Serving distribution with tertiary winding 1

[tlrols]

We are looking at using our tertiary windings to serve our 13.8 kV dsitribution system. Presently our tertiary's hardly do anything--they serve the station load and provide protection with their polarizing currents, etc.

Typically we use two 250 MVA 230/115 kV auto-transformers with the tertiary rating of 30 MVA and up.

I know we will need a grounding transformer, although I admit I don't have a good understanding for the need unless it is related to a grounded delta placing full line to line voltages on the distribution as measured line to ground.

Anyway, looking for some thoughts on this--I searched here and read up on this topic from its 2005 posting.

What I am really interested in is practical experience and perhaps a one-line drawing or two that show a system being fed by a tertiary winding.

We would like to add some distribution to existing 230/115 kV stations with minimal expense...is using the teritary a good idea or is it a waste of time?

Thanks!

 

[davidbeach]

Why would you want to place your bulk power system at that much risk? Normally a distribution fault doesn't show up on the transmission system, what about a fault on your tertiary? Tempting, sure, but there is a reason that it is rarely done. If it made economic sense it would happen all the time, utilities try to get the most benefit from the installed plant, but tertiaries as distribution sources don't meet the test.

 

[tlrols]

Makes sense...I had thought that might be the case given that faults on the distribution system tend not to be as clear as faults on the transmission. So, one could assume that the transformer differential protection would act before any distribtuion protection and trip the transformer first...not a good thing.

Thanks for the insight...

 

[davidbeach]

Why would transformer differential trip for faults out on the distribution?

 

[dpc]

If the 87T trips for fault on the distribution, it would be a misoperation.

I have seen the tertiary windings used for distribution inside power plants where the exposure should (theoretically) be less.

It's a tradeoff between purchasing transformer capacity, transformer losses, and system reliability.

Grounding transformer would be needed since the delta tertiary is ungrounded.



"The more the universe seems comprehensible, the more it also seems pointless." -- Steven Weinberg

 

[tlrols]

I'm not a protection guy...so if I understand correctly the concern is that the transformer protection would see the distribution fault and remove the transformer from service BEFORE the distribution protection had a chance to clear the fault--is this correct?

I recall seeing tertiary's used for distribution in "3rd world" countries--obviously the economics outweighed any reliability concerns in that situation.

So, to recap what I have learned the issue is that you run the chance of allowing the transformer protection to "see" distribution faults which could lead to loss of the transformer (i.e. breakers opening) for a distribution...is this correct?

 

[davidbeach]

Transformer protection is almost always differential protection and it only responds to faults in its zone; the distribution circuit would be outside its zone. The voltage sags associated with the distribution fault would be much more noticeable on the transmission system than if the distribution were served from its own transformer. The tertiary is also the winding most likely to be damaged during a through fault, why expose it to more through faults?