ABB RELION 620 Series Technical Manual page 885

1MAC504801-IB E
Section 13
13.1
13.1.1
13.1.1.1
620 series ANSI
Technical Manual
Requirements for measurement transformers
Requirements for measurement
transformers
Current transformers
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 protection relay should be defined in accordance with the CT
performance as well as other factors.
Appropriate 'C' class CT should be used based on the total resistances of the CT secondary
circuit. If other accuracy class CTs are used then refer the following discussions.
Current transformer accuracy class and accuracy limit factor
The rated accuracy limit factor (F
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 617: Limits of errors according to IEC 60044-1 for protective current transformers
Accuracy class Current error at
rated primary
current (%)
5P ±1
10P ±3
) is the ratio of the rated accuracy limit primary current
n
Phase displacement at rated primary
current
minutes centiradians
±60 ±1.8
- -
Section 13
Composite error at
rated accuracy limit
primary current (%)
5
10
879