7SG15 MicroTAPP Application Guide
Errors in the controlled voltage will be directly related to errors in the measured voltage and so these should be
minimised by using the most accurate VTs available. As a guide the measuring accuracy of the VTs should be
matched to the tap step resolution as a minimum requirement.
8 Current Transformers
As for a VT, the CT used for measurement of load current need not be related exactly to the transformer full load
rating. The actual value is entered in the ‘system settings/transformer’ menu together with the power transformer
details, from which the control algorithms determine the actual current value.
If the CT is mounted at the circuit breaker the CT current direction will be reversed, the setting menu allows the
current direction to be reversed.
The accuracy of the measuring CTs is not as critical as it is with the measuring VTs. However, metering class
CTs should be used if possible to minimise measuring errors.
©2011 Siemens Protection Devices Limited Chapter 5 Page 13 of 21
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