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My Grid tied installation


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At long last! It has started!

I've been researching solar PV systems for more than a year now, reading as much as I could get my hands on. Some time ago I've stumbled onto the Power Forum and it has really been a source of inspiration and a wealth of information. Thanks guys!

Let me give a bit of background as to what I have and what I want to do.

Some years ago I bought an old house and started renovating and improving it. In the process I've tried to make it more efficient and less costly to run and maintain. I've been able to do the following:

  • Replaced the old swimming pool pump with a Eco variable speed pump. The old one consumed 1600W, the new one uses 300W (Big monthly saving)
  • Replaced old stove with a gas stove (unfortunately it has an electric oven)
  • Replaced all Incandescent bulbs with LED down lights on a separate lighting circuit (Essential lighting circuit).
  • Added additional electrical outlets (plugs) to all rooms. (Essential plugs circuit).
  • Installed insulation into the roof.
  • Installed underfloor heating in the kitchen (Before 2008 when electricity was still relatively cheep, do don't use it much anymore)
  • The living room as an anthracite fireplace that keeps it nice and comfortable during winter, sometimes we also use gas heaters.
  • Installed a 300L solar split pumped vacuum tube geyser. (That was when you could still get a rebate from Eskom - that helped!) We have more hot water than we can use. Only on the coldest,cloudiest winter days does it use some electricity to bring the temperature up to 55.

What do I want to achieve with the solar PV system?

  • My main aim is to reduce my monthly electricity bill in a "cost effective way".
  • I don't want to change my life style. If I want to weld or grind I want to do it without worrying if the washing machine or other appliances are on or not.
  • The system should require as little maintenance as possible.

One of the first things I did was get an energy meter (Effergy) so I can monitor my consumption. (If you can not measure something you can not control it!) I was able to extract the data into a spreadsheet. With the help of Google, Nasa and other websites I was able to get a hold of formulas that I could use to calculate the solar radiation that would fall on my roof at a given latitude, angle (20 degrees east of north) and elevation (30 degrees) for any given date/time.  I used this spreadsheet to estimate the how much energy I could produce (ignoring cloud cover) and subtracted that from the actual usage I got from the Effergy data. After all this I could estimate that a 3kw grid tied system would reduce my electricity consumption between 30% and 50%.

I looked at the different ways to setup a PV system (Grid Tied, DC coupled, AC coupled, off-grid, etc.) In the end I decided to go for a AC coupled systems. I'll have a grid tied inverter that provide AC power to my house grid. Then have a inverter/charger that will draw AC from the house grid to charge the batteries. The inverter/changer will only feed the essential loads (LED lights, Fridge, TV, WiFi rooter etc). Since the inverter will only need to power the "essensial" loads it can be small and the battery bank can be small as well.  The reason I liked this solution is that I could implement it in phases.

Phase one I would implement the Grid Tied inverter to supply power while the sun shines. Any appliance would be able to use the electricity (poolpump, underfloor heating etc). If there are any shortfall between what the PV produce and what I consume it will be drawn from the Municipal connection.

Phase two I will implement an inverter/charger and battery bank that only feeds the essential loads (the ones I use at night). 

Batteries are expensive and have limited life span. From what I could gather they are not really cost effective in terms of the cost per kWh they will store during their lifespan. This may change in future if the Rand strengthens, battery technology improves or Municipal electricity prices increase drastically. (Anything is possible) but for now I'll hold back on it. 

One thing I forgot to mention is that our area are not affected by Load Shedding for some reason. (I am not complaining). If we were affected by load shedding I might not have gone for a Grid Tied systems. 

One of the draw backs of a grid tied system that the inverter shuts down if there is no grid power. So if there is a power interruption 3+kW PV on the roof and the inverter will we doing nothing for me! We do get power outages (typically once a year or so and it can be for anything from an hour to a day or two). 

