ABB RELION REF615R, Техническое руководство

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Section 13 Requirements for measurement
transformers
13.1 Current transformers
13.1.1 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.
For all ANSI applications, details are available in C37.110-2007, IEEE Guide for the
Application of Current Transformers Used for Protective Relaying Purposes with all other
reference documents listed therein. The application manual of REF615R provides a brief
introduction on CT requirements.
The following section provides basic inputs on CT requirements when CTs with IEC
standards are specified.
13.1.1.1 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 443: 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
1MRS240050-IB C
Section 13
Requirements for measurement transformers
REF615R 621
Technical Manual