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Building a relay system to auto switchover from inverter to grid


Ben Harper
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I've got an Axpert VM III (3KW), and about 2KW max solar, but a very small battery setup.

So to make use of the solar energy, I want to drive heavy loads off the solar during the daytime, but when the sun goes down, then I want those heavy loads to switch back to grid power. Of course the inverter kind-of handles this itself, but the reason I want to switch it back off the inverter, is in case load shedding kicks in. If load shedding kicks in at night time, then my batteries are insufficient to power the heavy things. So... my idea is this:

  • Build some kind of relay based switch-over system that will flip the "heavy" circuit between straight-from-grid and inverter-powered.
  • Use a raspberry pi to control that relay. Ideally, monitor the available PV power, to figure out when to flip the switch.

Has anybody got experience with anything similar?

I'm hunting around on sites like www.robotics.org.za for parts, but I'd love to hear any advice before I start cooking things up.

Ben

 

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Hi i have done something like that, but my system works only on utility not solar panels.

So I have installed a contactor between my house feed and my inverter. From the contactor it goes to my Victron BMV 712. And the BMV have a relay function built in that I have configured to trigger the relay when the batteries reach 55% works great. That will cut the house feed off the batteries

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3 hours ago, Ben Harper said:

Build some kind of relay based switch-over system that will flip the "heavy" circuit between straight-from-grid and inverter-powered.

It really depends on how fast you need the changeover to be. For some loads, such as water heaters, stoves, pool pumps, and so forth, a small delay in the switchover is not a problem. So then you simply design the changeover to incorporate such a delay, and you don't have to worry about the waveforms being synced.

You can look on youtube to find some examples on how to design a changeover switch with two contactors. Ideally you want to physically interlock them so they cannot both be on at the same time. Then you can use a simple DIN-mount timer between them to ensure that a number of seconds pass after one of them drops out before the other one engages.

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Thanks Plonkster - I'll take a look on Youtube.

I was actually thinking of using Solid State Relays (specifically https://www.robotics.org.za/SSR-1DA100A) to perform the switching. In a case like this, if I screw up the software, then I'm trouble, but software is what I do, so I'm OK with that. I see that many SSRs do zero-point crossover activation, which sounds like a good thing, but I'm finding it hard to discover datasheets for these things.

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27 minutes ago, Ben Harper said:

Hi Richard - You can read, but I'm not sure if you can write to them via the serial port. However, I'm not sure how that would help me achieve my goal.

As I see it the inverter is functioning as you want it to but during load shedding you want to disable the inverter..

So if you can establish how to do this remotely then all you need is to configure this (low power) gizmo to do the job (No heavy switchgear required..) 

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1 hour ago, Ben Harper said:

Solid State Relays

Transfer switches also have to comply with SANS/IEC, and it is required that they interlock in the prescribed manner. Also, solid state relays are usually triacs (aka two back to back thyristors/SCRs), and one thing I remember about them is there are conditions where they can be tricked into turning on despite not having current on the gate (for thyristors there are snubber circuits to prevent this sort of thing). I only remember this vaguely, very vaguely... but it makes me extremely uneasy about using SSRs to switch between two AC sources, especially if they are cheap eBay SSRs. Personally I would sleep a lot better with a good old contactor.

Edited by plonkster
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19 hours ago, Ben Harper said:

So to make use of the solar energy, I want to drive heavy loads off the solar during the daytime, but when the sun goes down, then I want those heavy loads to switch back to grid power.

I do basically that. My axpert switch to solar with a sonoff timer at about 07h30 in mornings. It than runs on solar the entire day and at night at about 18h00 it switches back to grid. This happens automatically everyday as long as inverter is switched on. If I decide to stay on grid rather due to rainy weather I just cancel the timer setting on my phone. Sometimes the weather clear up by say 12h00 I than switch to solar on my phone. With this I must add that I use my cctv cameras at home to monitor how the weather is, if I see sunshine I switch with my phone to solar.

I have two switches on my phone, one for essential and one for non-essential loads each with it’s own timers. I find this to work very well.

 

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16 hours ago, Ben Harper said:

The AC voltage rises and falls 50 times per second. What I'm wondering is whether the inverter output is on the "rise" at the same time that the Eskom power is on the "rise", and likewise for when the voltages are falling.

My relays switch in n matter of milli-seconds so you can see n slight dip on the lights, but this is more due to my grid is about 250V and my inverter is lower at about 230V. It can switch while heavy loads are on like dishwasher or washing machines. My two son’s have gaming pc’s and it can switch while they play without them noticing any switching.

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28 minutes ago, Ben Harper said:

you can use the app to switch between Input1 and Input2?

I think he does something different. He's switching the power to the Axpert off, which in turn forces the inverter to "go to solar".

I would definitely not use two sonoff's. In my own home I've seen terrible delays between switching, and sometimes it's like the message gets lost and it doesn't switch at all.

You can however use the sonoff to control two interlocked contactors. Probably cheaper than a DIN-mount timer too.

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35 minutes ago, Ben Harper said:

The sonoff dual has four inputs

No, it only has a supply that is 230V. And than two outputs that can be used to switch contactors or relays. I use one output to switch my essential contactor and the output to switch the non-essential contactor. I use the tomzn change over contactors for this. So it can switch essential on inverter and non-essential on grid, or vise versa essential on grid and non-essential on solar. It has been working perfect since last year about May, 

277DF44B-1DC2-4AAB-A6A5-BFC7F5A9375B.jpeg

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3 minutes ago, Ben Harper said:

Do you have two contactors?

That is another debate we've had in the past, whether you are allowed to use a single "double pole double throw" contactor or not. The regulations only require that it be properly interlocking, and I think it is quite clear that such a contactor is indeed interlocking.

The only point I do want to raise, which you seemed to be concerned about earlier, is that if the AC waveform of the inverter is not synchronised with the grid, a contactor like this will essentially move a load from one side to the other with total disregard of the phase shift between them. Some loads won't care (geysers), but others will. This is why it is sometimes advised that you insert a small delay of a few seconds in the middle, and if you do that, you will have to use two contactors.

It's going to totally depend on what kind of loads you want to move over.

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3 minutes ago, Ben Harper said:

Do you have two contactors?

And if so, is one of them NC and one of them NO ?

Yes, two contactors my one is a 63A and the other a 25A but both tomzn make. The normally close sides wired for grid power and normally open sides wired for inverter power. If contactors are energized and loads fed from solar, if solar should be switched of or trip for what reason the contactors just drops out and grid again supply loads. This in milli seconds without interruption of loads.

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14 minutes ago, Gerrie said:

is one of them NC and one of them NO ?

These contactors must not be confused with the general contactor that normally only have has 3xN/O  and 1x N/C contact for power switching, this has 2xN/O and 2xN/C. It’s exactly like a manual double pole hager change over switch the only difference you need to 230V power to switch it. 

On top of this I still actually have my manual hager change over switch in the DB. But I can’t recall when was the last time I had to use it, it’s now just gathering dust and will only be used if there is a problem with the auto system then it can still be used as normal.

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