Manual MRM3-2 GB
Woodward
TD_MRM3-2_09.08_GB_Rev.New
19
4.4 Requirement on the Main Current Transformers
The CTs chosen have a considerable influence on the accuracy of the protective system. In order
to select the right type of transformer, the requirements and conditions on site have to be consi-
dered carefully.
Type of Transformer
Current transformers have to be designed as protection transformers (P).
Over current Factor
:
To ensure precise operation of the protection unit even under full short-circuit current, the chosen
transformers must not saturate in this current range. This means that the overload factor must be
sufficiently large.
Class
For the nominal range or the lower load range it has to be taken into account that not only the basic
accuracy of the MRM3-2 has to be considered but also the transformer accuracy. This applies es-
pecially for cases where the Holmgreen circuit is used and for low earth fault currents in isolated
networks.
Power Rating
The transformer must be rated sufficiently to cover all measuring instruments and protective devic-
es connected as well as the losses on the transformer measuring line without becoming over-
loaded.
