Setting of Differential Protection of Motor

[SELEC]

Hi, Guys,

Could you direct me to the reference of settings of motor differential protection?

Thanks.

 

[HCBFlash]

Here's a couple;

text -

http://www.wiley.com/WileyCDA/WileyTitle/productCd-3895782343.html

online -

http://www.findarticles.com/p/articles/mi_qa3726/is_200504/ai_n13602099#continue

There is no standard formula or chart like lays out diff amps per phase per horsepower. (That I've ever found, or been told of)

That said, what size, voltage and winding arrangement are you working with in your present application? I'd approach it as a % of full load current.


Last I used one, I used a Square-D unit, and set it according to the literature enclosed with the unit. The general idea was "as low as you can get away with". Since the control panel was for use of a 3rd party I had no feedback as to what settings were settled on,or if there was any value at which nuisance tripping occurred.


.

remember: An opinion is only as good as the one who gives it!

 

[SELEC]

I just went throgh IEEE C37.96-1988 4.2.17. it says that 10% slope for percentage-differential relay setting, and the pick-up setting of the relay should be 10%-20% of motor FLA (for ordinary overcurrent relays in a diiferential circuit to provide differential protection to large motors). For flux-balancing differential schemes, relays should be set in accordance with instructions for seeting device 50G.

My understanding is, the latter scheme uses non-percentage differentila relay. Is that right?

 

[dpc]

The flux balancing scheme isn't even a differential relay, really. They just run all six leads through a CT and then to an overcurrent relay - often just an instantaneous element. You may need some time delay to avoid nuisance tripping, but not much.

Determination of the settings for motor differential is just a question of setting it as low as possible while avoiding nuisance tripping on inrush.

 

[davidbeach]

Flux balancing through 3 CTs, each connected to one phase of a three-phase relay is an even better means of fast motor protection.