DC to DC Converters / Optimizers

[Carultch]

I read through this... great article, thank you. is there a similar article for three phase inverters? the only one i can find is 3 phase but with the Canadian Electrical Code... looking for three phase with the NEC.... nothing on Google that I can find. If nothing i can try to call SolarEdge tomorrow.

The working principles of the DC side of the system are still the same for SolarEdge's three phase inverters as they are for their single phase counterparts. There are different specs and constraints that apply, but the fundamentals of the DC design are still the same. The 277/480V inverters that require you to use 1000V strings, while the 120/208V inverters are still 600V and less on their input. The 3-phase inverters also introduce the possibility of using dual-module optimizers. Expect them to be used on 277/480V designs, but don't expect them to be possible to use on most 120/208V designs.

The AC side of the system is designed exactly the same as the AC side for any other three phase PV inverter. If you are new to three phase, I'd recommend researching some of the fundamentals of 3-phase circuits. Some questions to get you started:
1. What is the meaning of the term "phase" in three-phase?
2. What is the ratio between the phase-to-phase voltage and the phase-to-neutral voltage? What is a geometric representation for why it is this ratio?
3. Given the phase-to-phase voltage and equal current on all three lines, and unity power factor, how do you calculate the kVA and kW?
4. Given equal currents on all 3 phase conductors, is there any current on the neutral? Given different currents on the three phase conductors, how would you calculate current on the neutral? What purpose does the neutral serve in a three phase circuit?
5. We talk about WYE and DELTA when discussing three-phase, which refers to the arrangement of the windings in a transformer. In general, this called the topology or the vector group. What do WYE and DELTA mean, and how did the two kinds of topologies get their names?
6. Can I connect a delta topology transformer primary, on a grid that starts with a wye topology? How about vice-versa? What is the essential difference between a grid formed by a transformer's wye secondary, and a transformer's delta secondary?

 

[Grouch1980]

The working principles of the DC side of the system are still the same for SolarEdge's three phase inverters as they are for their single phase counterparts. There are different specs and constraints that apply, but the fundamentals of the DC design are still the same. The 277/480V inverters that require you to use 1000V strings, while the 120/208V inverters are still 600V and less on their input. The 3-phase inverters also introduce the possibility of using dual-module optimizers. Expect them to be used on 277/480V designs, but don't expect them to be possible to use on most 120/208V designs.

ok, got it. So I have to be aware of the input voltage to the inverter, whether it's limited to 600 volts or 1000 volts, among other parameters. Makes sense.

I've been dealing with 3 phase for a while... I'm familiar with the calculations, whether or not the neutral conductor carries current, wye / delta wiring configurations in the transformers, etc. That part I got. However with a question here or there on other parts of the forum

Great... thanks!

 

[ggunn]

ok, got it. So I have to be aware of the input voltage to the inverter, whether it's limited to 600 volts or 1000 volts, among other parameters. Makes sense.

True, but with SolarEdge systems all you need concern yourself with is ensuring that your string length falls within the range shown in the chart at the bottom of the optimizer data sheet, though 30 is the upper limit for rapid shutdown.

 

[Grouch1980]

https://www.solaredge.com/sites/default/files/solaredge-system-design-nec.pdf

This part of the article confuses me: "For an inverter with a 240Vac output, the DC input to the inverter operates at 350Vdc. A 208Vac output system will maintain the DC input at 305Vdc."

Is the 350 / 305 Vdc across the board with all Solaredge inverters? Looking at cutsheets of some models, they indicate operating voltages that are higher than this. But the article seems to imply the 350 / 305 values are for all their inverters... or are they pointing to a specific model?

 

[Carultch]

This part of the article confuses me: "For an inverter with a 240Vac output, the DC input to the inverter operates at 350Vdc. A 208Vac output system will maintain the DC input at 305Vdc."

Is the 350 / 305 Vdc across the board with all Solaredge inverters? Looking at cutsheets of some models, they indicate operating voltages that are higher than this. But the article seems to imply the 350 / 305 values are for all their inverters... or are they pointing to a specific model?

The article is from 2015. They are either talking about a specific model or a hypothetical model. I cannot pinpoint which model it would've been. I would guess it is a hypothetical DC voltage, likely based on speculation about what the DC voltage would've been when reducing the AC voltage. 305V/350V is about the same ratio as 208V is to 240V, so it is a reasonable guess. Perhaps the white paper was written prior to the inverters getting tested for operation on the 208V grid.

This was before the HD Wave series became the standard for their single phase units. Here's a datasheet that shows their single phase series of inverters before HD wave technology became the standard, which shows 325Vdc corresponds to 208Vac, for an inverter that operates at 350Vdc on a 240Vac grid.

https://www.solaredge.com/sites/default/files/se-single-phase-us-inverter-datasheet.pdf

The HD wave inverters, that are the current generation of single phase inverters, have the same DC voltage, regardless of whether the AC side is 208V or 240V. You still have a power reduction from the nameplate power, since AC output current is a fixed value for the same model (typically the case for transformerless inverters in general). Only certain models are compatible with both grid voltages.

