ABB RELION REX640 Technical Manual page 814

Protection functions
Voltage Umax, ignoring the CT saturation during the fault is calculated using
First, voltage U
the equation
U
max
I
kmaxin
n
R
in
R
m
R
s
The peak voltage ȗ, which includes the CT saturation, is estimated by using
Equation 219
û =
U
kn
The VDR is recommended when the peak voltage û ≥ 2kV, which is the insulation
level for which the IED is tested.
For example, the maximum fault current in case of a fault inside the zone is 12.6 kA
in primary, CT is of 1250/5 A, that is, ratio n = 240, and knee point voltage is 81 V.
The stabilizing resistor is 330 Ohms.
U
max
˘
u =
As the peak voltage ȗ = 3.2 kV, VDR must be used. In some cases, VDR can be
avoided if R
current and stabilizing voltage. Thus, a higher setting in the protection relay must
be used or the stabilizing voltage lowered.
Flux-balancing principle
When the function block is used with the flux-balancing principle, there are no extra
requirements for the measuring devices. The core-balance transformers used in an
ordinary overcurrent protection are adequate here as well.
814
, ignoring the CT saturation during the fault, is calculated with
max
Equation 218
I
k max in
(
= × R + R +
in m
n
Maximum fault current inside the zone, in primary amps
Turns ration of the CT
Internal resistance of the CT in ohms
Resistance of the longest loop of the CT secondary circuit, in ohms
Resistance of the stabilized resistor, in ohms
(given by P. Mathews 1955).
( )
2 2 U U − U
kn max kn
Knee point voltage of the CT
12600 A
330 17325
= ⋅ Ω =
240
( )
2 2 81 17325 81 ⋅ ⋅ − ≈
is smaller. The value of R
s
.
I
k max in
)
R ≈ × R
s s
n
V
3 34 . kV
depends on the protection relay operation
s
1MRS759142 F
(Equation 218)
(Equation 219)
(Equation 220)
(Equation 221)
REX640
Technical Manual