How To Wire and how close to amp limit help please

[Subjated]

Hi All

I'm relatively new and still learning and been reading a lot on this forum, but a few things always crop up i don't quite get! I'm getting close to installing my system and have my hybrid inverter which has the following specs .......

• Nominal battery voltage: 48V
• Continuous power output: 8000W/8000VA (220-240V AC)
• Surge power: 16000VA
• Output frequency: 50/60Hz
• Output waveform: Pure sine wave
• Maximum solar input power: 4000W per input (8000W total)
• Maximum solar input current: 18A per input
• Maximum open circuit solar input voltage: 500V (open circuit)
• MPPT solar operating voltage: 90V - 450V
• Maximum solar charging current: 80A
• Maximum mains AC charging current: 120A
• Maximum combined charging current: 120A

i have found some solar panels with these specs

Pmax 410W
Imp 8.47A
vmp 48.41V
isc 8.95A
voc 60.32V

I'm not 100% on the exact definition of all of these, but i cant quite make the maths work with these panels i assume i can only get 8 panels in series (8 x 60.32 = 482.56) but in parallel 8.95 x 2 = 17.9 amps, is this not dangerously close to the limit? how close can i go, and does it even help? I'm looking at 18 panels which i assume is the limit 4s2p on one input and 5s2p on another, 19 panels would be under the 8000w limit but allowing for a could weather and I'm not sure how to wire that is that even possible?

id really appreciate the help as i may be able to pick those panels up this weekend and im struggling to get my head around it, many thanks in advance

 

[RCinFLA]

You are dealing with an all-in-one inverter/charger. I would not recommend allowing cold Voc of PV stack to get greater than about 470-480v. These inverters spec their max PV input voltage with almost no margin. The PV mppt charge controller is just a boost DC-DC converter so PV voltage input cannot exceed the internal high voltage DC level of inverter. Input electrolytic caps they use are 500vdc to 510vdc rated. If the inverter spec for max Voc is 500vdc then it is a 240vac inverter.

Vmp on panels is approximately 0.81 to 0.85 x Voc so with a max Voc of 500v you will have a Vmp of 405v to 425v, so you will never have a Vmp of 450v and stay with less than 500v Voc.

The panel design needs to be based on maximum Voc at coldest temp in your area. There is typically a -0.34%/degreeC rise in Voc for temp so figure that against panel stack Voc at 25 degC spec. Voc at -20 degs C (-4 degsF) is 15.3% greater than at 25 degsC. Worse case is usually cold morning just as sun rises.

As to the 18 amp max PV input spec, if the unit has two MPPT inputs, the 18A limit is usually reduced when both MPPT inputs are used. This is because the MPPT controllers share the same heat sink and there is a max PV power that can be used which is maximum battery charging power plus maximum inverter AC output power.

Another common issue with these All-in-One inverters is their capacitor storage is marginal. When operating at large AC loads there can be a lot of ripple voltage on internal HV DC. Running at high PV input voltage, if the internal HV DC ripple voltage valley drops below Vmp of PV input then the ripple voltage shows up on PV panels which reduces their output power.

The point of all this is don't think you have to run with highest possible PV input voltage. PV boost DC-DC converter is a little more efficient when PV input voltage is close to internal HV DC of inverter but not that much more. More chance to stress inverter and have ripple loading loss on panels when pushing max PV input voltage to its limit.

 

[sunshine_eggo]

I'm not 100% on the exact definition of all of these, but i cant quite make the maths work with these panels i assume i can only get 8 panels in series (8 x 60.32 = 482.56) but in parallel 8.95 x 2 = 17.9 amps, is this not dangerously close to the limit?

The 486 is way too close as over-voltage from low temperature voltage increase can kill the MPPT. 7S is likely better. Depending on your panels, they should be good on voltage down to about -20°C.

Current limit is more flexible. Going a little bit over the current limit is typically fine as the unit just wont pull more than its max.

how close can i go, and does it even help? I'm looking at 18 panels which i assume is the limit 4s2p on one input and 5s2p on another, 19 panels would be under the 8000w limit but allowing for a could weather and I'm not sure how to wire that is that even possible?

Power limits are also more flexible. They tend to represent what the unit can deliver, not what it can have on its input.

I wouldn't hesitate to put 7S2P on each MPPT.

 

[Subjated]

thanks for the speedy replies

As to the 18 amp max PV input spec, if the unit has two MPPT inputs, the 18A limit is usually reduced when both MPPT inputs are used. This is because the MPPT controllers share the same heat sink and there is a max PV power that can be used which is maximum battery charging power plus maximum inverter AC output power.

so would that mean you would suspect that these panels are unsuitable? as on both inputs the amps would be too high? also doesn't get crazy cold here (mid uk) but i have seen -10 in some winters but its not often in minus figures for long here

Power limits are also more flexible. They tend to represent what the unit can deliver, not what it can have on its input.

