Minimum Knee Point Voltage - Toshiba GRE110 Instruction Manual

a lower value of, say, (RCT + R2) = 5Ω is applied, then the practical overcurrent factor may be
increased by a factor of two, that is, to 40A.
In summary, the example given of a 5P20 CT of suitable rated burden will meet most applications
of high fault current and tight grading margins. Many less severe applications may be served by
5P10 or 10P10 transformers.

2.12.2 Minimum Knee Point Voltage

An alternative method of specifying a CT is to calculate the minimum knee point voltage, according
to the secondary current which will flow during fault conditions:
≥ I
V (R + R
k f CT
where:
V = knee point voltage
k
I = maximum secondary fault current
f
R = resistance of CT secondary winding
CT
R = secondary circuit resistance, including lead resistance.
2
When using this method, it should be noted that it is often not necessary to transform the maximum
fault current accurately. The knee point should be chosen with consideration of the settings to be
applied and the likely effect of any saturation on protection performance. Further, care should be
taken when determining R2, as this is dependent on the method used to connect the CTs (E.g.
residual connection, core balanced CT connection, etc).
2.12.3 Sensitive Earth Fault Protection
A core balance CT should be applied, with a minimum knee point calculated as described above.
2.12.4 Restricted Earth Fault Protection
High accuracy CTs should be selected with a knee point voltage Vk chosen according to the
equation:
≥ 2× V
V
k
where Vs is the differential stability voltage setting for the scheme.
)
2
s
 54 
6 F 2 T 0 1 7 2