ABB RELION 611 Series Technical Manual page 497

1MRS757454 D
Section 10
10.1
10.1.1
10.1.1.1
611 series
Technical Manual
Requirements for measurement transformers
Requirements for measurement
transformers
Current transformers
Current transformer requirements for 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.
Current transformer accuracy class and accuracy limit factor
The rated accuracy limit factor (F
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 481: Limits of errors according to IEC 60044-1 for protective current transformers
Accuracy class Current error at
rated primary
current (%)
5P ±1
10P ±3
The accuracy classes 5P and 10P are both suitable for non-directional overcurrent
protection. The 5P class provides a better accuracy.
The CT accuracy primary limit current describes the highest fault current magnitude
at which the CT fulfils the specified accuracy.
In practise, the actual accuracy limit factor (F
factor (F ) and is proportional to the ratio of the rated CT burden and the actual CT
n
burden.
) is the ratio of the rated accuracy limit primary
n
Phase displacement at rated primary
current
minutes centiradians
±60 ±1.8
- -
) differs from the rated accuracy limit
a
Section 10
Composite error at
rated accuracy limit
primary current (%)
5
10
491