Second system (plagiarism edition)

[Stinsy]

I hadn’t considered voltage as a metric by which to differentiate panels…

The insurance company are trying to find a replacement panel for one of mine that has an earth leak and it's the voltage that is causing problems as they are 60.5 VOC

My understanding is that it doesn’t matter! Voltage is additive in a series string so if you’re replacing a 60V panel with a 40V panel your string voltage will be lower by 20V but that is to no consequence. Current will be limited to the lowest panel in the string so if your string currently works at 11A then a 10A panel would limit the string to 10A and a 12A panel would run at 11A.

That is my understanding anyway, happy to be corrected by someone better informed.

I’m interested to hear the story though! How did you figure out something was wrong? how did you figure out an Earth leak in a panel was causing the problem? How come the insurance company is involved? (I guess they’re carrying the guarantee liability for a now defunct solar company).

[Tinbum]

My understanding is that it doesn’t matter! Voltage is additive in a series string so if you’re replacing a 60V panel with a 40V panel your string voltage will be lower by 20V but that is to no consequence. Current will be limited to the lowest panel in the string so if your string currently works at 11A then a 10A panel would limit the string to 10A and a 12A panel would run at 11A.

That is my understanding anyway, happy to be corrected by someone better informed.

I’m interested to hear the story though! How did you figure out something was wrong? how did you figure out an Earth leak in a panel was causing the problem? How come the insurance company is involved? (I guess they’re carrying the guarantee liability for a now defunct solar company).

The solar company say a different voltage won't work even with Tigo's on. The panels are also 1580 x 1062 which is a bit of an unusual size. Also they are Mono.

The inverter kept tripping when it was damp or wet. At first it was on the odd occasion then became more regular, fault showed as Riso value. I turned one string off and that did the trick- inverter would restart. I then measured resistance from + to earth for the faulty string and from - to earth. Resistance was about the same which meant fault was approx 1/2 way down string. Examination of the panels showed;



Re the other questions it's a real saga and I'll start a new topic on it. and

[Stinsy]

Right chaps! An update for you all and a quandary!

Where I left you: I'd taken advantage of an unseasonably warm/sunny day in October to re-felt my shed (garden office) roof and throw up 3x380W panels. Coincidentally Joeboy had bought an upgrade for his very high quality, very expensive Victron 150/35 that he'd been using for the exact same application. So I negotiated the purchase, sent the money and waited for Joe to get back from his holiday, remove the charge controller from his installation, and waited for the postman to deliver.

In the mean time I got to thinking about the unused 2nd MPPT on my existing inverter. I'd originally planned 2x 495W or 2x 540W panels with a string voltage in the 80-100V range but they all sold out while I hesitated (surging electricity prices prompted a flurry of demand for panels). The 3x arrangement was a bit more of a faff to get up there, overhangs, and cost a lot more, but it gives me a chunk more precious kW and a string voltage of 126Voc and 98Vmp.

The specs of the MPPT on my inverter are:

DC input Voltage Range	100-550Vdc
MPPT Voltage Range	120-500Vdc
Startup Voltage	140Vdc
Max DC Input Current	11A
Max Short Circuit Current	13.7A

I was pretty sure it wouldn't work, but Impatience got the better of me so I fitted some MC4 connectors and hooked it up. I didn't know if "open circuit" or "max power" voltage applied to the specs of my inverter, but that was irrelevant because there was no way my string could provide the 140V startup voltage and the "max power" voltage of 98V isn't enough to drive the MPPT.

But work it did!

How well?

Difficult to know! The new panels have a lot going against them. They're at the bottom of the garden and much lower down, than my main rooftop array so shaded much more by the houses behind when the sun is low. They are also angled at 15°from horizontal which is imperfect for winter sun. I have a habit of checking the web interface of my inverter whenever I notice the sun shining to see how well my PV is doing. While the new panels were indeed generating when I checked, they rarely seemed to be producing 25-30% of the power my main array as I hoped.

