Star-Star Differential Protection

[Power0020]

15 MVA 33/11 kV YNyn0 (solidly grounded) transformer to be protected. I propose the following:

1- REF HV
2- REF LV
3- HV OC/EF
4- LV OV/EF
5- Buchholz
6- Pressure Relief.

I think these are enough, nevertheless, I wonder if I need a full differential protection on the transformer? I think that restricted earth fault on HV & LV are enough and no need for additional protection.

any clue? anything I missed?

 

[cranky108]

My last company would use fuses for this. My present company would use a full transformer and wrap the switchgear for arc-flash.

 

[jghrist]

I would use full differential protection. What if there is a turn-to-turn failure without involving earth?

 

[cranky108]

Protection is like insurance. How much can you justify?

 

[EEfromOU]

If you already have REF elements on the HV & LV sides then your relay should be able to do phase differential too or at the least it has the CTs available and wired assuming it is one relay and not split between two. I do not see a reason not to omit it unless you would have to add another relay which in that case I might look at a different relay that can do all of the elements in one unit as an option.

 

[rajen1967]

As other mentioned you have missed the differential protection. Must do it.

 

[davidbeach]

With a really good differential relay you can possibly do without everything else on the list, but none of what you've listed can come close to what a decent diff relay will do for a broad range of fault types.

 

[Power0020]

thanks everyone, I just thought that the small size trafo doesn't justify a full diff. protection, but as you mentioned, one relay can do all that "7UT612 just jumped to my mind" with no additional CTs.

I have read that it is common to provide diff. protection for transformers greater than 5 MVA and some say it is crucial at 10+ MVA. Well, but I have seen some applications at 15 MVA without any. As cranky mentioned, it is like insurance.

 

[ZeroSeq]

Buy a SEL-487E or a GE-T60, everything is covered.

 

[Power0020]

Yep, I think they will go for one of that.

I have read somewhere that it is sometimes advisable to have separate relays for reliability issues. I see no much gain here as at the end you have the same CT so any failure in it will lead to same result with any number of relays!. The only gain will be to avoid issues if the one relay failed...

 

[DTR2011]

For a few extra dollars, a simple separate 3 Ph O/C relay could be added as a back up. I see SEL 501 (Dual O/C Relay) used for this often. Probably a slew of other manufacturers that make similar. Wired to the 86T (Lockout), in addition to SPR, etc, can allow for primary relay maintenance / modifications in the future, with the transformer in service. Beats fuses, seen in similar applications.

You may not need it now but also consider adding trip coil monitoring to HV/LV breakers and lockout relay (if used). The SEL and GE relays mentioned previously have sufficient inputs to these and other quantities. If someone wants SCADA in the future, you are ready for it.

 

[davidbeach]

I don't get the continued use of the "one good protection, one crummy protection" philosophy. It sort of made sense when things like differential relays were much more expensive than overcurrent relays, but now it just doesn't seem to make sense. For less than the cost of an SEL-487E and an SEL-501 you could get a pair of SEl-787 relays and have fully duplicate/redundant protection. In addition to the workhorse differential you'd get items 1-4 at no additional cost. The negative sequence differential could allow elimination of the sudden pressure relay (the Buchholz listed probably doesn't exist on a transformer this small). The only possible glitch is that the REF would be for the whole transformer, not the two windings individually. I'm sure there are other manufacturers that offer products to compete with the SEL-787.

 

[ZeroSeq]

Agreed with David, the 787 does everything that you need and is much cheaper than the 487E. You could go with two different manufacturers just to be *absolutely* sure you have redundancy if there are issues with the current firmware versions one family of relays are running on.

 

[slavag]

15MVA is nor small size of transformer.
Today, newer protective terminals is "zero" price.
Put full redundancy scheme with full functionality.

 

[krisys]

The protection for the transformer is many times dictated by the application of the transformer such as is it for a critical industrial process plant, oil and gas application, a municipality or for the power distribution to the remote villages.

It also depends on the system operating configuration such as down stream and upstream supply system configuration (bus tie-open/close etc.), how important to save the network itself, is it a distribution transformer or a generator transformer. Hence a generic solution may not be appropriate.