Underspecifying an inverter

[st3ve]

What implications are there from underspecifying an inverter?

(1) What actually results at the inverter, should the irradiance on the panels attempt to generate more than the peak rating of the inverter? (Design Volts are below max as are design amps) tripout? restart? smooth limit to max and continue uninterrupted?
(2) May be running near 100% for a longer period of time than usual - affect on reliability?
(3) Using eg SMA SB 3000TLs, can the max POWER be set in the software setup, or is this a CURRENT limit that is set this way? and (4) what range of setting is generally available (5%? or any%?)
(5) Because the total is over 16A/4kW, what additional specs do I have to meet and how do I achieve those specs?
I need to install a twin roof twin inverter (need to monitor each roof/direction separetely) system as follows:

3.12kW SSW plus 4.68kW NNE (ie. TIC = 7.80kW).
DNO has indicated that 5.96kW total max generation would be acceptable on this single phase domestic install at the installation address.

I intend to use a pair of SB3000TLs, each set to a max power of 2.98kW, say, to meet the DNO limit.

SSE roof of 3.12 will probably max at under 2.98 anyway, but the NNE roof of 4.68 looks severely underspec'd, until you consider the likely max irradiance around 70% of due south. So 0.7 * 4.68 giving 3.276kW.

Can this arrangement be safely and reliably achieved?

Thanks for looking,
Steve.

 

[screwdriver]

read the sma datasheet for temperature derating, it probably answers most of your questions, yes you can set the max kw output of each unit but I would prefer a larger twin tracker inverter such as a pvi 6000 which would be single phase and would be easier to set max power across BOTH inputs allowing you to balance between the two roof aspects, this should gain yield.

 

[TedM]

You need to model the likely maximum system output in much greater detail. Taking 70% (annual average) for a NNE roof would over-look the output in early morning in June, for example, when the sun is going to be pretty much from that direction and panel output is (I'm only guessing as I haven't modelled it and depends on other factors too that you haven't quoted) going to be much closer to 100%.

 

[nowty]

It depends very much on the roof angle. You can use PVGIS to give the max clear sky irradiance level for the max month of May or June. I ran it through PVGIS assuming NNE and a standard roof angle of 35 degrees. In June it only shows a max of a little over 500 w/m2, so you could use 50%.

However, if the angle is shallower, it will be more. For example I am planning on putting some panels on my NE facing shed roof which has an angle of less than 10 degrees. For that I get about 800 w/m2 or 80%.

Edit - My calcs are also based on the south coast, so it also depends on where the installtion is.

 

[st3ve]

Thanks all for your inputs so far.

I have modelled this as 70% MAX output as you say, [TedM], June am. Averaged over the year I reckon around 50/55% of due South.[nowty]

I was going for the 2 inverter option rather than the single larger one, to give me more options for monitoring the 2 roof outputs separately.[screwdriver]
Also, I may yet decide to only have the SSW roof done now , but am planning ahead to try not to cut off my future options.

A single inverter may prove less problematical to set up, no balancing/risk of tripping each other etc and easier for DNO to understand

Any comments on this part? ..."(1) What actually results at the inverter, should the irradiance on the panels attempt to generate more than the peak rating of the inverter? (Design Volts are below max as are design amps) tripout? restart? smooth limit to max and continue uninterrupted?"

Thanks again,

Steve.

 

[Del Boy]

Whereabouts are you roughly located? Near me, alot of installs which are between SE and SW facing have hit their peak output last month.

I found last year (albeit with 43.3p calcs) that a 3kW south facing with FIT of 43.3p would provide a much greater ROI than a 6kW split of 3kW South and 3kW North with 37.8p. So I expect although the lower tariff rates will bring the difference down, a 3.12kW and 4.68kW would be a worse ROI than just the 3.12kW on the South facing roof.

