Section 12 Requirements for measurement
transformers
12.1 Current transformers
12.1.1 Current transformer requirements for non-directional
overcurrent protection
For reliable and correct operation of the overcurrent protection, the CT has to be
chosen carefully. The distortion of the secondary current of a saturated CT may
endanger the operation, selectivity, and co-ordination of protection. However,
when the CT is correctly selected, a fast and reliable short circuit protection can be
enabled.
The selection of a CT depends not only on the CT specifications but also on the
network fault current magnitude, desired protection objectives, and the actual CT
burden. The protection settings of the IED should be defined in accordance with
the CT performance as well as other factors.
12.1.1.1 Current transformer accuracy class and accuracy limit factor
The rated accuracy limit factor (F
n
) is the ratio of the rated accuracy limit primary
current to the rated primary current. For example, a protective current transformer
of type 5P10 has the accuracy class 5P and the accuracy limit factor 10. For
protective current transformers, the accuracy class is designed by the highest
permissible percentage composite error at the rated accuracy limit primary current
prescribed for the accuracy class concerned, followed by the letter "P" (meaning
protection).
Table 663: Limits of errors according to IEC 60044-1 for protective current transformers
Accuracy class
Current error at
rated primary
current (%)
Phase displacement at rated primary
current
Composite error at
rated accuracy limit
primary current (%)
minutes centiradians
5P ±1 ±60 ±1.8 5
10P ±3 - - 10
The accuracy classes 5P and 10P are both suitable for non-directional overcurrent
protection. The 5P class provides a better accuracy. This should be noted also if
there are accuracy requirements for the metering functions (current metering,
power metering, and so on) of the IED.
1YHT530004D05 D Section 12
Requirements for measurement transformers
615 series 843
Technical Manual
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