Inverter max VOC

[electrofelon]

Ok so this is driving me nuts. Why don't inverter and charge controller manufactures make their products so that the equipment never sees the full VOC of the array? Let me elaborate. The general rule is to size the VOC of the array at the record low temp to not exceed the rating of the inverter or CC (I would argue that is in fact much too conservative in most applications, but my argument still applies even to a more realistic design). Here in central NY at low temps you could have 100 volts difference between VOC and VMP. If that 100 volts were "available" it could make a big difference in system layout and design options, wire size and costs. Why dont the manufactures make their products so they dont open circuit the array and/or incorporate a resistor to pull the voltage down if necessary or....hello....disconnect from the array? It really boggles my mind that they dont do this. We have 96% efficient dc-dc converters, pure sine wave inverters and controllers that can do MMPT from wind turbines, but we are stuck with having to worry about VOC? Fill me in on what I am missing, please.

 

[GoldDigger]

Well, whatever circuit was incorporated would have to work off the PV array power, since you could not count on line power being connected.
And you would not want to use a fixed resistive load for efficiency reasons. Pulling the voltage down substantially from VOC would require dissipating a significant amount of power.
I am not saying that it would be impossible, just difficult.
And the Midnite Solar HyperVOC design for CCs already goes a long way toward doing what you are asking for. For example, it will withstand with no damage an input voltage up to 48 volts higher than its maximum operating voltage when used to charge a 48 volt battery system.
I have not heard of inverters with similar abilities though.

FWIW, the Midnite support for wind turbine input actually involves programming a specific load current versus Voc curve rather than doing an MPPT sweep. The results are similar but actually better suited to the output characteristics of a turbine than a straight MPPT algorithm would be. The turbine has a time dependency in its output in that it can deliver more power while spooling down from a higher speed than the wind is delivering. Writing for the output to stabilize would make an MPPT algorithm designed around PV output characteristics impractical.
But by setting load current based on sampled Voc, you can allow the turbine to set its own running speed based on the available wind power.

Tapatalk!

 

[hurk27]

It seems with switch mode technology today it would be no problem to convert this over voltage into more useful output power, we do it at work with switch mode AC/DC to DC PS that will take in 90vdc through 385 vdc and still give us a steady 24 volts output, which is adjustable up to 48 volts.

We have LED lights on our cranes that use switch mode PS that will take anything from 75 volts AC/DC and keep the LEDs at the same level of brightness all the way up to 350 volts, once in a while the regen of an ore bucket going down will cause enough regen to peek at 400 vdc and take out the fuses in them as the MOV's start to clamp at 385 volts, but still were not talking about that much here, again these lights can be plugged into any 120 volt outlet or 240 volt ac, but we run them on our 250 volt DC crane buss, which is why we went to them because of the regen taking out the older 1500 watt quartz lamps as well as the other types of lamps we have on the cranes, which we replaced with LEDs with SM PS.

So it seems to me this would be the way to go with this application

We also have inverters that are designed to run on 250vdc but will run just fine from 90 vdc to 350 vdc so it is possible, just pricy

 

[jaggedben]

I'm not sure I understand what the issue is. Does the complaint mostly have to do with charge controllers for off-grid applications? Because if we're talking about grid-tie applications, the (typical) 600V limit is set by the UL standard, and is held in place by a lot of other factors such as the rating of wiring and equipment.

 

[electrofelon]

Sorry for the delayed reply. The theory is this: I am in the northeast so say I design a PV system such that I think it is practical for it to produce power when the modules are 0 degrees F. Now say I have a string of modules that has a VMP at 0 degrees of 530 volts which would be a VOC at the same temp of about 125 volts more or 655. To make my point maybe we could even add another modules in there but likely the inverter's MPPT range tops out at 550 plus there is no point in cutting things too close so lets leave it here. It seems that without much trouble, inverters could be designed so that the VOC doesnt happen - it doesnt "use" the VOC so who cares? That is 120 volts of valuable electrical real estate that could result in much more design flexibility in decreased costs. Granted the NEC probably wouldnt be thrilled to entrust the inverter to keep the VOC below the cables' rating, however we are not talking much over worst case, its a current limited source, and other electrical items like CT's can rise to very high voltage if open circuited. As far as the engineering of the inverter goes, I dont see that it would be a challenge. The highest voltage levels will be when there are very low power levels. In the case of a grid tie system we would need to provide for the grid going down but we could just short the lines. It is a similar concept to what is done with wind turbines where you have a "clipper" to keep the voltage in spec. Take my idea a little more conservatively if you want, just seems we could do a little better. What got me thinking about this is a severe case which is a system at my place I will be putting in this summer where I have a 1400 wire run and a 3.25 KW array going into a 600v charge controller (24V battery system). More panels in series are a free lunch and because of the length of the run and the cost of the charge controller, I was NOT going to run a second string and controller so I wanted to pack as much in as possible and it was frustrating seeing all the "real estate" eaten up providing for a once every 5 year cold event.

