PV voltage and more universal controller options?

fastline

Member
Location
midwest usa
I have been thrown in to help design an off grid PV system. Right now the system is being designed around 20 KW of panels, and 10KWH of Li battery storage. The distance between the array and service point will be approx 400ft. There is also a definite plan for a lot of expansion to the system so I need to drop spare conduits, and think ahead. I am looking for the highest practical DC operating voltage and have yet to comb through codes to see if there is much out there on it? I don't yet know where we will end up but looking like 500-550VDC will be a target, mostly to minimize conductor losses over the distance. Many charge controllers appear to be getting on board with the higher voltage, but there is still a LOT of low V stuff due to the DIYers.

Our battery bank voltage will have to be configured to play nice with the inverters, and that right now is being proposed at 48V.

I am curious if you guys are running at higher voltages?

Also, it would seem many equipment OEMs like to offer proprietary controls, monitor software, etc. There is a large push to find a solution that is more generic so we are not married to a brand of inverter and controls. Right now we are looking for Inet or at least PC based solutions because we need to be able to monitor the system remotely. It would be VERY preferred to control the system remotely as well, but that may not be required at the first stage.

Monitored systems would be the typical power, power trends, voltages, temperatures, etc.
 

tom08588

Member
Location
Florida
Occupation
Engineer
Option we've used (and arguably the most reliable) is an SMA setup


Check out page 18 of that PDF for a quick overlay. Their monitoring software and service is really easy to trend and seems to fit your needs. Plus SMA is a bigger name in the game and won't likely go out of business, so you'll get plenty of support.

Our arrays are never more than 600V DC either, I've only seen higher on utility scale fields.
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
I have been thrown in to help design an off grid PV system. Right now the system is being designed around 20 KW of panels, and 10KWH of Li battery storage. The distance between the array and service point will be approx 400ft. There is also a definite plan for a lot of expansion to the system so I need to drop spare conduits, and think ahead. I am looking for the highest practical DC operating voltage and have yet to comb through codes to see if there is much out there on it? I don't yet know where we will end up but looking like 500-550VDC will be a target, mostly to minimize conductor losses over the distance. Many charge controllers appear to be getting on board with the higher voltage, but there is still a LOT of low V stuff due to the DIYers.
It looks like you are going to be going with AC coupling, judging from the battery voltage you cited; why not 1000V if your inverter of choice can handle it?
 

fastline

Member
Location
midwest usa
Tom, I agree, SMA seems to have a solid product lineup. However, as mentioned, I believe their monitor system is proprietary to their equipment. The concern is if say a charge controller is later converted to another brand, we may lose the ability to monitor unless staying with SMA. I sort of feel like the monitor systems are working to lock people to a brand and I am finding the inverter markets to be highly mobile with very large companies selling out, and would not surprise me to see someone buy SMA, or even SMA going out due to competition.

For this reason, we are really hoping to find a stand alone system, but that might not be feasible.

ggunn, one of the most frustrating things in all of this is getting quality technical documentation. Most of these inverter companies will not give a good block or wiring diagram. They are a magic box! I am used to the world of VFDs where we can nearly get PCB schematics and know what we have.
It would seem most "hybrid" inverters seem to follow with AC coupling. For what reason, I really don't know. One would think inverting the DC PV panels to AC, just to go right back to DC to charge a battery would just add unnecessary complexity. I would not mind, and rather prefer to have discrete components such as a DC/DC charge controller for the battery., but getting down to actual efficiency differences will be difficult.

The system will have genset backup. Many hybrids include generator support but I want to know EXACTLY how the systems work. I am left to speculate that how, for instance, the Schneider can be tuned for charge amps from the genset. How is it modulating the power from the genset?
 

jaggedben

Senior Member
I have been thrown in to help design an off grid PV system. Right now the system is being designed around 20 KW of panels, and 10KWH of Li battery storage. The distance between the array and service point will be approx 400ft. There is also a definite plan for a lot of expansion to the system so I need to drop spare conduits, and think ahead. I am looking for the highest practical DC operating voltage and have yet to comb through codes to see if there is much out there on it? I don't yet know where we will end up but looking like 500-550VDC will be a target, mostly to minimize conductor losses over the distance. Many charge controllers appear to be getting on board with the higher voltage, but there is still a LOT of low V stuff due to the DIYers.

