ABB RELION 620 Series Technical Manual page 601

1MRS757644 F
620 series
Technical Manual
voltage dependent resistor. The value of the primary current I
protection relay operates at certain setting can be calculated using
I n I I m I
(
= ⋅ + + ⋅
prim rs u m
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I The magnetizing current per current transformer at U
m
I The primary current level at which the protection is to start
prim
Operate value setting
I The value of
rs
I The leakage current through the VDR at U
u
n Turn ratio of the current transformer
m The number of current transformers included in the protection per phase (=2)
The I value given in many catalogues is the excitation current at knee point voltage.
e
Assuming , the value of
U U
≈ ⋅ 2
kn s
109.
The selection of current transformers can be divided into the following steps.
1. The rated current In of the protected machine should be known. The value of I
also affects the magnitude of I
2. The rated primary current I
the machine. The choice of the CT also specifies R
3. The required U is calculated using
kn
enough, another CT has to be chosen. The value of the U
manufacturer in case of a Class X current transformer or it can be estimated using
Equation 110.
4. The sensitivity I
prim
is sufficient, the present CT is chosen. A CT with a bigger core is chosen if a better
sensitivity is needed.
If a Class X CT is not used, an estimate for U

U . F I R
0 8
= ⋅ ⋅ ⋅ +

kn n n in
2


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F The rated accuracy limit factor corresponding to rated burden S
n
I The rated secondary current of the CT
2n
R The secondary internal resistance of the CT
in
S The voltage-ampere rating of the CT
n
)
voltage
s
I
e
I
≈
m
gives an approximate value for
2
. Normally the I
kmax
of the CT has to be higher than the rated current of
1n
Equation
is calculated using Equation
is calculated using
kn
Sn 

2

I
n
2 
Section 4
Protection functions
at which the
prim
Equation
voltage
s
value for motors is 6 · I
kmax
.
in
106. If the U of the CT is not high
kn
is given by the
kn
109. If the achieved sensitivity
Equation
n
109.
(Equation 109)
Equation
n
.
n
110.
(Equation 110)
595