Fairly common questions are:

which side to star the CT?
which side to have the polarity?
are there any consequences or implications?

The answer, as always, depends on the application, and then to some extent on the capabilities and settings of the IEDs!

Non-directional Protection

If the protection elements "50" or "51" elements (IEEE C37-2008 are purely non-directional, they only operate on the magnitude of 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.

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.

As most modern directional-based relays have polarity settings. then as with non-directional relays, the polarity of the CT is arguably just a matter for company convention.

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.

P2 towards source

P1 towards source

CT
Star
source
side

CT P1 is connected to relay R1

Note direction of current through the relay

Phase relay: R2c to R1c

Earth Fault relay: R1n to R2n

CT P2 is connected to relay R2

Note direction of current through the relay

Phase relay: R1c to R2c

Earth Fault relay: R2n to R1n

This is opposite #1 as both CT polarity sense as well as relay star has swapped

CT
Star
source
side

CT P1 is connected to relay R2

Note direction of current through the relay

Phase relay: R1c to R2c

Earth Fault relay: R2n to R1n

This is opposite #1 as only relay star side has changed

CT P2 is connected to relay R1

Note direction of current through the relay

Phase relay: R2c to R1c

Earth Fault relay: R1n to R2n

This is opposite #5, but the same as #1 as you can see the wiring arrangement is identical

CT

Star

load

side

CT P2 is connected to relay R1

Note direction of current through the relay

Phase relay: R1c to R2c

Earth Fault relay: R2n to R1n

This is opposite #1

CT P1 is connected to relay R2

Note direction of current through the relay

Phase relay: R2c to R1c

Earth Fault relay: R1n to R2n

This is same as #1

CT

Star

load

side

CT P2 is connected to relay R2

Note direction of current through the relay

Phase relay: R2c to R1c

Earth Fault relay: R1n to R2n

This is same as #1

CT P2 is connected to relay R1

Note direction of current through the relay

Phase relay: R1c to R2c

Earth Fault relay: R2n to R1n

This is opposite #1

Clearly star side is irrelevant as #1, #4, #6, and #7 are identical,
whilst #2, #3, #5, #8 are identical but opposite the others.

It is critical however to match polarity of CT and relay connections ... noting many relays now allow you to "swap" the polarity sense of the relay by settings.

Differential Applications

This is arguably a little more complicated in the "it depends" consideration

Merz-Price Circulating Current High Impedance Differential
Low Impedance Differential

Current Transformer: Star side and Polarity