Our battery bank voltage will have to be configured to play nice with the inverters, and that right now is being proposed at 48V.

I am curious if you guys are running at higher voltages?
I agree with ggunn, sounds like you should AC couple. Such a relatively large PV array at such a distance does not lend itself to 48V DC coupling.

48VDC is something I regard as rather old-school, but the alternatives, such as the LG RESU10H with SolarEdge, are not really up to speed for the specs you're talking about either.

I also think the SMA Sunny Island is probably your solution. I'm not very familiar with it, but I'm familiar enough with most of the well known alternatives to know what the problems or drawbacks would be in your case.

Also, it would seem many equipment OEMs like to offer proprietary controls, monitor software, etc. There is a large push to find a solution that is more generic so we are not married to a brand of inverter and controls. Right now we are looking for Inet or at least PC based solutions because we need to be able to monitor the system remotely. It would be VERY preferred to control the system remotely as well, but that may not be required at the first stage.

Monitored systems would be the typical power, power trends, voltages, temperatures, etc.
My opinion is that you're probably going to cause yourself headaches by thinking this way. Inverters are listed to product standards that generally preclude Joe Electrician from messing with their internal controls, not to mention much of this stuff is proprietary stuff they don't want to share with you. Attempts to jury rig solutions that are not supported by the manufacturers are going to simultaneously risk breaking stuff and voiding warranties, which is not a good combination. Other than passive monitoring, your not going to find any third party off-the-shelf solutions that allow you to 'control' the system. What do you want to control anyway? Once these systems are set up they're pretty much supposed to work on their own.

As far as Schneider, I'm sure if you called the right tech support line you could find someone to explain how their algorithms and such work in basic terms. I don't think they are hugely complicated, just formulaic parameters that are adjusted for different battery types. If you enjoyed programming MIDI synthesizers in the early 90s then their interface is for you.

For any given product, ask tech support whether user settings can be changed remotely by internet. Usually this is limited (partly for security reasons, I think) but often tech support can do things for you remotely if you call, which still saves a trip to the site. That's the advantage of going along with the manufacturers features instead of trying to stay independent.
 

jaggedben

Senior Member
Hmm. Another product suite you might want to look at is Pika. They can really DC couple at high voltage without the same limits as Solaredge. But then, it sounds like your project already has its batteries, so it might be late for that.
 

pv_n00b

Senior Member
Location
CA, USA
Hmm. Another product suite you might want to look at is Pika. They can really DC couple at high voltage without the same limits as Solaredge. But then, it sounds like your project already has its batteries, so it might be late for that.
Pika was bought by Generac.
 

fastline

Member
Location
midwest usa
Let me ask a couple dumb questions that I have yet to get solid answers to. excuse any ignorance.

1. There seems to be some hazy information about "AC coupling" and most all information I find is regarding grid tie applications. This is not. It would seem that AC coupling (the term) is regarding the high voltage, series connected PV array in which digital DC conversion must occur in either a built in or discrete charge controller? Obviously you have to get down to appropriate battery voltage. Example 600VDC array down to 50VDC charge voltage.

2. Also, there is lots of sellers mentioning the shading issues of series connected arrays as it will drop the voltage. However, if you are series connected, you have a huge voltage delta and I would think a good charge controller could work this out?

3. I have also not every touched micro inverters as they are fairly new and I was asked specifically about them today for pros and cons. Personally, I don't see these as being practical or beneficial for a large system? I mean, I guess if 1-2 go down, you still have some output, but part of the design is to bring ALL sensitive electronics indoors so we can manage temperature, and we are also planning to parallel multiple inverters for some level of redundancy. Right now that is at least a pair so if one goes down, you still have 50% power and not a total black out condition.

As well, the design changed gears today with some new loads and calculations and battery storage is now at 40kwh and inverter capacity at 15kw continuous. The overall intention of the upsize is to reduce the current and cycles to the battery for a better life forecast and to ensure the inverters are not forced to work at full rated capacity.
 
Let me ask a couple dumb questions that I have yet to get solid answers to. excuse any ignorance.

