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2.1.8
CT Requirements
2.1.8.1 Phase Fault and Earth Fault Protection
Protection class current transformers are normally specified in the form shown below. The CT
transforms primary current within the specified accuracy limit, for primary current up to the
overcurrent factor, when connected to a secondary circuit of the given burden.
5 P 20 : 10VA
Accuracy
Limit (%)
Accuracy limit : Typically 5 or 10%. In applications where current grading is to be applied and
small grading steps are desirable, then a 5% CT can assist in achieving the necessary accuracy. In
less onerous applications, a limit of 10% may be acceptable.
Overcurrent factor : The multiple of the CT rating up to which the accuracy limit is claimed,
typically 10 or 20 times. A value of 20 should be specified where maximum fault current is high
and accurate inverse time grading is required. In applications where fault current is relatively low,
or where inverse time grading is not used, then an overcurrent factor of 10 may be adequate.
Maximum burden : The total burden calculated at rated secondary current of all equipment
connected to the CT secondary, including relay input burden, lead burden, and taking the CT's
own secondary resistance into account. GRD150 has an extremely low AC current burden,
typically less than 0.1VA for a 1A phase input, allowing relatively low burden CTs to be applied.
Relay burden does not vary with settings.
If a burden lower than the maximum specified is connected, then the practical overcurrent factor
may be scaled accordingly. For the example given above, at a rated current of 1A, the maximum
value of CT secondary resistance plus secondary circuit resistance (RCT + R2) should be 10Ω. If
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.
2.1.8.2
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,
Overcurrent
Maximum Burden
Factor
(at rated current)
)
2
⎯
⎯
72
6 F 2 S 0 8 4 2
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