Transformer overload capacity vs voltage

[electrofelon]

191005-2351 EDT

electrofelon:

I have a 50 kVA pole transformer, some wire to my meter, a small additional amount of wire, main fuses, and then one circuit breaker. With about a 10 A 120 V load the 120 loaded bus to neutral bus has a voltage change of about 0.9 V. This does not include the breaker.

A 240 V 10 A load would probably produce about a 1.0 V change. I can explain why if need be. The 50 kVA transformer will probably look like about 0.25 V change for a 10 A load change, but a good amount of this is at 90 deg.

If you pump back 80 A at 240 V, then for my parameters the hot to hot bus voltage will possibly increase by 8 V. My nominal at present is 123 V, which is quite normal, but sometimes rises to over 125 V. With either of these nominal values an 8 V increase would take me over 246+8 or 250+8, half at 120. Can equipment in my home tolerate voltage that high? Probably, but should not.

If I put an interposing transformer between the PV system and my main panel, then I can reduce the voltage the inverter sees, but it does not correct the voltage in my home. I can also put the whole home load and inverter on the output of the interposing transformer. This increases source impedance.

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I just did a measurement here and I get 1.5V drop when I turn on 2KW of 240V electric heat (8 amps). I have 25 KVA utility transformer -> 80' 4/0 AL -> 15 KVA 240-2400 transformer -> 1900' #12 CU -> 15 KVA 2400 - 120/240 transformer feeding house. There is a tap off the 2400 side of the house transformer which is 600' #10 CU -> 15 KVA 2400 - 240 transformer for PV system. So this gets to what you were saying : The house still "sees" the voltage drop/rise of some of the system, but not the PV transformer. I think it is good this way as it keeps a few more volts of voltage swing off the house and I can do what I want with the voltage at the PV system if needed.

I am surprised by your voltage change figures, that they are that high. I guess my setup isnt so bad considering the multiple transformers and wire runs. That is the thing with PV systems, it essentially doubles the voltage swings. I figure about 60 amps would be max load at the house, but I can have 60 amps going the other way so essentially doubling my voltage variation. I might have gone a bit tighter with everything when I designed it, but at the time I had a slightly different philosophy and was only going to have a small PV system and not as much electric loads. I should be fine if I keep my L-G voltage between 110 and 125 at the house.

 

[gar]

101006-1222 EDT

electrofelon:

I did a slightly more accurate measurement. Used one oven as a 240 V load. 240 is reading 246.4 bus bar to bus bar with oven off. Drops to 245.5, 0.9 V change with oven load of 12 A on. Calculated source impedance 0.075 ohms. Part way from poll to meter is wimpy power company wire, about 100 ft, 200 ft loop. Not hard to see a good part of that 0.075 ohms being the wire.

.

 

[gar]

191006-1511 EDT

electrofelon:

You need to do a temperature rise measurement on your PV transformer. It would be good, if the transformer was in a moderately constant temperature location, then the ambient would be moderately constant during the test.

I suggest measuring primary resistance. This will be the lowest current, and highest voltage. For a 15 kVA I will make a wild guess at 2 ohms. A DC measurement current of 0.5 ampere should provide good voltage, 1 V, and negligible self heating during the time to measure the voltage drop. Do as a 4 terminal measurement, That is apply current thru the coil, and make the voltage measurement directly on the transformer wires.

Do the initial measurement after the transformer has had an all night cool down. Measure ambient temperature at this time. At a later time in the day when the transformer has been at maximum power transfer for a long enough stabilization time, then read ambient temperature, and power input to transformer.

Depower the input to the transformer (probably means opening the circuit), and disconnect the 2400 V side (obviously meaning both wires). Again measure the resistance of the 2400 V winding as before.

Now calculate the average temperature rise, correct for change of ambient, and estimate maximum hot spot.

I quit for now this website keeps dumping me.

​​​​​​​.

 

[gar]

191007-1731 EDT

electrofelon:

I have a number of comments, but not all at once.

An ordinary magnetic power transformer is a bidirectional device. Power can flow in either direction and no switches or anything else is needed to accomplish this. There is nothing that prevents you from sending power one way or another thru a transformer.

The basic limitations on what you can do are: Voltage breakdown, maximum insulation temperature (an absolutre value, not rise), and core saturation. Indirectly frequency is a consideration. Temperature rise is important because it relates to ambient temperature, but absolute temperature is the real determinate.

Suppose the transformer that connects the PV system to the 2400 V lines can be in your basement, or where a moderate amount of dissipative average load exists. For example: refrigerators, air conditioning, furnace, and daytime lighting. And this load is less than the PV transformer rating, then, If this stuff is placed direct on the PV system before the PV transformer it soaks up its energy from the PV directly, and the PV transformer never sees that power. When the PV system is not producing power, then the transformer is supplying those loads. Since these loads don't exceed the PV transformer rating there is no problem.