Also, what size are your roofs? On one install it was possible to fit 3 rows of the smaller 240 Sanyos (+1 extra column) vs 2 rows of 250 Monos/Polys, the roof couldn't quite squeeze on 3 rows of 250 Monos/Polys in a landscape fitting. This resulted in an increase in the peak on the roof from 2.5kW to 4.3kW, and the roof being fully utilised. Obviously that is a more expensive option, but I think would give the greatest ROI is possible. The option of putting the north face was also looked into but the DNO restricted the install to 6kW, so it was either 4.2kW with Sanyo 235s South facing or 5kW split over North/South (which was cheaper for paneling but double cost for scaffolding). In the end the South facing only made the most financial sense. Sometimes spending less is better!

As for an inverter, if you have no real shading issues, then I agree with screwdriver, 1 x 6kW dual tracker would be better than 2 x 3kW, otherwise you are restricting each side to a peak of only 3kW. Your roof angle is important, with a near north facing roof, it is possible that your north side may not see direct sunlight at all. If it's over 35 degrees then I'd say forget about the north roof, even if you live in Penzance. How much sun does it see during the peak hours of the day (11am to 2pm)?

"(3) Using eg SMA SB 3000TLs, can the max POWER be set in the software setup, or is this a CURRENT limit that is set this way? and (4) what range of setting is generally available (5%? or any%?)"

Not had a SB 3000TLs but have been able to restrict the power output of other models using the SMA software. I think I needed to use my GridGuard code though.

"(5) Because the total is over 16A/4kW, what additional specs do I have to meet and how do I achieve those specs?"

The DNO should have informed you when they confirmed 5.96kW. Over 4kW, mine have wanted G59/2/.

Hope that helped rather than confused!

 

[st3ve]

V.Helpful post - cheers DelBoy.

Have to agree about the ROI potential. My main aim though, is to generate max power that DNO will allow, ROI may need to be secondary (though maximising what is poentially available!).

My NNE roof is plain and all usable , the SSW has a chimney incurring into one edge and a pseudo-dormer roof breaking up the area towards the other edge. Also want to leave a space (2 or 3 m^2) for Solar thermal eventually, perhaps. Additional requirement is aesthetics - don't want to spoil the front aspect (SSW) by shoehorning-in PV panels over every sq inch of the roof!

Whereabouts are you roughly located?

Location is South Wales

... Over 4kW, mine have wanted G59/2

I believe you are right on that.

Does anyone know if having a pair of inverters make G59/2 more difficult to achieve than with a single inverter?

Cheers,
Steve.

 

[Del Boy]

Does anyone know if having a pair of inverters make G59/2 more difficult to achieve than with a single inverter?

Not to my knowledge, have done a 2 x SMA SMC 6000A on G59/2. The inverters are G59/2 compliant, looked into SolarEdge for that install but would then need a G59/2 control panel... alot of extra cost which isn't worthwhile if you can get another make that doesn't require it.

Also want to leave a space (2 or 3 m^2) for Solar thermal eventually, perhaps.

I don't want to rant about this one but... if you have Gas, then you are far far far far better off saving the 3k rip-off price and having some extra panels with the FIT... check this out, the maths aren't perfect but give you a good idea Solar Hot Water

Also I've got a prototype working which will power an immersion heater instead of wasting the excess solar through export... there's another thread on it but I'm going to make a few for some members on here (as I have alot of spare parts from experimenting... doing this was more the fun of the challenge and the learning). I think this would be a better route as you are being paid to heat your water! I'm currently going through pvoutput.org to see how much you would actually save a year by heating this way... but this is taking a bit of time, so no results yet. Sorry for going off topic!

South Wales (well Cardiff I used), max height of sun on June 21 is about 62 degrees. So unless you have a flattish roof... say 20 degrees, you are going to see a big performance hit as you will only be generating from diffused light for most of the year and most of the day in the summer. Appreciate the aesthetics point... I prefer installs to look square rather than dotted all over the place but each to their own!

 

[Del Boy]

Whoops... forgot about this...

"(1) What actually results at the inverter, should the irradiance on the panels attempt to generate more than the peak rating of the inverter? (Design Volts are below max as are design amps) tripout? restart? smooth limit to max and continue uninterrupted?"

