Matching Panels and Inverter

[fastbike]

The final part of the design is starting to firm up having found a local supply of 440W panels at a good price and a suitable 3 phase hybrid inverter.

The layout on my roof requires two strings: 13 in one and 10 in the other. This is due to a variable pitch/heading.

The initial design used Longi panels, the final design will use Trina.

I'm figuring out if the Trina panels are going to work, here's the relevant specs.

	Trina	Longi
Panel Voc	52.2v	39.8v
13 string Voc	678.6v	517.4v
10 string Voc	522v	398v
Panel Vmp	44.0	33.6
13 string Vmp	572	436.8
10 string Vmp	440	336

And here's the inverter specs:

• Max. PV Input Voltage (V) 800
• Start-up Voltage (V) 160
• MPPT Voltage Range (V) 200-650
• Rated PV Input Voltage (V) 550

Is the 13 string max output voltage (572v) too high for the inverter rated voltage of 550V. What about the MPPT range, how does this compare ?

[fastbike]

To answer my own questions I've just watched Will Prowse explain the basics.

He says the open circuit voltage (V_oc) for the string has to be less than the max MPPT voltage for the inverter, or bang !

So my 13 panel string has a V_oc of 678V which exceeds the inverter's V_{MPPT_max} of 650V by 29 volts.

Maybe time to remove a panel or see if another physical configuration of the panels on the roof works !

[Siwastaja]

You need to temperature compensate the Voc:

Find the record low temperature of your region (so usually something between -20 to -50 degC based on where you live)
Find the Voc temperature coeff from the panel datasheet
Calculate Voc at that low temperature. It is usually significantly more than Voc at +25degC, maybe by 20%.

Even though exceeding the max MPPT voltage should not be a safety problem, you are likely to find that you are nearly exceeding even that 800V max rating with 13 panels. So use fewer panels (or lower voltage panels). Or get an inverter which can handle this. 1000V max is not uncommon, and you can still get 200V minimum MPPT voltage, so more range.

Range (relative difference between maximum and minimum MPPT voltage) is important if you have potential partial shading, which causes some panels to run through their bypass diodes. If temporarily losing production from 1-2 panels causes the whole string to drop significantly below minimum MPPT voltage, then you lose the production from the whole string (basically making the usually false claims of microinverter/optimizer sales folks true in this corner case).

[fastbike]

You need to temperature compensate the Voc:

Find the record low temperature of your region (so usually something between -20 to -50 degC based on where you live)

We're in a coastal zone so record low is -5 Celcius. This typically occurs in July just around dawn when the sun does not come onto the panels until another hour. At this time they will be running above zero. However even 25 degrees difference at -0.230%/℃ makes the V_oc 55v, so definitely something to factor in.

I'll consider the shading factor although I'm lucky that it is a large section with no other buildings close by, and trees can be managed.

[fastbike]

Of interest, local standards applicable in New Zealand (and Australia) limit the DC voltage to 600 in residential installations (but see below), so that is a hard limit that needs to be complied with in order to get certification so we can connect to the grid.

I'm sure it would also have insurance implications if exceeded and something went awry

So at V_oc of 55v that implies a maximum of 11 panels per string which is a shame as my roof has a slightly irregular shape and I can physically and aesthetically fit 13 on the lower part and 10 on the upper. The lower part is angled at 26 degrees tilt, the upper at around 13 degrees so without individual optimisers they should be on separate MPPT inputs.

Time to look at panels with a  lower V_oc.

P.S.
Voltage limits in residential  installations are covered by two conflicting standards.
AS/NZS 5033:2021 sets out general installation and safety requirements for electrical installations of PV arrays, including d.c. array wiring. The revision in 2021 increased the maximum PV array voltage from 600 V.d.c. to 1000 V.d.c. for domestic systems (clause 3.1)

However there is another standard,  AS/NZS 4777.1:2016, which specifies the lower voltage of 600 V dc for "domestic dwellings" (clause 2.3). This is deemed to override the other standard, so the limit as it stands is 600V.

[fastbike]

However there is another standard,  AS/NZS 4777.1:2016, which specifies the lower voltage of 600 V dc for "domestic dwellings" (clause 2.3). This is deemed to override the other standard, so the limit as it stands is 600V.

Funny how things progress, because the standards applicable to getting my system certified so it can be connected to the grid ( AS/NZS4777.1 ) has just been revised to AS/NZS4777.1:2024
And the good news is that the voltage limit has been raised to 1000 V dc, so the planning is now limited by the specification of the chosen inverter, rather than by confusion within the standards setting body
Onwards and upwards.

[nctnico]

Still you need to ask yourself whether you want your system to run at 1000V. The higher the voltage, the quicker insulation is deteriorating and thus more chance of electrical problems long term. And it is also not ideal situation for solar panels as well as high voltages do have an effect. I would aim for an operating (= solar panels loaded) voltage of 500V to allow for some margin.