As a contingency I plan to make use of a small Petrol Inverter Generator (pure sine wave). Just a small one will do (3.5 kVA). At night is powerfull enough to power the essential loads and during the day it can boost the 3KW Grid tied inverter to produce up to 6.5 kVA. That should be more than sufficient given power spikes rarely goes over 5kW. 

Since last year I've been sourcing the various components and last weekend I started the installation. The railings that will hold the PV has been installed on the roof. The only component that I am still waiting for is the high voltage combiner box. As soon as that arrives I'll put the PV on the roof and do the wiring. Here is what the system will look like. If you have any comments/suggestion please let me know.

 

Screen Shot 2016-04-08 at 12.42.19 PM.pngIMG_0898.jpg

 

 

 

 

 

 

 

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Welcome Janma!!!

Well, all I can say is ... uhm ... well done!!! You have done a lot of homework.

18 minutes ago, Janma said:

Then have a inverter/charger that will draw AC from the house grid to charge the batteries.

Tell me, why did you chose to do it this way, for I presume the batteries will only be charged daytime, and not at night using Eskom?

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1 hour ago, The Terrible Triplett said:

why did you chose to do it this way

The rule of thumb is that if you consume most of your power as it comes in (aka you are at home during the day), or if your objective is to make/save money by feeding into the grid, you bring your solar power in as AC with a GTI, and use an inverter/charger to charge the batteries. If you store most of the energy for later use (aka you are not at home during the day), it is more efficient to bring your solar in as DC with a charge controller.

What you do is just follow the electrons to the point where you want most of them to end up. With GTI, only one conversion step between PV and appliances, but two conversion steps between PV and batteries. With CC, only one conversion step between PV and bats, but two conversion steps between PV and appliances.

With charge controllers and inverters being 98% efficient (typically), your still ending up with an overall 96% efficiency (ignoring all the other inefficiencies such as that in the batteries, cable losses, etc... just looking at the ones influenced by this choice), so for a 2% hit, you do whatever makes sense. In this case... being able to do it in phases. In my case, I needed the backup first. Free lance programmer worried about load shedding... :-)

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Yes, the plan is to only change the batteries during the day while the grid tied is producing power. But that is still some time away. For now I'll be happy with just the grid tied during the day.

When I started investigating PV solar options I stumbled upon the MLT drives Powerstar system. What a nice piece of kit! but expensive! I just could not justify spending that amount of money on the system, but liked the idea that it could manage all the energy sources. Municipal feed, Generator, Solar, wind whatever and then manage the flow of energy from source to load. If money was not an option I would do something like that. The problem I had with it is that if you want to do this kind of thing with a "central power management system" that should be able to manage ALL the load (Lots of amps going trough it, large battery banks = lots of money.) Also if that one piece of equipment breaks the whole system is down. It also needs to be sized so that it can handle the entire load all the time.

 

Screen Shot 2016-04-08 at 3.39.28 PM.png

The other thing I looked at was to only do it for a subset of the load. The moment you want to power ALL the loads in the house the battery bank gets really large! So to keep the size of the batteries small enough one would only put the "Essential" loads on the inverter. Something like this:

Screen Shot 2016-04-08 at 3.45.57 PM.png

In this case the inverter is smaller (cheeper) the battery bank is also smaller (cheeper) but the PV you attach to it can only ever power the essential loads. If you have underfloor heating installed you would not want to attach that as a load to your inverter.

That's why I like this option:

 

Screen Shot 2016-04-08 at 3.49.41 PM.png

The PV via the Grid Tied inverter can be used to power any and all load in the house (Swimming pool pump, geyser, underfloor heating etc). If its large enough and the sun shines you have power. The PV has a life expectancy of 25 years. ROI of that part of the system is quite good (over time it will pay itself off). The batteries on the other hand is the "expensive part" as you would have to replace them every so many years. The thing I like about this AC coupled system is that you can "choose" the size of of the essential load and thus the size of the inverter/battery bank. You also "choose" the size of the PV/Grid Tied side of the system.  The smaller the inverter/battery bank side of the system is the better the overall ROI for the system. For now I will not have an inverter/battery bank, just the GTI and therefor optimal ROI. (Or so I'd like to belief ;-) )

 

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Quick side note about using a 3kw generator with the GTI: If the generator doesn't make clean power, the GTI will not tie with it. The GTI uses a combination of methods to detect islanding, usually: Voltage change, frequency change, rate of change of frequency (ROCOF).

