Si hoc legere scis nimium eruditionis habes.
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.
Ethan Brush - East West Electric. NY, WA. MA
"You can't generalize"
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.
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.
Ethan Brush - East West Electric. NY, WA. MA
"You can't generalize"
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.
Si hoc legere scis nimium eruditionis habes.
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.
For the 4,160V system, the conductor losses are calculated the same way (of course) and are negligible (about 60kWh). For the transformer, I got a few bids including a DOE efficiency for the step-down (as required) and a non-DOE for the step-up (doesn't fall under DOE guidelines). The main difference is the standby losses with the DOE transformer about 245W no-load and the non-DOE about 1200W no-load. I used DOE for both as the extra cost would quickly pay for itself in losses. The transformer no-load losses are there 24/7 and thus can be multiplied out over the year (245W * 8760 hours * 2 transformers = 4,292 kWh). As we can see the standby losses for the transformers are pretty much identical to the resistive losses of the 480V run. For the load losses, the transformer was quoted with load losses of 5,200W at 500kW. As load losses are resistive, we use the i^2r formula to estimate losses at lower outputs (take the output in kW over the rating of 500kW, square the ratio, multiply it by the 5,200W load loss).
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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