size of transformer for solar PV plant

[xernoxian]

Hello, I would like to know the factors to consider when one is choosing a transformer (or transformers) for a solar PV plant. In the design of utility scale plants (eg. 4MWp and above), some engineers choose several 1 MVa transformers and others choose one big transformer for their design. Please what should inform the choice of a transformer for a solar PV plant.

 

[Desrod2]

The choice of your transformation depend on a number of factors and how you manage risk. Do you want to carry-on operation if there is a transformers fault? If its a yes, you will have to build some redundancy into your system. Full redundancy might require to be able to stay connected even if one transformer is out. So for a 4 MW plant, 3 x 2 MVA transformer might do it.But considering that PV is not operating at full capacity 100% of the time, you might consider using only 2x 2 MVA units with some overload capacity. This can easily allow you to operate most of the time and sell most of what is produced.
If you don't want redundancy and want to save on infrastructures, a single unit is probably the lowest capital investment but you will have to factor how much money will be lost if the unit fails. A compromise is using a bank of 3 single phase units with a backup phase. If any phase fails, it is a matter of hours before restoring connection and this is likely a lot faster than ordering a new one.
Sometimes the transformer is supplied by the utility and that's an interesting solution since they are likely to carry spares that will bring you back in operation rapidly.
Run the numbers and then do a proper risk assessment (cost vs potential $ losses).

 

[crshears]

Agreed that one way to go would be with three single-phase units plus a spare ready for quick swap-out...but with solid fire-proof barriers between the units so the explosive failure of one is less likely to take one or more others with it.

Another way to go, depending on how the transformers are connected, is to run open corner delta at a reduced output with just two cans until the failed one is swapped out...and doing that overnight may be cheaper than doing it during dasylight hours and losing a day's production...unless the weather is predicted to be overcast...

Like Desrod2 said, quite a few factors.

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[xernoxian]

Thank you very much for your answers. I would like to know which of these two scenarios is better: 1. a 4MWp PV plant with 4 X 1MVa transformers or a 4MWp PV plant with 2 X 2MVa transformers.

 

[dpc]

A single 4 MVA unit will be less expensive to purchase and maintain than your two alternatives. "Better" is not really a meaningful term unless you define what you mean by it. For large turbine-generators - up to 800 MW, a single three-phase generator step-up transformer is by far the most common configuration for anything built in the last 40 years, at least in my experience. It's hard to imagine the need for reliability at a 4 MW PV facility would exceed these much larger installations.

 

[racookpe1978]

You should know this already, but just as a reminder:

A solar PV plant generates power only when the sun shines, and its power output peaks through the middle of the day with a short maximum at local apparent noon. 90% of your power comes between 09:00 oclock and 3:00 oclock (15:00 hours military time), and that peak power ONLY comes on a "perfectly clear" day (no clouds, but also no humidity, no haze, no pollen or dust, and onto "perfectly" clean panels. )

So, your calculation for maximum peak power can only come true through those few fleeting moments around noon on perfect days. Every other minute of every other day in a real weather, real output current WILL be less power out than peak advertised rate.

First: Size your transformer rather for that 11:00 to 13:00 hour (lower) but more likely "average" long term heating rate!

Further, PV panel degrade with time: rapidly at first to something like 90-87% max "sales" power output, then they stay static (slowly decreasing) for several years before dropping off at the 7-10 year point. Catastrophic loss (broken panels or burned connectors or an ice storm or hail storms will kill a few panels along the way - but those losses are unpredictable.

Regardless, you will never again see a "peak power output" again once the plant starts up - everything thereafter will be at lower power. Also: When considering that long-term 11:00 - 13:00 "generated" heat load across the middle of the day, recognize that "maximum generated power" (output) from the PV array IS maximum on a solar year and solar day - NOT on a real world heating day! Thus, the air temperature around the transformer WILL be hottest (northern hemisphere) between 13:00 to 15:30 hours (later in the day!) and later in the year (in July and August) than the maximum solar yearly exposure of May 22 through June 22 until July 22 each year and maximum daily exposure to the sun between 11:00 until 13:00 hours each day. What this means is: Your local air temperature through the fans cooling the transformer WILL be hotter each day when the heat generated by generated load in the transformer is NOT at maximum each day, and the maximum current generated inside the transformer will NOT be at maximum when the air temperature around the transformer is at maximum through the summer over all.

Further, the solar top-of-atmosphere radiation value will vary through the year as well: Solar top-of-atmosphere radiation peaks on January 5 each year at 1410 watts/m^2, but is its yearly LOW on July 5 at 1306 watts/m^2. Again: in the northern hemisphere, the PV panels will NOT be producing their rated maximum in the summer at the time of year when the air temperature (northern hemisphere!) is at its hottest.

So, be prudent in sizing the transformers, but DON'T buy too much transformer as long as the first few months of operation in the first summer during LATE July and August can get through without burning up. That first August will never again see that large a heat load.

Notice that i keep repeating "generated current" and not "maximum demand" - Load (grid demands) do NOT peak when your solar PV is outputting maximum current, regardless of time-of-day nor day-of-year. The electric industry has peak demand early in the morning, then again "after work" around 16:00 to 19:00 pm each day, when solar power plants CANNOT be putting out their maximum output. So, your output transformer at those times-of-day will also see below maximum currents and thus below maximum heat loads. Both peak demands also happen when air temperature is lower than its daily maximum of 14:00 to 15:00 hours.

The combination of everything means you face a high (but not maximum possible!) generated current around 15:00 hours during a higher (but still not maximum!) demand curve during a hot time of day on hot (but not maximum!) month during a not-maximum solar radiation period of the year.

What latitude is the proposed plant? What region?