Transformer loss and cost

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Electromatic

Electromatic

Senior Member
Location
Virginia
Occupation
Master Electrician
Hello all,

I have a customer with an unoccupied commercial space that still has an electric bill of about $250 a month. All of the HVAC is off as well as water heaters, lights, etc. The only thing I can see running is a few exit/emergency lights, an electronic door lock, ...and several transformers.
There are three 480--208 stepdown transformers constantly humming along. I took readings on the primary side of each in their normal
(almost no)load state and with completely no load.

With absolutely no load, I had:

75kVA with an average 3.8A per phase, another 75kVA with an average 4.1A per phase, and a 150kVA with an average 9.2A per phase
(they are all delta-wye, 150° rise)

First, is this normal?
My understanding is that transformers typically run best at around 50-60% load, and I've have been reading-up on no load losses. I've read several other threads here with differing estimates. My observations seem to be at about 5-6% loss.

Second, are there any suggestions to alleviate the situation other than just shutting off the feeds to the transformers?

Lastly, I'd appreciate any help on the math of converting these per-phase amp readings to kWh in order to confirm that these transformer losses are causing the high bill. The only way I can get it close to $250/month is with a power factor of 0.1.
Averages: 3.8+4.1+9.2=17.1A
P = 3 (VL-N) (I) (Pf)
3 (284V) (17.1A) (0.1) = 1457W = 1.45kW
1.45kW * 3 phases = 4.35kW
4.35kW * 24hr * 30day = 3132kWh/month
3132kWh * $0.08 = $250

It seems like I'm fudging things low with assuming a power factor of 0.1 and 8¢/kWh. I'm a better practical troubleshooter than a mathematical one!

Thanks everybody.
 
Transformer loss and cost

If I am reading your equation correctly, you are using a constant of 3 for 3 phase. It should be the square root of 3.

And you can't add the current of each phase. Use the average. Or are the 3 current values for 3 different transformers?
 
use 100 kva as a base
pu z of 5% in ohms ~ 0.115 Ohm
x/r of 10 typ in this range
pf ~ cos(arctan(10)) ~ 0.1
x ~ 0.11
r ~ 0.011

rated i at 480 120 A
losses ~ 0.05 x 120 = 6 amp
for 75 kva ~ 4.5

100 kva losses/month = sqrt3 x 480 x 6 x 0.1 x 24 x 31 x 0.08/1000 = $30/month per 100 kva of xfmr

~$100 for 300 kva

just a sanity check
rate is likely higher than 0.08
if 0.12 then $150 plus the other misc loads
 
Transformer with no load, .1 PF is probably close to the truth. I usually use a .2 PF when guessing.

Yes, the solution is to de-energize all of the unloaded transformers. If they need to maintain the emergency exit lights and door locks, it might be worth installing a small transformer big enough for just those loads and running temporary wiring in the unoccupied space. So find the service switchboard, install a small feeder to a small single phase transformer, then use that to power up these loads.
 
Thanks, all, for the replies.

@ retirede: The amperages I used were the averages of phases from each of 3 transformers, then I totaled the averages. Also, I thought if I used line-to-neutral voltage (VL-N) I multiplied by 3 and if using line-to-line (VL-L) I used sqrt3.

@Ingenieur: If I follow you, you're saying theoretically/mathematically I could expect about $150/mo for 300kVA of transformers. That still leaves me with $100/mo to explain, but I can kind of see that in little loads, older transformers, flat-rate fees, etc.

There's at least one door with a keyed lock. I might have to ask the AHJ about unoccupied emergency lighting. I have to ask the owner what he wants to do about HVAC for the winter. That will definitely affect what I can leave turned off.

Thanks again.
 
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