Siemens 7SR242 Duobias, Technical Manual

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©2015 Siemens Protection Devices Limited Chapter 7 Page 10 of 64
2.1.1 ICT Settings for Current Magnitude Balance
Internal current multipliers are used to adjust the CT secondary currents to accommodate for any mismatch
between the winding 1 and winding 2 CT ratios.
For load or through fault conditions the output of the ICT Multiplier for each winding must be equal,
notwithstanding variations in the OLTC position. Where possible the output of each ICT Multiplier is set to 1A at
transformer full load rating when the transformer OLTC is on its mid-tap position. At mid-tap a balanced relay
should have virtually no differential currents, the bias currents will vary with the load level.
Balancing ICT Multiplier outputs (ICT
OUT
) to 1A at transformer rating ensures that the relay operates at the levels
indicated by its differential protection settings. However achieving balance at ICT
OUT
= 1 is not always possible;
here the effects on settings must be taken into account. The effect of applying ICT
OUT
< 1 is to de-sensitise current
dependent differential settings, applying ICT
OUT
> 1 makes the effected elements more sensitive. To compensate
for the resultant ICT
OUT
value the following settings must be multiplied by ICT
OUT
:
87HS Setting
87BD Initial
87BD 1
st
Bias Slope Limit
See examples in sections 0 and 2.1.6.
2.1.2 ICT Settings for Vector Group Correction
Internal interposing current transformers are used to correct the CT secondary current phase relationships in line
with any phase change introduced by the transformer connections.
As a general rule, the phase angle ICT Connection setting to correct the phase angle difference is applied to the
star side winding. A table showing the settings to apply for all standard transformer vector groups is included on
the following page. The table assumes that all line CTs are ‘star’ connected.
Note that the choice of interposing CT vector group will modify the effective operating levels of the protection due
to the current distribution for the various fault conditions – the effects on settings must be taken into account - see
section 2.1.5.1.
Settings examples included in section 6 cover selected non-standard connection arrangements e.g. where the
primary connections within the protected zone are crossed.