[Siwastaja]

Yeah, if you have a choice, don't go close to maximum voltages, effects like PID degradation are real.

But 600V limit by legislation is stupid because nearly every inverter supports more and it's awkward if you have to change your plans because your system voltage would be 600.000001V due to that one extra panel which looks good.

In any case, that 1000V limit (if absolute maximum of the inverter, not just MPPT max) must be temperature compensated so is closer to 800Voc in reality.

If you have a suitable location where certain number of panels looks aesthetically good, then go for a tad over 600V seeing they have fixed the legislation. Splitting in two strings has a new type of risk of the strings going too low in minimum voltage and in some conditions produce poorly.

Anyway, when reading standards be careful if they talk about nominal/average/typical operating voltages, or actual voltage at any point in time (so worst case). There's a big difference here, 500V typical operating is approx. 1000V worst case open circuit or so.

[fastbike]

...The higher the voltage, the quicker insulation is deteriorating and thus more chance of electrical problems long term. And it is also not ideal situation for solar panels as well as high voltages do have an effect. I would aim for an operating (= solar panels loaded) voltage of 500V to allow for some margin.

Thanks for the feedback from some real world experience. I have two firms coming out this week to provide installation quotes - I wanted to be an informed consumer so can ask intelligent questions and see what is realistic.

The two preferred layouts I have sketched for our roof can fit either 26 or 27 panels (1762x1134mm 440W, temperature adjusted V_oc of 55V and Vmp of 44V).  The Fronius Gen24 has a MPPT range to 850V.
The first layout which is aesthetically the best  would be split into two strings of 13, so would be a max of 715Voc on each input, with an input of 572Vmp when producing the max output.
The other layout would be split into a string of 7 panels (385V) and two parallel strings of 10 panels (550Voc, 440Vmp). However due to the nature of the roof it will not appear as integrated.

[Siwastaja]

Don't do parallel strings, if you want three strings then get inverter with three MPPT inputs. With 7-panel string carefully check if minimum MPPT voltage gets satisfied in all conditions. I would probably go for the two 13 panel strings.

[nctnico]

About the 3 input inverters: I looked around but I have not been able to find models with 3 MPPTs. Only 2 MPPTs where one is connected to two string (in parallel I guess) to make for 3 inputs. So if you need to have 3, seperate strings, you'll probably end up with 2 inverters.

[Siwastaja]

E.g. Solis S6 hybrid series has 3 MPPT inputs for the 5kW model and 4 MPPT inputs for the bigger ones. So more than two is getting more common but indeed having exactly two is still the "industry standard" of today (single input inverters have nearly disappeared).

[fastbike]

E.g. Solis S6 hybrid series has 3 MPPT inputs for the 5kW model and 4 MPPT inputs for the bigger ones. So more than two is getting more common but indeed having exactly two is still the "industry standard" of today (single input inverters have nearly disappeared).

I had an installer here on Friday for my third quote. This time it was the two guys who will do the work (the others have been sales people). The recommended Solis.
I've been reading up on the S6-EH3P(3-10)K-H-EU models but can't see them on the approved inverter list - must be new and the paperwork is behind.

[fastbike]

I had an installer here on Friday for my third quote. This time it was the two guys who will do the work (the others have been sales people). The recommended Solis.
I've been reading up on the S6-EH3P(3-10)K-H-EU models but can't see them on the approved inverter list - must be new and the paperwork is behind.

I've put the application into the network operator using the localised version of the 10kW inverter (S6-EH3P10K-H-AU) and it has been approved. I had to supply the testing certificate to show compliance with AS/NZS4777.2 (Grid connection of energy systems via inverters, Part 2: Inverter requirements).

Funny how things progress, because the standards applicable to getting my system certified so it can be connected to the grid ( AS/NZS4777.1 ) has just been revised to AS/NZS4777.1:2024
And the good news is that the voltage limit has been raised to 1000 V dc, so the planning is now limited by the specification of the chosen inverter, rather than by confusion within the standards setting body
Onwards and upwards.

Regarding the maximum allowed d.c. voltage, apparently in Australia it is now 1,000V as revised 4777.1 has taken effect. But here in New Zealand the standard needs to be "cited" in the relevant regulations passed by the government, which have not been updated since 2010 !  So back to a maximum temperature adjusted limited for Voc of 600V.

Luckily the panels we're now looking to use (TW) are rated around 40V IIRC, and the inverter has 4 separate MPPT inputs so there is a lot more flexibility. It is likely we will end up with 2 strings of 5, 1 string of 7, and 1 string of 10 due to roof layout.

[nctnico]

Given your curved roof shape (as you showed in a different thread), it would make sense to connect panels at the same angle into 1 string in order to maximise output.