 

[Grouch1980]

The HD wave inverters, that are the current generation of single phase inverters, have the same DC voltage, regardless of whether the AC side is 208V or 240V. You still have a power reduction from the nameplate power, since AC output current is a fixed value for the same model (typically the case for transformerless inverters in general). Only certain models are compatible with both grid voltages.

Thanks! Do you select the voltage on these latest inverters? I'm assuming you can select the voltage, and that forces the optimizers to work at that voltage value. If not, how does one set the voltage?

 

[Carultch]

Thanks! Do you select the voltage on these latest inverters? I'm assuming you can select the voltage, and that forces the optimizers to work at that voltage value. If not, how does one set the voltage?

The inverter selects the voltage. This is model-specific, so you have to check the datasheet to know this value.

 

[ggunn]

Thanks! Do you select the voltage on these latest inverters? I'm assuming you can select the voltage, and that forces the optimizers to work at that voltage value. If not, how does one set the voltage?

Are you talking about AC voltage or DC voltage? You cannot set the AC voltage; it is fixed at the different service voltages for different inverters, and an inverter can only run at the service voltage for which it was designed. Likewise, you cannot set the DC voltage; that is determined by the particular inverter and optimizer combination you use. You cannot, for example, configure a 480V three phase inverter to run at less than 600V on a one or two family dwelling, not that it is likely that you would want to.

I respectfully submit that the answers to your questions are in the installation manuals and data sheets for SolarEdge inverters and optimizers. When you are designing a SolarEdge PV system, very important information for stringing is in the charts at the bottom of the data sheets for the optimizers. They will inform you of minimum string lengths, maximum string lengths, and maximum string kW for the different possible configurations of inverters, optimizers, and service voltages.

 

[ggunn]

Are you talking about AC voltage or DC voltage? You cannot set the AC voltage; it is fixed at the different service voltages for different inverters, and an inverter can only run at the service voltage for which it was designed. Likewise, you cannot set the DC voltage; that is determined by the particular inverter and optimizer combination you use. You cannot, for example, configure a 480V three phase inverter to run at less than 600V on a one or two family dwelling, not that it is likely that you would want to.

I respectfully submit that the answers to your questions are in the installation manuals and data sheets for SolarEdge inverters and optimizers. When you are designing a SolarEdge PV system, very important information for stringing is in the charts at the bottom of the data sheets for the optimizers. They will inform you of minimum string lengths, maximum string lengths, and maximum string kW for the different possible configurations of inverters, optimizers, and service voltages.

Correction: Single phase SolarEdge inverters can run at either 240V or 208V, but the DC voltage is still set by the inverter, optimizer, and service voltage. You cannot change it.

 

[jaggedben]

Thanks! Do you select the voltage on these latest inverters? I'm assuming you can select the voltage, and that forces the optimizers to work at that voltage value. If not, how does one set the voltage?

The components do it all for you. You only need to know the voltage to provide the code required labeling, and maybe for troubleshooting. Also the real voltage may vary somewhat depending on sunshine.

 

[Grouch1980]

Are you talking about AC voltage or DC voltage?

I respectfully submit that the answers to your questions are in the installation manuals and data sheets for SolarEdge inverters and optimizers.

I'm talking about the DC voltage. AC is what I mostly work with, I'm ok with that part.

I do look at the data sheets, but alas for me it doesn't come across too clearly at times. I'll show in the next post...

 

[Grouch1980]

The inverter selects the voltage. This is model-specific, so you have to check the datasheet to know this value.

For example, here's the data sheet for Solaredge's SE14.4 / 17.3 models... it's in the attachment. if you look at the input section, it states the operating voltage range at 370-600. How do I know exactly what voltage it will run at, if it gives me a range.

 

[ggunn]

For example, here's the data sheet for Solaredge's SE14.4 / 17.3 models... it's in the attachment. if you look at the input section, it states the operating voltage range at 370-600. How do I know exactly what voltage it will run at, if it gives me a range.

So, it varies. As long as the maximum is 600V, why do you care?

All inverters use MPPT to look for the voltage current product that maximizes power. Voltage goes up as cell temperature goes down, too. Voltage always varies.

 

[Grouch1980]

So, it varies. As long as the maximum is 600V, why do you care?

All inverters use MPPT to look for the voltage current product that maximizes power. Voltage goes up as cell temperature goes down, too. Voltage always varies.

Ok.. so in some of the inverters, the DC voltage is constant… and in other models, such as the ones I attached…. The DC voltage varies. Do I have it right?

 

[ggunn]

Ok.. so in some of the inverters, the DC voltage is constant… and in other models, such as the ones I attached…. The DC voltage varies. Do I have it right?

