16 minutes ago, Mark said:

40plus amps to power loads

Yes, but that's from the batteries, not from the panels. You can know beforehand, rather precisely, exactly what the worst case current is that you have to break. It's written on the back of the panels :-)

5 minutes ago, Jaco de Jongh said:

You might create surges (Transients) if you want to call it that, and I have seen on much bigger VSD's (Basicly the same consept) that they tend to fry the electronics when you cut the supply while under load especialy on the DC buss side. I just dont think its safe to do this uncontrolled.

I suppose anything is possible, but I rather doubt you'll be able to kill an MPPT by cutting the PV. It's a buck converter. It's very operation relies on cutting the power between 40 000 and 100 000 times per second and making a big old inductor throw back-EMF :-)

Thanks sir, then I guess it's safe for noobie to do it then.. I hope it works for him.

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4 hours ago, The Terrible Triplett said:

Never EVER disconnect panels under load ... ever, unless it is a dire emergency and you had no choice at all.

Don't agree with this
I use DC rated Noark breakers on each of my strings and have no issues turning them on and off 1 by 1 to test the strings individual performance under load conditions.
 

35 minutes ago, plonkster said:

I suppose anything is possible, but I rather doubt you'll be able to kill an MPPT by cutting the PV. It's a buck converter. It's very operation relies on cutting the power between 40 000 and 100 000 times per second and making a big old inductor throw back-EMF :-)

Think Plonkster gave the very reason why this should not be a problem

If I did this, I'd want to turn the power down before I switch. Having just completed a project where we turn the power on an MPPT down based on external signaling, I know you can do this with any BlueSolar MPPT by writing a one to /Link/NetworkMode (remote control) and then writing a zero to /Link/ChargeCurrent. It doesn't shut down immediately, it ramps it down. But after two minutes or so, you can then switch completely off-load.

This is how I'd prefer to do it. But you guys with your Axperts are on your own there :-)

Thanks@plonkster, it took quite some time to find out we actually agree.

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Thanks for all your input guys.

Just to sum it up... I want to set the axpert to UTI so that it only inverts from DC to AC when there is an Eskom power failure. This is so that I can reduce the amount of switching the inverter does during the day, also the fans will not run as often as they do when the unit is set to SBU or SOL

When the Axpert is set to UTI it will only use the only PV panels and batteries to power the load when there is an Eskom power failure. So if we don't have a power failure for a month the PV panels will be doing nothing but collecting dust!

Since I already have 1500w of PV panels installed I don't want to see them go to waste

I want the PV panels to connect to a DC geyser element directly (there will be a geyser wise control system). If there's an Eskom power failure I want to disconnect the PV panels from the geyser element and connect them to the Axperts PV input. This way we could run the load connected to the inverter longer than just relying on the battery bank capacity.

So there would be a relay which is controlled by Eskom with one input PV + and -, and then 2 outputs: output 1 would be geyser + and -, output 2 would be inverter PV input + and - (DPDT SSR)

@viper_za I also use DC circuit breakers to test my PV strings under load without any issues. I guess the DC breakers are designed to extinguish the arc?

@plonkster I have no clue what a buck converter is, Google will have to teach me

Because I have no way of anticipating when Eskom will have a power interruption I am not able to ramp down the load before disconnecting the geyser element (900w), like wise when Eskom power comes back on the relay would disconnect the inverter from the PV panels under full load (1500w).

Once again' appreciate everyone's assistance, this forum is awesome 

3 hours ago, Noobie said:

 I have no clue what a buck converter is, Google will have to teach me

Buck converter is a "step down" converter. It's a "charge pump", so to speak. It opens up the tap and lets energy flow to the batteries, but that energy flows through a large coil (aka inductor). Then when the inductor is more or less saturated with magnetic energy, it closes the tap. Now inductors hate it when you do that, and they retaliate by kicking back electricity in the opposite direction. Luckily, you placed a reverse-biased diode right there to catch that back-EMF and channel it into the battery. That's the basics. Now, by carefully modulating the mark-space ratio of opening and closing that tap (usually a FET), you can control the magnetic flux in the inductor, and so you can control the energy flow. Controlling the energy flow is really the same as controlling the impedance, and if you can control the impedance, you can influence the voltage of the PV array and make it run at Vmp. That's MPPTs 101 :-)

An every day example would be a holding dam for irrigation purposes. If your water supply cannot sustain the rate at which your irrigation pumps runs, you may use a holding dam to collect water, and then start the pumps once the dam reaches a certain level. Buck converters do something similar, some energy is temporarily stored in an inductor and channeled into the battery at the correct "rate" :-)

https://en.wikipedia.org/wiki/Buck_converter

... and here's a video made by yours truly -- including horrible stereotypical South African accent -- where I have an MPPT rigged to an oscilloscope, showing the principle.

