phil.g00

The reason I am unconvinced about fuses, is not that they aren't a legal or safety requirement. Yes, I'm convinced to install them, but I am unconvinced that they actually blow and notify me that I have a circulating current issue. A PV panel will only conduct in reverse after Voc is exceeded, and at Voc the parallel strings provide zero current by definition. Now a blown bypass diode could provide sufficient voltage mismatch, but I think you'd still be splitting hairs whether you'd get enough current to blow a fuse in say a 3 string array. Maybe even a four string array would be pushing it too. Granted, if there is 1.5X the rating the fuses will blow, I just think the likely-hood of this condition carrying on merrily under that current level is high. A blocking diode would stop this, but I wouldn't use one because of power loss ( .7V x whatever current in each string ( say 8A) , about 5Watts/string). So put fuses in, but can I suggest this as well. This would cost no power during normal operation, use lightweight wiring, and indicate if you had a problem and where it was. It uses two pin bidirectional two colour LED's, during normal operation they would be off. In a circulating current condition the combination of LED colors would indicate in which string the issue is. The resistors would be sensible values found by initially adjusting a pot.

Considering Weber's quite correct diagram as to the diode orientation. i'll have to think about things, still not convinced about fuses.

Then you would have a short circuit of that entire panel. (Assuming 1 bypass diode per panel) It would be a 3||3||2 panel situation. Still wouldn't blow a fuse.

Which diode blew a bypass diode or an internal cell diode?

I know they are forward biased in normal operation otherwise they wouldn't work. Think of a diode as a non-return valve. So you connect the +ve of all the parallel strings together and all the -ve legs. All of the internal non-return valves point in the same direction. The convention is current flow from +ve to -ve outside of the source terminals. ( think of a battery). Conversely current flows from negative to positive inside the source. If one string was to drive current through another string it would try a drive in the opposite direction to normal operation to make its circuit. In other words it would try and cause current against the non-return valves. It would try to drive from positive terminals to its negative terminal inside the source thereby reverse biasing a whole bunch of diodes, so current wont flow. The above description of a Zener diode is correct, but Zener diodes are used in electronic circuits for special voltage clamping purposes and not just used willy-nilly. You wont find them on a PV panel, never, ever, not a variable. Diodes do blow short circuit, but we have to consider their statistical failure rate. Lets just consider rectifiers, for example they do fail every now and again, but they are forward and reversed biased 50 times a second, 60 times in the states. Probably more times in the time its takes me to write this, than they'll be expected to do on your roof in 25 years. Now factor in that you need multiple series failures to create a short circuit through a series on panels. Lightning aside, this is borderline statistically impossible in normal operation.

The bypass diode purpose is to allow other panels in the same series string to "bypass" a shaded panel, which when shaded has a high impedance. No, a PV panel doesn't take as much current in reverse as forward, you will not pass any current in reverse through a pv panel. It is a series of diodes that will block current for many multiples of it output voltage applied in reverse. As will the bypass diodes. The bypass diode is a pn junction in parallel with a string of series pn junctions within the panel. A pn junction is a diode. A diode needs a 0.7V forward bias to conduct in forward direction, and will not conduct when reverse biased. Panel strings in parallel will provide a voltage in the polarity that would attempt to reverse bypass all neighboring strings. For a string to conduct in reverse (and then presumably blow a fuse) a series combination of diodes (pn junctions) would have to blow. (Highly unlikely). A single blocking diode in a string would prevent this better than a fuse. But if you aren't going to believe that multiple diodes is series are already blocking any reverse current, why add an extra one?

Nope, the OP described a string as panels in series, all that does is increase the voltage. If 8 amps is the panel rating, then shorting out 10 panel in series will still only provide the same SC current of 8 Amps as shorting out 1 or shorting out a string of 100. If they were in parallel then the current would increase multifold. The question here is if a parallel string went short circuit would you want its fuse to blow so you could limp along probably unknowingly with 1 string less production or whether you'd prefer to know straight away as you'd have no production straight away. (Remember nothing will get further damaged as its quite permissible to SC PV panels.) I submit I'd prefer I'd want to know about it straight away, however its never going to happen. Why? Now, I've just spent 5 min googling and I couldn't find an instance of a panel going short circuit. The little cells themselves are like diodes. Now when a diode blows it does go short circuit, but for series column of them with a panels to blow that would be winning the bad luck lottery. For a column of series cells in every panel in a string to blow, borderline statistically impossible. As to shading of cells that doesn't become lower impedance but a higher impedance. That lowers current and wont increase current which is what you need to blow a fuse.