Voltage always varies. With some inverters more than others, but it always varies. Except for SolarEdge we set up our PV systems so that the DC voltage can never be more than 600V, or 1000V, or 1500V, depending, by calculating the maximum Voc at the lowest expected temperature, and then we just let it be what it is. For SolarEdge systems we set up our strings according to the ranges shown in the charts at the bottom of the optimizer data sheets; as long as we do that correctly we can trust the inverter never to let the DC voltage go over the top of its range. Other than that we don't worry over what the DC voltage exactly is at any point in time, because, as I said, it varies.

 

[Carultch]

For example, here's the data sheet for Solaredge's SE14.4 / 17.3 models... it's in the attachment. if you look at the input section, it states the operating voltage range at 370-600. How do I know exactly what voltage it will run at, if it gives me a range.

That's a good question. I haven't explored the reason why it gives this range, instead of the single nominal value that their datasheets usually give.

Historically, the similar 3-phase inverters built for the 208V grid have had nominal operating voltages of 400V. If you look at the optimizer datasheets for string sizing, then for the 208V inverters, it indicates that 6000W max per string on 15A output optimizers, which is consistent with a 400V operating voltage (400V*15A = 6000W). It doesn't make a distinction as to which of the 3-phase 208V inverters this applies, so my guess would be that the 400V value is still a likely operating input voltage of the inverter. This is from the datasheet for the P505 single module optimizer datasheet.

There's a fine print note on the same optimizer datasheet allowing 6500W in some circumstances, having to do with the maximum power difference per string. The reason it works this way, is the inverter increases the voltage above the 400V nominal value when the power exceeds 6 kW, in order to avoid increasing the amps per string above the 15A limit of the optimizers. The optimizer strings are less flexible for paralleling strings of dissimilar power ratings, when they need to add up to a higher voltage.

So, it varies. As long as the maximum is 600V, why do you care?

All inverters use MPPT to look for the voltage current product that maximizes power. Voltage goes up as cell temperature goes down, too. Voltage always varies.

Reasons you would care:
1. Voltage drop calculations
2. Labeling the system with NEC2014 and earlier requirements
3. String sizing. The max

SolarEdge externalizes the MPPT functionality to the optimizers, so the theory of operation is different from what other brands do. In general, the inverter aims for a nominal operating voltage, and may increases it between that value and the maximum voltage when necessary.

Because the optimizers "re-format" the mix of voltage and current, the adjusted Voc value of the modules doesn't get past the optimizer and doesn't add up with all the other optimizers in series. Voc only needs to be considered prior to the optimizer, and it is rate (if ever) that this governs a design decision.

 

[ggunn]

Reasons you would care:
1. Voltage drop calculations
2. Labeling the system with NEC2014 and earlier requirements
3. String sizing. The max

But with the exception of Voc max for traditional strings we don't care what the exact voltage is. We figure voltage drop on approximate voltage, and we label the system with approximate voltage for "operating voltage". We don't care what the precise DC voltage is when the system is running, a long as it is high enough to power the inverter and low enough so that it doesn't damage the inverter.

At least I don't.

Factoid: the early SMA transformerless inverters had such a narrow operating voltage window that with some modules it was impossible in Texas or any other place that gets as hot and as cold as it does here to design a system that could start up in the heat of the day.

 

[Grouch1980]

There's a fine print note on the same optimizer datasheet allowing 6500W in some circumstances, having to do with the maximum power difference per string. The reason it works this way, is the inverter increases the voltage above the 400V nominal value when the power exceeds 6 kW, in order to avoid increasing the amps per string above the 15A limit of the optimizers. The optimizer strings are less flexible for paralleling strings of dissimilar power ratings, when they need to add up to a higher voltage.

Thanks guys. Lots of great help and analysis.

I do have a follow-up question... Carultch mentioned the fine print saying 6500 watts per string is allowed. I've seen this fine print as well, it's a footnote towards the bottom. Let's say the difference between string A and string B is 500 watts... can you make each string a max of 6500 watts? or does the difference between the strings have to be exactly 1000 watts, instead of 500 watts? By saying 'maximum power difference of 1000 watts' seems to imply anywhere from 1 to 1000 watts.

 

[ggunn]

Carultch mentioned the fine print saying 6500 watts per string is allowed. I've seen this fine print as well, it's a footnote towards the bottom.

Where, on which data sheet? Inverter or optimizer? I have a bunch of SolarEdge cut sheets here on my desk and I don't see where you are finding that language in a footnote.

...does the difference between the strings have to be exactly 1000 watts

Nothing in solar is "exact"; we build systems that operate within ranges. I can't imagine that they mean exactly 1000W.

 

[Grouch1980]

Where, on which data sheet? Inverter or optimizer? I have a bunch of SolarEdge cut sheets here on my desk and I don't see where you are finding that language in a footnote.

Optimizer data sheet... I attached it. scroll down on the 2nd page, all the way to the bottom... it's footnote #(10).