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Grid tie inverter?


cvzyl
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Hi guys

Here is my current situation: I have a 5kVA Axpert inverter with 3kW panels and 200Ah battery bank which is supplying most of the loads in my house (I split the DB, everything except geysers and oven on inverter). The problem is that the battery bank is too small, ideally I probably need another 200Ah. So around 02:00 in the morning the bank is at 70%SOC and switches to utility till around 08:00 in the morning. During the day the battery bank is charged at 20A but is full around 12:00. From then on the PV panels are idling, doing only the

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I know that the Victron multiplus can be used with the Fronius. Then you put all your panels on the Fronius GTI. The victron is connected to your battery bank. The fronius will then push back against the victron, which will use the excess energy to charge the battery bank. Once the battery bank is full, it will switch and push back into the grid. I don't think this is possible with other inverters. It will also give you access to a lot more power during the day, since you will be able to draw both the power of the sun and 4.5kva from the Victron.

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cvzyl, I feel your pain :(

 

If I was allowed to push back power onto the grid, I would be a very happy man right now.

To date I am still very happy using the 3kw Infini, but yes, it might be a bit small for you.

The real advantage is that I am only using the battries as standby power when the grid is off.

 

For me the cheapest option right now would be to replace the Infini with 2 x 5kva Axpert units, but then my battery bank size might become an issue.

I am still working on a solution to have the inv switch to mains as soon as PV output is low, before starting to drain the battries.

 

Next option for me were the 3kva microcare GTI with grid limiter.

 

Also as I have 2 PV strings facing diffrent directions, ideally I would have liked a unit with 2 x MPPT inputs.

So, currently my 2 options is either the Goodwee or the 5kw Infini, but I am waiting for confirmed pricing first.

 

You might look at the Imeon, but the cost per watt is nuts.

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Wetkit, can the Infinisolar be run without a battery bank?

If the 5/6kW Infinisolar is finally released I would like to run it with my Axpert. Connect the main utility feed (from MCB) into the Infini and the output goes to the DB board to feed all consumers, also the input to the Axpert. In that case I don't want to add another bank to the Infini, just operate it kind of like a grid tie inverter. Is that possible?

Can the 3kW Infinisol be connected only to the grid (input) and PV panels? That means no output from the Infini to consumers and no battery bank. That way the Infini will push all PV energy back into the grid to operate like a grid tie inverter. As I understand it the Infini software allows you to set times of the day when it is allowed to grid feed and also how much?

 

Jaco, your software looks brilliant. My problem remains, what do I do with the energy. All the consumers are running during the day and we are only drawing around 500W. No sense switching all the lights on just to load the PV panels? One option is to get my geyser element (solar but with electric backup) on the inverter. If I can then switch the element on to heat the water it could make sense? I will however require a smaller element, currently have a 3kW and would need to drop it to 1.5kW or I will start running the geyser off batteries!

 

Anyone on here with experience on the Microcare, Fronius or Steca?

 

C

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Hi Jaco 

 

Just an idea.

 

What if all your PV panels were all on the Infini and it charged batteries and supplied minimal load (separate)and excess energy went to grid. The 2 Axperts  run off batteries or Eskom. In the current configuration any excess PV energy on the Axpert side is lost. One is introducing an inefficiency charging and discharging batteries so that might be worse than the energy "lost" by the Axpert PVs.

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One option is to get my geyser element (solar but with electric backup) on the inverter. If I can then switch the element on to heat the water it could make sense? I will however require a smaller element, currently have a 3kW and would need to drop it to 1.5kW or I will start running the geyser off batteries!

 

 

 

That is  my current thinking. I want a geyser with two ports into which I can put two 1000W elements. I have stripped out an old 1970s solar  geyser and it has a copper vessel about 150l but obviously no ports for electrical elements. Might need to go with 2 geysers in series each with its own element. If the excess from the PV is not great we will heat today's hot water and if it is a bumper day (or when the sun is directly overhead) we can heat tomorrow's water too.

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I'm with McWidowmaker. I'd go with a Fronius, because 1) it works well with a Victron and I'm a bit of a Victron groupie, and 2) even when not used with a Victron, they have an open protocol you can use to control it, which is what is used by some grid-limiters (such as the one made by ExSolar... but that one costs almost 7k).

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Also, wrt what Chris said about the geyser. One thing I really want to experiment with is a way to run a geyser element at part-capacity and basically balance surplus by running the geyser at the same power. There is a good 5kwh room in a 150 liter geyser between 55 and 85 degrees. Add on a tempering valve... and you have WAY too much hot water in summer. This would actually work brilliantly up North where they actually have sunshine in winter :-)

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You can also look at the ABB GTI's they are well priced and i would also agree that the Fronius is a good unit - i just don't like having to take the whole plastic casing apart to do connections on them - look at the Kaco GTi as well. Both the ABB and Kaco work well with the Victron. When i started out i used a Microcare 3kw GTI - worked well with 3kw of pv but display etc misted up when the panels were really producing.

