Ct Requirements For Overcurrent And Earth Fault Protection; Overcurrent Protection Cts; Earth Fault Protection Cts; Ct Requirements For High Impedance Restricted Earth Fault Protection - Siemens Solkor 7PG2113 Technical Manual

4.2 CT Requirements for Overcurrent and Earth Fault
Protection
4.2.1

Overcurrent Protection CTs

The requirements for Current Differential protection are generally more onerous than for overcurrent protection.
If the internal fault level is much greater than the through fault level, the saturation during internal faults must be
considered.
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 overcurrent factor has only a minimal effect. However, this must be
taken into account in establishing the appropriate setting to ensure proper grading.
Knee point voltage does 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
overcurrent 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 overcurrent factor must be raised
accordingly, by increasing the kneepoint voltage requirement.
4.2.2

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 overcurrent accuracy limit factors are therefore already determined and for both these
factors the earth fault protection requirements are normally less onerous than for overcurrent.
4.3 CT Requirements for High Impedance Restricted Earth
Fault Protection
For high impedance REF it is recommended that:
Low reactance CTs to IEC Class PX are used, this allows a sensitive current settin to be applied.
All CT's should, if possible have identical turns ratios.
The knee point voltage of the CTs must be greater than 2 x 64H setting voltage Vs.
Where the REF function is used then this dictates that the other protection functions are also used with class PX
CTs.
A full explanation of how to specify CTs for use with REF protection, and set REF relays is available on our
website: www.siemens.com/energy.
©2010 Siemens Protection Devices Limited
7PG2113/4/5/6 Solkor Applications Guide
Chapter 7 Page 37 of 49