ABB RELION 620 Series Technical Manual page 602

Section 4
Protection functions
596
The formulas are based on choosing the CTs according to
106 which results an absolute stable scheme. In some cases, it is
possible to achieve stability with knee point voltages lower than the
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
111.
I
k max in
U R R
(
= ⋅ +
max in
n
GUID-1367D437-AC4D-4323-AFC9-114CE2472CFF 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
112 (given by P. Mathews 1955).
˘
u U U U
2 2 (
= −
kn max kn
GUID-C1127EC9-FB6D-4898-BC03-CCABAA62344C 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.
≥ 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
)
+ ≈ ⋅
m s s
n
)
1MRS757644 F
, the relay operation can be slightly
Technical Manual
Equation
Equation
(Equation 111)
Equation
(Equation 112)
620 series