Protection Relay - Current Display Units

[jokerman]

Help

What units do Voltage and Current Protection Relays display there voltage and current readings in?

Confronted with an SEL 351R I cannot find any information regarding the current display units - is it Peak or RMS Amps?

I am aware of the measurement math to derive the Magnitude and Phase but is the Magnitude (which equates to a peak value) then scaled to give an RMS value?

Does anybody know the answer

Regards, Mike.

 

[DanDel]

Typically, by convention, these values are always in rms units, even if magnitude and angle are available.

 

[dpc]

Values are rms.

 

[buzzp]

Some of the SELs may not show the correct current as far as whether your using a delta or Y connected set (so our relay experts tell me-although the protection schemes use the correct current it will not display delta currents correctly). This is a 351A SEL. Anyone confirm this?

 

[Bung]

The relay can only tell you the currents in the CT circuits connected to it. If you have some "odd" connection of CTs which does not map the phase currents you want to know directly to the CT inputs on the relay, you can't get the right answer. The relays are smart, but they ain't that smart. And this applies to all measuring devices, not just SEL or even just protection relays.



Bung
Life is non-linear...

 

[buzzp]

Well there is a mathematical relationship between delta phase currents and Y phase currents. They assume all are Y connected, so if you have the CT's connected in a delta (common in older set-ups to offset the delta-Y configuration of the transformer), then the unit will not display the correct current, although it will use the correct current in its tripping circuit. It would seem easy enough for them to display the correct current. (remember our relay 'expert' is telling us this; is it true for SEL?)