Step-Up to Step-Down Transformer Winding Configurations

[Milo902]

Now the monkey wrench. Up to this point, my calcs have been from the AC combiner panel to the interconnection point. If we stopped there then it's clear the 480V system is the winner. The next step is considering the inverter to combiner panel run. With the shape of the array, I have inverters from 150 to 500' away from the combiner panel.

The 60kW solectrias can accept up to a 2/0 conductor, maxing this out at 2/0 Cu yields a 0.4% VD at 150', 0.66% at 250' and 1.32% at 500'. Based on the previous post's analysis, the actual kWh loss for these runs shouldn't be a concern, but the voltage at the inverter terminals may.

If the whole plant is cranking, I can expect a 2.65% voltage rise at the 150' inverter, 2.91% at the 250' inverter, and 3.57% at the 500' inverter. I have traditionally tried to keep my AC voltage drop under 2% w/ the occasional stretch to 2.5%. Solectria specifies a maximum VD of 2% to the interconnect, and we have blown right past that. Keep in mind these percentages are calculated based on L-N voltages as the inverter is connected in a Wye config (based on their neutral connection). If I consider L-L voltages, the 500' inverter will see a 2.06% voltage rise. I plan to follow up w/ Solectria next week to get their input. If the interconnection voltage is running high, we may have unhappy inverters under these conditions.

Alternate ideas? How about a voltage regulator at the AC combiner panel to buck voltage? I'm not familiar with these in practice and not sure what the cost implications would be, but it could solve the voltage issue and even allow smaller cables ran to the interconnect. Does anyone have any experience with this approach?

 

[Besoeker]

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.

Don't disagree with any of the above except that I think there would more than two extra pieces of kit. Switchgear and protection for the transformers, additional power cable joints, more controls. As you say, additional headaches.........

 

[Milo902]

Don't disagree with any of the above except that I think there would more than two extra pieces of kit. Switchgear and protection for the transformers, additional power cable joints, more controls. As you say, additional headaches.........

Actually it should be pretty simple. Liquid-filled transformers include a load-break switch and fusing on the HV side, meaning the MV system has protection and switching built in. On the 480V side, the transformers would be protected by the interconnection breaker and the combiner box breaker on each end. For the MV cable, just elbow or bolted terms, not any different than 480V with respect to complexity in my mind. The key would be to get long enough reels to avoid splicing mid-run (shouldn't be a problem). It's still additional headache regardless.

Side note, brain fart on the voltage drop, it wouldn't be correct to simply apply the voltage drop in the line to 480V, as the line currents are based on a balanced wye system (no neutral needed for return current). If I wanted to apply it to a delta system, I would have to consider return currents through the adjacent phases (1.732) and the % voltage drop equals out, I know better.

 

[electrofelon]

Thanks for sharing your analysis. Very good info. I am very surprised at how low those no load loss figure are. Just a couple ideas:

1. What we have done in the past is put a junction box next to the inverter and spliced to large conductors. Hate that extra time and material, but sometimes that is what you have to do. Also that may get you into being able to use aluminum so the whole affair would pay for itself and then some.

2. Regarding the voltage window, perhaps you can get lucky and the utility can just change the tap position on the serving transformer?

 

[Besoeker]

Actually it should be pretty simple. Liquid-filled transformers include a load-break switch and fusing on the HV side, meaning the MV system has protection and switching built in.

None that I have used on projects did. Nor any, not of our supply, that I've seen on sites. It is usual here for switchgear to be mounted in a separate enclosure.
But, whether integral or otherwise, it is addition parts. Whatever, you seem to be favouring the LV solution so this is probably no longer pertinent to the ongoing discussion.

 

[jaggedben]

...

The 60kW solectrias can accept up to a 2/0 conductor, maxing this out at 2/0 Cu yields [too much voltage rise]...

Alternate ideas? ...

I don't see any reason you couldn't splice to a larger conductor at the inverter, with a very short 2/0 pigtail to properly terminate at the inverter lugs.

[Edit: Oh, electrofelon already said that two posts ago.]