 

Thanks @plonkster much appreciated! This leads me to believe that I should not have any issue if I connect and disconnect the PV panels from my inverters PV input.

Now I just need to convince the Mrs that spending more money on the solar system is a good idea 

@TinkerBoy I will touch base with geyser wise and see if they have any issues with me running their DC element directly off the PV panels and bypassing any MPPT. Judging by the manual I downloaded off their website I should be OK. I am not against MPPT's I know that they are very useful. My concern is that this will be mounted in my roof next to my geyser, it gets bloody hot up there and I wouldn't want the MPPT's fans getting jammed up with dust and failing, hence why I would like to keep it out of the setup. Thats kind of why I have set my Axpert to UTI, the fans dont run unless the unit is inverting from DC to AC and the relays in the unit switch less often. I am hoping that this will prolong the life of the unit.

Something that I've actually considered is using a boost converter. The principle is the same, you still pump charge into an inductor, but the switching is slightly different, so it boosts the voltage rather than the other way round. You can still apply the duty cycle trick to influence the impedance of the circuit and do power point tracking. Eg, this three-part video of a China special, linked below. The idea with that would be, use a normal 3kw 230V element, but just run it at 115V or even less. Power is proportional to the square of the voltage, so running at half the voltage would give a quarter of the power, but you could still heat 70 liters of water in about 5 hours if you wanted to...

This is just a wild idea. No idea how practical it is. If you just connect the panels directly to the element? It would work, I'm pretty sure there won't actually be smoke or anything (though of course the man who spend 200k in R & D probably knows something I don't, so we should ask him what he meant), but quite simply, the impedance isn't necessarily matched, the panel won't run at Vmp unless you're just plain lucky of make a custom element that precisely matches the current of the panel so IR = Vmp (roughly), so in one sense the maker of the Geyser Pro is absolutely right: This won't work properly without an MPPT.

 

Right, in from Morningstar support desk, getting down to supplier recommendations ... man, ain't it just schweet that suppliers respond.

The video I posted had no breakers to isolate the panels as I would assume some newbies would do starting out on their own. So, how do you get their attention? You put the fear of sparks in them!

Which means that the advice does not go for the @viper_za and the rest who have all the safeties in place, who understand the potential problem, only thinking of their own systems. 

We need to think like newbies here.

Hello,
If you are using a breaker/fuse to isolate PV from the controller then you should not need to worry about anything become damaged.
If you are physically adding/removing wires while your panels are under load (sun is shining) you may see an arc which may cause damage.

Having a proper disconnect will be quite important.  Having a circuit breaker seems to be the easiest way.  You will have over-current protection and a safe disconnect option.  I would not recommend just pulling wires from inside the controller as you may see an arc.

Regards,
MS Technical Support
Morningstar Corporation

So I think that we all can agreed that if you have a combiner box with the right breakers, that you are safe to disconnect under load.

IF you have fuses only, do not use them as breakers by pulling the fuse. Disconnect the wire, yes, but do not pull the fuse to break the circuit. Jy kan lekker brand.

Ps. I still switch off the inverter having had unsavory fortune of making a few things go poof.

So Geyserwise said that you can connect their DC element directly to the PV panels as long as it doesn't go over 72volts and you use a solid state relay controlled via the Geyserwise controller to turn power to the element on and off.

I have had a look and think I have found a relay that could work to connect and disconnect the panels from the geyser and inverter: http://za.rs-online.com/web/p/contactors/7447346/ 

Its contacts are rated at 690v AC/DC 40amps AC-1 

This may be a dumb question but would this be ok for 120v DC switching at 30amps? 

 

2 hours ago, Noobie said:

Its contacts are rated at 690v AC/DC 40amps AC-1 

It's not clear to me. The datasheet seems to indicate the 40A is for AC, but it also indicates large DC breaking capabilities. So it's probably fine... but I'd feel better if an actual engineer told me so :-)

8 hours ago, plonkster said:

It's not clear to me. The datasheet seems to indicate the 40A is for AC, but it also indicates large DC breaking capabilities. So it's probably fine... but I'd feel better if an actual engineer told me so :-)

Thanks Plonkster, I will get hold of Schneider Electric and see what they have to say

@Noobie The TeSys D series can be used for switching DC although the design was done for ac by default, If you want to switch 40A AC, that contactor will do, but if you need to switch 40A DC, you will have to buy  1 size bigger. Schneider can check it for you quickly, they are normally very helpful.