Golddigger, Yes the midnite controllers are very sweet and the hyper voc capability is super handy. I would use one for my array but I need to go all the way to 600 volts because of this long run and there is only one that does it, the schneider mppt 600 (actually morning star has one too but it has a lower current rating). I hear that midnite is working on "wind learn" software that will allow the CC to over time figure out the MPP curve of a turbine. It sounds like an interesting problem, basically a matter if figuring out how to know the turbine's inertia i guess.

 

[jaggedben]

... Granted the NEC probably wouldnt be thrilled to entrust the inverter to keep the VOC below the cables' rating,...

You're right. They would never, ever do that. What if the inverter is turned off? What if the there's a grid outage? What if there's a ground fault? Do you want to service a system that has wiring and equipment that's only rated for 600VDC when the actual voltage may be higher? How 'bout if you're there to service it because there might be an intermittent fault?

I think you are missing the fundamental point that the VOC is a property of the panels, not the inverter, and that the max voltage rating for all the wiring and equipment, including the inverter, must be for the VOC. It's true that the 600V limit is fundamentally arbitrary, but as long as the vast majority of available wiring is rated for 600V then it makes sense for the UL inverter standard to put in there.

You'll be glad that the 2014 NEC raises the limit for some systems (not residential!) to 1000V. But you're still going to have a gap between the the VMP and the max rating of the wiring and equipment. That will be true long as you're using crystalline silicon panels.

 

[electrofelon]

You're right. They would never, ever do that. What if the inverter is turned off? What if the there's a grid outage? What if there's a ground fault? Do you want to service a system that has wiring and equipment that's only rated for 600VDC when the actual voltage may be higher? How 'bout if you're there to service it because there might be an intermittent fault?

I addressed those issues in my previous post.

I think you are missing the fundamental point that the VOC is a property of the panels, not the inverter, and that the max voltage rating for all the wiring and equipment, including the inverter, must be for the VOC. It's true that the 600V limit is fundamentally arbitrary, but as long as the vast majority of available wiring is rated for 600V then it makes sense for the UL inverter standard to put in there.

no I understand that, just saying that the "VOC" can be controlled/clipped. My wind turbine will go way above 600 volts if it is unloaded. A typical safety set up for turbine is to have a NC contactor across the line terminals and held open by the controller so that if it fails, it will short out and stop the turbine. Just saying I think a little more allowance would be justified because the inverter could "clip" the voltage so that the equipment would never see the voltage above 600 and even if it did happen, it would be very rare and not that much above 600 anyway. Currently we are generally limited to 550V max for mppt so say we set that as our worst case vmp, The VOC at 0 degrees calculated from the stc ratings is extremely unlikely.

You'll be glad that the 2014 NEC raises the limit for some systems (not residential!) to 1000V. But you're still going to have a gap between the the VMP and the max rating of the wiring and equipment. That will be true long as you're using crystalline silicon panels.

Yes that is interesting. Are you aware of any plans by inverter manufactures to make higher rated products?

 

[GoldDigger]

And what others are trying to tell you is that relying on active clipping or a shorting contactor to hold the voltage within the upper limit of either the wiring insulation or downstream equipment is not going to be allowed by Code.

 

[SolarPro]

Yes, inverter manufacturers are making higher rated products. There are listed 1,000 Vdc-rated inverters, combiners, modules, fuses, wire, etc. available for non-residential applications right now.

 

[jaggedben]

no I understand that, just saying that the "VOC" can be controlled/clipped. My wind turbine will go way above 600 volts if it is unloaded. A typical safety set up for turbine is to have a NC contactor across the line terminals and held open by the controller so that if it fails, it will short out and stop the turbine. Just saying I think a little more allowance would be justified because the inverter could "clip" the voltage so that the equipment would never see the voltage above 600 ...

Your wind turbine control might be allowed because it is a single device whose voltage can be limited outside the listed assembly. That isn't possible for a string of PV modules connected in series, where you have wiring in between them, which may even be field wiring rather than part of the listed module.

The only way I see anything like your idea being allowed by the code is with module level electronics that limit the voltage unless they receive a signal that the system is operating or ready to operate. Something like SolarEdge. It's not something that can be done with the inverter or charge controller alone.

