ABB RELION 611 Series Applications Manual page 66

Section 4
Requirements for measurement transformers
60
voltage-dependent resistor (VDR). The value of the primary current I
protection relay operates at a certain setting can be calculated using the formula
I n ( I I m I
= × + + ×
prim rs u
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I the primary current at which the protection is to start
prim
n the turn ratio of the current transformer
Operate value setting
I the value of the
rs
I the leakage current flowing through the VDR at the U
u
m the number of current transformers included in the protection per phase
I the magnetizing current per current transformer at the U
m
The I value given in many catalogs is the excitation current at the knee point voltage.
e
Assuming U ≈ 2 × U , the value of
kn s
5.
The selection of current transformers can be divided into procedures:
1. The rated current I
n
how high I is.
kmax
2. The rated primary current I
the feeder.
The choice of the CT also specifies R
3. The required U is calculated using
kn
enough, enough, another CT has to be selected. The value of U
manufacturer in the case of Class X current transformers or it can be estimated
using Equation 6.
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 U
)
m
I
e
I
≈
m gives an approximate value for
2
of the feeder should be known. The value of I
of the CT must be higher than the rated current of
1n
.
in
Equation
is calculated using Equation
1MRS757455 E
at which the
prim
voltage
s
voltage
s
also affects
n
2. If U of the CT is not high
kn
is given by the
kn
5. If the achieved sensitivity is
is calculated using the equation:
kn
Application Manual
(Equation 5)
Equation
REB611