A Down Side to String Inverters

[LowGear]

Wow!  My electric bill jumped up above $300 again.  Running the pool pump too much?  Too many hot showers?  No, a couple of trips out to the SMA inverter and it's obvious that one of two panel strings is not working. 

I've cleaned all the rats nests out but still three KW at noon prevails.  The inverter has two three KW strings so no math majors required.  One of the strings is down.

So I'm sure three KW at about 270 volts is pretty dang dangerous.  You're advice - warnings - heads-ups would be appreciated.

Casey

[mike90045]

a clamp on DC ampmeter is handy for cases like this.
be sure the one you get has DC amps for the clamp.
http://www.sears.com/craftsman-digital-clamp-on-ammeter/p-03482369000P

[glort]

Going out and checking the meter once a week isn't a bad idea either.

I used to screw with the mongeral neighbour at my last place.  The isolator switch for his solar was just over the fence on his wall. It was a rotary type so I cut slots in a piece of PVC pipe that would engage them  and used to just stand on my side of the fence and turn it off.  I only did it about twice in 14 months because he never checked it and being a new house, probably had no idea what his bill should be. I know one time it was turned back on was by a sparky who came to do some work and turned the power off then turned the solar back on . It was off again that night.

[LowGear]

I have one of those but upon checking I see it's AC only.

Yes, checking it each morning is the new routine.

[David Baillie]

When you do figure out the bad string maybe look into optimizers for the panels. They will give you panel by panel readings and a heads up if one or more go bad. We use them for partially shaded locations where we have to use the central inverter. More and more we are switching over to micro inverters due to all the rapid shut down requirements and the inherent dangers of high voltage DC strings. Not sure what you have installed forgive the intrusion if its too obvious.
Cheers, David

[glort]

I have a set of Micros and frankly find them to be a pain in the backside. I also think Casey would not enjoy installing them onto an existing system. More cabling, have to mount the things under the panels or close by, need other connectors, end up with a whole load more wiring and the things are not cheap either.

The thing about High voltage DC strings I find a little bit of a moot point when you are going to have high voltage AC cables as soon as the things leave the panels.  Running a DC string for me has been a lot easier than Hooking up each and every panel to a DC and an AC connection. Mounting the inverters is a whole other issue and you multiply the failure points of the AC connectors as well.

I actually have my micros for sale right now and a couple of regular string inverters on order to replace them.
I don't have any shading issues so to me the positives are nil but the drawbacks are several.

Casey,

I have a few of these watt hour meters.

http://www.ebay.com.au/itm/100A-AC-Digital-LED-Power-Meter-Monitor-Voltage-KWh-Watt-Voltmeter-Ammeter-OK/261671605849?hash=item3cecd7d659:g:U3kAAOSwkNZUcFnP

You would just run the active of your inverter going back to the grid through the little torrid so it does not interfere with the wiring itself.  You tap to the AC just to power the meter which only takes a couple of watts.
They tell you the voltage of the line, the instant power in volts and watts and keep a total of the KWH which is re settable.  There are a few other functions like setting power factor and other things I don't bother with but are there.

You could run some light wire from the torrid back to the meter so you could see all the output the inverter was doing at a glance. You soon get an idea of what is normal and can spot any anomalies in the power produced.
I find scrolling through some of the inverter menus tedious and annoying when you have to look at screen that tells you the brand and then the make of the inverter, the date and timeyearly total and other things in about 10 screens when you only want one. Some of my inverters go back to the start screen after a period of tinme so there is no just leaving them at the screen you want. Pain in the arse!
With these little meters,  you have all the info at a glance.

I have a couple built into small project boxes that I wired up with a short extension cord I cut in half and wired in.  I can plug in any appliance and see what the thing draws.  I can be an interesting experiment to see what that little bar fridge pulls over the course of a day or even just what the total power consumption is to boil the kettle. 

I'm building a diversion box atm to run the hot water heater off the solar feed. I have a PWM controller incorporated so I can regulate the amount of power I send to the heater. One of these little meters tells me exactly how much power that is which is useful to balance the load with the output of the solar.  Without the meter I wouldn't be able to tell if I was feeding 200W or 2 KW.
They make the meters in a straight though setup with no induction coil which is fine for low amperage uses like the test power meter but for something like your solar feed which is going to be heavy wire you don't want to be diverting through tiny little terminals, these meters with the torrid are great because you are not interfering with the cable in any way. Just disconnect the wire, slip the inductor over it and re connect.

