ABB REF611, Applications Manual

Page: 175 / 188

Section 4 Requirements for measurement
transformers
4.1 Current transformers
4.1.1 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 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 26: 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.
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
1MRS757456 D
Section 4
Requirements for measurement transformers
REF611 169
Application Manual