# Thread: Step-Up to Step-Down Transformer Winding Configurations

1. Originally Posted by Milo902
I'll report back with some numbers after I get lost in excel for a few hours.
Good idea.
One other thing to consider, and you probably have, is how the insolation level varies throughout the day and to zero after sunset. How would you deal with transformer light load/no load losses under those conditions?

2. Originally Posted by kwired
I missed that there were multiple units.
Maybe you, I and Jaeref need to go to the same optician...........

Sorry mods - couldn't resist.
Last edited by Besoeker; 12-29-17 at 02:14 PM.

3. Originally Posted by Besoeker
Good idea.
One other thing to consider, and you probably have, is how the insolation level varies throughout the day and to zero after sunset. How would you deal with transformer light load/no load losses under those conditions?
Right I am not exactly sure how to calculate that. I Assume there is some software that can model those loading conditions. Basically though, using the max inverter output current for your voltage drop calculations will give unrealistically high energy loss numbers. Also, as I mentioned in an earlier post, there was an article about VD and solar systems in one of the trade publications a while back and they concluded that most systems are designed with excessively low VD numbers to the point that it is not the best economically overall.

Edit: "voltage rise" is also an issue though and can result in the voltage rising above the inverters voltage window, so that needs to be taken into account.

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Originally Posted by electrofelon
Right I am not exactly sure how to calculate that. I Assume there is some software that can model those loading conditions. Basically though, using the max inverter output current for your voltage drop calculations will give unrealistically high energy loss numbers. Also, as I mentioned in an earlier post, there was an article about VD and solar systems in one of the trade publications a while back and they concluded that most systems are designed with excessively low VD numbers to the point that it is not the best economically overall.

Edit: "voltage rise" is also an issue though and can result in the voltage rising above the inverters voltage window, so that needs to be taken into account.
Voltage drop and voltage rise are the same thing. Due to resistance in the wire the voltage is higher at the inverter than it is at the service when the inverter is producing. Whether it is rise or drop depends on which end of the conductors you take as the starting point for your measurement. Conductors that are too small can cause the voltage at the inverter to rise above the top of its operating voltage window and shut down your system.

FWIW, the voltage at the service can be at the top of the POCO's window of operation as well, which will exacerbate the situation and may even cause inverters to shut down where you didn't think there would be a problem. In a few cases we have had to install buck-boost transformers to fix it.
Last edited by ggunn; 12-29-17 at 03:32 PM.

5. Originally Posted by ggunn
Voltage drop and voltage rise are the same thing. Due to resistance in the wire the voltage is higher at the inverter than it is at the service when the inverter is producing. Whether it is rise or drop depends on which end of the conductors you take as the starting point for your measurement. Conductors that are too small can cause the voltage at the inverter to rise above the top of its operating voltage window and shut down your system.

FWIW, the voltage at the service can be at the top of the POCO's window of operation as well, which will exacerbate the situation and may even cause inverters to shut down where you didn't think there would be a problem. In a few cases we have had to install buck-boost transformers to fix it.
Yes I am aware that VD and VR are the same thing. I was just trying to say that VD isnt just about economics, it may need to be considered for voltage window also. One could have a set of circumstances where 6% VD is the most economical, but that would cause voltage window issues.

6. Originally Posted by electrofelon
Right I am not exactly sure how to calculate that. I Assume there is some software that can model those loading conditions.
Probably there is. I have had to incorporate transformer losses in bids where I had to provide guaranteed efficiency. I divided it into Cu loss and Fe (magnetising) loss.
My assumption at the bid stage was 2% total loss of which 0.7% was Fe loss and the rest Cu. The Fe loss was assumed to be constant and the Fe varied as the square of the current.
Don't know if that helps.

7. Originally Posted by electrofelon
Yes I am aware that VD and VR are the same thing. I was just trying to say that VD isnt just about economics, it may need to be considered for voltage window also. One could have a set of circumstances where 6% VD is the most economical, but that would cause voltage window issues.
I meant to say "could". Was not trying to generalize that 6% would never work.

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Originally Posted by electrofelon
I meant to say "could". Was not trying to generalize that 6% would never work.
Gotcha. Conversely, if the service voltage is on the high side, even a 2-3% Vd can cause voltage window issues.

9. Originally Posted by Milo902
......(6) 500 Cu conductors/phase.......it would need 500 Cu as well to keep voltage drop around 2%. .....

If I was paying for this, I would tell you to use aluminum conductors

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Alright, I did some spreadsheet work to get a picture of losses over the year. My PV modeling software puts out hourly data which makes it easy to build a spreadsheet to tally losses based on actual plant loading.

As there will be a main AC combiner panel located at the array regardless of interconnection method, started my calculations at that point. For the 480V system, this is simply calculating current based on real power output (as the output does not include voltage, nominal system voltage was assumed for all calculations) and using that to estimate i^2r losses for the long run. Since this is calculated on an average hourly output, watt losses = watt hour losses, simplifying calculations. I revised my conductor to (6) 1000 kcmil AL/phase to keep prices in check.

480V system: Maximum voltage drop = 2.25%, yearly losses = 4,432kWh. This represents a 0.52% yearly loss, quite acceptable.

4,160V system: Maximum voltage drop = 1.33%, yearly losses = 16,150kWh. This represents a 1.9% yearly loss and is in line w/ the rule of thumb for transformers of about 1% loss/transformer yearly.

Based on this alone, it leans pretty heavily in the favor of the 480V system as rough estimates put the install cost between the two options within a few kilobucks of each other. Trenching in (6) sets of conductors for that distance isn't a huge concern, but there are a few spots we need to directional bore which introduces some headache. On the other hand, two more pieces of equipment introduce their own headaches.

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