I've pulled the data for the total generated by each array:

Date	EPV1	EPV2	PV1 kWh/kWp	PV2 kWh/kWp	VAR
17/10/2021	1.8	0.6	0.44	0.53	119%
18/10/2021	0.5	0.2	0.12	0.18	143%
19/10/2021	1.4	0.5	0.34	0.44	128%
20/10/2021	5.3	1.2	1.30	1.05	81%
21/10/2021	13.6	2.0	3.33	1.75	53%
22/10/2021	1.9	0.6	0.47	0.53	113%
23/10/2021	3.1	1.0	0.76	0.88	115%
24/10/2021	1.2	0.5	0.29	0.44	149%
25/10/2021	4.5	1.2	1.10	1.05	95%
26/10/2021	2.0	0.6	0.49	0.53	107%
27/10/2021	2.2	0.7	0.54	0.61	114%
28/10/2021	1.4	0.5	0.34	0.44	128%
29/10/2021	2.0	0.6	0.49	0.53	107%
30/10/2021	7.3	1.4	1.79	1.23	69%
31/10/2021	3.4	0.8	0.83	0.70	84%
TOTAL	51.6	12.4	12.65	10.88	86%

This data is from 15 days in a really terrible October for PV generation. But clearly my reclined panels do better than my main array in bright-overcast conditions and worse in winter sunshine, as you could expect from their positioning/orientation. Overall I was surprised they did as well as producing 86% of the kWh/kWp compared with my main array!

Questions:

• Do I risk damaging the MPPT in my inverter running it outside it's spec as I am? I know over-voltage is bad, but is under-voltage bad?

• Would I get more kWh in the same conditions using the Victron MPPT?

I'm tempted to leave it as-is over winter then evaluate again in March/April. Input from you chaps much appreciated...

[Stan]

This might help Stinsy.
https://www.solarpowerworldonline.com/2 ... gradation/

[Joeboy]

Great that you have comparison figures for the two strings. I'd run for a week to 10 days as is. Work out the percentage in comparison then run new string the same volume of time and compare that 2nd set of figures. The percentage will give you your path I think?

[nowty]

I was pretty sure it wouldn't work, but Impatience got the better of me so I fitted some MC4 connectors and hooked it up. I didn't know if "open circuit" or "max power" voltage applied to the specs of my inverter, but that was irrelevant because there was no way my string could provide the 140V startup voltage and the "max power" voltage of 98V isn't enough to drive the MPPT.

But work it did!

Oh, I have experimented for a decade with my SMA inverters doing all sorts of things which should not work but they do.

I don't know what inverter you have but my SMA's work way out of their spec range. The MPPT says 175v to 500v, with a min working voltage of 125v but I run one of the strings at max power point of 130v and it works fine and generates what I expect. You might find yours is running at the min input spec of 100v so might be a little off its max power point.

The advantage against the charge controller is once the batteries are full, no more power from the charge controller. The advantage of the charge controller is its more suited for its max efficiency point and DC coupled is more efficient than AC coupled for charging the batteries.

The other thing to mention again with my SMA inverters, is if its a dual MPPT and the other higher voltage string has started the inverter, then the second MPPT does not need get to the min start up voltage for the string to work.

And finally check how much the small string is generating in bright cloudy conditions, you may find it generating more pro rata than the main string as my 8 degree slope ones on my shed facing NE do. In Summer they may do very well indeed. With my flatter shed ones, the cross over is mid March to mid Sept, i.e. they generate the same pro rata in mid March and mid Sept, more pro rata in summer and less pro rata in winter.

[nowty]

This might help Stinsy.
https://www.solarpowerworldonline.com/2 ... gradation/

That article is very misleading as it uses an inverter with an alleged working MPPT of 460v to 700v. Its giving an extreme example to make a point of the shape of the power curves.

In practice most inverters MPPT specs will go much lower than that and even Stinsy's inverter officially goes from 120v to 500v.

[nowty]

This data is from 15 days in a really terrible October for PV generation. But clearly my reclined panels do better than my main array in bright-overcast conditions and worse in winter sunshine, as you could expect from their positioning/orientation. Overall I was surprised they did as well as producing 86% of the kWh/kWp compared with my main array!

Assuming your main string your comparing with is at the standard 35 degree roof angle, then they are probably working fine.

I checked with PVGIS for October and the difference in yield at 35 degrees slope verses 15 degree slope is about 15%. So if your saying your getting 86% of the pro rata of the main array, then it looks OK.

It will drop to 75% in November though and maybe more if it shades.

[Stinsy]

Thanks everyone.

Interestingly 10 out of the 15 days the new array outperformed the old one kWh/kWp-wise!

I'm going to keep the setup as it is and wait until spring before making any decisions to fit or sell the charge controller. Even though I got the charge controller for a good price, the payback time for my new array will be much shorter if I can sell it on eBay for a similar price to what I paid.

[Joeboy]

Thanks everyone.

Interestingly 10 out of the 15 days the new array outperformed the old one kWh/kWp-wise!

I'm going to keep the setup as it is and wait until spring before making any decisions to fit or sell the charge controller. Even though I got the charge controller for a good price, the payback time for my new array will be much shorter if I can sell it on eBay for a similar price to what I paid.

I reckon you will easy shift it and might even turn a profit.