Safety concern for MPPT over paneling

[brum]

My inverter has two MPPT trackers that are rated for 15A max current per input and 20A DC short-circuit current. The MPPT voltage range is 130V to 500V. Currently, I have six 535W panels (Jinko Tiger Pro 535W) in a single string. The panel specs are:

• Maximum Power Current (Imp) STC 13.17A
• Short-circuit Current (Isc) STC 13.79A
• Maximum Power Voltage (Vmp) STC 40.63V
• Open-circuit Voltage (Voc) STC 49.34V
• Temperature coefficients of Isc 0.048%/°C

The string is connected to both MPPT trackers and they are splitting the current load. This is by design supported by the inverter (SMA Sunny Boy 3.6-1AV-41). Today I got peak current output of 15.4A from the panels (7.7A per MPPT tracker). This is despite the angle that the panels are mounted - optimal for June/July, not for March. The inverter max power output was even a bit above the total STC power rating of the panels.

I'm considering to over-panel the inverter without exceeding its max input ratings based on the panels. This will be accomplished by two strings connected to separate MPPT trackers. The maximum MPPT tracker voltage is 500V and the string open circuit voltage will be around 300Voc. The maximum MPPT tracker current is 15A and the string STC specs say 13.2A max current. Unfortunately connecting the existing PV string to just one of the outputs can produce higher than the rated max current under certain conditions. And here comes the concern.

Should I expect problems with the inverter if I'm having a PV string that can exceed the max current input? I kind of expect the answer to be no and the inverter to be smart enough to limit the current it draws from the panels, but if so - why are there the following two specs for the inverter:

• Max. input current input A / input B is 15A / 15A
• Max. DC short-circuit current input A / input B is 20A / 20A

 

[timselectric]

Current is not pushed, it's drawn.
The MPPT draws what it needs, never more than it can handle.

 

[brum]

This is what I expect and hope for. But then the question of why are these in the specs remains. The only thing that these would be useful for would be as a guide. A guide that helps you choose the panels and the string configuration.

 

[zanydroid]

There was active discussion on a necro thread for this subject last week

Exceeding voltage or amps on SCC

But for the sake of my understanding (and for sure others) can you explain the difference between: You are making a heavy assumption that the strings are operating at a specific Vmp. That is dependent on the whims of MPPT algorithm and lighting conditions. If you have bypass diodes activated...

diysolarforum.com

And here is a forum guide on it

Overview of Over-paneling an MPPT controller

To get to the paper, click on the orange button at the top of the screen. There is a misconception that putting more panels on an MPPT controller than the controller is able to use will damage the controller. However, in almost all cases it is...

diysolarforum.com

 

[zanydroid]

SMA is tier 1 so I presume they are more likely than not going to have proper specs for this, or respond to tech support inquiries on the limits

 

[zanydroid]

Sunny Boy 3.0 / 3.6 / 4.0

manuals.sma.de

Should be pretty clear? There are multiple overpaneling limits specified up here. Max short circuit current and max PV array watts

 

[brum]

Max watts will be exceeded. That is the purpose of over-paneling. Max voltage will not be.

The Max current will be exceeded under certain rare conditions, but the max short circuit PV current will likely not be exceeded. The guide in the forum does not provide details on the input current, only on the output which is kind of clear.

 

[zanydroid]

There's two watts ratings. One on DC input and one on AC output.





ISC rating in addition to DC input. This limits how much you can overpanel by paralleling.


So don't exceed either. The part that I'm not sure about is where you apply things like NEC's 125% rule that addresses over STC situations. If you have to do that for the SMA then the overpaneling ratio is a lot lower. For converting Isc to OCPD amps you do have to apply 125%. When comparing Isc between solar panels you don't. For microinverter panel W-dc to W-ac ratio you don't have to worry about > STC situations. You pick a panel within the recommended module wattage range, and you are allowed to pick one at the max end.

So I would guess you don't have to apply 125% buffer to Watts or Isc.

All the input sections I can find right now in the guide:


Page 10 & Page 11 for BlueSolar and Smart Solar. Isc limit specified on inverter

Page 13. Epever ratio of 1.5 = W-DC / W-AC

Page 16. Array Isc limit specified on inverter.

 

[brum]

Both strings are with almost identical panels. South with Jinko 535W, 6 pcs, north with Canadian Solar 540W 6 pcs.

South facing is on a 17 degrees slope and its max power considering the STC ratings is 3200W. Its short circuit is less than 14A.
North facing is on a -17 degree slope and with some math considering the STC ratings the max power will be 2400W. The short circuit current is again less than 14A.