You can also get DC versions which use a shunt but from what I have seen they only are good to 100V which is probably going to be way too low for your string voltage. They would probably be fine for measuring input in to a battery bank however.

[LowGear]

Hi David and glort,

Way back in 2010 when we designed the array and system micro inverters were around $200 each and I knew eventually we would need 30 of them.  Yeah, that's $6000 in inverters alone vs 1 SMA Sunnyboy for about $1500.  Not a tough decision.  I'm still good with the decision.  Being about to track each panels output would be nice, espectially today, and I'll look into the individual panel reporter systems.

The unpleasant mother of my situation at this time is access.  I'll take some photos as my helper, that's a person with long arms and a strong back, pulls the panels off as we look for the problem.  I'm thinking rats chewing through a connector somewhere.  I'll report back.

And never worry about intruding on my knowledge base.  If I don't know what you're talking about I'm gonna learn something and if it's basic even to me I really like reinforcement.

The Sunnyboy does report - line voltage - string voltage (up to four strings) - string current (up to four strings) and accumulated production.  About 2 megawatts at this time.  A very nice cast aluminum housing well built unit.  Some person, names withheld to protect the guilty, put a lot of panels on a rather small roof.  In fact, besides a two foot path up the center there is only about one foot on one end that is not covered by panels.

Aloha

[glort]

Some person, names withheld to protect the guilty, put a lot of panels on a rather small roof.  In fact, besides a two foot path up the center there is only about one foot on one end that is not covered by panels.

Aloha

How Did you Know? Google earth?
And let me tell you, that's not a 2 foot path up the middle that's only one foot. as in one foot width so I can get one foot in front of the other and that's it. Not sure where you got the foot at the end either, I have over hang there!
Oh, wait, You're talking about your roof not mine..... Oops!   
Nothing wrong with efficiency I say.

I got the micros I have with a set of used panels I bought along with all the rail and fittings.  I have set them up on the lawn to test it all but I personally found that hooking the micros up even like that was a pain and as I mount the panels I have straight to the tin roof, Fitting the micros would be a huge pain. Bad enough I think when one would have rails to put them on.  They are still available here but not sure how they get round regulations which state that the DC and AC sides must be able to be isolated from a position that a normal person could access without aid. IE, not on the roof.

I might re do my panel layout yet.  As I learn more and understand the possibilities better I come up with more ideas.
Riht now I have hit a brick wall. I can't run the potential power I can generate down the wiring I have and need to significantly upgrade it.  My plan is to put in 3 dedicated Solar circuits. One for the shed on one phase, one for the main house array on another phase and a the 3rd on the house on the 3rd phase.  This will allow me to run the house as well as have the 3 phases needed for the AC and the Hot water.
I have built a control box for diverting power to the hot water over the weekend and will hook it up today or tomorrow.

I had to turn my solar setups off yesterday. We are due a meter reading this week and I was in credit on my useage.  I found out that what I'm doing in back feeding is not illegal but the power company would Not want to miss out on revenue and would change my meters if they thought something was wrong with them. I have the old spinny mrers I can rewind with my input and they would give me these smart arse meters which I don't want apart from the fact they can't back feed.
Terrible thing to have that power going to waste but I just have too much atm.  Come winter when the solar radiation drops off may well be a very different story.  My meter reads will put all the winter low generation months together which is less than ideal but guess I have to look at the annual cost and be very happy with that.


Really funny you mention rats.  I picked up a huge one  right out the front of the shed yesterday morning.  First one I have seen since we have been here.  Not a good idea for rodents to hang around a place with 6 cats. Mind you, 4 of them probably wouldn't know what to do if they saw one but 2 of them would be really interested and one is just deadly with them. Shes not a young cat at all now but still looks like a kitten.
Like a 70 Yo woman that looks like a teenager and does not seem to have slowed one bit. I I have seen that cat with one rat or mouse over the years I must have seen her with 100.
Easy to see her with them. She brings them to the backdoor and crys till you come see them usually. 

When she cleaned them all up round our old place she went and cleaned them up at the neighbors. I was out the front one day and she came wandering up the road with a 4 legged catch in her mouth half as big as she was.  A lady that lived well down the next block was sitting at the bus stop and told me that was the cat that had been coming to her place and catching all the mice.  She said she had seen she had a collar and tag but was leaving out a saucer of milk in the hope he would come back and catch more.  I said no worry about that, she will keep coming back whenever she thinks there are any there and won't need the milk to bring her back.
Lady from the street behind knew her too and told us she had wiped out all the rats in her place and she was happy to see her around her yard as well.

