Stepdown transformer used as stepup transformer

[hidalgoe]

Hello:

A 12.470kV/4.16kV, 3750kVa trafo is being used at our campus facility to supply sundry loads at 4.16kV through a secondary VCB with 50/51 relay protection. It was decided by some of our technicians to use it as a step up transformer to supply a 12.47kV load while a new feeder is being installed. It will take approximately 12 hrs to install and commission. Two question have been asked.

* Is it prudent to use a step down power transformer as a step up transformer?

* Will the 50/51 relay protection still work if the current flows in the opposite direction? Can merely changing the leads on the CT secondaries provide proper operation as far as polarity is concerned?

Respectfully
Hidalgoe

 

[karalahana]

So I think your question is a bit weird or I just have misunderstood, ?f you have a 12,470 KV grid so why would u use a transformer to supply a 12,470 kV load?
secondly, if it is this very same transformer which transforms the grid to load-side voltage so how can u use the "same" transformers to transform load-side voltage to grid one? are there parallel working transformers which supply 4,16 kV?

 

[rbulsara]

hidalgo:

1. Transformer is a bidirectional device so it can carry power in any direction.

From system grounding point of view, for a permanent installation, ideally you would like to get wye- or delta on appropriate sides for system grounding. But for a temporary installation as long as you are aware of type system and understand the implications, it is not an issue.

2. Yes, 50/51 function would work fine. Typical 50/51 relays (or such functions in a multi-function relay) are non-directional, meaning they cannot decipher the direction, unless there is a directional element installed or used. So there should not be a need to change the polarity.

Only issue is that if this relay ends up on the secondary side, it will not protect the transformer from a fault or short circuit but will afford the overload protection. It , of course, will provide protection from faults on the load side of the relay.

Also, if this relay has a metering function (multifunction relay), it may read kW backward but for temporary issue it should not be an issue. But still over-current protections should work just fine.









Rafiq Bulsara

http://www.srengineersct.com

 

[jghrist]

If it is a delta-wye 12.47-4.16 kV transformer, the 12.47 kV line fed by the transformer will be ungrounded. No Ø-N loads will be able to be served. All equipment will have to be rated for the full 12.47 kV voltage, including arresters. The 50/51 relay will not protect for a ground fault on the secondary.

 

[waross]

And further to jgrist's comments. If the transformer is used as in wye to delta configuration and the wye neutral is connected any lost phase or unbalance supply voltages wil cause serious circulating current issues. If the wye neutral is floated the circulating current issues will be replaced with switching voltage transient issues. The accepted connection is to float the wye point and live with any resulting transients.
Other than that, the transformer will work fine in either direction.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[davidbeach]

Echoing the past few comments - the transformer won't give a darn which direction the power flow might be, but if the high side is delta you could do great mischief by feeding from the low side.