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New Offgrid Solar Installation 2 * Axperts MKSII 5KW advice needed


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Hi Guys,

need some advice and recommendations for an Off grid system. Planning a system with 2 * Axpert MKSII 5k off-grid inverters in parallel, 14 * 330W Panels and  8 * 240AH 12V batteries.

Still deciding on a generator for the system, no idea what would be suitable and compatible. Any recommendations would be welcome.

Looking at increasing the PV panels and battery backup after a few years. Will split the 14 panels now for the inverters which have MPPT range 120-430VDC.

Also clueless on how I would connect the gennie to the system and if it would be manually operated. Advice will be highly appreciated.

Edited by mojive
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Welcome to the forum.

 

Are you planning on installing 7 panels on each inverter? This inverters's input voltage is between 60V and 145V. Ideally you should get as close to the 145V limit, with about 20% margin is possible. So if your 330W panel has a Voc of 45.6V, 2 panels would run at 91.2V and 3 panels would run at 136.8V. Running 2 panels in series would probably be safer. This means you should rather start with 16 panels so you have 8 panels per inverter.

Do you need a generator?

Adding PV later is fairly easy. Adding batteries is a bit more tricky. Lead Acids don't like it when you mix new and old batteries (to put it blatantly). It would be best to try and start off with as many batteries as you need + can afford right now. Or add more when you need to replace the current bank. Some, cheap, batteries have 300 cycles at 50%, some have 1200 cycles, some have 1600 cycles, and LifePo4 batteries generally have 7000 cycles at 50%. It all depends on what you really need, and can afford right now. For my personal needs I went for the smallest, cheapest bank I could get since I only use it for standby, which is probably 5 times a year.

 

The generator is connected on the grid feed side, using a changeover switch.

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Hi mojive,

First we can see the nominal Vmp of the series will be round 260 Vdc. In hot days Vmp reduces 20%, so Vmp in that case would be round 208 V, which is in Vmp range.

Nominal capacity of batteries is 48Vx480 Ah, so max charging current shouldn´t exceed 72 A. That´s important and you can configure it on Axpert.

 

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9 hours ago, SilverNodashi said:

Welcome to the forum.

 

Are you planning on installing 7 panels on each inverter? This inverters's input voltage is between 60V and 145V. Ideally you should get as close to the 145V limit, with about 20% margin is possible. So if your 330W panel has a Voc of 45.6V, 2 panels would run at 91.2V and 3 panels would run at 136.8V. Running 2 panels in series would probably be safer. This means you should rather start with 16 panels so you have 8 panels per inverter.

Do you need a generator?

Adding PV later is fairly easy. Adding batteries is a bit more tricky. Lead Acids don't like it when you mix new and old batteries (to put it blatantly). It would be best to try and start off with as many batteries as you need + can afford right now. Or add more when you need to replace the current bank. Some, cheap, batteries have 300 cycles at 50%, some have 1200 cycles, some have 1600 cycles, and LifePo4 batteries generally have 7000 cycles at 50%. It all depends on what you really need, and can afford right now. For my personal needs I went for the smallest, cheapest bank I could get since I only use it for standby, which is probably 5 times a year.

 

The generator is connected on the grid feed side, using a changeover switch.

Hi SilverNodashi,

New axperts models can support high PV voltage. 

axpert mks II 500v (2).jpg

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Hi  everyone.

Firstly thanks for all the replies.🤗

Its my first time using a forum so forgive me if I don't quote or answer  a person directly.

The inverter being used is the second last column on the spec sheet posted by Javi. The panels have Voc = 45.6 * 7 panels = 319.2V which is Ok as the inverter can take 450V.

We're looking at a generator 7.5 kVA gennie . There is a second hand GRIP gennie available.

Would the generator have to be manually operated?

Is there a specific cost effective changeover switch?

Still figuring out the circuit breakers and isolators and battery fuses. 

Did see an Axpert Pre-wired connection box, but it is for a single inverter. Would I need say double of what the box contains. Trying to keep the costs down.

Thanks again guys looking forward to some more advice.

Appreciated

axpert prewired_db_1.jpg

axpert prewired_db_2.jpg

axpert prewired_db_3.jpg

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1 hour ago, SilverNodashi said:

Do you need a generator?

The generator is connected on the grid feed side, using a changeover switch.

Off grid you need a generator. 2 or 3 days of clouds are not uncommon, even up to a week.  A generator for standby is cheaper than batteries. You can have a smaller battery bank if you use the generator for the odd occasion you need it, i.e you don't need 5 days battery standby, if you can just power up the genny if required.

