ABB RER620

1MAC309294-MB F Section 11
Requirements for measurement transformers
RER620 549
Technical Manual
Section 11 Requirements for measurement
transformers
11.1 Current transformers
11.1.1 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 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.
11.1.1.1 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 427: Limits of errors according to IEC 60044-1 for protective current transformers
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 relay.
The CT accuracy primary limit current describes the highest fault current magnitude at
which the CT fulfils the specified accuracy. Beyond this level, the secondary current of the
CT is distorted and it might have severe effects on the performance of the protection relay.
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