More later.

.

 

[gar]

191007-2051 EDT

A little additional comment on my last post for those not familiar with circuity theory.

When my power load that is directly on the PV system needs more power than than PV can provide, then the grid via the PV transfomer provides the needed power. The PV will always provide the maximum power it has available to provide. That is inherent in the design of a grid tied PV inverter. The PV inverter is a constant power source with a power capability determined by the available solar energy.

If the power available from the PV system is greater than needed by my load, then that excess power goes to the grid via the PV transformer. Voltage and frequency are defined by the grid. Frequency stays locked to the grid. Voltage is basically that derived from the grid via the transformer ratios. The voltage at any particular point will be dependent upon power flow, the various impedances to that point, and currents. So voltage at any particular point varies up and down, but moderately related to grid voltage.

.

 

[electrofelon]

191006-1511 EDT

electrofelon:

You need to do a temperature rise measurement on your PV transformer. It would be good, if the transformer was in a moderately constant temperature location, then the ambient would be moderately constant during the test.

I suggest measuring primary resistance. This will be the lowest current, and highest voltage. For a 15 kVA I will make a wild guess at 2 ohms. A DC measurement current of 0.5 ampere should provide good voltage, 1 V, and negligible self heating during the time to measure the voltage drop. Do as a 4 terminal measurement, That is apply current thru the coil, and make the voltage measurement directly on the transformer wires.

Do the initial measurement after the transformer has had an all night cool down. Measure ambient temperature at this time. At a later time in the day when the transformer has been at maximum power transfer for a long enough stabilization time, then read ambient temperature, and power input to transformer.

Depower the input to the transformer (probably means opening the circuit), and disconnect the 2400 V side (obviously meaning both wires). Again measure the resistance of the 2400 V winding as before.

Now calculate the average temperature rise, correct for change of ambient, and estimate maximum hot spot.

I quit for now this website keeps dumping me.

.

So I am a bit confused. Am I measuring winding temperature indirectly, by noting the change in winding resistance? IF so I dont know if the winding are copper or aluminum. I am sure the temperature coefficient of resistance is different for CU and AL.

191007-1731 EDT

electrofelon:

I have a number of comments, but not all at once.

An ordinary magnetic power transformer is a bidirectional device. Power can flow in either direction and no switches or anything else is needed to accomplish this. There is nothing that prevents you from sending power one way or another thru a transformer.

The basic limitations on what you can do are: Voltage breakdown, maximum insulation temperature (an absolutre value, not rise), and core saturation. Indirectly frequency is a consideration. Temperature rise is important because it relates to ambient temperature, but absolute temperature is the real determinate.

Suppose the transformer that connects the PV system to the 2400 V lines can be in your basement, or where a moderate amount of dissipative average load exists. For example: refrigerators, air conditioning, furnace, and daytime lighting. And this load is less than the PV transformer rating, then, If this stuff is placed direct on the PV system before the PV transformer it soaks up its energy from the PV directly, and the PV transformer never sees that power. When the PV system is not producing power, then the transformer is supplying those loads. Since these loads don't exceed the PV transformer rating there is no problem.

More later.

.

Yes, and this is why i have only expressed concern about the one transformer, not the one that steps back down near the utility (recall in post #21 I described the multiple transformers). The PV transformer serves only the PV (and a few insignificant things like a few lights and a convenience outlet) so it will get no relief from local loads. The transformer serving the house only sees house loads. The transformer nearest the utility does see the Pv load, but not all of it because whatever the house is using comes off the top, further, the PV transformer chews up 400-500 watts at max power, and the high voltage run chews up a few hundred watts. Its pretty cool having my own little mini Mv distribution system. I like thinking about all three transformers and the current flows changing all the time. I like it when I use a heavy load at the house and its sunny and its like I am using power "directly" from the Pv system.

 

[gar]

191007-2229 EDT

electrofellon:

To know if the transformer is copper or aluminum: Possibly you can determine that from the manufacturer, if not, then do an experiment. Things I type disappear. You measure winding resistance after transformer has reached a stable temperature. Then externally raise the transformer ambient tempertaure a known reasonable amount, let it stablize, then remeasure resistance, calculate temperature coefficient from this resistance and temperature change, and lookup material from a reference.
https://www.engineeringtoolbox.com/resistivity-conductivity-d_418.html

Since the PV transformer is 600 ft from home and you don't want to move it to home. Then what would another 600 ft of *10 from the PV to home to parallel the output of the home transformer do. I have not tried to estimate, but if a balance could occur that you bleed off 10 A of the 240 V PV current from going to the PV transformer, and the #10 was copper, then voltage drop is about 12 V at 240 or 6 V at 120.

But assume none of the above is useful, then we are back to the PV transformer, and its analysis.

