Originally Posted by

**Milo902**
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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