The accuracy limit factors corresponding to the actual burden of the phase current
transformer to be used in differential protection fulfill the requirement.
F K Ik T e
a r dc
T T
m dc
> × × × × − +
−
max
/
( ( ) )
ω
1 1
GUID-DA861DAD-C40E-4A82-8973-BBAFD15279C0 V1 EN (Equation 201)
Ik
max
The maximum through-going fault current (in I
R
) at which the protection is not allowed to trip
T
dc
The primary DC time constant related to Ik
max
ω The angular frequency, that is, 2*π*fn
T
m
The time-to-saturate, that is, the duration of the saturation free transformation
K
r
The remanence factor 1/(1-r), where r is the maximum remanence flux in p.u. from saturation flux
The accuracy limit factors corresponding to the actual burden of the phase current
transformer is used in differential protection.
The parameter r is the maximum remanence flux density in the CT core in p.u. from
saturation flux density. The value of the parameter r depends on the magnetic material
used and on the construction of the CT. For instance, if the value of 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 87T is half fundamental cycle period (10 ms
when fn = 50Hz).
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
Section 12 1MAC059074-MB A
Requirements for measurement transformers
1136 615 series ANSI
Technical Manual
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