Mixing MV Voltage Ratings

Status
Not open for further replies.
B

bwat

EE
Location
NC
Occupation
EE
Customer has 13.2kV site.
Customer bought too many 13.8kV to 480V 2MVA transformers for a different site and wants to use a couple of those 13.8kV transformers for this 13.2kV site. Prices and lead times are crazy right now for transformers, so I understand the thought.

I've seen this out in the wild, but always seemed liked a case of something was upgraded after the fact that made it turn out like this. This would be a new install in this state. It's within 5% difference, so I want to say it could be ok if connected equipment had appropriate tolerances to allow, but this just seems too odd to me. Maybe I'm being overly conservative... thoughts?
 
bwat said:
Customer has 13.2kV site.
Customer bought too many 13.8kV to 480V 2MVA transformers for a different site and wants to use a couple of those 13.8kV transformers for this 13.2kV site. Prices and lead times are crazy right now for transformers, so I understand the thought.

I've seen this out in the wild, but always seemed liked a case of something was upgraded after the fact that made it turn out like this. This would be a new install in this state. It's within 5% difference, so I want to say it could be ok if connected equipment had appropriate tolerances to allow, but this just seems too odd to me. Maybe I'm being overly conservative... thoughts?
Click to expand...
A pic or data plate information would be great. We use transformers at every substation that are rated for more than our voltage. We tap them appropriately and/or regulate.
If it has taps then simply adjust them for the TTR.

there shouldn’t be any issues
 
Taps would be your best bet if they are available.

Yes, the 13.2 vs 13.8 is within the 5% tolerance of each other, but remember the utility’s 13.2 can go to -5% from there, so down to 12.54kV, which on a transformer set up for 13.8kV, will put your 480V at -10%, then any VD from the service to the point of use may put you too low at machines. In other words you are looking at the difference between the two as if the transformer cares, it does not. The voltage tolerances are meant to allow for safe levels at the point of use.
 
Thank you both. Waiting for nameplate still, but it sounds like they may have appropriate taps.

Jraef - my thoughts were similar to yours. It might be within tolerance for most connected equipment, but your starting point could be right up against that tolerance, so you lose the ability to drop any lower than that on the MV side which would certainly be a real possibility.
 
Your 480 Volt 2nd side may look a little "low." If you have multi-tap options on the 2ndary, you may need/or want to adjust them upward. So if you have long feeder runs out to pumps, or motors, or AC equipment -- you may want check the actual observed voltage at the distant disconnect(s) with those motors / loads turned on. And then tag the equipment with long-term durable notes of what you have done for the sake of the "next guy."

It is not uncommon to see such mixes on older sites --- but usually the other way around. Older 13.2kV equipment and transformers now being fed with 13.8kV due to Utility upgrades. Makes the 2nd Voltage (nominal 480V) well over 500V. Makes me cringe a little -- but it tends to work fine.

Only conditions I would really severely no-go it would be if it were critical or emergency power circuits (data-center, medical, etc.) as I would not want to answer for "why" if something went wrong (because the boss wanted to save a few hundred dollars is NEVER the right answer), or if were part of a double-ended (A-side / B-side) substation where selecting either end or both (tie-bus) would put different voltages to the equipment down the line.
 
Status
Not open for further replies.
Top