1. In principle, the highest through-fault should be known. However, when the
necessary data are not available, approximates can be used:
• Small power transformers: I
kmax
= 16 x I
n
(corresponds to z
k
= 6% and
infinite grid)
• Large power transformers: I
kmax
= 12 x I
n
(corresponds to z
k
= 8% and
infinite grid)
• Generators and motors: I
kmax
= 6 x I
n
Where I
n
= rated current and z
k
= short circuit impedance of the
protected object
2. The rated primary current I
1n
of the CT has to be higher than the rated current
of the machine.
The choice of the CT also specifies R
in
.
3. The required U
kn
is calculated with
Equation 61. If the U
kn
of the CT is not
high enough, another CT has to be chosen. The value of the U
kn
is given by the
manufacturer in the case of Class X current transformers or it can be estimated
with
Equation 65.
4. The sensitivity I
prim
is calculated with Equation 64. If the achieved sensitivity
is sufficient, the present CT is chosen. If a better sensitivity is needed, a CT
with a bigger core is chosen.
If other than Class X CTs are used, an estimate for U
kn
is calculated with the equation
U F I R
S
I
kn n n in
n
n
= × × × +

ï£






0 8
2
2
2
.
GUID-AFA68232-5288-4220-845E-40347B691E29 V2 EN (Equation 65)
F
n
the rated accuracy limit factor corresponding to the rated burden S
n
I
2n
the rated secondary current of the CT
R
in
the secondary internal resistance of the CT
S
n
the volt-amp rating of the CT
The formulas are based on choosing the CTs according to Equation
61, which results an absolutely stable scheme. In some cases, it is
possible to achieve stability with knee point voltages lower than
stated in the formulas. The conditions in the network, however,
have to be known well enough to ensure the stability.
1. If U
k
≥ 2 x U
s
, fast IED operation is secure.
2. If U
k
≥ 1.5 x U
s
and < 2 x U
s
, IED operation can be slightly
prolonged and should be studied case by case.
If U
k
< 1.5 x U
s
, the IED operation is jeopardized. Another CT
has to be chosen.
1YHT530004D05 D Section 4
Protection functions
615 series 439
Technical Manual
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