Page 281 - Practical Power System and Protective Relays Commissioning

Protection Relays Chapter | 18 275

FIGURE 18.12.4 Auto-transformer differential protection.

will be minimal due to the high CT ratios that the protection requires. If
more sensitive earth fault protection is required for the delta winding, a
separate earth should be used, together with a suitable CT and relay. The
protection is also stable under over fluxing conditions, and magnetizing
inrush. If CTs are not available on the neutral, a biased differential relay
can be used in the conventional way. The line CTs should be connected in
delta (as for the star/star transformer). The biased differential relay will
give a measure of interturn protection and will also detect delta tertiary
faults.

18.12.6.4 Earthing Transformer Protection

If the earthing transformer is inside the zone of differential protection then
no separate protection is required. Overcurrent protection can be provided by
three overcurrent units connected to delta-connected main CTs, as shown in
Fig. 18.12.5.
The protection will not operate on zero-sequence currents due to system
earth faults because of the CT delta connection.

18.12.7 ZERO-SEQUENCE CIRCUITS OF TRANSFORMERS

18.12.7.1 Two-Winding Transformers
Consider, for example, a two-winding transformers delta/star connection, as
shown in Fig. 18.12.6.
The zero-sequence currents will be there in the star connection side of
the transformer but will not be there in line with the connection of the delta
side (primary side) of the transformer.