Siemens 7SR242 Duobias, Technical Manual

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©2015 Siemens Protection Devices Limited Chapter 7 Page 44 of 64
87 Inrush Setting (0.1 to 0.5 x Id)
This defines the levels of inrush used in each of the above methods.
The setting applied will determine the level of even harmonic (second and fourth) content in the relay operating
current that will cause operation of the relay to be inhibited. The lowest setting of 0.1 x Id therefore represents the
setting that provides the most stability under magnetising inrush conditions.
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.