Above design volts or current is big no-no. However, it's possible to have more than the max inverter rating. I was looking at the PVI 6000 and I think you can put upto 4kW into either tracker (Spec sheet: http://www.power-one.com/sites/power-one.com/files/pvi-5000_6000-outd_us_na1.0.pdf). Max voltage on that is 600V and current is 36A... 21.6kW... but it can only take 8kW if I've read the spec right. So you have a big leeway with the voltage and current. I normally plug the stuff into Sunny Design for SMA or Aurora Designer for Power-one. Link here: http://www.power-one.com/sites/power-one.com/files/re/aurora_designer_3.8.9_2012-03-31.zip

The SB3000TL will take upto 3.2kW of DC input. Not 100% sure what happens if you put more in but I have seen somewhere on the internet where someones is shutting down for a few mins, but this might have been a heat issue. Have also seen some that plateau during peak hours, giving you a daily readout that looks like table top mountain!

Sorry for the lack of help on that one... hopefully someone with more knowledge will answer.

 

[st3ve]

Cheers Del Boy . Gives me a little more to be going on with! Have looked at SMA 6kW inverter, doesn't have the twin trackers plus 2 string inputs I would need. Will investigate the aurora asap. Cheers.

 

[st3ve]

It depends very much on the roof angle. You can use PVGIS to give the max clear sky irradiance level for the max month of May or June. I ran it through PVGIS assuming NNE and a standard roof angle of 35 degrees. In June it only shows a max of a little over 500 w/m2, so you could use 50%.

However, if the angle is shallower, it will be more. For example I am planning on putting some panels on my NE facing shed roof which has an angle of less than 10 degrees. For that I get about 800 w/m2 or 80%.

Edit - My calcs are also based on the south coast, so it also depends on where the installtion is.

You need to be careful on this one, nowty...
PVGIS may give the stats you quoted (50%) for May/June, but this is the overall average return compared to for example facing South.
What I need to be mindful of, is the maximum possible output at any one time on any one date as I need to know the maximum possible power generation to specify my underspec'd inverter correctly.

The equivalent %age for this is around the 70% mark, in, as you say May/June.
Additional complication is that the peak power on each of my 2 roofs will occur at different times of day and maybe even dates!

I have therefore had to model the 2 daily and hourly output curves and combine them 'out of synch' so to speak, to deterrmine the maximum possible combined output to spec the inverter correctly whilst not wasting peak capacity. Phew.

 

[nowty]

No !,

I used the Daily Radiation tab in PVGIS to find the MAX clear sky irridance level in w/m2, not the usual average PV estimation tab to give average return. As the max amount I found at a particular day and time in May/June was circa 500w/m2, that is 50% of the 1000w/m2 that solar panels are rated at.

But yes, you will have to model both aspects and add them together to find what the max peak of both roofs in w/m2 will peak at.

 

[st3ve]

Nowty,

Am about to find out what the actual output is across the 2 roofs..... Installation and commissioning due to begin today Weds 18th July....

Will be interesting to compare against the theoretical values

 

[Earthstore]

Looking out of my window you will get very little...
but at least they will get the dust washed off them...

 

[st3ve]

I now know what happens when the input power exceeds the inverter capacity... See this link for a 5k280 panel system (South facing, no shade, in Exeter area) into a 4000W Sunny inverter... (NB. not my system)

Dan's PV 5.280kW | Live Output

:teeth_smile:
Steve.

 

[Worcester]

They must have put the (optional) 16 A limit on. I'd be suprised if the 4000TL would limit like that otherwise

 

[SolarCity]

The 4000TLs can't be limited to 16A - the power can be limited, but not the amperage.

 

[Gavin A]

They must have put the (optional) 16 A limit on. I'd be suprised if the 4000TL would limit like that otherwise

why not?

it's limiting at the maximum output limited for the inverter that's set as standard, not 3680W, so is exactly what I'd expect it to do.

 

[Worcester]

You are of course correct. it an be set at 3680W manually (or any other figure) so I was just suprised to see it so flat topped at exactly the specified output.