I think they would have a hard time getting onto my roof where the panels are and lying flat and screwed down onto the roof. Not much room for them up there and it would get way too hot during the day. Still, they are vile little creatures and I hope to heck my  super mouser Puss gets them before they get up there.

[David Baillie]

Rodents chewing pvc wiring I think was the number one reason all the rapid shut down rules came into effect. I think when you are talking micro inverters you mean the older enphase ones. They were pricey. We are using the ap systems ones. 2 panels each and you can daisy chain the ac from 7 of them all with built in connectors both ac and dc. Price wise if you need optimises on a string inverter it works out cheaper to micro. If you strings are all fully exposed to sun a single inverter is the way to go but you can't diagnose single panel faults.
Cheers, David Baillie

[glort]

I sold the last of my Micros last friday.
I paid $550 for the 16 panels, 16 Micros and all the racking. I sold 8 micros for $250 and the other 8 for $220.  Worked out my panels cost me $5 ea.
I didn't specifically test it but I really got the gut feeling I was getting less out of the array I put the micros on to test them than I did with a string inverter. Had the same panels in the same place and switched the inverters and seemed to get significantly less output from the micros according to the meter I ran both  inverters back through.  Even on clear sunny days when all my other arrays were pushing what I'd expect, the micro  array was way down. I tested them all individually before I sold them and they all worked but just did not seem to give the cumulative output I'd expect.
I had a 1.5 set on a string inverter on the roof facing west and 2kw set on the micros facing north ( our optimum orientation) and a better angle than the others and I got a lot less power from the micros.

I was going to use them on my Dads place as he gets shading where I want to put them but in the end I decided to go with a string and just put on more panels to compensate for the morning and afternoon shadowing.
Bought another 4.5Kw string inverter yesterday for $75 thats not too old and got another 2.0 Kw inverter with a 1.7KW set of panels I bought for $200 today.

I also found the extra connectors on the micros were a failure point. I got dew in them a couple of times which tripped the RCD breaker on the circuit board.  That does the whole house atm  So i'm thinking to change all the breakers to individual RCDs so only one circuit will go down at a time.
I'm sure the micros are beneficial in the right location but failed to impress me.

Not sure how you would find a single panel fault with the micros I had. Not only have you the panels that could go wrong with them, you have double the failure potential in the inverters themselves.

[LowGear]

Not sure how you would find a single panel fault with the micros I had.

The micros I see have Apps that report output in real time and various histories.

[glort]

The ones I had must have been too old. No communication ports on them unless there was some other way.
Spose that could be a plus on newer ones. Then again, if you had to connect each inverter with a wire/ connector for that..... more mucking round and installation cost.
  I would have to wonder what the in service failure rate of modern panels was though.

[David Baillie]

It's really hard to argue with those prices on the used gear. The comms on the micros run through the ac wiring. You just plug in the monitor to an outlet near the main panel and hook it up to the Internet.  Setup an account and you get panel by panel monitoring. I've heard less than great things about the older micro inverters. I really like the new ones. The price has come down as well. Like everything it's application specific. For us  In ontario between critter guard and rapid shutdown requirements split arrays, partial shading, snow cover we use them a lot. They usually match my modelling. We use the sol metric software for estimating.

[LowGear]

Wow glort,

Those are great prices.  Nothing like that has happened here in Hawaii and I suspect the rest of the US.  Not quite as much government "help".  If it wasn't for the government early obsolescence program in your country I suggest you'd be shopping a used market that would reflect something like 60-70% of retail. 

As for direct roof mounting - That's not playing nice with the panels.  It would be interesting to see how much production is reduced by the increased heat from the lack of ventilation.  I have a neighbor that just foams the panels to his roof and tells me I overthink everything.  He shops the used market as well.

The tester you linked is for AC.  My challenge is DC. 

I have some volunteers coming to the farm in a couple of weeks.  just the kind of intellectual ability I'm hoping for - mostly in their backs.  ;~)

[glort]

Wow glort,

Those are great prices.  Nothing like that has happened here in Hawaii and I suspect the rest of the US.

Since I wrote that I have bought a few more systems.  I'm paying less for them now. I picked up a system on wednesday. Asked the guy if he had much interest and said he had only had 3 enquirys and my offer being one of them, the only offer he got in a fortnight. The other systems I have got are the same, just not a lot of interest in them atm. System I got the other day is 3KW with a 3.6 Inverter. Still haven't got to my fathers so I'm thinking he might get the  3kW of panels with a 2.5 Inverter I got off another system.