I'm not sure how all inverters work, but my generator is connected to the inverter which detects when the genny starts and then uses it to charge the batteries and power the load. There is an auto start relay, but I have never used it.

Manual start for the genny is better, as you don't want it starting by itself in the middle of the night, or if there is a fault. Dangerous IMO. I find it is better to manual start as you can anticipate the loads, rather than waiting for your system to be under strain before the genny auto starts. I.e., going to weld? Start the genny. Clouds coming in and batteries a bit low? Start the genny.....

Once you get a feel for how your system behaves, you will know when the genny is required and when not.

Edited by DeepBass9
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So it probably doesn't need a changeover switch, the inverter should detect the generator input.

It is also better to use a diesel generator, rather than a petrol. Safer to store diesel for one, and the diesel generators are more stable and sync with the inverter better. There have been lots of problems reported with cheap petrol generators and inverters.

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32 minutes ago, DeepBass9 said:

Off grid you need a generator. 2 or 3 days of clouds are not uncommon, even up to a week.  A generator for standby is cheaper than batteries. You can have a smaller battery bank if you use the generator for the odd occasion you need it, i.e you don't need 5 days battery standby, if you can just power up the genny if required.

I'm not sure how all inverters work, but my generator is connected to the inverter which detects when the genny starts and then uses it to charge the batteries and power the load. There is an auto start relay, but I have never used it.

Manual start for the genny is better, as you don't want it starting by itself in the middle of the night, or if there is a fault. Dangerous IMO. I find it is better to manual start as you can anticipate the loads, rather than waiting for your system to be under strain before the genny auto starts. I.e., going to weld? Start the genny. Clouds coming in and batteries a bit low? Start the genny.....

Once you get a feel for how your system behaves, you will know when the genny is required and when not.

I suppose it depends on your needs + location. We don't have a generator on the farm and so far haven't needed one. In the city I use Eskom prepaid for night time use.

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If you are completely off grid all the time, then it is necessary, also for redundancy. If your inverter packs up what are you going to do while it is fixed? I've had an MPPT which was hit by lightning so I ran on generator for a week. I now have 2 MPPTs, and I'll buy a spare inverter one of these days.

BTW, on a generator, you can never have too big a flywheel....:D

Edited by DeepBass9
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3 hours ago, DeepBass9 said:

If you are completely off grid all the time, then it is necessary, also for redundancy. If your inverter packs up what are you going to do while it is fixed? I've had an MPPT which was hit by lightning so I ran on generator for a week. I now have 2 MPPTs, and I'll buy a spare inverter one of these days.

BTW, on a generator, you can never have too big a flywheel....:D

Yes you have a valid point. We just don't have a generator on the farm, probably cause the one my dad brought for his farm hasn't been used in at least 8 years now. And mine at home has also been gathering dust since 2012 . So far, no issues ;)

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What about grounding in an Off Grid situation?

I read something I think by Plonkster but that applied to grid-tied systems

Quote

The short version is >>

Connect all earth terminals of EVERYTHING that has an earth terminal or requires earthing to your main municipal earth, using an earth wire of at least 1/2 the size of the main conductor, or 4mm for things that you are not sure of (TV antenna, sat dish, PV panels etc). After that do nothing because it is now earthed.

But in an off grid situation we do not have munipality(Eskom) grid cos we're going to disconnect from them. Advice on this front will be appreciated.

Hoping to get all the components together this week and installing on Sunday.

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The smaller gensets 6kva and the likes don't usually work well with inverters or UPS the frequency and voltage is too eratic. Using the genset will it be needed for back up on low battery days or if the inverter fails. If it's for when the inverter fails then use the genset and a change over/bypass switch. I'm tempted to make a 48v DC genset , I  have 2  24v truck alternators which I want to alter to use to my charge batteries. But it's been on my to do list for a while.

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14 hours ago, mojive said:

What about grounding in an Off Grid situation?

Ideally you should do it the same way as in city installations. Create a suitable ground (which usually involves burying lots of copper), then connect your protective earth conductor to that, and bond your earth and neutral together on the inverter output. In such a setup the bond can be permanent, no need to bother with a bonding relay and all that stuff.

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20 hours ago, DeepBass9 said:

Manual start for the genny is better, as you don't want it starting by itself in the middle of the night, or if there is a fault. Dangerous IMO. I find it is better to manual start as you can anticipate the loads, rather than waiting for your system to be under strain before the genny auto starts. I.e., going to weld? Start the genny. Clouds coming in and batteries a bit low? Start the genny.....

