Note:
Class x C
T
s should be used for high impedance REF applications.
2.7 HIGH IMPEDANCE BUSBAR PROTECTION
The high impedance bus bar protection element will maintain stability for through faults and operate for internal
faults. You should select Vk/Vs based on the X/R of the system. The equation is:
V
s
=K*I
f
*(R
CT
+R
L
)
For X/R <= 40
V
k
/V
s
>= 2
Typical operating time = 25 ms
For X/R > 40
V
k
/V
s
>=4
Typical operating time = 30 ms
Note:
K is a constant affected by the dynamic response of the device. K is always equal to 1.
2.8 USE OF METROSIL NON-LINEAR RESISTORS
Current transformers can develop high peak voltages under internal fault conditions. Metrosils are used to limit
these peak voltages to a v
alue 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.
P14x Chapter 9 - Current Transformer Requirements
P14xEd1-TM-EN-1 203
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