ABB RELION 620 Series Technical Manual page 893

1MAC504801-IB E
620 series ANSI
Technical Manual
Requirements for measurement transformers
I
k max in
(
V R R
= ⋅ +
max in
n
GUID-A9BE50E9-5524-4A5A-8749-9FCAD94E16BB V1 EN
I the maximum fault current inside the zone, in primary amps
kmaxin
n the turns ration 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 estimated with
(given by P. Mathews, 1955).
( )
˘
u V V V
= 2 2 −
kn max kn
GUID-E8979454-4121-4FC8-9C09-28BC668E714E V1 EN
V the knee point voltage of the CT
kn
VDR is recommended when the peak voltage û ≥ 2 kV, which is the insulation level for
which the protection relay is tested.
The maximum fault current in case of a fault inside the zone is considered to be 12.6 kA
in primary, CT is of 1250/5 A (ratio n = 240) and knee-point voltage is 81 V. Stabilizing
resistor is 330 Ohms.
A
12600
V
= ⋅ 330 Ω = 17325
max
240
GUID-25402D7E-7E44-493C-BD9D-3C438B454ED0 V1 EN
( )
˘
u
= 2 2 81 17325 81 ⋅ ⋅ −
GUID-2DD6330F-674F-4F05-A15A-1A9C95049E5B V1 EN
As the peak voltage û = 3.2 kV, VDR must be used. If the R
avoided. The value of R
s
operation relay. Therefore, either a higher setting in the protection relay or a lower
stabilizing voltage must be used.
I
k max in
)
R R
+ ≈ ⋅
m s s
n
V
. kV
≈ 3 34
depends on the operating current and stabilizing voltage of the
Section 13
(Equation 139)
Equation 62
(Equation 140)
(Equation 141)
(Equation 142)
is smaller, VDR can be
s
887