This to me is one of the more important aspects to being off-grid. I concur with DB9 manual start.

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42 minutes ago, plonkster said:

Ideally you should do it the same way as in city installations. Create a suitable ground (which usually involves burying lots of copper), then connect your protective earth conductor to that, and bond your earth and neutral together on the inverter output. In such a setup the bond can be permanent, no need to bother with a bonding relay and all that stuff.

In my case i am still connected to the earth on the Eskom pole. The transformer and everything else has been removed, but the earth is still there. I used the aluminium eskom conductors they left behind to do the earthing net for my lightning conductor. I have few 100m of it still lying around the farm if any one wants it. 12mm aluminium conductor with a steel core. I can't really take it to the scrap metal place as it would probably be flagged as stolen, but eskom just cut it off the poles and left it on the ground.

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 I presume you have a current Eskom connection? Go and look at the pole with the transformer, there are two pipes going into the ground with some conductor going down the middle. Those are your earth spikes. The transformer and your house DB etc are all to earthed there.

When Eskom disconnects you, they will take the transformer but everything else will remain. You just need to make sure everything is still connected to that earth on the Eskom pole.

I took one of those old analogue meters from the Eskom box and connected it to my generator so I can see how much power it has generated.

 

Edited by DeepBass9
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On 2018/11/21 at 10:05 AM, DeepBass9 said:

 I presume you have a current Eskom connection? Go and look at the pole with the transformer, there are two pipes going into the ground with some conductor going down the middle. Those are your earth spikes. The transformer and your house DB etc are all to earthed there. 

When Eskom disconnects you, they will take the transformer but everything else will remain. You just need to make sure everything is still connected to that earth on the Eskom pole. 

I took one of those old analogue meters from the Eskom box and connected it to my generator so I can see how much power it has generated.

 

Yes still have the Eskom connection, but not for long😀. So the current home wiring is using the eskom earth.

So I guess I just take the Inverter output and replace Eskom input on DB board? And as @plonkster advised bond inverter N output to eskom earth.

How does one bond earth and neutral and at what point in the circuit?

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13 hours ago, mojive said:

How does one bond earth and neutral and at what point in the circuit?

Normally the bond is made where the supply enters the premises (assuming TN-C-S), which in this case would be at the inverter or as close as possible to it. You also connect earth to neutral at the same point.

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On 2018/11/24 at 12:20 AM, mojive said:

Yes still have the Eskom connection, but not for long😀. So the current home wiring is using the eskom earth.

So I guess I just take the Inverter output and replace Eskom input on DB board? And as @plonkster advised bond inverter N output to eskom earth.

How does one bond earth and neutral and at what point in the circuit?

For curiousity's sake, what is Eskom charging for power these days? When I went off grid it was about R1200 connection and R1 something per kWh. That was 3 years ago.

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On 2018/11/24 at 1:43 PM, plonkster said:

Normally the bond is made where the supply enters the premises (assuming TN-C-S), which in this case would be at the inverter or as close as possible to it. You also connect earth to neutral at the same point.

@plonkster

Can you maybe explain this a bit in layman terms. Will be appreciated. Would I still use the old Eskom earth , and do I connect the neutral to the earth?  

Does the battery also need to be grounded in this offgrid setup?

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3 minutes ago, mojive said:

Can you maybe explain this a bit in layman terms. Will be appreciated. Would I still use the old Eskom earth , and do I connect the neutral to the earth?  

Let's see if I can make this really simple.

1. On the inverter output, put a permanent link between the earth and the neutral terminal (note for anyone finding this thread in the future: In the context of this answer you can do this on the inverter in question... but on some inverters you will blow things up if you do this).

2. In the cable that comes from the Eskom transformer there will be a number of wires, possibly as much as 5 (earth, neutral, and three phases). Ignore all wires except the earth wire (the other wires will be disconnected when Eskom fetches their transformer). The earth wire is connected at the transformer to a buried earth spike/structure. Connect this earth wire to the combined earth/neutral terminals on the inverter.

3. Also connect the case of the inverter to this earth.

4. When Eskom fetches their transformer, make sure they leave the cable with your earth connection untouched.

So basically it's a two step thing. Step 1 is to tie the neutral TERMINAL of the inverter to the earth TERMINAL of the inverter. This is the so called N-E bond. The second step is to actually earth the thing to an earth spike, and here you use the very good earth that is already there.

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