1. There seems to be some hazy information about "AC coupling" and most all information I find is regarding grid tie applications. This is not. It would seem that AC coupling (the term) is regarding the high voltage, series connected PV array in which digital DC conversion must occur in either a built in or discrete charge controller? Obviously you have to get down to appropriate battery voltage. Example 600VDC array down to 50VDC charge voltage.
I agree the term "AC coupling" is typically used for grid tie application. Its kinda a silly vague term IMO, but generally refers to having two AC systems synced together, neither of which is the grid. One is a typical GTI, the other provides a grid for the the GTI to sync to and removes or adds power to the system as needed. So for off grid, its really the same thing, but likely things are just sized differently. So this varies from a "traditional" off grid system where the PV connects to a charge controller and charges batteries. Then an inverter provides the AC power. I cant really comment on the advantages of each. I did live off grid for years and did or worked on a handful of grid systems, but these were all smaller and of the traditional type. The AC coupled type seems to be better suited for larger systems.

There are several 600V charge controllers available (morningstar and Schneider), but even for a 48 V bank, that is only about 5KW on the battery side, but remember the PV side could be considerably more considering max charge rates and over-paneling for less than ideal days and winter.

IF you went the AC coupled route, you can go up to 1000V strings with fronius inverters which would help with you 400' wire run, but 20KW @ 400V really isnt that bad (#2 AL would probably do it).

Either way, make sure you have generator support functionality built into your equipment. I assume something like a sunny island has it, but not all off grid inverters do. Know how to use it. The key thing about off gird is dont be scared or running the generator. Many avoid it at all costs until the batteries are way down, then you are stuck running it for hours and hours. Proper generator support will bring the genny online strategically for short periods of high loading - that is what its there for, not to charge your batteries when they are dead.

Good luck , and IMO dont get involved with off grid. Its hard to keep people happy as most do not understand the limitations and lifestyle required. These systems can be finicky, have bugs, and/or take lots of experience to get them right.
 

fastline

Member
Location
midwest usa
Much appreciated and I agree with all points. The reason I am involved is I know these people and the owners are technically minded and there is some drive on all sides to "learn, understand, and break new ground" at least in our area. Usually when people say "live off grid", it conveys a primitive lifestyle. The reality here is they are building in an area that would cost huge dollars to get grid power only to have the "privilege" of paying them each month. I was contacted and I mentioned "invest half of that install expense in solar and live a normal life". Now I am trying to make that a reality.

Right now the bench mark is 50kwh/day usage. Switching a couple things like an air compressor to a VFD and going to a gas dryer will be HUGE. Once it is running, it will be monitored for a year or so and then make improvements.

Agree on the genset. Offgrid people seem to live in this dream where the generator is a last resort. NO, if you know you are doing laundry, washing dishes, while running the tanning bed, kick the genny! A good size generator can actually be a very comparable cost to grid power if loaded. This will reduce the cycle rate on the battery, make it live longer, and make everyone happy. the trick is getting really good genny controls. What I see most common is "set batt voltage for when to turn on genny". No, I want to set several protocols because I would rather not cycle the battery if not needed.

Here is my wonder with the AC coupling thing. They show having two inverters basically, then you have an "all in one" hybrid inverter (very common) with MPPT charge controller, with/without grid tie, gen start, etc, etc. Its all in there, but it comes down to how well each function actually works and if even one section goes down, you are hosed, in which this has to be designed to completely bypass the PV (likely manual) and let the genny take over while things get repaired or figured out.

Obviously all this complexity is why people don't get into solar but I think some things are getting sorted and somewhat standardized.
 
How much are we talking to get utility power? Can they do some some sort of step up step down scheme? Utility power is a wonderful thing, and dont underestimate the full cost of an off grid system considering expensive appliances, batteries, fuel, maintenance, problems, decreased house value, inconvenience, etc.
 

jaggedben

Senior Member
AC coupling means that instead of trying to match PV and battery DC voltages, you invert the PV to AC and have a charge controller that draws AC from the micro-grid to rectify it back to DC to charge the batteries. It's becoming the more common method in residential energy storage for backup purposes (e.g. Powerwall, Enphase, Sonnen). It generally works by shifting the AC frequency, which knocks the PV inverter offline when there's more power than needed and the batteries are already charged.

Again, to my knowledge the SMA Sunny Island is the most comprehensive solution. SMA had at least at one point a feature to actually throttle the solar output with frequency, so that the PV control isn't just on/off. This allows more flexible pairing of different size systems. That product definitely includes generator support as well.

