Two typical cases are considered for the determination of the sufficient
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. Re-energizing the transformer against a bus fault
leads to very high fault currents and thermal stress and 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 10 I
r
for a
substation main transformer. 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
10 I
r
T
dc
100 ms
ω 100π Hz
T
m
10 ms
K
r
1
When the values are substituted in
Equation 421
, the result is:
F
a
> × × × × − + ≈
−
K Ik T e
r dc
T T
m dc
max
/
( ( ) )
ω
1 1 40
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, 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 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* 10 = 7 ( I
r
)
T
dc
50 ms
ω 100π Hz
T
m
10 ms
K
r
1/(1-0.4) = 1.6667
When the values are substituted in the equation, the result is:
F
a
> × × × × × − + ≈
−
K Ik T e
r dc
T T
m dc
max
/
. ( ( ) )0 9 1 1 40
ω
If the actual burden of the current transformer (S
a
) in
Equation 420
cannot be reduced low enough to provide a sufficient value for F
a
, there
are two alternatives to deal with the situation:
- a CT with a higher rated burden S
n
can be chosen (which also means
a higher rated accuracy limit F
n
)
Requirements for measurement
transformers
1MRS759142 F
1920 REX640
Technical Manual
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