Buck-Boost Transformer / Magnetizing Current

[KY Solar]

I need to install (2) 3 kVA buck-boost transformers to slightly lower the grid voltage as seen by a single phase 250VAC / 30kW solar array.
They will most likely be SquareD / Schneider. Max design amps is 150 but nominal load is about 120 amps.

1.) How much energy will the transformers consume through magnetizing current?

2.) How much effeciency is lost when you have to introduce a small auto transformer?

Thanks!

 

[topgone]

I need to install (2) 3 kVA buck-boost transformers to slightly lower the grid voltage as seen by a single phase 250VAC / 30kW solar array.
They will most likely be SquareD / Schneider. Max design amps is 150 but nominal load is about 120 amps.

1.) How much energy will the transformers consume through magnetizing current?

2.) How much effeciency is lost when you have to introduce a small auto transformer?

Thanks!

I don't follow your choice of transformer sizes.
If you wanted to feed back into the grid, your transformers are quite small.
Here:
Assuming your primary voltage is 480V, a 2 X 3 kVA autotransformer bucking transformers in parallel will only give you 37.5A at the secondary (240VAC).
Secondary amps = 3,000VA/480V + 3,000VA/240V = 18.75A!

Your transformer bank is capable of delivering (paralleling the two 3kVA transformers configured in autotransformer) = 18.75A X 240V X 2 = 9 kVA.
Or did I miss what you meant?

 

[jaggedben]

I don't follow your choice of transformer sizes.
If you wanted to feed back into the grid, your transformers are quite small.
Here:
Assuming your primary voltage is 480V, a 2 X 3 kVA autotransformer bucking transformers in parallel will only give you 37.5A at the secondary (240VAC).
Secondary amps = 3,000VA/480V + 3,000VA/240V = 18.75A!

Your transformer bank is capable of delivering (paralleling the two 3kVA transformers configured in autotransformer) = 18.75A X 240V X 2 = 9 kVA.
Or did I miss what you meant?

Yeah, you missed that he said buck-boost and 'slightly' and that the utility voltage is 250VAC. What he forgot to say is what he needs to buck the voltage down to, but I think it's 240VAC. (Because I think I recall seeing his other thread, and it's also a good assumption for a solar inverter.)

I'm no expert whatsoever on this but I believe the transformer nominal VA rating does not need to be higher than the nominal load for a buck-boost application like this.

 

[gar]

170419-2346 EDT

Check with the manufacturer for efficiency at full transformer load. I might guess at 3% loss for for a 3 kVA unit at full load. Or about 100 W, possibly somewhat higher. Power loss will drop to some minimum that occurs at zero load current. This zero load power loss is made up of core loss and I^2*R loss from the magnetizing current.

Ball park if you consider 30 kVA total load, then at 250 V this is 120 A. A voltage of about 25 V (10% change) at 120 A requires a transformer of about 3 kVA.

A loss of 100 W out of 30,000 W is a loss around 0.3%.

.

 

[KY Solar]

Buck Boost

Buck Boost

Yeah, you missed that he said buck-boost and 'slightly' and that the utility voltage is 250VAC. What he forgot to say is what he needs to buck the voltage down to, but I think it's 240VAC. (Because I think I recall seeing his other thread, and it's also a good assumption for a solar inverter.)

I'm no expert whatsoever on this but I believe the transformer nominal VA rating does not need to be higher than the nominal load for a buck-boost application like this.

Yes, around here, the nominal voltage at the meter is 250V. When the solar array comes up to power, the voltage at the meter base goes to 255. Then we get voltage rise at the inverters (reverse voltage drop). Now we are approaching 260 volts and the inverters start dropping out.

I did not specify how much we need to Buck voltage down but 5% would be plenty.

 

[topgone]

Yes, around here, the nominal voltage at the meter is 250V. When the solar array comes up to power, the voltage at the meter base goes to 255. Then we get voltage rise at the inverters (reverse voltage drop). Now we are approaching 260 volts and the inverters start dropping out.