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 One thing I really want to experiment with is a way to run a geyser element at part-capacity and basically balance surplus by running the geyser at the same power. There is a good 5kwh room in a 150 liter geyser between 55 and 85 degrees. Add on a tempering valve... and you have WAY too much hot water in summer. This would actually work brilliantly up North where they actually have sunshine in winter :-)

 Hi Plonky

 

What is your idea?  Perhaps I can get away with an ordinary geyser with only 1 element. Remember you explaining this to a farmer and some concepts enter my balding head SLOOOWLY.

 

Chris

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 Hi Plonky

 

What is your idea?  Perhaps I can get away with an ordinary geyser with only 1 element. Remember you explaining this to a farmer and some concepts enter my balding head SLOOOWLY.

 

Chris

 

Okay, so the element inside a geyser is a large old resistor that gets hot. If you put half the normal energy into it, it will still heat the water, it will just take twice as long. That is the basic idea. I'm using "half" for the sake of argument, that should ideally be some variable fraction.

Now there's two ways of putting half the energy into it. The one way is to feed it with a lower the voltage. That can be calculated of course: I = V/R (and P = VI), so P = V^2/R, half the voltage is 25% of the power. Ignore for now.

 

The second way is to turn the switch full on and full off at a certian duty cycle. Suppose you turn it on for a second and off for a second, then you have a 50% duty cycle, and you're putting in half the power. If you take your time unit down to milliseconds, you end up with the equivalent of a dimmer switch. You can of course lower this right down to 0.01 seconds, because at 50Hz that's about half a cycle. Make it shorter than that, and you get all sorts of radio interference.

 

Now I am concerned that using the second method with an inverter is a bad idea. Lets assume your surplus (that you want to use and not push back to the grid) is less than the rating of the heating element, and suppose you are using a GTI: If you switch at 0.02 seconds at 50% duty cycle... well then every second cycle you're pushing surplus to the grid, and every other cycle you're pulling the difference from the grid. I'm sure the kind of noise you'll be generating there is going to be a bad idea.

 

It might not matter if you have an old style disc meter. But I'm sure it will cause bad things with a prepaid meter.

 

So this leaves you with the first option, which means efficiently stepping down 220V to something lower. The geyser isn't going to mind if you feed it with DC instead, so if you rectify your 230VAC to 325VDC and step it down, that would allow true and clean variable control. However... switching components for high voltages like that aren't cheap, and I half suspect you'll want to use more conventional transistors/diodes rather than the more fancy schottky/mosfet setups (I'll tell you below why, just to keep this simple) and those conventional components make rather a lot of heat compared to their modern counterparts.

 

So basically... I'm still wondering about the best way to cleanly chop off some of the power.

 

More technical reason around choice of components: MOSFETS and Schottky diodes have the property that they have a low forward resistance, which means they don't get that hot. But the downside is that they don't handle high voltage as well, especially Schottky diodes (and schottkys also leak more in reverse... but ignore that for now). That means you'll probably opt for a more conventional bipolar/silicon setup. Those drop 0.6V. So a geyser that will draw up to 15A, times 0.6V, 9W of heat you have to dump somewhere at full power :-)

 

From what I've seen though, many MSW inverters use bipolar transistors in their high voltage stage... so perhaps I'm overthinking this.

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Okay, that's probably still way too technical.

 

What I want to do is put a dimmer switch on a geyser. The question is: What kind of dimmer switch :-)

 

Will an ordinary dimmer switch work? Well yes, it will. From an electrical perspective, a geyser's heating element is no different to the filament of an incandescent lamp. If you have a dimmer that can handle the power (3kw for argument's sake), then a dimmer will allow you to turn the power down.

 

But what actually happens on the wire? For this, you have to understand how a dimmer switch works. What a dimmer does is turn the lamp on and off really quickly. So for a fraction of the time it is at 100%, and for a fraction it is at 0. With a good old magnetic rotating disc meter, this will average out and should work just fine. With a prepaid meter like mine... I'm not so sure.

 

Further to that, and now I will get tecnical again: A dimmer switch uses an electronic component called a triac. Now a triac, in turn, is two thyristors back to back. So you have to understand how those work.

 

A thyristor is a diode with three legs, the usual anode and cathode, and also a gate. If you apply power to the gate, it starts to conduct power between the anode and cathode. So it behaves a bit like a switch, much like a transistor. The difference is that a thyristor latches on: it stays on even if you remove the power from the gate. It only switches off when the current through it drops to zero.

