ABB RELION 611 Series Applications Manual page 67

1MRS757455 E
REB611
Application Manual
Requirements for measurement transformers

U 0 8 . F I R
= × × × 
kn n 2 n in


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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 selecting the CTs according to
results in an absolutely stable scheme. In some cases, it is possible to
achieve stability with the knee point voltages lower than stated in the
formulas. The conditions in the network, however, must be known
well enough to ensure the stability.
1. If U
2. If U
slightly prolonged and should be studied case by case.
If U
Another CT has to be chosen.
The need for the VDR depends on certain conditions.
First, voltage U , ignoring the CT saturation during the fault, is calculated using the
max
formula
I
k max in
(
U R R
= × +
max in
n
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I the maximum fault current inside the zone in primary amps
kmaxin
n the turns ratio of the CT
R the internal resistance of the CT in ohms
in
R the resistance of the longest loop of the CT secondary circuit in ohms
m
R the resistance of the stabilized resistor in ohms
s
Next, the peak voltage û, which includes the CT saturation, is calculated using the
formula (given by P.Mathews, 1955):
(
û = 2 2 U U − U
kn max kn
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U the knee point voltage of the CT
kn

S
n
+ 
2

I
2 n 
≥ 2 × U , faster protection relay operations are secure.
kn s
≥ 1.5 × U and < 2 × U
kn s
< 1.5 × U , the protection relay operation is jeopardized.
kn s
I
k max in
)
R R
+ ≈ ×
m s s
n
)
Section 4
n
Equation
, protection relay operation can be
s
(Equation 6)
2,
(Equation 7)
(Equation 8)
61