I did not specify how much we need to Buck voltage down but 5% would be plenty.

I just wanted to be clear with your situation.

solar panels -->inverter --->isolating transformer---->grid meter
-----is that the correct arrangement that you have there?

1) Is your problem about keeping your inverter voltage higher than the grid voltage (bucking transformer -->you wanted to maintain your inverter voltage higher than the grid voltage of 250V) or
2) you wanted your inverter voltages to be maintained low so you won't trip on overvoltage (boosting transformer-->inverter voltage lower while the output is maintained at 250V)?

 

[Phil Corso]

Gentlepeople

Following are the three basic tenets of power transfer that seem to have been forgotten:

1) Power is transmitted if the two Sending and Receiving "Bus-Angles" differ!

2) VAr is transmitted if the two Sending and Receiving "Bus-Voltage" magnitudes differ!

3) Power and VAr are transmitted simultaneously if both Magnitude and Angle, differ!

Regards, Phil Corso

 

[gar]

160420-0955 EDT

I believe the operation of a typical solar inverter is to make itself look like a constant current source within some voltage range, and with its current output synchronized to be in phase with the voltage at its terminals.

By introducing a transformer of non-unity ratio between the inverter and the grid tie point shifts the operating point of the inverter. The output voltage at the inverter will be lower and the current higher to stay below the inverter's over voltage limit.

Inverters are designed to be maximum power devices to extract the maximum power from the solar panel. That is the knee of the curve, between approximately constant voltage and constant current, of the solar panel. Of course that point varies as the solar energy on the panel varies.

Use of a transformer between the solar inverter and the grid works as long as the system stays within the inverter's dynamic range.

.

 

[jaggedben]

I just wanted to be clear with your situation.

solar panels -->inverter --->isolating transformer---->grid meter
-----is that the correct arrangement that you have there?

There's no isolating transformer.

1) Is your problem about keeping your inverter voltage higher than the grid voltage (bucking transformer -->you wanted to maintain your inverter voltage higher than the grid voltage of 250V) or
2) you wanted your inverter voltages to be maintained low so you won't trip on overvoltage (boosting transformer-->inverter voltage lower while the output is maintained at 250V)?

Number (2). The inverter(s) is specified for 240V so it trips on overvoltage because the utility runs their grid intentionally 10V or so higher.

Here's the OP's previous thread on what appears to be the same project.

 

[topgone]

There's no isolating transformer.



Number (2). The inverter(s) is specified for 240V so it trips on overvoltage because the utility runs their grid intentionally 10V or so higher.

Here's the OP's previous thread on what appears to be the same project.

With all due respect, I only asked the original poster for a clearer description of his problem, not anybody else.

 

[GoldDigger]

With all due respect, I only asked the original poster for a clearer description of his problem, not anybody else.

So you would rather force the OP to answer the question again instead of being pointed to the OP's own answer in another thread?
That sounds pretty selfish to me.


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[topgone]

So you would rather force the OP to answer the question again instead of being pointed to the OP's own answer in another thread?
That sounds pretty selfish to me.


Sent from my XT1585 using Tapatalk

Duh. I don't like wasting my time digesting something that the OP will say is wrong in the end. Nobody's forcing anybody. One comes for help, he needs to be clear on what help is needed. That simple.

 

[Smart $]

Duh. I don't like wasting my time digesting something that the OP will say is wrong in the end. Nobody's forcing anybody. One comes for help, he needs to be clear on what help is needed. That simple.

If he could be perfectly clear on what help he needs, a simple confirmation would satisfy his needs. :happyyes:

 

[KY Solar]

160420-0955 EDT

I believe the operation of a typical solar inverter is to make itself look like a constant current source within some voltage range, and with its current output synchronized to be in phase with the voltage at its terminals.

By introducing a transformer of non-unity ratio between the inverter and the grid tie point shifts the operating point of the inverter. The output voltage at the inverter will be lower and the current higher to stay below the inverter's over voltage limit.

