ABB RELION 620 Series Technical Manual page 488

Section 4
Protection functions
482

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 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
kn
2. If U
kn
slightly prolonged and should be studied case by case.
If U
kn
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 m
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):

S
n
+ 
2 
I

2 n
≥ 2 x U , faster protection relay operations are secure.
s
≥ 1.5 x U and < 2 x U
s
< 1.5 x U , the protection relay operation is jeopardized.
s
I
k max in
)
+ R ≈ × R
s s
n
1MAC504801-IB E
n
, protection relay operation can be
s
(Equation 60)
Equation 56,
(Equation 61)
620 series ANSI
Technical Manual