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DC changeover switch


gooseberry
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I'm rather new to solar power systems but I've been doing some reading as I'm building a new house this year and want to do it right, to be as energy efficient as possible and no load shedding. The area I'm in also experience quite frequent power cuts so having a backup that can last up to 12h for lights, fridge and electronics is a must.

Now I've been looking at ways to do this piecemeal so I can increase capability over time... so starting with a pure UPS backup system makes sense to me. This would probably be an Axpert and a Lithium Ion battery, possibly with a few solar panels and a changeover switch on the DB to comply with the regulations. Later I would like to have geysers and pumps etc also benefitting from solar generation and reducing my energy bill and here I would perhaps add Solis inverter and charge the batteries with AC through the Axpert. 

Obviously the problem with this is that the batteries can't charge when the grid is down. Now my question is, would it be possible to install some switch, even manual, that disconnects the solar panels from the Solis and connects them to the Axpert to charge my battery?

Or is this not feasible and would it be a better option be to go for a hybrid grid tied inverter from the start?

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This article explains it better. It seems there is something like this and it's called a DC transfer switch.

https://www.solarpowerworldonline.com/2016/03/using-a-direct-dc-transfer-solution-to-better-back-up-solar/

I'm also told the Axpert can't be connected to the DB in any way, is that true? Would I need to install a separate charge controller?

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Hi,

Yes, a DC transfer switch would be what you are looking for, simplest (cheapest too) being a manual switch, and then an automatic being more ideal, albeit at a higher cost.

21 hours ago, gooseberry said:

I'm also told the Axpert can't be connected to the DB in any way

Who told you this? Most residential installations of Axpert inverters have it connected to the DB board.

In my setup, I have a main DB which contains breakers for all the non-essential loads, and this feeds through to a sub-main DB, where I have breakers to and from the Axpert, as well as an AC changeover/bypass switch, and then all the essential loads which are normally powered from the Axpert.

This is the main DB, please excuse the gaping hole. Pic was taken during installation.
IMG_20200127_223623.thumb.jpg.6676ce230fc45ab578b60b0617e606f9.jpg

 

This is my second DB which is connected to the Axpert.
IMG_20200127_223821.thumb.jpg.a146dfee09d2237e0bfb61f4240af1ae.jpg

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Nice setup! So are your panels only used for the critical loads and charging the battery? I.e. your critical loads are basically completely off-grid?

I've been trying to figure out what DC transfer switches cost but haven't seen any advertised online.

The main reason for the grid tied inverter in my case would be for the geysers. I will have two 100l geysers (one at either end of the house) and we're not allowed geysers on the roof. Adding two collectors just seems very expensive and fragile for what you get, especially if there is no backup for the geyser pumps.

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At the moment, yes, the panels are only charging the batteries and running the critical loads, and yes, critical loads are off grid, although the Axpert will switch to grid if the batteries get below 40% (setup in ICC).

Together with the gas stove, and evacuated tubes for the geyser, it's working well...BUT...once the batteries are charged, the panels are 90% wasted during the day, and I've also started investigating adding a hybrid inverter. My first thought was to split the panels between what I have and the hybrid inverter, but this solution is inflexible, and at times wouldn't fit my needs.

Seeing your post and the possibility of switching the panels between inverters really appeals, as I could have 100% of the panels dedicated to the grid tied inverter while the batteries are being charged, and then switch them over to the hybrid to run the house for the house for the remainder of the day.

Could also get a bit more complicated as I'd also want to bypass the grid-tied inverter and run all loads through the hybrid inverter during the day, making as much use of the panels as I can.

There seem to be a few automatic switches locally, but I'm not sure at the moment if all or any would be totally suitable.

http://www.giga.co.za/ocart/index.php?route=product/product&product_id=423 <- drop shipping

https://www.epicsolar.co.za/Items/Transfer-switches-and-DC-distribution

https://www.atisystems.co.za/electrical-enclosures/normand-automatic-manual-transfer-switches-ats-mts <- actually looks like it could do the trick

Edited by viceroy
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25 minutes ago, gooseberry said:

Won't you get as much use of your panels just leaving it on the grid tied inverter? The critical loads will just run from the battery, which is continually topped up with the AC output of the grid tied.

No, as the system stands now, the batteries are fully charged by 11am, from that point on, the essential loads are drawing maybe 200W until the kids get home at 4pm and start using computers, tv, etc...quite a waste I think.

By switching the panels over to a hybrid inverter at the point the batteries are charged, their power will be blended with grid power, but giving precedence to the panels, in effect allowing me to maximize their use during the day.

21 minutes ago, gooseberry said:

One issue I see is that for grid tied the solar strings is usually connected to form a high (>250V) voltage whereas with off-grid the charge controller likes it to be much lower - close to battery voltage. So you may need a separate DC-DC step down charge controller too. 

Yep, from when I was looking at the various hybrid inverters, their startup volts seem to be around 100/120V while my current string configuration is between 85 and 95V.
So you are correct that, I'd need to reconfigure the string to provide higher volts, and get a separate MPPT charger.

--edit--
I see the Kodak i2.5 has a startup voltage of 60V, but nominal is still 250V, and I don't see any locally.