Inverters are designed to be maximum power devices to extract the maximum power from the solar panel. That is the knee of the curve, between approximately constant voltage and constant current, of the solar panel. Of course that point varies as the solar energy on the panel varies.

Use of a transformer between the solar inverter and the grid works as long as the system stays within the inverter's dynamic range.

.

Exactly….
There is no isolating transformer and we are only seeking to buck the voltage down 10 to 12 volts (5%).
The transformers will be connected as autotransformers, carrying only a portion of the total load.
There is no specific target, we just need to get away from the inverters' high voltage shut down limit.

The Schneider / SquareD rep told me today…..
The losses are approximately: 90 watts @ NO LOAD + 135 Watts @ LOAD = 225 Watts fully loaded and operating.

Thanks!

 

[GoldDigger]

Although the cost is small in the big picture, you might want to consider a contactor to disconnect the autotransformer during non-solar hours.

But far better working reliably than not working, in terms of energy production!

Sent from my XT1585 using Tapatalk

 

[topgone]

Exactly….
There is no isolating transformer and we are only seeking to buck the voltage down 10 to 12 volts (5%).
The transformers will be connected as autotransformers, carrying only a portion of the total load.
There is no specific target, we just need to get away from the inverters' high voltage shut down limit.

The Schneider / SquareD rep told me today…..
The losses are approximately: 90 watts @ NO LOAD + 135 Watts @ LOAD = 225 Watts fully loaded and operating.

Thanks!

Thanks for coming back to clear things up.

FYI, you don't even need 2 transformers to be able to transfer 30kW. You just need 1:

Rated amps at 12V winding (series winding) of your transformer = 3,000/12 = 250A
So, your rated kVA at 250V side = 250 X 250 = 62.5kVA!

Your voltage at the inverter side will be = 250VAC X 240/(240+12) = 238.1 VAC

Checking:
rated power = 238.1 X [250A + (3,000/240)] = 238.1V X 262.5A = 62,500 VA~ 62.5 kVA -> checks out!

On the original query re losses, please be reminded that when opting for an autotransformer configuration, your transformer impedance will not be the nameplate value as a single-phase unit. The autotransformer impedance will be a lot smaller:

%Z_{autotransformer} = %Z_single-phase x [1-1/TR], where, 1-1/TR = co-factor and TR =250V/240V = 1.0417.
%Z_{autotransformer} = %Z_single-phase x (1-0.96) = %Z_single-phase X 0.04
The losses when connected as an autotransformer will then be a lot smaller. That 225W losses is too high, (3,000-225)/3,000 = 92.5% efficiency!
Lastly, be aware that the available fault current of the autotransformer will now be greater than it was connected as two-winding transformer!

 

[GoldDigger]

If, like most grid tie inverters, the OP's inverter requires a center point connection to 120/240 and monitors for voltage imbalance from L1 to N and L2 to N, then a single autotransformer will not work unless it has a center tap and two 6V end windings.

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[KY Solar]

Autotransformer Connection

Autotransformer Connection

If, like most grid tie inverters, the OP's inverter requires a center point connection to 120/240 and monitors for voltage imbalance from L1 to N and L2 to N, then a single autotransformer will not work unless it has a center tap and two 6V end windings.

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What if the transformer only connects to the 240V legs and the Neutral passes straight through?

Thanks!

 

[Smart $]

What if the transformer only connects to the 240V legs and the Neutral passes straight through?

Thanks!

Your neutral will not be "neutral" to either of the load side legs.

 

[gar]

170424-0013 EDT

Suppose we wind a single autotransformer with a main winding designed for niminal 240 V, and place a tap at each end that is 12 V from the end. Then connect the solar to the two 240-12 taps. Then the service neutral is still a good neutral for the solar system. And solar sees an apparent utility voltage of 216 V. Or 108-0-108 relative to neutral.

You can use two individual 240 to 228 buck transformers and do the same thing.

Or use two individual 120 to 108 buck transformers with neutral used.

All the above produce the same result.

.