Puzzled - PV switched off

[Leonc]

I can't find the reason for my PV switching off, no error codes

My system - 5kva Axpert inverter, 2 strings of JA solar 330w panels and 2 Pylontech 2400w batteries and using the ICC system to monitor.

The system has been performing well until yesterday at about 12:00 when the panels were producing about 1650w and the load was about 25% on the inverter and then the watts from the panels dropped to zero, the sun was out no clouds at all and I could not get the PV back. After shutting the system down and then up again the PV came back. 

Any ideas or experience like this

Thanks 

[Chris Hobson]

Hi Leon

The Axpert's MPPT (same with other MPPTs) does sometimes reset. The conditions where this occurs most often is either with variable light conditions (which you were not experiencing) or variable loads. How long was the PV off. Sometimes it takes a whole minute - how long was the PV off for?

 

[Leonc]

Thanks Chris

it went off at around 12:00 and when i got home after work it still was off 15:00

[___]

It could be a couple of things. Things I've seen include:

1. You blew a fuse. You do have inline fuses, right? :-)

2. The device reset due to changing light conditions, changing loads or an overcurrent event (this is completely normal). It failed to restart.

  2.1. One reason it might fail to restart is a high DC ripple on the battery. This happens if the battery bank is too small for the loads, or if the battery bank is on the way out.

  2.2. It could also just be a software bug. Turn it off and back on again.

3. Something blew up. It will need repair.

[DeepBass9]

Probably 2.1. As I understand it lithium batteries have a much smaller voltage range than other batteries, so it may be the battery voltage was too low for the system, and so it didn't switch on again. Maybe look at those settings on the inverter?

[Leonc]

Thank you very much for the info, I will check the settings. The system is running again after the manual restart

But looking at what Plonkster said 2.1 looks like the issue

[Leonc]

Plonkster, I do not have inline fuses from the panels to the inverter, I have now searched the net and cannot find any. Where can i get some and what size do I need. I have two strings of 3x330w panels.

thanks 

[Guest]

5 hours ago, Leonc said:

I do not have inline fuses from the panels to the inverter, I have now searched the net and cannot find any. Where can i get some and what size do I need. I have two strings of 3x330w panels.

Leon, I once had a situation where the panels where preforming, and then not. Can you check each panels connector box, is it deformed, open it, are the inside diodes still ok?

This one has 3 diodes:

And get fuses in quick. The guys will be around to help.

Quick search, to give you an IDEA, don't buy this one, just an starting point to Google: OmniPower Fuse 80A 22 x 58

Also highly recommended to have a disconnect, that you can switch the panels "off".

[___]

6 hours ago, Leonc said:

inline fuses from the panels to the inverter

Just some form of fuse. I use DIN-mount units similar to these:

[Leonc]

Thanks so much

I will check the diodes and get some of these din mount fuses. Do I need one for pos and one for neg or one for each string. Where do i mount these?

I do have a disconnect switch for the panels.

Can i use these type of fuses?

Edited August 8, 2018 by Leonc

[___]

5 minutes ago, Leonc said:

Do I need one for pos and one for neg or one for each string

I think you only need one (in the positive line usually), but it seems the industry is headed in a direction that requires that you fuse both. There are no official regulations yet in South Africa, only best practices. Closest we got is the green card stuff from SAPVIA. Here is a presentation of theirs that suggests you need fuses and breakers in both lines.

In my case, I have external MPPTs, so I have a fuse before the MPPT (both lines) and a breaker after it (again, both lines). The reason for that is also that you get double the DC voltage rating if you have two contacts that open, so a breaker rated for 55V DC becomes 110V.

[Chris Hobson]

1 hour ago, Leonc said:

Thanks so much

I will check the diodes and get some of these din mount fuses. Do I need one for pos and one for neg or one for each string. Where do i mount these?

I do have a disconnect switch for the panels.

Can i use these type of fuses?