[fastbike]

Given your curved roof shape (as you showed in a different thread), it would make sense to connect panels at the same angle into 1 string in order to maximise output.

We're going with three rows (I started installing rails at the weekend), see the diagram.
The tilts (from the bottom row) are 26,17,12 degrees. There is some potential shading late in the afternoon, in the middle of winter at the bottom right.

So a 4 MPPT inverter is quite attractive as I will have the top seven on one string, the next middle ten on another, and the then two strings of five each from the left and right of the bottom row. This row will provide the most power in the winter so gets VIP treatment to avoid shading issues.

[nctnico]

Sounds like a plan 

Have fun on the roof. Take some time to enjoy the view as well 

[fastbike]

Sounds like a plan 

Have fun on the roof. Take some time to enjoy the view as well 

It was 23 years ago that my Dad and I installed the roof when I built the house - I'd forgotten the views !

I've been doing it after work, starting around 4pm and working through to 7pm. So far I have installed all the pucks/feet for the rails for the bottom two rows and will install those rails after work today. I'm awaiting more pucks (the piece that sits on the top of the corrugation to spread to the load and has an EDPM membrane to seal the screw hole. They should be here on Friday.

I have one final installer coming out to quote on Friday, he is very relaxed about letting a home owner provide "sweat equity" to help keep costs affordable.

[fastbike]

I've been doing it after work, starting around 4pm and working through to 7pm. So far I have installed all the pucks/feet for the rails for the bottom two rows and will install those rails after work today. I'm awaiting more pucks (the piece that sits on the top of the corrugation to spread to the load and has an EDPM membrane to seal the screw hole. They should be here on Friday.
(Attachment Link)
I have one final installer coming out to quote on Friday, he is very relaxed about letting a home owner provide "sweat equity" to help keep costs affordable.

Gosh, I started this thread 2 months ago, and am finally installing stuff !

The installer came today, he's pretty relaxed about allowing me to do work up until we do the actual wiring of the panels into the inverter, so very happy for me to finish the rails, run the earthing, install the actual panels ... we've agreed the layout will be one string for the 7 landscape panels at the top, and the ten panels to the left of the skylight (two rows in portrait) are the second string, with the panels to the right being the third string. This makes wiring a whole lot easier.
I've used the excellent Niwa SolarView tool to check how this impacts the output, as the ten lower panels will be throttled at the output of the ten above them - it looks like I'll lose about 400wH per day, assuming I was exporting all of this, that around $25 per year.

I've just ordered a pallet of 36 TW panels as it is much cheaper to buy them wholesale, they should be here later next week so it looks like a weekend job for next week to get them on the roof.
The installer will supply the inverter and all switchgear/sub panels etc.

Now, back to work, levelling the top horizontal rail and then tomorrow I'll install the 14 short vertical rails for the top row.

Edit: added link to panels

[fastbike]

Just had an email from the installer (who was to supply the inverter), the one we selected is not available in NZ at the moment

Requirements were:
- 3 phase
- unbalanced loads to 50% of rating
- handle 12kW of panels => inverter sized at 10kW
- HV DC battery capable
- 3 independent MPPT

So I'm looking for any recommendations for an alternative to Solis S6-EH3P10K-H ?

[fastbike]

So I'm looking for any recommendations for an alternative to Solis S6-EH3P10K-H ?

Found another local wholesaler who can supply mid November. By the time I get the panels on the roof and the electrician to finish the wiring, it will be about a one week delay.

[fastbike]

Look what arrived today ....

[fastbike]

Now I'm looking for a home for the remaining 9 panels. I have room for 2 rows of 7 on my stand alone garage (I can buy an additional 5 for about the same per panel cost), so this would add approx 6kW of peak capacity.
This roof is not optimal but gets useful afternoon sun.  In winter the insolation is about 1kWh/m^2 per day (main roof is approx 1.7), and in summer it is roughly the same as the main roof (6kWh/m^2 per day). It is also subject to seasonal and time of day shading, which is not evenly across the roof.

So I'm thinking of going the optimiser or micro inverter route for this part of the install.

What are the  gotchas I should be aware of. Installer has said we should be connecting to all 3 phases to avoid phase imbalance. I can see that Enphase do a 3 phase combiner cable. Or would I just use optimisers and a cheap 5kW 3phase inverter ?

[nctnico]

The extra panels are a good excuse to build an extra shed in your garden to put them on. 

[fastbike]

The extra panels are a good excuse to build an extra shed in your garden to put them on. 

The garage is bigger than the garden shed
For the garage, in theory I could fit 2 rows of 7 in portrait, with another row of 5 in landscape (total 19 panels) but the mounts for the additional 5 would be more effort than justified.
By putting 14 on the garage I will have 41 panels in total, tied into my electrical installation: that is 18kW. The local network operator has a limit of 15kW feedin without going to a different contract aimed at commercial installations.