 

Fortunately, with AC, the current drops to zero 100 times a second. So you know it always turns off at the end of the cycle.

 

Now as I said, a Triac is two thyristors back to back. But because of the latching behaviour, you can't turn it off halfway through a cycle.

 

Further to that, it is a bad idea to turn it on halfway through a cycle, because that causes radio noise. So many triacs have zero-crossing detection and turn on at the start of a cycle.

 

The long and short of that is that dimmer switches works in multiples of cycles. And that really limits what you can do. If you have a 3kw element in there... you essentially generate 3kw spikes in such a way that the average is lower... and that won't do for me.

 

So I need to build a step down converter, essentially a Switch Mode Power Supply. And that's where the stuff about mosfets vs old bipolar transistors come in :-)

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Hi Mike

Who supplies ABB and Kaco?

Any news on Infini units? The guy on Gumtree mysteriously removed the 5kW Infini's from his ad.

C

ABB you can get from Rubicon, Kaco from Green Star Energy Products. I think the main issue at the moment with stock on the 5kw Infini's is that most of the suppliers went and bought tons of stock of the Axperts and are now sitting with them.... i know of two that will not import till they sell current stock. There is also a new law coming into place, think it abbreviated term is R.O.L. this law is going to stop all non certified NRS097-2-1 inverters from entering the country - so i think i can see a big problem arising trying to bring in.

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Thanks Mike.

 

Is the proposed R.O.L. law only applicable to grid tie installations? Are there already NSR 097 approved inverters available or do they still have to be certified? You mention that you see a problem bringing them in, I assume you are referring to the 5kW Infini's. Is there a reason why they would have trouble passing the NRS 097 standards?

 

Last question, when this law kicks in, will systems that are already installed be exempt?

 

Thanks

Cobus

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Eish, this thread has gone waaaaay off grid very quick :/

 

@cvzyl, yes, the Infini can run without battries and/or without PV. It can also run as pure GTI.

The single biggest problem for me right now is that the grid tie output cannot be remotely controlled.

If we can get that working I will buy more right now!

The Infini can run in parallel, but only on the grid tie side. You cannot combine battery backed up outputs like on the Axperts.

 

 

@jdp, your first drawing will work, but not the 2nd.

The Infini output is limited to 3kw, so it will not support the 10kva load.

You also do not need to connect the Infini to your battries on the first drawing, as it does not support any backup load.

Also, the Infini can run way more than 18 PV panels.

The only real limitation is the upper volt limit of 500V.

 

@plonkster, yes, the Victron is a great unit, but it will cost way too much to implement now together with a Fronius as well.

You have to look at Rand per Watt ratio as well.

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Hi Wetkit, thanks for the info, very interesting.

When you say it cannot be remotely controlled, do you mean that you cannot switch on/off grid feed or alter the amount that is fed? Are you referring to something like a grid tie limiter that control the Infini?

 

C

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@plonkster, yes, the Victron is a great unit, but it will cost way too much to implement now together with a Fronius as well.

You have to look at Rand per Watt ratio as well.

 

I whole-heartedly agree. I love their hardware but man.... it is expensive. Someone once said that the difference between an engineer and the rest of us, is that they can do with R50 what any idiot can do with R5000. :-) Likewise... any idiot can solve this problem by buying more hardware (well, maybe not ANY idiot...), the challenge is to do it with less.

 

It is technically possible to do it with just a Fronius. There is an open protocol for communicating with  Fronius hardware, and I believe this is what is used by the grid-limiter made by ExSolar.

 

http://www.exsolar.co.za/products/grid-tie-limiter/

 

Only issue is that that product isn't exactly cheap, but at least it is cheaper than almost anything with a Victron badge on it :-)

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jdp, I think his comment re not being connected to the battery bank is related to a question I asked. In my case I would still have the Axpert which can keep the battery bank charged. I would then set the Axpert to charge using utility (as I will move the PV panels to the Infini) and run the Axpert simply as an emergency backup. That is, I will used grip power all the time but use the grid tie inverter and the network as a battery.

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OK when you say it can take more than 18 panels why is it limmited at 3Kw. I have 18 X 300 watts of panels. that is 5400 watts. So if I put all my panels on it where does the other 2400 watts then go ? Am I missing somthing ?

If you say I dont need to connect it to my batteries then how will the bank get charged should it need to. I see they are grid tied with battery back-up. So I want the bank to charge and then when full then push the power to the grid.

 

The Plus model infini can utilize up to 4500W from the PV panels.  That is 3KW output + battery charging + internal consumption + losses.  You can always add more panels to ensure that you have more than enough PV power available under less ideal circumstances, but the inverter will only utilize up to 4500W of it.

 

The infini is a true hybrid, it will charge the batteries while supplying the load and be able to feed the balance to the grid - all at once.

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