With traditional distributed generation, you might have a mechanical inverter that runs synchronised to the grid. When the grid disconnects, it causes the load to change on your generator, and with a mechanical generator the result of that is that the engine speeds up or slows down. Usually the engine governor compensates for it fairly quickly, but it is still enough for the frequency to change for a few cycles. What the anti-islanding does is feed the frequency through a phase locked loop (PLL), so it knows when to expect the next zero crossing in the waveform. When the engine speed changes, it "misses" the zero point and the control circuit knows that something just happened...

Small petrol engines are terrible in this regard and your system will island every time a fridge turns on :-)

If you use an inverter generator... now that might work :-)

Edit: Now when I first started reading up on anti-islanding, and every explanation explained it in terms of a mechanical generator, I was skeptical. Surely, I thought, the rules are different for electronic generators? Then one day I read a paper where the author went through the math in detail. I wish I could find that paper again. Basically, when the load changes suddenly, the voltage will change, even if just momentarily. And when the voltage change, because of all sorts of laws that says the total energy must stay the same, the frequency must change too, even if just subtly. So even with an electronic generator, islanding equipment still use the same techniques.

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Thanks Plonkster,

I have thought of that as well. I do have a 6.5kVa petrol generator (modified sine wave - ugly wave form!) I am in the process of replacing because of that. I doubt that the GTI would appreciate all the harmonics that it produce, it may just blow out a lot of smoke if you try to do it. The plan in to get a inverter generator (pure sine wave). I hope it will work :-) I'll test it and let you know. One thing I am concerned about is feedback from the GTI to the generator. That is why I have the GTI Feedback Limiter in place. What I do not know is how accurately and quickly it blocks feedback. If it does allow some feedback momentarily the generator might have a problem. 

I can think of  two ways around this.

One, the GTI feedback limiter has some dip switches you can set to how much power it will allow to be drawn from the grid. You can set it to 0W or 50W or 100W. (In other words, if set to 100W the GTI limiter will try to force the GTI to produce 100W LESS than the load consumes, thereby allowing 100W to be sourced from the Eskom all the time. I really don't like to use this setting because if I set the dip switch I will always consume power from Eskom (50W or 100W) even if there is plenty of Sun/PV to power the full load.

Two, I put two (redundancy) small incandesent bulbs/resistors in parallel between the Generator supply connection and the Grid Tie Limiter. In other words the incandescent bulbs should draw power BEFORE the Grid Tied Limiter. Since the Grid Tied limiter only measures the power (Amps) going through it, the amps going through the bulbs will be excluded from the calculation. When I run on Generator it should be enough to prevent feedback (I hope).

 

 

 

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Why do you want to run the genset through the inverter? I have a 2 X changeover switch that I can choose my inputs and I then choose if the house will run off generator, eskom or inverter. I can automate everything using SSRs but I have not used the generator since we went solar so I have not even looked at it. In fact I have now a smart battery charger to keep the genset battery full or else it runs flat.

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Mumble, Mumble - Think I might have to leave this forum as I am driving the equivalent of a 1984 Ford Escort compared to all your Porches and Ferraris :D

Nice work Janma - Glad to see you have done your homework so far, and welcome to the forum.

PS: I have absolutely no idea what an SSR is that JDP mentioned above, but we will learn, that is the point of the forum... Enjoy...

Edited by KLEVA
Quoted wrong Forum member
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56 minutes ago, KLEVA said:

Mumble, Mumble - Think I might have to leave this forum as I am driving the equivalent of a 1984 Ford Escort compared to all your Porches and Ferraris :D

Don't worry KLEVA (is that said with a clichéd KZN accent - U wena kleva?) most of our installs would pass as a bakkie, I even know of an oke who has a draadkar (apologies Mark I could not resist)!