Jip you can use those fuses. I have some in my store. What I don't like about them is they do not click closed and you not quite sure that it is making good contact.

Just a pointer often you will be supplied with 20A or 15A fuses. For a single string of PV they are a bit on the big side. 12A is more than sufficient to avoid nuisance blowing and low enough to actually provide you with some protection.

[Leonc]

6 minutes ago, Chris Hobson said:

Jip you can use those fuses. I have some in my store. What I don't like about them is they do not click closed and you not quite sure that it is making good contact.

Just a pointer often you will be supplied with 20A or 15A fuses. For a single string of PV they are a bit on the big side. 12A is more than sufficient to avoid nuisance blowing and low enough to actually provide you with some protection.

Chris Thanks

I have 2 strings of 3 x 330 w panels. Just to be sure what i am doing do i need to fuse the string or each panel and what size of fuse would i need. If i put a fuse after the 2 strings are combined what size of fuse would i need then.

Really appreciate everyone's input.

I have learned a lot so far

[Chris Hobson]

2 minutes ago, Leonc said:

I have 2 strings of 3 x 330 w panels. Just to be sure what i am doing do i need to fuse the string or each panel and what size of fuse would i need. If i put a fuse after the 2 strings are combined what size of fuse would i need then.

Hi Leon 

It always helps if we understand why are doing something. We fuse the individual strings of the PV to prevent a panel going short circuit drawing current from the other PV strings and causing a fire. Under normal  circumstances 60 and 72 cell panels will deliver 8 or 9 A. Since they are connected in series this value does not change per string. We now combine the strings either in a combiner box or using Y connectors and the Amps become cumulative. Current always take the path of least resistance and a short circuit in one PV string has the potential to draw current from the other strings. In my case 3 other strings so a potential 28A. 

With only 2 strings there is a chance under certain circumstances that the fuse will not blow but install them anyway as it is better than nothing at all.

If you fused the cable after combining the two string that would be to protect the cable itself. Each string would produce roughly 9A and a combined 18A. With our high summer temperatures you would derate your fuses by 25% to prevent nuisance blowing. That gets you to 22.5 A - so a 25A fuse. 

[Leonc]

Chris, Thank you very much for the detailed explanation.

If I understand you correctly, it is best to fuse each panel and then a fuse after Y connector.

Fuses for the panels can be 10 A and then the one after the Y connector 25 A.

Sorry for all the questions 

Thanks again 

Leon

[Chris Hobson]

41 minutes ago, Leonc said:

 it is best to fuse each panel 

Each string of panels ...so you have 2 strings of 3 x 330W so two fuses (12A including the 25% derating) one for each string before the Y connector. Those should protect the combined cable from currents over 25A. You can fuse the combined cable but to my mind it is not necessary. You do still need a PV disconnect and I have bought a 40A NOARK DC circuit breaker and use it as a disconnect (It is cheaper and provides an additional layer of protection). With two strings if you go this route a 25A breaker would be great but make sure it is a DC breaker - I cannot stress this enough. Some suppliers say Schnieder breakers are DC rated and they are not. A DC breaker has a snuffling mechanism to prevent an arc forming.

[___]

35 minutes ago, Leonc said:

fuse each panel

No, you don't need to fuse each panel. You have 6 panels arranged in two strings. You will have one fuse per string, not one per panel. So in the positive cable that comes from the last panel, ghat gets one fuse. Then the two come together with a Y- connector (or even better, in a combiner box on the ground), and then you can optionally have another fuse in the main run there if you want to (though it is unnecessary with just two strings that are already fused).

[___]

3 minutes ago, Chris Hobson said:

Some suppliers say Schnieder breakers are DC rated and they are not

I specifically researched the Gewiss breakers. They are DC rated up to 55V per contact. If you have two contacts, then double that, and so forth. I eventually found the specification pdf for this breaker (which is listed as an isolator), which shows a 110V rating for two poles, up to 6kA. On page 15 they show you how to connect it for DC applications. Page 16 shows the tripping characteristics (about 20 seconds at twice the rated current, instantaneous at 4x).