I won't get into that tonight.

.

 

[gar]

191008-1631 EDT

electofelon:

We need to know as much about your PV transformer as possible. Some questions are:

1. Full load rating? 15 kVA.
2. At what still air ambient temperature?
3. Temperature rating of winding wire? Thus, means to get temperature rise.
4. Resistive temperature coefficient of winding wire?
​​​​​​6. Conductive material in winding wire?
7. Thermal time constant?
8, What physical orientation and mounting? Including heat sinking.
9. At what secondary voltage and current? Meaning the 240 winding.
10. For a specific primary tap the primary voltage and current for above secondary voltage?
11. Maximum secondary voltage?
12. Where is transformer located? Can it be underground?
13. Is there sun shading?
14 To what extent does some amount of forced air cooling do to reduce the temperature rise?
15. Outside surface temperature of transformer?.

Possibly some of this information can be obtained from the transformer manufacturer.

I fully believe your off hand judgement that the transformer will work is probably correct. But some of the above information, and experiments should provide more confidence.

.

 

[electrofelon]

191008-1631 EDT

electofelon:

We need to know as much about your PV transformer as possible. Some questions are:

1. Full load rating? 15 kVA.
2. At what still air ambient temperature?
3. Temperature rating of winding wire? Thus, means to get temperature rise.
4. Resistive temperature coefficient of winding wire?
​​​​​​6. Conductive material in winding wire?
7. Thermal time constant?
8, What physical orientation and mounting? Including heat sinking.
9. At what secondary voltage and current? Meaning the 240 winding.
10. For a specific primary tap the primary voltage and current for above secondary voltage?
11. Maximum secondary voltage?
12. Where is transformer located? Can it be underground?
13. Is there sun shading?
14 To what extent does some amount of forced air cooling do to reduce the temperature rise?
15. Outside surface temperature of transformer?.

Possibly some of this information can be obtained from the transformer manufacturer.

I fully believe your off hand judgement that the transformer will work is probably correct. But some of the above information, and experiments should provide more confidence.

.

gar,

Unfortunately I have very few details of the transformer. I am attaching a picture of the data plate. I believe solomon is transformer refurbisher/re-seller and they have put their own label on it. I do not even know who originally made the unit or any details other than what you see. This thing is no prize, its older and has rather high no load losses (about 180 watts vs 60 for my newer ones built under doe2016). I dont mind if the thing just burns up, it is outside so it wont hurt anything. My only hesitation to just throwing whatever I want at it and use it until it blows up is if something upstream trips while I am away and I loose heat and refrigeration.

So the only other thing I can tell you about is it is a typical oil filled padmount. It has +/- 2.5% and 5% taps. It is located where it gets direct sun - I should do something about that. It does not have any external fins or forced cooling. I bet welding on some cooling fins, blocking the sun, and maybe adding a small fan would greatly extend its capabilities.

EDIT: I hadnt noticed before but it does say the manufacturer. Looks like Howard industries. They are a big name.

 

[Hv&Lv]

gar,

Unfortunately I have very few details of the transformer. I am attaching a picture of the data plate. I believe solomon is transformer refurbisher/re-seller and they have put their own label on it. I do not even know who originally made the unit or any details other than what you see. This thing is no prize, its older and has rather high no load losses (about 180 watts vs 60 for my newer ones built under doe2016). I dont mind if the thing just burns up, it is outside so it wont hurt anything. My only hesitation to just throwing whatever I want at it and use it until it blows up is if something upstream trips while I am away and I loose heat and refrigeration.

So the only other thing I can tell you about is it is a typical oil filled padmount. It has +/- 2.5% and 5% taps. It is located where it gets direct sun - I should do something about that. It does not have any external fins or forced cooling. I bet welding on some cooling fins, blocking the sun, and maybe adding a small fan would greatly extend its capabilities.

EDIT: I hadnt noticed before but it does say the manufacturer. Looks like Howard industries. They are a big name.

They are, but Solomon rebuilding had had there share of problems. Transformers may be OK. I’m very hesitant about OCRs or three phase reclosers from them.

 

[electrofelon]

They are, but Solomon rebuilding had had there share of problems. Transformers may be OK. I’m very hesitant about OCRs or three phase reclosers from them.

Do these outfits actually rewind them or is it just a coat of paint and new bushings? This summer I priced out some new vs refurb units (from the same vender), 15 KVA padmounts, $1500 for refurb, 1800 new. IMO its a no brainer to go new.

 

[gar]

191008-1940 EDT

electrofelon:

To what extent do you want to play with and learn about transformers? This can provide you a nice learning experience.

If you have no desire in that direction, then do something simple. Provide ventilation, and totally shield from solar energy. Provide moisture (evaporation) by shade bushes around the transformer. Do IR measurement of the transformer surface, and make judgements.

.