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On 2016/04/09 at 8:57 PM, Chris Hobson said:

Don't worry KLEVA (is that said with a clichéd KZN accent - U wena kleva?) most of our installs would pass as a bakkie, I even know of an oke who has a draadkar (apologies Mark I could not resist)!

You go away for a few days and the "draadkar" is mentioned again... ja ja ... :rolleyes:

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On 2016/04/08 at 7:23 PM, jdp said:

Why do you want to run the genset through the inverter? I have a 2 X changeover switch that I can choose my inputs and I then choose if the house will run off generator, eskom or inverter. I can automate everything using SSRs but I have not used the generator since we went solar so I have not even looked at it. In fact I have now a smart battery charger to keep the genset battery full or else it runs flat.

Hi JDP. The grid tied inverter only works if it has a grid to attach to. So if the grid drops, the grid tied inverter will shutdown as well. I'll have no power in the house.

I only have 2 sources that can act as a grid. The municipal power and the generator. I have a changeover switch which I can use to select which "grid" I want to use.  In the rare event when out municipal power goes out (we do not get load shedding) I have to option to run the house solely from the generator (switch off the grid tied inverter), or I can run the generator and supplement it with the grid tied inverter.

This is one of the drawbacks of the grid tied system. You need something to provide the "grid" for it to attach to.

 

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So this inverter does not have battery backup?? The InfiniSolar will grid tie and if the grid falls away will carry the laod from battery bank ? If not I would then just run the house on generator on its own and bypass the inverter set-up all-together. I would not run it via the inverters as how will you stop the power going back to the grid ? If you want to stop it via changeover switch then I would just bypass the invert and run the house on its own when you have a power failure. Simpler and safer.

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39 minutes ago, jdp said:

how will you stop the power going back to the grid ?

Because while the generator is running, you physically disconnect the grid and connect the generator in its place. The GTI then ties with the generator instead of with the grid.

As I said earlier, this only works if the generator makes really clean power, because the GTIs anti-islanding will disconnect when it senses anything that might suggest an "islanding event"... and small petrol generators basically produce islanding events as the main output, with electricity as a side-effect :-P

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5 minutes ago, plonkster said:

Because while the generator is running, you physically disconnect the grid and connect the generator in its place. The GTI then ties with the generator instead of with the grid.

As I said earlier, this only works if the generator makes really clean power, because the GTIs anti-islanding will disconnect when it senses anything that might suggest an "islanding event"... and small petrol generators basically produce islanding events as the main output, with electricity as a side-effect :-P

But won't it just keep disconnecting the generator when it senses the "islanding event" from the generator?

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4 minutes ago, viper_za said:

But won't it just keep disconnecting the generator when it senses the "islanding event" from the generator?

A GTI connects "in parallel" with your AC powering the house. When the GTI disconnects from "the grid" (which is really the generator) it leaves the generator to carry all of the load.

The GTI reconnects if the voltage is stable for a certain time period. 60 seconds or something, it's in the official standard.

So what will likely happen is the GTI will tie with the generator for a few minutes, and then the fridge will turn on... causing a momentary speed change on the generator engine, which will cause an islanding event, and the GTI will disconnect for a minute... come back... then disconnect again for another load... come back, etc.

Come to think of it... there is another problem here. The GTI needs to back off if it's making more power than you're using.

In general, I'd say it is better to tie the GTI to a point BEFORE the generator transfer switch. Agree with JDP here, much safer that way.

If you have an actual battery-backed inverter to tie with, things change. Then you can use GFPR.

I wonder sometimes how well GFPR works in this country. I've heard of people out Knysna area who's seen their local grid run at 53Hz. At 53Hz, most GTIs reduce their output to zero.

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2 minutes ago, viper_za said:

But won't it just keep disconnecting the generator when it senses the "islanding event" from the generator?