The sum is exactly 5.5kW. I'm in the limits here. The concerns come from the observations yesterday - a panel with Isc of 13.8A was producing peak power at 15.4A current for a short time window.

We'll see if a tire 1 inverter can handle these. If not - I'll get the 5kW one. But I kind of expect after reading the details above and considering that SMA produces tier 1 inverters that the 3.6kW Sunny Boy will handle these.

 

[zanydroid]

Hmm.
Your Isc on each string is lower than the max operating current of the string.

I get 6450W STC when I add you your panels. That's above the 5.5kW limit SMA wants you to stay under. Did you run something like PVwatts on the second string? If so I don't think you're allowed to use that.

Max input current is the max current the inverter is designed to draw under normal conditions. Think of that as an "appliance nameplate current rating". Isc is for fault conditions, think of that as max circuit overcurrent production on "appliance nameplate".

 

[timselectric]

This is what I expect and hope for. But then the question of why are these in the specs remains. The only thing that these would be useful for would be as a guide. A guide that helps you choose the panels and the string configuration.

For the same reason a container has its capacity listed. So that you can know what it will hold.

 

[TorC]

A max input wattage is a guide, likely to be an approximation of how much the controller can actually use without generating complaints from people saying, "I put 8kW of panels on it, but it's only giving me 5.2kW out in perfect sun." Because of battery voltage curves and current output limits, I consider panel wattage limits useful for nothing more than back of the envelope design.

The Voc and Isc limits are "if you exceed these, you will almost certainly damage or destroy the controller." If you know what you're doing and why your location makes it sensible, I wouldn't hesitate to run both of these approaching (but not exceeding) the limits. Exceeding the input current, but remaining under Isc just means it will clip at the input current limit even with a low voltage string that needs, say, 18A to max out the output stage — another form of overpaneling.

 

[zanydroid]

A max input wattage is a guide

Then what is the difference between max DC input wattage rating and max AC output rating? If I saw both, as is the case with the Sunny Boys, I would not complain if I clipped at AC output rating.

 

[timselectric]

VOC is all that matters. Short circuit current is not an issue with the correctly sized OCP.
I ran several months with over 80a worth of panels connected to a single SCC with an 18a max input.
A 20a fuse made it perfectly safe.

 

[TorC]

Then what is the difference between max DC input wattage rating and max AC output rating? If I saw both, as is the case with the Sunny Boys, I would not complain if I clipped at AC output rating.

That's going to depend in part on where the output can go. Given the spec sheet snippet, it looks like something near 3-4kW inverter tied to two SCCs that can together feed it enough PV to max it out plus 1.4-3kW into the batteries if you have them. Looking at the rest of the spec sheet will make it clear what it can do. Once you have those numbers you can look at your power usage and likely generation for your area and see what makes sense.

 

[zanydroid]

Given the spec sheet snippet, it looks like something near 3-4kW inverter tied to two SCCs that can together feed it enough PV to max it out plus 1.4-3kW into the batteries if you have them.

It's a Sunny Boy, grid tie inverter. It can only go out the AC output.

 

[TorC]

Ah. In that case, there's probably not much advantage to more than the 5.5kW of panels, which already clips quite a bit in high sun. That already looks to me like they threw darts to pick a number about as high as they thought consumers would tolerate for clipping in bright sun and put it in the spec sheet. Strings in two orientations may let you get a bit more nameplate Wmpp that will actually do something.

That said, if experimental evidence says you need more power in the winter only and it's not reaching clipping, you may be a candidate for further overpaneling.

 

[brum]

With the current setup the produced energy is far away from sufficient during the winter. I'm consuming ~30 to 50kWh daily (heating is with a heat pump mostly and gas from time to time), and the PV system produces 10 to 12kWh daily on sunny days.

Overpaneling will be useful, as for 20% additional cost I expect to get ~60% more energy. Clipping is expected from June to July and is pretty acceptable.

And yes - the SMA Sunny Boy is PV to 230V AC inverter, no battery here. The SMA Sunny Island is a 230V AC battery inverter, no PVs here. The system transfers energy over the AC wires and can easily scale up.

 

[brum]

It's done. SMA Sunny Boy 3.6 with 6.4kWp panels - 3.2kW on the south roof and 3.2kW on the north roof, both are at ~17 degrees.

Now I'll wait and see if issues will pop up. But I'm within the safe limits of the voltage and the inverter is in a cool place with sufficient air circulation around it. So no issues are expected based on the SMA guide for over paneling.