I wouldn't hesitate to put 7S2P on each MPPT.

contrary to the above this would total 11,480w, significantly over the 8000w max stated at least but this doesn't matter? i doubt ill ever draw that much power through the unit maybe 4000w continuous at any one time max, i just wanted to get as close to an 8kw system as possible and didn't fancy parrelling two inverters! the plot thickens! very interesting though.

 

[sunshine_eggo]

so would that mean you would suspect that these panels are unsuitable? as on both inputs the amps would be too high? also doesn't get crazy cold here (mid uk) but i have seen -10 in some winters but its not often in minus figures for long here

I think they're great. It's hard to find panels that almost perfectly hit the current limit. While the heat sink think may be true, I would tend to trust the current specs.

contrary to the above this would total 11,480w, significantly over the 8000w max stated at least but this doesn't matter?

No. The MPPT is output limited, i.e., it can't charge at more than 80A. At peak voltage, 80A * 57.6V = 4608W (4000W limit imposed). The charger just can't pull more than that. The charger is PULLING current/power from the panels. The panels aren't pushing their current power to the MPPT.

This is called over-paneling, and it's very common.

i doubt ill ever draw that much power through the unit maybe 4000w continuous at any one time max, i just wanted to get as close to an 8kw system as possible and didn't fancy parrelling two inverters! the plot thickens! very interesting though.

Given that you're in the "land of the reluctant sun," that's all the more reason to over-panel the system.

 

[Subjated]

No. The MPPT is output limited, i.e., it can't charge at more than 80A. At peak voltage, 80A * 57.6V = 4608W (4000W limit imposed). The charger just can't pull more than that. The charger is PULLING current/power from the panels. The panels aren't pushing their current power to the MPPT.

This is called over-paneling, and it's very common.

ah so that would explain why higher volts is more dangerous than higher amps in this instance? so how do you go about fusing these and surge protecting them from lighnting strikes? can you put the whole lot on a 20amp breaker and protect the inverter? (its expensive and in short supply so id like to keep it safe!)

love to have a big system but these panels are 1300 x 1950mm so 28 of those in nearly 71 m2! i will certainly run out of space as i have about 48m2 max to play with

 

[sunshine_eggo]

ah so that would explain why higher volts is more dangerous than higher amps in this instance? so how do you go about fusing these

Arrays with 2P do not require fusing. 3P requires a fuse per string. If you want to fuse them, they likely have a max fuse rating on them. There are MC4 fuses that can simply be connected between two panels.

and surge protecting them from lighnting strikes?

Not a pro here. Generally speaking, the array itself (panel frames and mount poles should be grounded with a grounding rod. There are also devices sold specifically for lightning protection.

can you put the whole lot on a 20amp breaker and protect the inverter? (its expensive and in short supply so id like to keep it safe!)

20A is too low. You want to fuse/break for 1.25X run current. A breaker is also a handy way to quickly disconnect the entire array.

If you want to fuse/breaker to the max, stick a single MC4 fuse in series with each string and a breaker at the inverter. Fuse is same rating as indicated on panel label, breaker is 25A.

 

[Subjated]

If you want to fuse/breaker to the max, stick a single MC4 fuse in series with each string and a breaker at the inverter. Fuse is same rating as indicated on panel label,
breaker is 25A.

and this should protect the inverter from any unfortunate events?

 

[sunshine_eggo]

and this should protect the inverter from any unfortunate events?

That's a very general statement. I can narrow it with "it will prevent the inverter from being exposed to >25A from the solar panels for a significant period of time.

Lightning protection is another matter as fuses/breakers don't mean much to lightning. I'm not qualified to comment, but generally speaking, following the national electric code for grounding arrays and PV systems in general provide lightning protection to dwellings and people - not necessarily equipment.

 

[Subjated]

that's fair enough, i understand that, i wondered if there even exists a fool proof way to protect the inverter from any damage?

 

[cinoaz]

Current limit is more flexible. Going a little bit over the current limit is typically fine as the unit just wont pull more than its max.

Are we sure about this? I hear conflicting reports on inverters. If an MPTT input says 15A max input, if you have a source capable of 30amps, with the inverter draw as much current as possible until it goes POP?

 

[TorC]

If it's properly designed, the MPPT will automatically reduce output as needed to keep from overheating. In which case, 30A and a load that could draw those 30A from MPPT input won't burn it up in normal operation.

HOWEVER: I'd be surprised if part of that rated limit isn't for what the MPPT can handle in a fault situation. Therefore, if there's an Isc limit specified for the MPPT, I would feel comfortable going up to it, but be cautious about exceeding it in normal operation. Cold weather pushing it over by a small margin wouldn't bother me.