We normally work on 1 size up, but with bigger loads, the Table might show something else.

Edit: I had the conversion table somewhere, but did not use it in a very long time, misplaced it and cant find it, wanted to load it here.  

Edit2: Just do yourself a favor, if you decide to buy that contactor, first check the price with an Electrical Wholesaler. Just phoned our local Electrical Wholesaler and they can get me the same contactor for R770 inclusive. You might save a little.

Thanks @Jaco de Jongh, I will touch base with Schneider.

@TinkerBoy these are awesome but I need a NC contact and as far as I am aware solid state relays do not come in a NC configuration.

This 4 pole contactor has 2 NC and 2 NO, so I could bridge the inputs and it would act as a DPDT relay

4 hours ago, Jaco de Jongh said:

@Noobie The TeSys D series can be used for switching DC although the design was done for ac by default, If you want to switch 40A AC, that contactor will do, but if you need to switch 40A DC, you will have to buy  1 size bigger. Schneider can check it for you quickly, they are normally very helpful.

We normally work on 1 size up, but with bigger loads, the Table might show something else.

Edit: I had the conversion table somewhere, but did not use it in a very long time, misplaced it and cant find it, wanted to load it here.  

Edit2: Just do yourself a favor, if you decide to buy that contactor, first check the price with an Electrical Wholesaler. Just phoned our local Electrical Wholesaler and they can get me the same contactor for R770 inclusive. You might save a little.

@Jaco de Jongh thats a good price! 

So this is my plan - attached simplistic layout using the Schneider contactor 

PV switching .pdf

On 8/12/2016 at 10:33 PM, Noobie said:

Program 38 displays NEC and gives you the option to enable or disable it 

I can't find option 38 in the manual or online. Does anyone have any idea wh

This is a new setting to avoid floating neutral. Basically there is an addon accessory called grounding box that would be connected to dry contact of the inverter and it would avoid the floating neutral problem. You should not enable this option if you do not have grounding box connected to the system.

@flltech Awesome, thank you.

How did you come across this? 

On 8/14/2016 at 6:02 PM, plonkster said:

I am puzzled by that. My first charge controller, a Microcare, has a breaker on each side. When you turn it on, it's battery first, solar last, and when you turn it off, it's the reverse, solar first, then battery. This is required because PV side might be up to 150V, and if you do it the other way round... well things might burn up, as a forum member discovered when a BMS disconnected a Lithium battery.

So if you really cannot disconnect the solar panels under "load" (which is like 16 amps on my setup, that's what the panels do at Vmp), do you have to wait for night time? Surely, if you rate the breaker for 16 amps at the relevant DC voltage ... "never" is a bit too exclusive? :-)

There is however an interesting feature on the blue solar chargers. You can programatically turn the charge current off, ie, remove the load before you disconnect stuff. Now isn't that cool :-)

There's a difference between "under load" and "with high volts". "Under load" means there's current involved as well, so if you switch off your inverter, there's no / little load and the damage is much less, or non at all.

On 8/15/2016 at 8:42 PM, Noobie said:

So Geyserwise said that you can connect their DC element directly to the PV panels as long as it doesn't go over 72volts and you use a solid state relay controlled via the Geyserwise controller to turn power to the element on and off.

I have had a look and think I have found a relay that could work to connect and disconnect the panels from the geyser and inverter: http://za.rs-online.com/web/p/contactors/7447346/ 

Its contacts are rated at 690v AC/DC 40amps AC-1 

This may be a dumb question but would this be ok for 120v DC switching at 30amps? 

 

Yes that will work but you need a 220V source to control it. 

13 minutes ago, SilverNodashi said:

Yes that will work but you need a 220V source to control it. 

Good news, I was planning on having it switch with the council 220v supply. It would essentially be a mains failure relay

2 hours ago, SilverNodashi said:

There's a difference between "under load" and "with high volts". "Under load" means there's current involved as well, so if you switch off your inverter, there's no / little load and the damage is much less, or non at all.

I know I know. Rate the "switch" for the voltage and current and you can safely switch. But as I alluded above, I would still prefer to take the load off. I would prefer that, because even a relay/contactor that is rated for the current and voltage, it has a switching life, a little bit of pitting that happens every time it is operated, a little (or a big!) spark is carefully dealt with every time.

Hence also the cost of the contactor :-)

Now I would probably sing a different tune if I had no way of doing that (shutting the power down), but as it turns out, my MPPT of choice can do it. It will probably cost even more than the contactor to implement all that stuff... but it sort of feels right to me.

As long as you do it safely, more power to you :-)