The typical generators produce a modified sine wave and as a AVR. When you switch on a heavy load you can hear the RPM changing (Hz also changes). So the power it produce is not very "clean". If I were to try and connect the GTI to it, it will most likely either disconnect because the power it sees on the "grid" does not fall within acceptable ranges (Volts, Hz and harmonics), or in the worst case it may blow smoke. That's why I am looking at an inverter generator. Thy work differently. It effectively has an inverter built in that produce 220V 50Hz sine wave. Its a lot cleaner than the typical generators (and more expensive).

To prevent the GTI of pushing power back into the generator there is a Grid Tied Limiter. It monitors the direction of the Amps passing through it. If it detects that the Grid Tied Inverter is producing more power than the load is using it sends a message to it to reduce the amount of power it produces. It will try to balance things so that the Grid Tied inverter only ever procude as much power as is consumed by the load. The net power flow from the grid/generator will be 0 Watts. (This is all in theory). If the Load is consuming more power than the GTI can produce it will pull power from the grid/generator.

Most likely I will err on the safe side and switch off the GTI if I have to run from the Generator. I really don't want to blow up the GTI or the generator. Most likely I'll only have to use the generator one or twice a year anyway.

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So if you have to disconnect the grid anyway to not push any power back to the grid I still dont get why you want to run it via the inverter. It seems like a complicated design just for that. I used to run a generator for years without any problems. It carries the load without any problem. It is just expensive as fuel is expensive. Look it is your system and you can do as you like. I just dont get why you want to run it via the inverter and risk everything. Just bypass it all. 

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I won't take the chance to connect an inverter generator to the grid tie limiter, a normal generator perhaps, but not the inverter type.

Under normal circumstances when the grid tie limiter is connected between the GTI and the grid and the GTI's load suddenly drops, the grid tie limiter will sense that some power is going into the grid and then it will send the instruction to the GTI to alter its output in order to only generate as much as required by the load.

If you now switch over to a generator or inverter generator instead of using the grid, there is a good chance that power will flow back to the generator before the grid tie limiter manage to alter the GTI's output and there is a good chance that the power flowing into the inverter generator will kill it - a standard generator might still survive it.

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Oo and btw I was told you are not allowed to have a generator autostart at a residential property. I baught all the componants and when I wanted to install it I was told I am not allowed. So how will your generater auto start to take the place of the grid?

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4 minutes ago, jdp said:

So if you have to disconnect the grid anyway to not push any power back to the grid I still dont get why you want to run it via the inverter. It seems like a complicated design just for that. I used to run a generator for years without any problems. It carries the load without any problem. It is just expensive as fuel is expensive. Look it is your system and you can do as you like. I just dont get why you want to run it via the inverter and risk everything. Just bypass it all. 

The reason why he wants to run the generator and GTI together is to use the make use of the power generated by the GTI (PV) to supply the load and if the GTI cannot supply enough power to the load, it will draw the remainder of the required power from the generator. If he uses the generator only he wastes the available PV power at that moment.

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7 minutes ago, superdiy said:

The reason why he wants to run the generator and GTI together is to use the make use of the power generated by the GTI (PV) to supply the load and if the GTI cannot supply enough power to the load, it will draw the remainder of the required power from the generator. If he uses the generator only he wastes the available PV power at that moment.

So are you telling me this inverter can only make power from PV if the grid is there? If so then that is a crappy design of a inverter. I wont buy it. Why does it need the grid to make power from PV to carry the load ?. I can understand that if the grid is not there then it should not push back into the grid the extra power the panels make. But why will it shut down if there is no grid and drop the load as well?

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6 minutes ago, jdp said:

Oo and btw I was told you are not allowed to have a generator autostart at a residential property. I baught all the componants and when I wanted to install it I was told I am not allowed. So how will your generater auto start to take the place of the grid?

@jdp I'm going to switch over manually. It's a small generator that I will store in the garage, but will have to carry it out and plug it in before I can start it up. Its small and not that heavy.

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