For some REC panels I have experience with, I have seen real output as reported by my Outback SCC noticeably greater than STC rating where I am, even at ~80°F+ ambient.

 

[sunshine_eggo]

Are we sure about this? I hear conflicting reports on inverters. If an MPTT input says 15A max input, if you have a source capable of 30amps, with the inverter draw as much current as possible until it goes POP?

I assume you're talking about an AiO. Unfortunately, AiO are referred to as inverters instead of AiOs.

An inverter inverters DC to AC power.

A MPPT converts high voltage PV voltage/current to battery voltage/current.

The MPPT is a load to the array. The MPPT operates within all published limits, i.e., if the unit can only OUTPUT 30A to the battery and can only draw 20A from the array, it will never exceed those because it can't. It doesn't matter what the inverter is pulling from the same battery. The MPPT will only supply what it can.

In many cases, it's a grey area. Using my 250/100 Victron as an example, they list a max PV input current of 70A. However, when pressed, Victron will confirm that the only reason to NOT exceed that current is reverse polarity protection may fail above that, i.e., as long as you don't reverse polarity, you can exceed that value. Even without exceeding that value, I could easily hang an 11kW array on a MPPT only capable of outputting 5800W @ 48V and never have any worry the MPPT will exceed 5800W.

Some MPPT actually break it out - they will give a max usable current and a max Isc current. They can be substantially larger. The DEYE 16k allows 13A on each MPPT, but the array can be up to 22A Isc. In those cases, they're establishing an over-panel limit, and one should likely respect it.

Some MPPT say nothing at all about it. In those cases, the input current is assumed to be the same as the output current.

 

[RCinFLA]

The most commonly used MPPT algorithm starts out by measuring Voc and begins its MPPT search at 0.6 to 0.7 x Voc, stepping up to find Vmp. This usually improves the initial MPPT discovery time over a blind search.

During the initial Voc measurement there is usually a check for its maximum level before starting up the switching on DC to DC converter. MOSFET's have greater margin on their breakdown voltage rating when not switching.

If you push the limit on PV array Voc maximum you might get a delay in wakeup of charge controller on a cold morning, until sun illumination warms up the panels enough to bring Voc down.

 

[sunshine_eggo]

The most commonly used MPPT algorithm starts out by measuring Voc and begins its MPPT search at 0.6 to 0.7 x Voc, stepping up to find Vmp. This usually improves the initial MPPT discovery time over a blind search.

I was surprised to see that my neighbor's Outback FM80 controllers actually have this target as a programmable setting. We changed it to .85 as that's the Voc/Vmp ratio of his particular panels. IIRC, the default was around .77.

 

[RCinFLA]

I was surprised to see that my neighbor's Outback FM80 controllers actually have this target as a programmable setting. We changed it to .85 as that's the Voc/Vmp ratio of his particular panels. IIRC, the default was around .77.

Be careful setting it too high. It can extend MPPT search time because it assumes it will initially land below actual MPPT point so it walks upward. If you set initial target too high and it walks upward, it discovers it is pass MPPT point and goes to a wider search starting at a much lower voltage point.

It has to see output rise, then fall, to be sure it found peak.

Once it finds MPPT point it typically just wobbles around the point to track it. A sudden shadow, like a cloud going by, may cause tracking wobble refinement to abort and go back to a Voc referenced initial search again.

 

[RCinFLA]

When you over panel an array and it can produce more input current then controller allows, it will do the initial Voc measurement go to 0.6-0.7 Voc target and see too much current.

It will immediately stop and start a new walk down search from about 0.90-0.95 x Voc where current should be less. It walks down from there until it reaches its maximum allowed input current. So when you over panel, the PV array run voltage will be greater than MPPT Vmp voltage, somewhere between actual Vmp and Voc.

Since this area of PV curve has a steep V-I slope, the control may have more active adjustments with overage and underage variance. Don't be surprised if you see charge controller output current jump around more.

 

[sunshine_eggo]

Be careful setting it too high. It can extend MPPT search time because it assumes it will initially land below actual MPPT point so it walks upward. If you set initial target too high and it walks upward, it discovers it is pass MPPT point and goes to a wider search starting at a much lower voltage point.

It has to see output rise, then fall, to be sure it found peak.

Once it finds MPPT point it typically just wobbles around the point to track it. A sudden shadow, like a cloud going by, may cause tracking wobble refinement to abort and go back to a Voc referenced initial search again.

Thanks. I don't think there's an issue, but I'll ask some questions and revisit the manual.

The array routinely outputs 90%+ of rated (excellent sun, clear skies, thin atmosphere), so I doubt we've created an issue.