I respectfully disagree. Solar panels can be shorted without damaging them, secondly how are you going to rate the fuse,when you want panels to deliver to their maximum. Thirdly if a fuse blows (with age as sometimes happens) you may not know about it until your batteries are flat. And fourthly you are going to have to go on to the roof to fix something that is not providing any value. Stick an appropriate rated MCB in your ground level DB that can act as functional isolator. Then put surge protection on the panel side of this MCB. ( for lightning) For surge protection I recommend buying cheap ferrite cores of say 15mm ID and putting at turn or two of your dc cable through this. This makes a choke that becomes high impedance to lightning, causing the voltage to rise at that point. A ferrite core looks like this: ferrite core Do this on the +ve and the -ve cable, then directly on the panel side of this choke fit cheap axial surge arresters that breakdown at say 145V between each cable and ground. Surge arrester Hopefully, the cheap surge arrester sacrifices itself before your batteries or MPPT depending on your setup. Lightning jumps air all the way from the sky, a further 1 inch fuse is not going to stop it. The MCB will protect the cable to your panels from your batteries, in the event of lightning allowing the batteries to backfeed to your now blown surge protection. The batteries are the only DC component with enough oomph to blow a fuse anyway and fuses wont save you from lightning. All you can do is stop the batteries continuing to feed the fault that lightning will create. So I recommend no fuses between panels (especially on the roof) in any configuration.

And some further reading leads me to believe that this assistant would allow me control AC2 out , while not even having the prerequisite of AC in, even in the conventional parallel configuration. The conventional behavior is forgotten when an AC2 assistant is used. Which solves load-shedding requirement, without having to re-invent the wheel. Thanks Plonkster for your assistance.

Further to my last it would seem the programmable relay assistant is tailor-made for this without a contactor, that presumably will allow the use of the PV inverter assistant as well.

It looks like a virtual switch operating a contactor is the way to go to shed AC2 load for the time being. Although the VS route excludes assistants, and that would exclude PV inverters assistants in the future. So I'll have to have a think about that. Victron themselves have indicated the power assist will do the job.

Yes, that sounds great. Will the downstream inverter still work from the DC if the AC is lost in this scenario? I know I would lose AC2 out if I lost the AC in on the downstream inverter, but could I set the AC2 out to switch off at a certain battery voltage with the AC in still on? In other words do I now have the ability to load shed non-essential loads and keep 10kVA available? And one more question please while I am picking your brains, am I correct in thinking that the 10kVA available on the downstream inverter is the sum of AC1 out and AC2 out? in other words, the 10kVA could come entirely from either ACout, if nothing was being drawn on the other ACout of the downstream inverter.

Yes, you're right I did say that. And I can see your logic, that if the downstream inverter is set to export into the grid, and if upstream there is insufficient load and it's battery bank is charged there is no mechanism to curtail the power backfeed. Unless zero backfeed power is set on the downstream inverter. Which would mean that only 5 kva would be available at the ac out of the upstream inverter. 10 kva should still be available from the downstream inverter. I don't like the idea of feeding back onto the ac input of the upstream inverter, that would seem to create the chance of a run away loop. So maybe that's a genuine limitation of this configuration: A.) The upstream inverter has to charge its own batteries and can only feed 5kVA. And B.) The downstream battery bank still gets charged by both and 10kVA is available. I would still consider this a more flexible configuration, as I would still get a two battery choice and two ac outs and possibly two gen starts. I still wouldn't get that from a straight parallel in an off-grid set up.

Ok, lets take this step by step. Forget about PV inverters for the moment. Given: In stand alone off grid mode, only fed from a battery a quattro matches its output to the load, at a stable frequency. Even on grid, the grid frequency is not used to curtail ouput power. It is the load that pulls the current. Not that I've personally tested, but from what I've researched, an inverter is pretty good at maintaining 50Hz. The fundamental operation is that it doesn't push electricity at a load thereby raising the frequency, it maintains the frequency/voltage relationship whilst varying the current within its rated range. Why do you say the frequency will head to 54Hz? In other words, why will the upstream inverter not act as the grid up to its rated output? I am asking because although I have a thorough knowledge of electrical fundamentals, inverters are not my area of expertise. There may very well be some kind of attempt to raise the frequency of the grid by the second inverter to let the inverter know the grid is actually there. Then again I believe there is a allowable configuration that will allow a PV inverter on the AC in side, and if a third party PV inverter is permissible I don't see how an own brand inverter would be unsupported. Please don't think I am being testy, I have read this forum top to bottom and I know that you are an authority on this stuff, and thank you for taking the time to discuss this.

