Section 3: Ct Requirements; Ct Requirements For Differential, Over-Current And Earth-Fault Protection; Over-Current Protection Cts; Earth-Fault Protection Cts - Siemens 7SR18 Solkor Manual

Section 3: CT Requirements

3.1 CT Requirements for Differential, Over-current and Earth-
fault Protection
3.1.1

Over-current Protection CTs

a) For industrial systems with relatively low fault current and no onerous grading requirements - a class
10P10 with suitable VA rating greater than the relay CT burden.
b) For utility distribution networks with relatively high fault current and several grading stages - a class
5P20, with suitable VA rating greater than the relay CT burden.
Note: if an accuracy limit factor is chosen which is much lower than the maximum fault current it will be necessary
to consider any effect on the protection system performance and accuracy e.g. grading margins.
For idmtl applications, because the operating time at high fault current is a definite minimum value, partial
saturation of the CT at values beyond the over-current factor has only a minimal effect. However, this must be
taken into account in establishing the appropriate setting to ensure proper grading.
c) For dtl applications utilities as for (b) above - a class 5P10 (or 20), with rated burden to suit the load.
Note: Over-current factors do not need to be high for definite time protection because once the setting is
exceeded magnitude accuracy is not important. Often, however, there is also the need to consider instantaneous
HighSet over-current protection as part of the same protection system and the settings would normally be of the
order of 10x the CT rating or higher. Where higher settings are to be used then the over-current factor must be
raised accordingly, e.g. to P20.
3.1.2

Earth-fault Protection CTs

Considerations and requirements for earth-fault protection are the same as for Phase fault. Usually the relay
employs the same CT's e.g. three-phase CTs star connected to derive the residual earth-fault current.
The accuracy class and over-current accuracy limit factors are therefore already determined and for both these
factors the earth-fault protection requirements are normally less onerous than for over-current.
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7SR18 Applications Guide
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