Differential current protection 2

[Freshsurfer]

Hi, I´d like to know about the use of trafo´s for differential current protection, I´m working with a 6600 HP @ 6000KV AC 3 phase-60hz ring motor and it have one 100:5A trafo in each power supply line as a differential current protection, my question is, why I have to use this 100:5A trafo since the nominal current is arround 600A?

many thanks!

 

[electricpete]

Are you asking why a ct is used as input to protective relays? Or why that ratio was selected?

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[Freshsurfer]

Exactly my question is about the reason to use a 100:5A trafo, and the conditions to actuate the protection

 

[aolalde]

For differential protection the current getting into the motor stator is compared with the current exiting the stator. The very same Current Transformer (CT) will have the in and out lines of each phase (E.g. Motor lines #1 and #4). The result is zero current under normal condition. That is why your CT is rated 100 Amps and ratio 100:5 in spite of 600+ Amperes at full load. When an internal failure is developed into the stator windings, the currents in and out are different and the total current in the CT increases tripping the power supply.

 

[busbar]

Good info by aolade… A couple of side notes are, 1) for a window CT, it’s important to keep the stator-coil external leads {1+4} routed symmetrically through the per-phase cores so currents cancel in normal operation. [100:5 ratio provides significantly more current sensitivity than a pair of 600:5 CTs.]

Note though, that differential-mode {87M} CTs are worthless for independently sensing motor running-overload [51/49] current, so can’t be applied in “all-in-one” motor protection.