Section 4
1MAC050144-MB C
Protection functions
330
615 series ANSI
Technical Manual
1.
A fault occurring at the substation bus.
The protection must be stable at a fault arising during a normal operating situation.
Re-energizing the transformer against a bus fault would lead to very high fault
currents and thermal stress, therefore re-energizing is not preferred in this case. Thus
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
120π Hz
T
m
8.33 ms
K
r
1
Substituting the values in Equation 33 the result is;
≈
24
2.
Re-energizing against a fault occurring further down in the network.
The protection must be stable also during re-energization against a fault on the line.
In this case, existence of remanence is very probable. It is assumted 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 a maximum fault current being 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
120π Hz
T
m
8.33 ms
K
r
1/(1-0.4) = 1.6667
the result is;
≈
24
If the actual burden of the current transformer (S
a
) in
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
) or
)
1
)
1
(
(
max
dc
m
T
T
dc
r
a
e
T
Ik
K
F
)
1
)
1
(
(
9
.
0
max
dc
m
T
T
dc
r
a
e
T
Ik
K
F
Содержание 615 Series ANSI
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Страница 29: ...1MAC050144 MB C Section 615 series ANSI 22 Technical Manual ...
Страница 389: ...Section 4 1MAC050144 MB C Protection functions 382 615 series ANSI Technical Manual ...
Страница 485: ...Section 6 1MAC050144 MB C Supervision functions 478 615 series ANSI Technical Manual ...
Страница 635: ...Section 10 1MAC050144 MB C Other functions 628 615 series ANSI Technical Manual ...
Страница 707: ...Section 15 1MAC050144 MB C Applicable standards and regulations 700 615 series ANSI Technical Manual ...
Страница 711: ...Section 16 1MAC050144 MB C Glossary 704 615 series ANSI Technical Manual ...
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