Specifications

7SR242 Duobias Applications Guide

The recommended settings for each method are:

Phase – 0.18 x Id

Cross – 0.20 x Id

Sum – 0.15 x Id

These setting provide a good compromise between speed of operation for internal faults and stability for inrush

current. Generally the above values will be stable for most cases, but in rare cases may not prevent relay

operation for all angles of point on wave switching, and the setting may require being lower slightly. If the relay

operates when the transformer is energised, the waveform record should be examined for signs of fault current

and the levels of harmonic current.

Set to 0.20 x Id unless a very rare false operation for inrush occurs. In which case a lower setting should be

adopted after checking the waveform record for the presence of fault current.

5.2 Overfluxing Detector (81HBL5)

An increase in transformer or decrease in system frequency may result in the transformer becoming over-excited.

The 81HBL5 element can be used to prevent protection operation e.g. prevent differential protection operation

during acceptable over-excitation conditions.