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The relay itself only operates the protection elements "50" or "51" elements (IEEE C37-2008 are purely non-directional, they only operate on the magnitude of the current so it does not care about which side of the CT is the star or which side is the polarity terminal.

Specifically the relay does not care about the direction of current flow through its elements so all four scenarios operate the same.

Directional

current so at least in that respect, polarity and star are immaterial.

The two independent choices star side and polarity sides is only a question of standardisation in order to be consistent across the various locations simply to ensure there is no confusion for the technicians.

Directional Current, Distance and Power- based Applications

However, if the "50" or "51" elements (IEEE C37-2008) are now directional as "67" devices, or perhaps a distance function "21", reverse/over power function "32", or any other power related function including metering etc, the direction of power flow becomes significant.

Scenario 1 and 4 are the same as each other, whilst scenarios 2 and 3 are also the same as each other, but 180 degrees opposite 1 and 4.

If the IED allows changing of the polarity seen by the IED as part of its available settings, the direction can be set to suit the application as part of commissioning.

Consequently the star side and As most modern directional-based relays have polarity settings. then as with non-directional relays, the polarity of the CT is arguably insignificant in most modern applications and becomes just a matter for company convention in order to be consistent across the various locations simply to ensure there is no confusion for the technicians..

If we consider directional overcurrent and earth fault relays connected to CTs in a Holmgren connection, there are eight possible combinations requiring consideration of the protection element forward/reverse setting.

This can be shown if we consider an earth fault "downstream" of the CT point and the direction of current flow in the relay earth fault element that will cause operation.

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A fairly common question is Fairly common questions are: