ABB Relion 620 Series, Technical Manual

Page: 511 / 1224

1. 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
max
is typically 6 I
R
for a motor. At
a short circuit fault close to the supply transformer, the DC time constant T
dc
of the fault current is almost the same as that of the transformer, the typical
value being 100 ms.
Ik
max
= 6 I
R
T
dc
= 100 ms
ω = 100π Hz
T
m
= 10 ms
K
r
= 1
Equation 67
with these values gives the result:
F K Ik T e
a r dc
T
m
Tdc
> × × × × − + ≈
−
max
( ( ) )
ω
1 1 24
2. 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
dc
) of the fault current is now
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
max
= 0.7 × 6 = 4.2 (I
R
)
T
dc
= 50 ms
ω = 100π Hz
T
m
= 10 ms
K
r
= 1/(1-0.4) = 1.6667
Equation 67 with these values gives the result:
F K Ik T e
a r dc
T
m
Tdc
> × × × × × − + ≈
−
max
. ( ( ) ) 0 9 1 1 24
ω
If the actual burden of the current transformer S
a
in the accuracy limit factor
equation cannot be reduced low enough to provide a sufficient value for F
a
, there
are two alternatives to deal with the situation.
1. A current transformer with a higher rated burden S
n
can be chosen (which also
means a higher rated accurate limit F
n
).
2. A current transformer with a higher nominal primary current I
1n
(but the same
rated burden) can be chosen.
1MRS757644 H Protection functions
620 series
Technical Manual
511