4.3.2.3 STABILISING RESISTOR CALCULATION
Assuming that a setting of 0.1A is selected the value of the stabilizing resistor, R
ST
, requir
ed is
R
ST
= V
s
/ (IREF> Is1 (HV)) = 45.5 / 0.1 = 455 ohms
To achieve an average operating time of 40 ms, Vk/Vs should be 3.5.
The Kneepoint voltage is:
V
K
= 4V
s
= 4 x 45.5 = 182 V.
If the actual V
K
is greater than 4 times V
s
, then the K factor increases. In this case, V
s
should be recalculated.
Note:
K can reach a maximum value of approximately 1.
4.3.2.4 CURRENT TRANSFORMER CALCULATION
The effective primary operating current setting is:
I
P
= N(I
s
+ nI
e
)
By r
e-arranging this equation, you can calculate the excitation current for each of the current transformers at the
stability voltage. This turns out to be:
I
e
= (0.3 - 0.1) / 4 = 0.05 A
In summary, the current transformers used for this application must have a kneepoint voltage of 182 V or higher
(note that maximum Vk/Vs that may be considered is 16 and the maximum K factor is 1), with a secondary winding
resistance of 0.5 ohms or lower and a magnetizing current at 45.5 V of less than 0.05 A.
Assuming a CT kneepoint voltage of 200 V, the peak voltage can be estimated as:
V
P
= 2
Ö
2V
K
(V
F
-V
K
) = 2
Ö
2(200)(9004-200) = 3753 V
This value is above the peak voltage of 3000 V and therefore a non-linear resistor is required.
Note:
The kneepoint voltage value used in the above formula should be the actual voltage obtained from the CT magnetizing
characteristic and not a calculated value.
Note:
One stabilizing resistor, Alstom part No. ZB9016 756, and one varistor, Alstom part No. 600A/S1/S256 might be used.
Chapter 7 - Restricted Earth Fault Protection P14x
180 P14xEd1-TM-EN-1
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