Instantaneous Differential Relay - Why do you need it?

[stretched2thin]

If you have a differential relay (GE Multilin 745) that has time/overcurrent elements, then why do you need the instantaneous differential element? Wouldn't the overcurrent instantaneous element perform the same function?

 

[DanDel]

The Inst Differential device can be safely set to a much lower value than the O/C Inst device, since differential currents should be much less than any currents seen by an O/C device.

 

[gordonl]

The main benefits to this more sensitive protection given in previous post is:
1)Reduced damage due to lower currents and likely less time. (Faults generally take time to develop to reach the IOC level)
2)Less bump on the system due to lower amplitude faster clearing
Point 1 is the the major justification.

 

[stretched2thin]

Thanks for you help.

 

[busbar]

Simple overcurrent functions can falsely operate for inrush and through faults. Lead time for power transformer repairs or replacement is often quite pricey, {to say nothing about consequent lost production} so differential protection can economically limit damage.

 

[Bung]

The key point here is "through faults". Differential protection is a unit protection, ie it protects a specific unit, or has a defined, closed zone of protection essentially between the sets of CTs. Overcurrent protection has an open ended zone of discrimination - any fault anywhere downstream of the CT can potentially cause a trip. You cannot say that overcurrent protection will operate for faults between point X and point Y, only that it will operate for faults beyuond point X. Differential operates only for faults between X and Y.


Bung
Life is non-linear...

 

[hsj]

Differential protection is selective (it operates only if te fault is within its zone) and therefore is set to operate faster(that means no need to delay). Therefore the fault is cleared quickly and rest of the system can continue to operate normal. Normal o/C and e/f protection is time delayed because it is not selective. Therefore a system suffers longer the effect of a fault.

 

[OldPfart]

I do a lot of generator work and differential protection is usually specified by the engineer. It is usually preferred to earth fault protection, in this case, as earth fault protection is notorious for false tripping.