 

[ggunn]

no I understand that, just saying that the "VOC" can be controlled/clipped.

No, it can't. Voc is the voltage on the wiring when the inverter is off, and when it's off it cannot control the voltage. On a cold winter morning when the sun's rays first hit the modules, they produce temperature corrected Voc with virtually zero current and the inverter will not come on until there is enough current available to run. That voltage needs to be kept below the max voltage that the inverter and wiring can tolerate, and limiting string length is how you do it.

It is pointless to argue about how inverter companies could build their machines differently to avoid this condition, or even whether or not it would be possible. They are what they are. If you can build a better mousetrap, then have at it.

 

[dereckbc]

Problem is solar panels are current sources from Vmp to Isc, and voltage sources from Vmp to Voc. There is no useful power operating as a voltage source above Vmp as current falls off a cliff and thus little power.

 

[electrofelon]

No, it can't. Voc is the voltage on the wiring when the inverter is off, and when it's off it cannot control the voltage. On a cold winter morning when the sun's rays first hit the modules, they produce temperature corrected Voc with virtually zero current and the inverter will not come on until there is enough current available to run. That voltage needs to be kept below the max voltage that the inverter and wiring can tolerate, and limiting string length is how you do it.

That is why i had VOC in quotes, that means i am using it in a non literal meaning, i.e there is some resistance connected to pull the voltage down. Yes it wouldnt actually be VOC in this case I understand that. When the sun comes up the irrandiance will be lower too so I think the worst case that is often calculated for would be extraordinary rare. I understand how it is done, just throwing around ideas here and that maybe a little more allowance could be considered. Let me give an example of how this might work and what got me thinking about it. I am off grid and I am putting another PV system at my house and the array will be 50% larger than the CC can handle. It is a 1400 foot wire run. The extra capacity is so I will have more output on cloudy days and so I can heat water. There will be some voltage controlled relays controlling water heating elements on the line side of the CC to extract the power that is beyond the CC capacity. Because of the long wire run, it is most efficient, cost effective, and I get the most bang for the buck to run the voltage as high as possible. Because of this I may want to run the VMP close to 550 for say 95% of the operating time even if it means using controls to disconnect and/or short out the array if the voltage exceeds 600 for that other 5% of the time. Anyway that what got me thinking about this and if perhaps something analogous could be applied to "normal" systems. Us off griders are on the cutting edge and way ahead of current codes and practices

It is pointless to argue about how inverter companies could build their machines differently to avoid this condition, or even whether or not it would be possible. They are what they are. If you can build a better mousetrap, then have at it.

Sorry I didnt know commenting on improvements to equipment and changes in the codes was off limits here :roll:

 

[SolarPro]

Why don't inverter and charge controller manufactures make their products so that the equipment never sees the full VOC of the array?

Doing so just requires more components and complexity. You can get away from dc string sizing limitations by using dc-to-dc converters. For example, the Solar Edge solution is designed such that the inverter always sees a fixed input voltage of 350 Vdc, regardless of whether you have the minimum or maximum number of modules per string. In effect, over-voltage is no longer your limiting design factor, but rather current and the allowable conductor ampacity. As a result, Solar Edge systems can accommodate 50% longer strings as compared to a traditional system designs.

I bet someone out there is ac coupling Solar Edge inverters with traditional battery-based inverters.

 

[GoldDigger]

But if the panels and optimizers are some distance from the string inverters, the lower (350V) voltage is still less than optimal.

Tapatalk!

 

[SolarPro]

Right. Satcon used to run its string optimizers up around 550 Vdc, but we all know how well that worked out for them in the end. Once we start seeing more 1,000 Vdc PV arrays in the US, I suspect we'll start seeing more solutions that will do what the OP asking for. It's technically possible. But no one is really designing equipment for the confluence of issues the OP is dealing with, especially for the relatively small and cost sensitive off-grid market segment.

 

[Barbqranch]

Regarding wire insulation, shouldn't wire rated at 600 VAC be designed to handle almost 850 VDC?

 

[jaggedben]

Regarding wire insulation, shouldn't wire rated at 600 VAC be designed to handle almost 850 VDC?

If you got that number by calculating the peak voltage of 600 VAC RMS waveform, and you are implying that the wiring could handle steady (non-pulsed) DC voltage at that level, then I'd say the answer is no, it's not designed for that. There's a reason that RMS voltages are used in the engineering of things. If you had a pulsed DC power source, you could have a peak well above 600VDC, but the RMS voltage would still have to be 600VDC or below. Solar panels are not a pulsed source, so the voltage simply has be less than the wiring is rated for.