As for direct roof mounting - That's not playing nice with the panels. It would be interesting to see how much production is reduced by the increased heat from the lack of ventilation. 

You are absolutely correct and this was not lost on me.  I looked up the fall off from temp on the panels and I'd strongly suspect mine are going to be in the falloff range probably 9 months of the year and certainly 6 no matter how they are mounted or even if they were force cooled with moving air. 
In the lowest generation times, June -Aug They might run in the under temp range. The rest of the time..... Forget it.  I have seen how hot the things get with ANY sun on them. In our spring and autum as it is now, clear day and those things are going to be way into the fall off zone.

I looked up the dropoff and once again, as always, it comes into the " not worth your while trying to make them efficient" range.  The diff between 50oC and 100o which I think they could easily get to is bugger all.
The only way I can see to make the things run in their ideal temp range would be to water cool the backs of them. I can't even see that forced air of a volume that would not use more power than it saved ( and that isn't much) would cool them sufficiently not to loose the majority of what they are going to fall off.

The panels are rated at 40oC. Given nearly any sunny day here is going to be 20o ambient, a 30o Rise on a Shiny black surface isn't going to take much.  Give it a 35o+ day. These things ARE cooking eggs, ventilated or not.
For me it's the cost / trouble factor. I'm talking to a guy now about 190W panels.  He wants $30 ea for them. I'll buy them at $20. 6.8 Kw worth with inverter thrown in.
If I have 5 Kw of the things on the roof and loose 10% generation, that's only 500W. Say I make that up and put an extra 800W of panels up. Cost to me with direct mounting is $80. No way I am going to rack them for that but the thing more significant to me is how secure they are. The racking is designed to withstand wind but that " Ventilation " underneath in a storm to me is a weak point. This place is known for strong winds and Cyclonic gusts. I think there is much less risk of them going anywhere flat than rack mounted. I fully accept that may not be the case but it's trade off I'm willing to make with heat efficiency fall off.

The other thing that should be kept in mind is the panels WILL fall off the efficiency curve in our weather most of the time ventilated or not. The question is how much MORE are  you loosing having them flat than Racked?
From what I saw, 5% would be really erring on the high side.  IF they are sitting at 80 or 120o, don't make much difference.  Other thing is, I have overclocked my inverters anyway so in peak sunlight like that, it's a given the inverter will be clipping the  output in peak times and the other losses in the shoulders will be insignificant.

I grant that if one were buying a new $5 or 10K System, you would want to rack them. I would and they couldn't be installed legally without being racked. In my case, it's a bootleg system to start with and I am paying as little as $5 per panel and getting 100% return on what I generate. I do have all the proper mounting clips that I secure the panel with and being a corrugated roof, the panels are not virtually sealed to the surface.  Ventilation would be minimal but then again, when those winds come up which happen on hot days and really dry everything out,  the airflow may not be all that bad. Another factor would be my Shed roof where the main array is has a pitch of only 5o and 13o. Probably not going to be a lot of induced draft at that shallow angle and given the panels all have a35mm lip round them, I'd suspect the air is gong to he heated in a pocket and stay there pretty much anyway.  Even on the 35o angles pergola, I have noticed the roof underneath is still pretty warm below the panels so I Imagine they are getting plenty hot.  It would be interesting to mount 4 panels on racking and 4 flat and see what the temp differences were. I think if I stuck a sensor under each directly on the back, I can't imagine them being a lot different TBH.

Inverters also fall over when they heat up and have inbuilt circuity to pull back their output when they get hot.  Their fall off can be a lot more significant. I don't like hot electronics at any time so I have installed a fan and temp switch on my main inverter so when it hits 40o, the fan comes on and blows air through the heat sink.  That fan runs on almost any clear day and I am not in the least worried about the maybe 40-50W the thing uses because I'm far more content the inverters is running cooler and therefor should run longer. When the inverter is cranking at near 5 Kw, i'm using 1%.
Maybe this too is a false economy given I got a nice 3.6 Kw inverter with the 3kw of panels I paid $300 for earlier in the week and replacing the inverter with another used one would not be expensive. Cooling it is something that makes me feel " Content" in the setup rather than it being not as it should be.

What I'd like to do if practice were as easy as the idea, would be to have the panels boxed in and have strong fans blowing underneath the panels and into the house in the cooler months. I am sure the BTU output would be huge with that amount of surface area. For summer I'd remove the edge of the box and leave the fans in place and try that given it would be no extra trouble.