Voltage Umax, ignoring the CT saturation during the fault is calculated using
First, voltage U
max
, ignoring the CT saturation during the fault, is calculated with
the equation
Equation 218
.
U
I
n
R R R
I
n
R
k in
in m s
k in
smax
max max
= × + +
( )
≈ ×
(Equation 218)
I
kmaxin
Maximum fault current inside the zone, in primary amps
n Turns ration of the CT
R
in
Internal resistance of the CT in ohms
R
m
Resistance of the longest loop of the CT secondary circuit, in ohms
R
s
Resistance of the stabilized resistor, in ohms
The peak voltage ȗ, which includes the CT saturation, is estimated by using
Equation 219
(given by P. Mathews 1955).
(Equation 219)
U
kn
Knee point voltage of the CT
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
A
V
max
= ⋅ Ω =
12600
240
330 17325
(Equation 220)
˘
.u kV= ⋅ ⋅ −
( )
≈2 2 81 17325 81 3 34
(Equation 221)
As the peak voltage ȗ = 3.2 kV, VDR must be used. In some cases, VDR can be
avoided if R
s
is smaller. The value of R
s
depends on the protection relay operation
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.
Protection functions
1MRS759142 F
814 REX640
Technical Manual
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