 

[jaggedben]

That is why i had VOC in quotes, that means i am using it in a non literal meaning, i.e there is some resistance connected to pull the voltage down.

VOC is a real property of solar panels. The wiring between them must be rated for the VOC to prevent a fault, period. The only way to get a VMP that goes all the way up to the wiring rating would be to have cutoff controls that are within the listed solar module assembly. The code will never allow anything else. Give up on that. (Yes, there are safety factors that mean this is overkill, but safety factors exist for a reason and the CMP will never change their minds on that.)

Yes it wouldnt actually be VOC in this case I understand that.

No, it could actually be VOC, in case of system failure. I think that's what you're not getting.

When the sun comes up the irrandiance will be lower too so I think the worst case that is often calculated for would be extraordinary rare.

The code needs to assure that installations fail safely, not just that they operate safely. The rarity of failure doesn't change this one iota. This is true across the board, not just in PV systems.

I understand how it is done, just throwing around ideas here and that maybe a little more allowance could be considered. Let me give an example of how this might work and what got me thinking about it. I am off grid and I am putting another PV system at my house and the array will be 50% larger than the CC can handle. It is a 1400 foot wire run. The extra capacity is so I will have more output on cloudy days and so I can heat water. There will be some voltage controlled relays controlling water heating elements on the line side of the CC to extract the power that is beyond the CC capacity. Because of the long wire run, it is most efficient, cost effective, and I get the most bang for the buck to run the voltage as high as possible. Because of this I may want to run the VMP close to 550 for say 95% of the operating time even if it means using controls to disconnect and/or short out the array if the voltage exceeds 600 for that other 5% of the time. Anyway that what got me thinking about this and if perhaps something analogous could be applied to "normal" systems. Us off griders are on the cutting edge and way ahead of current codes and practices

This system design you've described makes little sense and would work badly. One does not run an array directly to multiple loads where one of them is a MPPT CC and the others are water heating elements. The latter will mess with the former and cause it to work suboptimally. More than one MPPT device on an array will not work either.

I believe the 2014 code would allow you to have up to a 1000VDC ground mounted array, as long you don't have more than 600VDC entering a single family home. (Disclaimer: haven't read the new code in all that much detail yet). In other words, perhaps you could have a relay cut power to the house if the temperature dropped so low that the VMP was over 600V. Now, this would mean losing power on the coldest days of the year, which seems ... dumb. But in the particular unusual case you have described above, it might be that code gives you the 50V of wiggle room you are asking for.

Generally, I feel that either
a) you are still not really getting how the technology and engineering really works, OR
b) you are trying to argue, without really saying so, that "the code should really allow us to ignore VOC because that is not the operating voltage." Regarding this, there isn't a chance in heaven that the Code Making Panel will ever agree.

Sorry I didnt know commenting on improvements to equipment and changes in the codes was off limits here :roll:

I think what ggunn was trying to tell you was about staying within reality, not current equipment and codes. :happyyes:

I think you should be happy that both myself and SolarPro have given you an answer that is code-compliant and already more or less exists: module level electronics. There's no technical reason that SolarEdge-style power optimizers couldn't be manufactured to operate at higher steady voltages than 350VDC. There's also probably no reason that such electronics couldn't be made to string voltage to 600V output, which could mean being off the actual max power point, but only on rare occasions.

Maybe you should contact SolarEdge and make the suggestion.

 

[ggunn]

That is why i had VOC in quotes, that means i am using it in a non literal meaning, i.e there is some resistance connected to pull the voltage down. Yes it wouldnt actually be VOC in this case I understand that. When the sun comes up the irrandiance will be lower too so I think the worst case that is often calculated for would be extraordinary rare.

Voltage has virtually nothing to do with irradiance. When the first rays of the sun hit the modules in the morning, the voltage goes to Voc for whatever temperature the modules are at the time, which is pretty much the low temp for that night. It's Voc because oc means open circuit, and when the inverter is off there is no current flowing through a series resistance to pull the voltage down. There is no "VOC" as you use it; it's either open circuit or it's not.

Sorry I didnt know commenting on improvements to equipment and changes in the codes was off limits here :roll:

C'mon man. I wasn't beating you up because I think your comments are "off limits". You can say anything you want but don't be surprised when folks criticize it because it is unrealistic. It isn't impossible that you would think of an inverter improvement that hasn't occurred to any of the inverter designers in the world; it's just highly unlikely.