Differential Relay On A NEMA E2 Starter?

[nightfox1925]

We have a 5kV NEMA class E2 motor starter feeding a 4kV induction motor. The short circuit protection is handled by the fuse while the O/L, Locked Rotor, Negative Sequence, Mech. Jam, Overtemp. protection etc are handled by a GE 469 and is set to trip the contactor.

The 4.16kV system is HRG grounded and hence the ground fault protection (via core balanced CT) is set to send an alamr only. The 87 phase differential element (along with the 50) of the relay is disabled owing to the presence of the fuse and the contactor is not rated to interrupt phase fault currents.

However, the client is insisting on enabling it to trip simply because their standard specifies that if the motor has phase CTs, they shall be used for differential protection. We are technically explaining to them why we are disabling it.

Is there any possibility that this differential trip will work with a fused controller since I don't see any possibility on my end?

 

[JMatt]

You can simply set up your trip equation to block on phase currents that exceed the contactor rating. I don't recall how the logic in a Multilin would read, but a SEL would be:
TR = 87P AND NOT 50P1T # where 87P is asserted for a differential trip but if phase current exceeds 50P1P, the TR equation would be forced to logical 0.

I know there's a way to do this using FlexLogic in the Multilin. This will meet your client's request for differential relaying, but won't melt their contactor either.

 

[slavag]

Hi.
Overcurrent is overcurrent, differntial is differntial.
50/51 and 50N/51N and fuses are only back up protection of 87M function, if you have this option, why not, you can use it. Setting is about 10% of rated current, IMHO, of course.
Best Regards.
Slava

 

[nightfox1925]

Thanks slavag. My concern in using the differential is that I do not have any other switching device to trip this function with aside from the contactor. Since the contactor interrupting capacity is not rated for the available phase fault current, using this element on this feeder is not a possible set-up.

Thanks JMAT for the suggestion. The Multilin 369 does not have FlexLogic Elements which we can easily program. What you are refering is a "supervised" differential protection wherein the 50 element will be permissive to the 87 element via output relay contacts. Since the GE 369 protection elements can be assigned with specific output relays, it is possible to utilize R2 and R3 where 50 =R2 and 87=R3 and R2 will be NC in series with R3. 87 element will have a time delay sufficient to allow 50 to pickup as necessary. 50 element may be set to pickup at overcurrents below 10kA (contactor rating). This would mean that at faults greater than the 50 setpoint (above 10kA), the differential element will pickup but will be blocked by the 50 element. I have attached a simplified sketch:

Even with this set-up, during a phase fault, the fuse could have blown up ahead of the differential trip on the contactor (due to time delay).

On a personal opinion, I prefer disabling the 87 for this application (Fused Starter) unless someone can suggest better options. JMAT's suggestion is one feasible idea.

 

[Handlin]

The differential function of the 469 is for differential protection of the motor windings. This protection can detect motor problems before motor failure. You must have six current transformers for this function (one on each side of each motor winding)

 

[nightfox1925]

Hi Handlin, we do have the CTs except that in our application, we have used 3 x core balanced CTs.