The other comment I have is that if the target is 50kWh usage per day, you don't have nearly enough storage capacity. Well, that is unless the generator is going to run a lot. You're talking about not being able to store even a tenth of the likely PV energy output in the summer, but you need to store something like 30-40% of it. Meanwhile in the winter or bad weather, you don't have enough storage buffer to get through half a day.

BTW, it's not super relevant, but micro-inverters are not really new anymore.
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
Good luck , and IMO dont get involved with off grid. Its hard to keep people happy as most do not understand the limitations and lifestyle required. These systems can be finicky, have bugs, and/or take lots of experience to get them right.
I agree. Some offgridders do their homework/research and know what they want and how to maintain it, but, unfortunately, many assume that they will be getting the same "set it and forget it" operation as if they were buying a simple grid tied system. If your customer is in the latter category, be prepared for questions and truck rolls for years.
 

fastline

Member
Location
midwest usa
Jaggenben, the storage was already upgraded to 40kwh and 15kw of inverters.

ggunn, the owner of the system is a mechanical engineer that I know. He is very hands on and once he is up to speed, things should go smooth. One of the reasons we are doing this is we can learn from each other a bit. This can help us when doing other installations in the future.
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
ggunn, the owner of the system is a mechanical engineer that I know. He is very hands on and once he is up to speed, things should go smooth. One of the reasons we are doing this is we can learn from each other a bit. This can help us when doing other installations in the future.
That's all well and good, but if you decide to get into the business of building off grid PV systems, you will undoubtedly run into the other kind of customer, and it doesn't take many of them to eat up a tremendous amount of your bandwidth. If for some reason I had no alternative, I think I'd have the customer perform his own load analysis and battery/PV requirements so I could be confident I could hand it over to him and walk away when I 'm done.
 
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So with AC coupling, one could of course keep the GTI fully loaded at all times using battery charging, and then when the batteries are full, diversion loads, but it seems with some of these the GTI is able to throttle back its output. How does that work? I just looked at a SMA system walk thru on you tube and the sunny island seems to be able to lower the output of the sunny boy inverter. How does that work? I have sunny boy inverters and have not seen such capability described in the manual. :unsure:
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
So with AC coupling, one could of course keep the GTI fully loaded at all times using battery charging, and then when the batteries are full, diversion loads, but it seems with some of these the GTI is able to throttle back its output. How does that work? I just looked at a SMA system walk thru on you tube and the sunny island seems to be able to lower the output of the sunny boy inverter. How does that work? I have sunny boy inverters and have not seen such capability described in the manual. :unsure:
It's in the Sunny Island manual. When the PV output approaches the demand of the load and the batteries are full, the SI will start to increase or reduce (it alternates between occurrences) the AC frequency and in response Sunny Boy inverters throttle back their output as the change increases to the point where they will shut down completely when the delta f reaches a minimum or maximum (I don't remember what it is). You have to get into the parameters of the SB to turn on this feature.
 
It's in the Sunny Island manual. When the PV output approaches the demand of the load and the batteries are full, the SI will start to increase or reduce (it alternates between occurrences) the AC frequency and in response Sunny Boy inverters throttle back their output as the change increases to the point where they will shut down completely when the delta f reaches a minimum or maximum (I don't remember what it is). You have to get into the parameters of the SB to turn on this feature.
Ok, cool.
 

jaggedben

Senior Member
So with AC coupling, one could of course keep the GTI fully loaded at all times using battery charging, and then when the batteries are full, diversion loads, but it seems with some of these the GTI is able to throttle back its output. How does that work? I just looked at a SMA system walk thru on you tube and the sunny island seems to be able to lower the output of the sunny boy inverter. How does that work? I have sunny boy inverters and have not seen such capability described in the manual. :unsure:
As far as I know it's a proprietary SMA feature. Of course it requires special programming of the PV inverter.[/QUOTE]
 
As far as I know, the gateway supplied with the PW (I don't see a brand on it but I think it's a Tesla manufacture) does the same thing regarding shifting the Hz of the PV array up to lower output if (a) the battery is charged and (b) there's more solar available than needed. My computer's UPS monitors powerline Hz and it's easy to see the shift upwards to 61-62 Hz around 1:30pm when when the battery is charged and the sun is out. PV output will lower and raise (if available) to meet just the house demand. Around sunset, when there's no PV juice available, the Hz settles back to 60 as the battery takes over.
 
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