ABB RED615 Technical Manual page 445

1YHT530004D05 D
615 series
Technical Manual
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
infinite grid)
• Large power transformers: I
infinite grid)
• Generators and motors: I
Where I = rated current and z
n
protected object
2. The rated primary current I
of the machine.
The choice of the CT also specifies R
3. The required U is calculated with
kn
high enough, another CT has to be chosen. The value of the U
manufacturer in the case of Class X current transformers or it can be estimated
with Equation 65.
4. The sensitivity I
prim
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

U = 0 8 . × F × I × R

kn n 2 n in


GUID-AFA68232-5288-4220-845E-40347B691E29 V2 EN
F the rated accuracy limit factor corresponding to the rated burden S
n
I the rated secondary current of the CT
2n
R the secondary internal resistance of the CT
in
S the volt-amp rating of the CT
n
The formulas are based on choosing the CTs according to
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
2. If U
prolonged and should be studied case by case.
If U
has to be chosen.
= 16 x I
kmax
= 12 x I
kmax
= 6 x I
kmax n
= short circuit impedance of the
k
of the CT has to be higher than the rated current
1n
.
in
Equation
is calculated with Equation

S
n
+


2
I

n
2
≥ 2 x U , fast IED operation is secure.
k s
≥ 1.5 x U and < 2 x U
k s
< 1.5 x U , the IED operation is jeopardized. Another CT
k s
Section 4
Protection functions
(corresponds to z
n k
(corresponds to z
n k
61. If the U of the CT is not
kn
is given by the
kn
64. If the achieved sensitivity
is calculated with the equation
kn
n
, IED operation can be slightly
s
= 6% and
= 8% and
(Equation 65)
Equation
439