 797

1MRS759142 F
1.
2.
If the actual burden of the current transformer S
equation cannot be reduced low enough to provide a sufficient value for F
are two alternatives to deal with the situation.
1.
2.
REX640
Technical Manual
A fault occurring at the substation bus.
The protection must be stable at a fault arising during a normal operating
situation. The reenergizing of the transformer against a bus fault leads to very
high fault currents and thermal stress. Therefore, reenergizing is not preferred
in this case. The remanence can be neglected.
The maximum through-going fault current Ik
a short circuit fault close to the supply transformer, the DC time constant T
the fault current is almost the same as that of the transformer, the typical value
being 100 ms.
Ik = 6 I
max R
T = 100 ms
dc
ω = 100π Hz
T = 10 ms
m
K = 1
r
Equation 210
with these values gives the result:
F K Ik ( T
> × × × × ω
a r max dc
Reenergizing against a fault occurring further down in the network.
The protection must be stable also during reenergization against a fault on the
line. In this case, the existence of remanence is very probable. It is assumed to
be 40 percent here.
On the other hand, the fault current is now smaller and since the ratio of the
resistance and reactance is greater in this location, having a full DC offset is not
possible. Furthermore, the DC time constant (T
smaller, assumed to be 50 ms here.
Assuming the maximum fault current is 30 percent lower than in the bus fault
and a DC offset 90 percent of the maximum.
Ik = 0.7 × 6 = 4.2 (I
max
T = 50 ms
dc
ω = 100π Hz
T = 10 ms
m
K = 1/(1-0.4) = 1.6667
r
Equation 210
with these values gives the result:
F K Ik 0 9 . ( T
> × × ×
a r max
A current transformer with a higher rated burden S
means a higher rated accurate limit F
A current transformer with a higher nominal primary current I
rated burden) can be chosen.
T
−
m
T dc
( 1 e ) 1 ) 24
− + ≈
)
R
T
−
m
T dc
( 1 e ) 1 )
× ω × − + ≈
dc
).
n
Protection functions
is typically 6 I for a motor. At
max R
) of the fault current is now
dc
24
in the accuracy limit factor
a
can be chosen (which also
n
(but the same
1n
of
dc
, there
a
797