I dont understand the KVA rating on pole transformers

[Jerome Tews]

Over the holiday weekend, a local bar & grille had a power outage. This building has been running off a 15KVA pole transformer, which also feeds a few nearby homes. That bar normally runs several large walkin coolers, lots of lighting, and electrical cooking devices.

On this weekend the outdoor temperature soared close to 100deg F. This put a high demand on the AC for that building and more ACs on the homes on that same transformer. Plus the bar opened their beer garden, which meant powering a few more cooolers, and more lights. Add to that, they had a live rock band using power for amplifiers and stage lights.

The load was too much for that transformer and it overheated and cut off. The power company came and said that transformer was hot enough to fry an egg. They said they would need to install a much larger transformer, but first they would have to go get one, and if the bar wanted to serve food and drinks, and have that rock band play, they would have to shut off the AC in the bar and in the homes on that transformer. Once that was shut off, the power company was able to reset that transformer.

It became very hot in that building, but the band played and beverages were sold. Halfway thru the evening, the POCO came back with a new 37.5 KVA transformer and as soon as the bar turned on some temporary lights, the band took a break, and the POCO did an amazing 15 minute transformer swap. Once completed, everything went back to normal and the rest of the evening went fine.

I watched the guys swap that transformer and noticed the old one was

15KVA and new one 37.5KVA. (much bigger).

But that left me wondering whgat those numbers actually mean. I pay for my electric usage based on KWH (kilowatt hours). So why are transformers rated at Kilovolt/amps? What I dont understand is what does that mean in actual watts, or amps?

For example, lets say the bar has 300 amp service and the homes have 100 amp service each. So, if there are 3 homes and the bar, that is a total of 600amp capacity. Ok, using that example, how many amps can a 15KVA transformer actually supply (at 120/240 Volts - Single phase - 60 cycle). And how much can the new transformer rated at 37.5 KVA handle?

(Obviously the 37.5 KVA trans can handle one and one half times more capacity, but in actual amps or watts, how much are we talking? Thats where it gets confusing. Why dont they just rate them transformers at 600amps, or use Kilowatt hours?

[hubops]

[None]

We could but would have to ban all loads that are not purely resistive.

[Clare Snyder]

On a straight resistive load, Killoatts and KVA are the same, as they are on DC. Capacitive or inductive loads shift the phasing of the voltage vs current - which is referred to as power factor. That is the difference between Kva and Kw.

That is the amount of power being used or provided at any instant in time.

The meter adds up the amount of power used OVER TIME - giving you the AVERAGE poer per hour times the hours of use.

If you buy 150KWH of power you could have used 150 KH for 1 hour, or

1KH for 150 hours, or.1Kw for 1500 hours. It does not tell you what your peak power requirement is.

The rating of the transformer is it's steady state output capacity. 1 15Kva transformer with a 240 volt output can provide a maximum of only 62.5 amps while a 37,5kva transformer can supply 156.25 amps - and double that at 120 volts. They can stand significant overloads for a relatively short period of time

Assuming 40C ambient, generally speaking, an oil-filled transformer can tolerate a 200% overload for 30 minutes or 150% for 60 minutes provided the preceding 2 hours as well as the following 2 hours were at 50% load or less.

At 30C ambient the transformer can handle 110% of the 40C rating, so

220% for 30 minites.

That is still a pretty small transformer for a commercial bar enterprize and several houses.

[trader_4]

KW is a measure of power, how much energy is being consumed per sec, min, hour, etc. KWh is a measure of energy, which is how much power is used over how long a period. As an example, if we had a motor that had to lift a 100 pound rock, we need one that can develop sufficient power. Let say that's a 1.3 hp motor, which is about 1.3 KW. That's the power it needs. How much energy it needs depends on how high it has to be lifted or how long the motor runs. If it took it 6 minutes to raise the rock, that would be

1.3KW x 6/60 hrs = .13KWh

That would work if the current and voltage were always exactly in phase with each other. Then you could multiply the amps times the volts and get the power. But with loads like motors, power supplies, other transformers, etc, the current can lead or lag the voltage waveform. For example if you looked at when the voltage peaked at 120v and you looked at the current peak, the current peak would be before or after the voltage peak. So, the power in that case is less than what you would get if you just multiplied based on the current and voltage. You have to multiply them instantaneously at all points on the curves then integrate them. Fortunately you don't have to do that each time, there is a power factor correction that you can multiply in based on how far ahead or behind the current curve is from the voltage curve. If there is a power factor of .8 involved, then while the volt meter tells you 120V and the ammeter tells you 5a, the actual power being used is 120 x 5 x .8 = 480 watts.

[gfretwell]

I agree that seems pretty small. Most of the transformers they set here are 50KVA and that will serve 3 houses, maybe 4. There are places that do more on one transformer but they are generally older neighborhoods that outgrew the original installation. I have never even seen a 15KVA around here. 25 seems to be the smallest they will hang. The replacements I see on the trucks are usually 50s.

[Tekkie®]

[e-mail] posted for all of us...

I think we need the overpressure pop off for some posters here...

[hubops]

Here are a couple old favs :

My favourite personal experience transformer blow-out was a huge 500 kv 750 mva auto-transformer failure that split it open ... explosion vents couldn't handle the sudden massive energy apparently.. Another one where the big oil circuit breaker < 230 kV > spit out bushings as it exploded and burned .. .. poop happens .. in the control room, at times ! :-) John T.