Transformer Differential Protection (87) 1

[sky2020]

Should the transformer differential protection (87) be responsible on detecting single phase to ground faults inside the transformer? If there is a SLG on transformer secondary, should 87 detect it?

 

[RRaghunath]

Transformer differential protection can detect SLG faults.
Only thing is that the zero sequence filtering that is adopted in the protection settings desensitises the protection to some extent (makes it less sensitive in other words) against SLG faults.

 

[davidbeach]

Need to back up to the "It Depends" stage of answering the question. What's the transformer? What are the "primary" and "secondary" winding configurations? (I like high-side and low-side much better than primary and secondary, they don't change with application.) Where in the winding is the fault?

Assuming that the "secondary" is a solidly grounded wye, most SLG faults will be detected by a phase 87 element. More can be detected by a negative sequence 87 element (called 87Q sometimes). But the closer you get to the neutral the harder it is for a phase 87 to detect. If the transformer is high impedance grounded, or capacitively grounded (aka ungrounded) then a SLG may not be picked up by the 87.

I’ll see your silver lining and raise you two black clouds. - Protection Operations

 

[jghrist]

Many new microprocessor differential relays have a Restricted Earth Fault (REF) function that can detect faults close to the neutral.

 

[davidbeach]

That’s the theory anyway. Just found out the hard way just how susceptible REF can be to CT saturation on external L-L faults with a bit of neutral current. There is certainly a class of fault for which REF is the most definitive protection element, but to date I’ve seen more REF mistrips than correct operations. A bit jaded at the moment. But I must not be the only one, a major relay supplier recently revamped the REF in their advanced transformer relay. Firmware revision time…

I’ll see your silver lining and raise you two black clouds. - Protection Operations

 

[jghrist]

David,

That's interesting. It would be a good subject for a technical paper. I haven't seen any on the subject.

 

[Palletjack]

That’s the theory anyway. Just found out the hard way just how susceptible REF can be to CT saturation on external L-L faults with a bit of neutral current. There is certainly a class of fault for which REF is the most definitive protection element, but to date I’ve seen more REF mistrips than correct operations. A bit jaded at the moment. But I must not be the only one, a major relay supplier recently revamped the REF in their advanced transformer relay. Firmware revision time…

Maybe a dumb question, but isn’t that what the stabilizing resistor is supposed to stop misoperations due to CT saturation??

 

[davidbeach]

Low impedance REF, one of many functions of a transformer relay all using the same set of CT inputs. No way to include a resistor.

I’ll see your silver lining and raise you two black clouds. - Protection Operations