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A fairly common question is which side to star the CT or which side to have the polarity and if there are any consequences?
The answer, as always, depends on the application, and then to some extent on the capabilities and settings of the IEDs!
Non-directional Current Applications
If we consider "general" overcurrent and earth fault relays connected to CTs in a Holmgren connection, there are four possible combinations.
| Polarity towards load | Polarity towards source | |
|---|---|---|
| Star source side |
|
|
Star load side |
|
|
The relay itself only operates 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, 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 the polarity of the CT is arguably insignificant in most modern applications and becomes a matter for convention in order to be consistent across the various locations simply to ensure there is no confusion for the technicians.
Differential Applications
This is arguably a little more complicated in the "it depends" consideration
Merz-Price Circulating Current
Holmgren or Low Impedance Connection
- Protection Systems Engineering
- IEC 61850 Engineering
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