I guess if there's no fundamental runaway or an some sort of unstable ringing ringing effect, this configuration will far more flexible. Sure there will tricking around with settings and inputs and outputs, but that just detail tweaking. Another advantage of doing this is the ability to mismatch equipment, like the stringent requirement of identical quattros etc. Maybe a Multigrid and a quattro?

"it must reduce power if the frequency shifts up." I can understand an ac-coupled Fronius PV inverter having to to this, and if I added ac-coupled PV to the system it would still work as essentially all the AC would be synched to Quattro 1. What I'd have to make sure of is that the 1:1 rule wasn't broken inadvertently, when the Quattro to Quattro AC was disconnected if I added PV inverters. This would happen say when there was the disconnection of a heavy load. However, I see the Fronius as a generator with inertia pushing power onto a grid as opposed to the Quattro supplying power that is being sucked out of a battery. So on the face of it I don't see a problem that is specific to down-streaming an inverter.

Lets say priority load AC out on Quattro 1 onto some load 1 the AC in of Quattro 2. Quattro 2 set to export back to Quattro 1 as well. ( What country Grid setting now?) Other AC out of Quattro 1 onto another load 2 Quattro 2 in turn has a two stage priority AC out. ( say load 3 and load 4) Quattro 1&2 either fed of one battery or two different batteries with two different charging curves. The idea being that load could be ditched or picked up at a more granular level during a power cut. Say, Granny flat high priority load, sister high priority , gen start, granny flat low priority, sister low priority, something like that. Normal usage: maybe based on different SOC's of a lithium bank and a LA bank, so as to deliver normal by light usage of the LA's but to use the lithiums deeper discharge and possibly avoid the downside of the lithium switching off. Backup Gen if first Gen fails to start etc. Can more than 1 Generator be configured on an AC in of one Quattro. Just an idea at the moment....

The context of system is that was put together by my father, and I'd like to optimize this system. There is 13.5kW of PV panels. 1200AH of 4 x 12V silver calcium sealed torque batteries ( Yes, I know wrong type, but due to a previously incorrect charging voltage seem to lasted nearly 3 years). 2 outback 60's & 2 outback 80's One connected 48V 5KW Quattro purely fed by the battery. This all supplies their granny flat on my sister's property rather nicely. The granny flat is also equipped with LED's, gas & EV geyser. They can chop over to a an ESKOM single phase unmetered supply that tee's off my sister's metered 100 kVA 3 phase supply, only do during heavy usage in prolonged inclement weather. They have no desire to ever parallel with the ESKOM supply. My sister has a 6kW 1ph generator that they would like to auto-start for her though. They would also like to pick up some of the sister's 200kWh/day usage to ease her electric bill, but rather by supplying certain specific loads.( Maybe an entire phase?). They are in the Drakensberg, on a rural supply so power cuts for every thunderstorm. They have acquired a second 48V 5kW Quattro, but I have to commission it, (my mother's stipulation). I am in Ireland, but visit my parent's in SA annually. I have been reading up on paralleling quattro's, and conditional on them having the correct models so that I can match the firmware, I am pretty confident I can do it. Now for my question, should I do it? I think that 1 quattro supplying the other quattro is a far more flexible arrangement. It could allow for two standby generators ( granted only one at a time, but two independent gen starts). It could allow for two separate battery banks, (which is important if one wants to gradually ease into the pylon tech route), but still use this battery bank until it starts to die. Or alternatively, use pylontech batteries as the cycling batteries, with a really low discharge cycle of the lead acids, but their when you need them. I stand to correction but I also think it would allow for 3 independant stages of load prioritizing, as if there was an AC1, AC2 & AC3out. Also think that because of the 2 Quattro's ability to pass through and back current there would still be 10kW available at any/all of the AC outs if they were on. It seems such an obvious way of configuring two quattro's, that I think I must be missing something. What shortcomings can anyone else see?