Section 3: Ct Requirements; Ct Requirement For Differential Protection - Siemens 7SR242 Duobias Technical Manual

Section 3: CT Requirements

The specification of CTs must meet the requirements of all protection functions utilised e.g. overall differential,
REF and backup over current protections.
The relay has 1A and 5A rated terminals for each CT input and any combination of these may be used. 1A rated
CTs can be used on one winding and together with 5A rated CTs on the other.

3.1 CT Requirement for Differential Protection

The quality of CTs will affect the performance of the protection system. The CT knee-point voltage (V
in assessing protection performance.
If a high level internal short circuit occurs the dc offset in the primary fault current may produce transient CT
saturation. This is more likely to occur if the CT knee-point is low and/or the connected burden is high. Saturated
CTs produce high levels of even harmonics which may increase the operate time of the biased differential
function where harmonic restraint or inhibit is applied.
A highset differential element (87HS) can be used without harmonic restraint this can reduce the overall operating
time of the differential protection.
Restricted Earth Fault protection helps ensure fast tripping as its speed of operation is not affected significantly by
CT saturation.
For high speed operation:
Vk 4 x I x (R
FS
Where:
Vk = CT knee point voltage.
I = Max. secondary 3-phase through fault current (as limited by the transformer impedance).
FS
R = Secondary winding resistance of each star connected CT.
CT
R = The CT secondary loop lead resistance.
L
Where the CTs used have a lower knee point voltage e.g. half that calculated in the expression above the biased
differential elements may have a slightly longer operate time. To ensure stability during through fault conditions
the biased differential settings should be increased by 10%.
Advice on CT Selection.
1A rated CT secondaries are preferred to 5A CTs as the CT VA burden is reduced by a factor of 25.
Line current transformer ratios should be selected to match the main transformer rating and ratio. However the
ICT Multiplier adjustment can be used to compensate for non matched ratios. Choose a CT ratio that produces
at least 0.33 x nominal secondary rating, when based on the transformer is at nameplate rating i.e. within the
range of the ICT Multiplier setting.
Where long secondary lead lengths are required measures can be taken to reduce the burden imposed on the
CTs:
Use high CT ratios to reduce the secondary current. Compensate the low secondary current using the
ICT Multipliers..
Parallel CT secondary cable cores to reduce lead resistance.
©2015 Siemens Protection Devices Limited
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Chapter 7 Page 39 of 64