ABB Relion 615 series - Section 4 Requirements for Measurement Transformers; Current Transformers; Current Transformer Requirements for Overcurrent Protection; Current Transformer Accuracy Class and Accuracy Limit Factor

Section 4 Requirements for measurement
transformers
4.1 Current transformers
4.1.1 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. The
protection settings of the protection relay should be defined in accordance with the CT
performance as well as other factors.
4.1.1.1 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 39: 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 protection relay.
1MAC109016-MB C Section 4
Requirements for measurement transformers
REF615 ANSI 197
Application Manual