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The parameter r is the maximum remanence flux density in the CT core in pu from the
saturation flux density. The value of the parameter r depends on the magnetic material
used and also on the construction of the CT. For instance, if the value r = 0.4, the
remanence flux density can be 40 percent of the saturation flux density. The manufacturer
of the CT has to be contacted when an accurate value for the parameter r is needed. The
value r = 0.4 is recommended to be used when an accurate value is not available.
The required minimum time-to-saturate T
m
in 87G, 87M is half-fundamental cycle period
(10 ms when f
n
= 50 Hz).
Two typical cases are considered for the determination of the sufficient actual accuracy
limit factor F
a
:
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
with these values gives the result:
F
K
Ik
T
e
a
r
dc
T
m
Tdc
>
×
×
×
×
−
+
≈
−
max
(
(
)
)
ω
1
1
24
GUID-C510CA46-3857-40D3-B40A-3C237BCE6E79 V2 EN
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.
Section 4
1MAC059074-MB A
Protection functions
578
615 series ANSI
Technical Manual
Содержание Relion 615 series
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