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GE P642 User Manual

GE P642
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In summary, the current transformers used for this application must have a kneepoint voltage of 182 V or higher
(note that maximum Vk/V
s 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 r
esistor, Alstom part No. ZB9016 756, and one varistor, Alstom part No. 600A/S1/S256 might be used.
5.3.3 USE OF METROSIL NON-LINEAR RESISTORS
Current transformers can develop high peak voltages under internal fault conditions. Metrosils are used to limit
these peak v
oltages to a value below the maximum withstand voltage (usually 3 kV).
You can use the following formulae to estimate the peak transient voltage that could be produced for an internal
fault. The peak voltage produced during an internal fault is a function of the current transformer kneepoint voltage
and the prospective voltage that would be produced for an internal fault if current transformer saturation did not
occur.
Vp = 2
Ö
(2VK(V
F
-V
K
))
Vf = I'f(R
CT
+2
RL
+R
ST
)
where:
Vp
= Peak voltage developed by the CT under internal fault conditions
Vk
= Current transformer kneepoint voltage
Vf = Maximum voltage that would be produced if CT saturation did not occur
I'f = Maximum internal secondary fault current
R
CT
= Current transformer secondary winding resistance
R
L
= Maximum lead burden from current transformer to relay
R
ST
= Relay stabilising resistor
You should always use Metrosils when the calculated values are greater than 3000 V. Metrosils are connected
across the circuit to shunt the secondary current output of the current transformer from the device to prevent very
high secondary voltages.
Metrosils are externally mounted and take the form of annular discs. Their operating characteristics follow the
expression:
V = CI
0.25
where:
V = Instantaneous voltage applied to the Metrosil
C = Constant of the Metrosil
I = Instantaneous current through the Metrosil
P64x Chapter 8 - Restricted Earth Fault Protection
P64x-TM-EN-1.3 187

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GE P642 Specifications

General IconGeneral
BrandGE
ModelP642
CategoryRelays
LanguageEnglish

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