Truth be told, 110V is a tad low still, but perfect for my 24V system.

[Leonc]

Thanks guys, appreciate your feedback. Make sense now

[Sidewinder]

Morning @Leonc,

One thing I have noticed on the Axpert's 5kVA units, they don't like being loaded "light" on the PV side. 

You have 2 x 3 x 330W = 1980W.

I've done this experiment on the InfiniSolar V 5kW, which has the same PV portion as the Axpert.

You can read the whole story here 

The short story is that i my opinion, the Axpert's likes being over supplied with PV. I run 3600W per MPPT, which means they effectively get just over 3000W in practice each, right on the money. And symptoms like yours have become less of a nuisance. Even on broken cloudy days, my system does not "loose" the PV , like when I was running 4 x 1800 PV setup.

Probably just some software tweak that is required to fix, but the Voltronics guys are notorious for not bringing out regular updates to all their range of Inverters, never mind the Axpert 5kVA one, which is well supported/fixed s/w wise by @Coulomb.

[___]

1 hour ago, Sidewinder said:

don't like being loaded "light" on the PV side

It's a buck converter. The range that it can comfortably support depends on the granularity of the PWM bit that drives it. Other factors are the frequency it runs at, the size of the inductor and the size of the capacitor on the output side. This affects your output voltage ripple and current ripple.

As a practical example, if you have in input of 100V and you're bucking down to 50V, the PWM mark/space ratio will be 50%. If your input is 60V and you're bucking down to 50V, now your ratio is 84%. As you get closer to 100%, you get closer to just having the panels tied directly to the batteries, and we all know what happens then.

Vmp should ideally be 30% or more higher than Vbat.

The reverse is also true. As the voltage goes up -- assuming its within the range of the controller -- the mark/space ratio becomes smaller. For example, on a 500V input, charging down to 50V, that's an 0.1 ratio. Now imagine you're using a PWM driver with an 8-bit register, aka 1024 discrete steps. If you want to cover 80V to 500V (as I've seen some do!), it essentially means it has 1000 steps of 0.5V that it can settle on, plus some error, so it can run about a volt off the real Vmp, which of course can be significant in the lower voltage ranges.

[KLEVA]

4 hours ago, Sidewinder said:

One thing I have noticed on the Axpert's 5kVA units, they don't like being loaded "light" on the PV side.

I did not know that, shows you can always learn something. I was using 3+3 configuration of 250W panels (roughly 80V in Winter - 110V in summer, that the Axpert was telling me). Although I didn't experience (or didn't think I did) reduced Wattage from the panels. I often got more in Summer, than rated (up to 1650W logged at some points).

@plonkster - Sorry, thats Greek. Umm, what does that mean on an Axpert, should we be putting 250-300W panels in a different combination?

[___]

10 minutes ago, KLEVA said:

what does that mean

It means try not to let the panels work at less than 80V, give the MPPT so meat to work with.

[Chris Louw]

That is how I find it also . Even if you exceed the mppt max with 750 watt I rarely get to the maximum . Makes a lot difference to available power . Is this normal or do my panels under perform .

[KLEVA]

Ok, so if I under-sand (pun intended), a min of a 4 string should be used on an Axpert (roughly 30x4 under load for 250-350W panels). Is the ultimate max voltage not too high or have I completely lost the plot? If the panels give it more (let's say for example 160V, when only specced at 150V), won't that create a bigger problem? I do understand that the MPPT part of the system will try reduce to an acceptable range, but where do we find a break point? If I parallel all 6 panels, isn't that Voltage at now roughly 240V when the system is only designed for 150V?

I am serious, no-one has explained that part to me properly. I thought I always had to keep the max PV voltage BELOW the Invertor/Axpert max.

Edited August 9, 2018 by KLEVA