ABB RELION REX640 Technical Manual page 735

1MRS759142 C
REX640
Technical Manual
voltage stated in the formulas. However, the network conditions have
to be known well enough to ensure the stability.
• If U
• If U
prolonged and should be studied case by case.
• If U
k
to be chosen.
The need for voltage dependent resistor (VDR) depends on the insulation level for
which the protection relays are tested.
Voltage U , ignoring the CT saturation during the fault is calculated using
max
172.
I
k max in
U R R
(
= ⋅ +
max in m
n
GUID-88A31663-F3F3-44AD-AA83-B1292B57253B V1 EN
I Maximum fault current inside the zone, in primary amperes
kmaxin
n Turns ratio of CT
R Internal resistance of CT in ohms
in
R Resistance of the longest loop of the CT secondary circuit in ohms
m
R Resistance of the stabilized resistor in ohms
s
The peak voltage ȗ, which includes the CT saturation, is estimated by using
173 (given by P. Mathews 1955).
˘
u 2 2 U U U
(
= ⋅ ⋅ −
kn max kn
GUID-6FA85D81-C49E-4B02-AAF6-07997B010A16 V1 EN
U Knee point voltage of CT
kn
The VDR is recommended when the peak voltage ȗ ≥ 2 kV. This the insulation level
for which protection relays are 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.
A
12600
U
330
= ⋅ Ω =
max
240
GUID-AFDF1190-84A2-4BF6-A75B-9EA5181D6582 V1 EN
˘
u 2 2 81 17325 81
(
= ⋅ ⋅ −
GUID-618CF8E6-DC8D-4359-B1E8-F9FAE9F7BE84 V1 EN
≥ 2 · U , the fast relay operation is secure.
k s
≥ 1.5 · U and < 2 · U
k s s
<1.5 · U , the relay operation is jeopardized. Another CT has
s
I
k max in
R R
)
+ ≈ ⋅
s s
n
)
V
17325
3 34 . kV
)
≈
Section 4
Protection functions
, the relay operation can be slightly
Equation
(Equation 172)
Equation
(Equation 173)
(Equation 174)
(Equation 175)
729