Edited by viceroy
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11 minutes ago, viceroy said:

By switching the panels over to a hybrid inverter at the point the batteries are charged, their power will be blended with grid power, but giving precedence to the panels, in effect allowing me to maximize their use during the day,

Oh it seems you want to do it the other way around. I want to keep the panels all on the grid tied inverter the majority of the time (this can be a Solis or a Sunny Boy type inverter) and only when there is a grid outage during daytime switch them to the off-grid inverter to charge the batteries. So during normal operation the batteries are charged with AC (grid+solar) - probably with some timer so they don't try to charge at night.

Edited by gooseberry
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I may actually be over complicating things too.

Doing like you suggest, and leaving the panels on the hybrid inverter, I'd still essentially be charging the batteries from solar.

Only if the grid goes down, would I then move the panel output to the Axpert if required.

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8 hours ago, viceroy said:

I've also started investigating adding a hybrid inverter. My first thought was to split the panels between what I have and the hybrid inverter, but this solution is inflexible, and at times wouldn't fit my needs.

You could also try to maximize your Axpert usage so that you don’t waste so much pv solar during the day. You could still run your essentials like currently, but also add a second change over switch to switch all your other loads like fridges and microwave etc. also to solar during the day when you have all that spare solar power, that way you will also benefit quicker on the return on investment.

I have two change over switches on my Axpert and have wired the system to work automatic with sonoff timers so there is no need for manual switching. I only switch when there is lots of clouds or if I’m going to use some heavy load like a dish washer that will drain my batteries.  (This is reasonably cheap compare to buying a grid tie inverter) Also the switching is done on my phone from anywhere. The spares will cost about R1500 + wiring labour. This way you minimize that waste solar pv and rather charge your batteries with that extra power and run those high power loads if possible.

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9 hours ago, Gerrie said:

You could also try to maximize your Axpert usage so that you don’t waste so much pv solar during the day. You could still run your essentials like currently, but also add a second change over switch to switch all your other loads like fridges and microwave etc. also to solar during the day when you have all that spare solar power, that way you will also benefit quicker on the return on investment.

I have two change over switches on my Axpert and have wired the system to work automatic with sonoff timers so there is no need for manual switching. I only switch when there is lots of clouds or if I’m going to use some heavy load like a dish washer that will drain my batteries.  (This is reasonably cheap compare to buying a grid tie inverter) Also the switching is done on my phone from anywhere. The spares will cost about R1500 + wiring labour. This way you minimize that waste solar pv and rather charge your batteries with that extra power and run those high power loads if possible.

It would be great if you could provide a list of parts you used to achieve this?

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+1

 

This sounds like a really good plan. I don't really see that big an advantage of a hybrid setup then when you can achieve the same with an off-grid (Axpert) setup and smart switches.

The one question I had with the Axpert and the CoCT regulations (which Stellenbosch municipality seems to follow too) is if it's allowed to connect the grid AC to the Axpert for charging batteries? Reading the regulations this is only allowed if the critical loads normally supplied by the Axpert is also switched to the grid at the same time. 

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It is something to consider, but in my house, aside from the stove and geysers, the only circuits still in the main DB powered only by grid are the kitchen counter, and garage/patio.
The loads applied to these circuits all high amp draw and/or inductive, and I don't really want these with a direct connection to the Axpert.

Can you imagine, the gardener cutting the grass, while at the same time the maid has the dishwasher going, the washing machine going and needs to heat something up in the microwave. The poor Axpert is going to switch to bypass mode, or just trip (if bypass disabled).

Yes, I know it's a bit of an exaggerated situation, but one entirely possible, and despite being more expensive, a hybrid inverter solution would simply add grid power to assist the PV. The Axpert wouldn't know about any excessive loads, and everyone would be happy.

If loadshedding happened, a DC transfer switch, would move PV over to the Axpert, and again, everyone happy.

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1 hour ago, gooseberry said:

Also the smart switches need to be interlocked change-over switches, not a reverse power flow blocking relay, whatever that means.

Basically interlocked change-over switch, physically disconnects one source before connecting the other. Safer, with almost no chance of failure in such a way that both sources are connected.

Reverse flow blocking relay, doesn't really disconnect either source, but rather blocks flow from one to the other. Less safe, and a much higher chance of both sources being connected in the case of a failure.

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4 hours ago, viceroy said:

It would be great if you could provide a list of parts you used to achieve this?

The parts i used were:

1x63A change over contactor see picture

1x25A change over contactor see picture

1x two channel sonoff wifi switch

1x 2A miniature mcb for control circuit

I ordered the tomzn’s online and sonoff from communica. 

AC/DC also have there own brand of contactors but quality is a bit low.

628B818A-6821-44A4-A491-F4BC345CF3F3.jpeg

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2 hours ago, viceroy said:

Can you imagine, the gardener cutting the grass, while at the same time the maid has the dishwasher going, the washing machine going and needs to heat something up in the microwave. The poor Axpert is going to switch to bypass mode, or just trip (if bypass disabled).

