ABB RELION 620 Series Technical Manual page 892

Section 13
Requirements for measurement transformers
886
The choice of the CT also specifies R
3. The required V is calculated with
kn
another CT has to be selected. The value of the V
the case of Class X current transformers or it can be estimated with
4. The sensitivity I
prim
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 V

V . F I R
= 0 8 ⋅ ⋅ ⋅ +

kn n n in
2

I

GUID-695E5AB5-7AF9-4990-B686-333FA8476C7C V1 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 equations are based on choosing the CTs according to
which results an absolutely stable scheme. In some cases, it is possible to
achieve stability with the knee point voltages lower than stated in
equations. The conditions in the network, however, must be known well
enough to ensure the stability.
1. If V
k
2. If V
k
prolonged and must be studied case by case.
If V
k
CT has to be chosen.
The need for the VDR depends on certain conditions.
First, voltage V , ignoring the CT saturation during the fault, is calculated with
max
Equation 61.
.
in
Equation
is calculated with Equation

S
n


2

n
2
≥ 2 · V , fast protection relay operation is secure.
s
≥ 1.5 · V and < 2 · V , protection relay operation can be slightly
s s
< 1.5 · V , the protection relay operation is jeopardized. Another
s
1MAC504801-IB E
56. If V of the CT is not high enough,
kn
is given by the manufacturer in
kn
Equation
59. If the achieved sensitivity is
is calculated with
kn
n
60.
Equation 60.
(Equation 138)
Equation 56,
620 series ANSI
Technical Manual