 

[ggunn]

Now the monkey wrench. Up to this point, my calcs have been from the AC combiner panel to the interconnection point. If we stopped there then it's clear the 480V system is the winner. The next step is considering the inverter to combiner panel run. With the shape of the array, I have inverters from 150 to 500' away from the combiner panel.

Why not have long DC runs and group the inverters? 1000V runs are cheaper than 480V runs

 

[electrofelon]

Why not have long DC runs and group the inverters? 1000V runs are cheaper than 480V runs

That is usually my thinking as well, however when you consider that really it ends up being about 700 volts, the higher cost of PV wire, and the transitions to and from the large conductot size most of the savings get chewed up. If he could move to 1500 volt strings, that might be the ticket.

 

[Milo902]

That is usually my thinking as well, however when you consider that really it ends up being about 700 volts, the higher cost of PV wire, and the transitions to and from the large conductot size most of the savings get chewed up. If he could move to 1500 volt strings, that might be the ticket.

That's my experience as well. I have been mulling over the idea of grouping the inverters at the switchgear as a possible alternative. Although wire costs and line losses will probably be a wash, it moves some of the AC losses to the DC side and solves my concerns about excessive voltage drop/rise on the AC side. It would require combiner boxes at the arrays with a disconnect, but if I'm considering a voltage regulator on the AC side to help with voltage drop, it's probably a cheaper option (and certainly a simpler and more robust option).

I'll get lost in a little more excel today and see what flushes out. Then I can get some firm quotes this week and do a full cost/benefit analysis. Thanks for all the feedback, it's really useful to bounce ideas off the hive mind, hopefully this discussion will been helpful for others as a reference as well.

 

[ggunn]

I'm considering a voltage regulator on the AC side to help with voltage drop, it's probably a cheaper option (and certainly a simpler and more robust option).

What voltage regulator is that?

 

[electrofelon]

Yes please share your conclusions, it is very useful information.

 

[electrofelon]

That's my experience as well. I have been mulling over the idea of grouping the inverters at the switchgear as a possible alternative. Although wire costs and line losses will probably be a wash, it moves some of the AC losses to the DC side and solves my concerns about excessive voltage drop/rise on the AC side. It would require combiner boxes at the arrays with a disconnect, but if I'm considering a voltage regulator on the AC side to help with voltage drop, it's probably a cheaper option (and certainly a simpler.

Having combiners at the array may not be such a bad thing. I am an installer, so I look at things largely from a ease of installation standpoint. One idea I have contemplated and would like to try is combining in the array. When we do several meg groundmounts, we pull a ton of #10 and #8 strings through the array. To the inverters. Last system we had 28 wires pulled through, and that was only half of a east west row. The other half was piped underground. It's just a big hassle dealing with so many wires, keeping track of them, tangles, setting up and moving the spools, etc. So I thought about combining say one inverters worth of strings then running a single pair back to the inverter. Another advantage besides ease of management, is you could use aluminum pv wire for the large combined dc run which would lower wire costs substantially. So i think that would lower material and labor substantially. That strategy flows nicely into a long dc run. Anyway, another idea.

 

[electrofelon]

Any updates OP??

 

[Sahib]

What voltage regulator is that?

Instead of step up and down transformers, use a LV voltage regulator at POCo end to çompensate for voltage drop. The conductors only need be sized for load current and no need for up sizing to compensate for voltage drop, a substantial saving thereby.

 

[Sahib]

It is interesting to calculate which one has lower life cycle cost: Step up and down transformers set up or voltage regulator set up.

 

[electrofelon]

Instead of step up and down transformers, use a LV voltage regulator

What is that? Can you provide a link to such a product?

 

[Milo902]

Instead of step up and down transformers, use a LV voltage regulator at POCo end to çompensate for voltage drop. The conductors only need be sized for load current and no need for up sizing to compensate for voltage drop, a substantial saving thereby.

I had considered this idea at one point as well. Unfortunately I got a budgetary quote and a 500kVA voltage regulator was about $80k, it just doesn't pencil. This was only reaching out to one company, but this would have to be less than 1/2 to be competitive.