This is true, but a bit of training for all the family members is a must. We manage during the week without any issues as my geyser is a ev tube and we use a gas hob, my wife normally warns me when she switches on the electric oven and than I switch to grid for a hour or so until the oven is switched off again. Should I forget to switch to grid and oven drains the batteries than the inverter just switches back to grid on low battery setting, this is instant so the computers and all electronics are uninterrupted.

On weekends we use a bit more power, but I still maximize usage by using my batteries to help buffer against the high power items like a kettle, hair dryer or vacuum cleaner. This power is normally quickly charged back to the batteries with pv solar. This past weekend I’v hammered my Axpert a bit as I was using my oil bath welder and some power tools in the garrage, while the vacuum, fridges, washing machine, two gaming pc’s and bunch of small stuff were on in the house. I only remembered we were on solar because everytime I welded, I heard the vacuum cleaner in the house changed its tune a bit and my inverter in the garage had its fan revving up hen I strike the rod. 

I was away on holiday December for 4weeks and my system operated without any hiccups. I just checked my phone every now and then to see the status of the sonoff switch. What also helps me, is I use my cctv cameras at home and check on my phone if lights or on or off at home.

Some guys on this forum complains about the Axpert’s but to me it’s all about how you wire the system to work for you. If you don’t put in the planning and training of the family on how to keep the solar happy than you definitely need to go for the expensive stuff with all the bells and whistles.😎

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5 hours ago, gooseberry said:

The one question I had with the Axpert and the CoCT regulations (which Stellenbosch municipality seems to follow too) is if it's allowed to connect the grid AC to the Axpert for charging batteries? Reading the regulations this is only allowed if the critical loads normally supplied by the Axpert is also switched to the grid at the same time. 

In my opinion it is best to wire the Axpert to be off-grid, try and charge the batteries with pv only. Only charge batteries with grid if it is cloudy or you have drained batteries and need to charge them urgently. I have a 3pin plug top on my Axpert to charge from grid only if it’s been cloudy with no pv, I hardly have to use it.

Wiring for off-grid excludes the engineer sign off, because it applies to grid-tie only.

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  • 7 months later...
On 2020/02/02 at 11:11 PM, gooseberry said:

would it be possible to install some switch, even manual, that disconnects the solar panels from the Solis and connects them to the Axpert to charge my battery?

Have a look at this DC changeover switch for transferring solar panels from a water pumping inverter to a high voltage solar charge controller.

The wiring diagram is attached.

Cellcronic DC-Changeover-Switch.pdf

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  • 2 weeks later...

It seems to me to much search for complications. My concept is to be as much as possible off grid. So I wired everything to the Axpert MKS 5K hybrid inverter, including electric cooking and geyser. I must admit I underestimated max simultaneous power. The house lady managed some time to overload the inverter. But the Axpert takes care of all situations automatically if properly programmed. It switches to utility bypass at overload as well as at empty battery (47V) and switches back to normal after overload is gone. Charging priority is given to PV panels, utility charging only if at empty battery situation and PV is not available. I would however re-conceive the system with 2 4kW MKS 4K units in parallel. As battery I have 3 packs of 4.8kWh LFeLi batteries from Averge in parallel. I oversized it somewhat in order to have some reserve for a cloudy day. Each battery pack comes equipped with a 125A breaker. In the feeder to the inverter are 125A fuses. The DC from the PV panels is wired directly to the input of the MPPT with only a 25A fuse. I have 9 335W Canadian panels 3s3p. They are supposed to deliver together 3kW at optimal sun exposure but if I choose to believe the Axpert display it showed sometimes 3.2kW with a peak of 3.5kW.

The 150l 3kW geyser is equipped with a 2m² solar heating panel. The pump is controlled by GeyserWise. I stopped all programmed electric heating. I can switch it on manually at the GeyserWise control panel. I do it when batteries are full early in the afternoon.

I rewired the distribution board as shown in below schematic. I only needed 3 additional 32A breakers. Luckily the builder of the house installed a generously large DB leaving space for them. The two breakers for the bypass are mechanically linked together with a 2mm pin (nail) in their handles. They can only be actuated together. This bypass is only used when cutting out the inverter completely for repair or replacement. Yes there is theoretically the risk of having the main and bypass on at the same time. But I think that the earth leakage detector witch detects unbalance between line and neutral current would immediately trip.

111295640_Powerpanelscheme.jpg.f4efaee21f5d4a522990d94d49ad5f15.jpg

PICT1302.thumb.JPG.199dc7d2a579367a919f0365954780b6.JPG

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On 2020/10/04 at 2:15 AM, Kilowatt Power said:

Have a look at this DC changeover switch for transferring solar panels from a water pumping inverter to a high voltage solar charge controller.

The wiring diagram is attached.

Interesting! Do you have one of these installed?

DC switching isn't a pushover (and switchgear is hard to find) so it's good to see a real live solution!

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22 hours ago, Richard Mackay said:

Interesting! Do you have one of these installed?

Not yet but just about to. Getting the actual switch (without IP66 enclosure) directly from the manufacturer in Austria, BENEDICT GmbH

You could probably get the same switch from the South African distributor, DEEBAR (PTY) LTD

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