Tying two different inverters in parallel to my home & connected to the electricity network

[Spanish Flyer]

Hi to all; this is my first post, so I'll try to explain myself the best I can and also wish you all the best in these very troubled waters...

I live in Spain and not in the most sunny of all places we have over here. It's a bit like the weather you have in Seattle.
Anyway, two years ago I installed my pv plant on my house roof. It consists of 16 solar panels (EX 260W/24V) and a 4.2 kW inverter (Kostal Piko 4.2 with only 1 DC input)
Problem is that due to the weather, my solar production during the year is lower than I expected and want to upgrade the system by installing 6 more panels on another roof with different orientation.
My doubts are many, but the most important ones are: 1).- Can I install another smaller inverter to handle the 6 new panels and connect it to the grid in parallel?
2).- I could also fit all panels (22 in total) to the first roof and then go for a new inverter that can handle all the panel, but that would possibly be a much more expensive option, since my 2nd hand inverter in a swap with the supplier would mean a very small deduction in the new one price.
3).- Could I install a new Kostal Piko 4.2 cheaper.. exactly the same as my old inverter to handle half of the panels (all on the same side roof) and connect it in parallel to the house and electricity network?
Here we use 230 V AC and 50Hz
I attach an image of my actual 16 panels
Best regards,
Spanish Flyer

 

[GXMnow]

You will not have any technical problem having 2 grid tie inverters. They will both sync to the grid and supply power to feed loads in your house. Now the electric utility and building codes might have an issue about how much you back feed into your breaker panel.

Solar panels that are aimed in a different direction should be on their own MPPT input. One exception is having two or even 3 strings of the same number of panels that are then in parallel. East facing panels will start making current in the morning, then the south facing panels will start to add in, as the east panels fade, then west facing panels start to add. By doing something like this, you don't increase your peak power by much, but you get the power for more time of the day. A separate MPPT on each string might make a bit more power, but this is a way to add a fair number of panels and not need another inverter. Do you know how the panels in the south facing array is wired? Are some in series and then 2 or 3 parallel groups?? Does the existing Piko 4.2 have 1, 2, or 3 MPPT inputs? Are they true separate controllers, or just a parallel buss?

 

[Spanish Flyer]

Thanks a lot GXMnow,
I'm very relieved to know I can connect two inverters in the same grid; basically I was worried about the synchronisation of both and the AC current coming from the power distributor.
I understand that the panels on another roof with different orientation must be connected to a separate MPPT, and the Kostal Piko 4.2 only has one DC or MPPT input : (
My 16 panels are facing South-East and the other roof is facing South-West (northern hemisphere, of course), so if I install 6 more on the second roof, I would clearly need two separate MPPTs.
My 16 panels are connected in series, producing in excess of 380 DC V.
What would you suggest I do, because I'm no real expert in these matters and don't want to make any mistakes....
Thanks again and best regards.
I include a "photoshopped" pic of what my roof would look like using both roofs and the gral orientation of the house

 

[GXMnow]

Adding just 6 260 watt solar panels is only 1,560 watts at perfect sun peak. If that is on a second inverter, you would want to get a smaller inverter. Exactly which model Piko inverter do you have? I don't see one listed on the Kostal web site as just a Piko 4.2

Depending on the MPPT voltage range, maybe you could add 8 panels on the west roof, and wire the 24 panels as 3 parallel strings of 8 in series. But you would need to check the inverter data sheet to make sure it can handle over paneling to that level. When the sun is straight out between the two roofs around 11 AM in a clear sky, you might exceed 4200 watts DC with the 24 240 watt panels. Many inverters will just clip and only produce up to their rated power. That is not always a bad thing, as it ensures you won't need to upgrade wiring or have a second feed to the panel. When the sun is directly over your existing east panels, you will get them to nearly 4,000 wats, but the other panels are far off angle and won't add much current. Past noon, your east facing panels are clearly going to be down on output, but the new 8 panels on the west roof will start to reach their maximum output after 3 PM by the looks of the angles. Your existing inverter will still top out at 4,200 watts, but it should make power for longer, later into the day and early evening before sunset. For this to be a benefit though, the inverter needs to be able to track MPP down to 180 volts or so. And you may need to add a fused combiner for the 2 halves of the existing array, as well as adding the new array of 8.

This will not be quite as efficient as adding a second inverter but it will work quite well. If you look at the typical Voltage / Current graph for most solar panels, you will see the amount of light hitting them changes the current, but the voltage is not as critical. As the amount of light hitting the panel falls off, the maximum power voltage stays nearly the same as the current falls. So this is a very economical way to add power to your system, if the inverter can work at the voltage and the possible increase in current. I doubt the current will be an issue, looking at how much my power falls off at those angles.

 

[Spanish Flyer]

Hi again GXMnow,
I fully agree with you that I could go with a smaller inverter for the 6 panels, but I'm thinking more towards the future when I might be expanding my solar surface (if I can afford it)...
The Kostal Piko 4.2 is no longer available from the manufacturer.
What I'm going to do right now is install a new 5 kW Kostal MP Plus to handle the remaining 6 panels and whatever I decide to install in the future.
You have made me realise the many options I have (after you dissipated all my worries about installing inverters in parallel), so now it's a question of getting things rolling, before the winter sets in, with snow and all...
Thanks a lot, and more so, thanks a million for your help and advice, since I would't have moved an inch without it.
Will keep you posted.
I must also thank this great forum for allowing me to actively participate and hope you keep up the good work
Best regards from northern Spain
Spanish Flyer

 

[GXMnow]

The minimum MPP tracking voltage on that label is 350 volts. typical 60 cell panels have a VMP of about 30 volts. That means you would need at least 12 panels for it to work at it's best. Make sure the VMP of then new panels added up is above the minimum MPP tracking voltage of the new inverter.

 

[Jugy]

You will not have any technical problem having 2 grid tie inverters. They will both sync to the grid and supply power to feed loads in your house. Now the electric utility and building codes might have an issue about how much you back feed into your breaker panel.

Hello, new to the forum and found your comment while searching for my issue. I have recently connected my new 5kw system to our electric connection. I already had 4kw on a different inverter in different position. Now when its not clear sky, both inverters work fine. But as soon as the 5kw system gets above 3600w coming in , Inverter calls a Grid Volt Fault and resets. This is not happening if the other 4kw system is turned off.
electrician, who does the mains connections for me, reckons the inverters are fighting and the one shorter distance to the mains makes the new one (further away) to go over volt limit. Any idea what could be happening here? Any comments Much appreciated .

 

[MajicDiver]

Hello, new to the forum and found your comment while searching for my issue. I have recently connected my new 5kw system to our electric connection. I already had 4kw on a different inverter in different position. Now when its not clear sky, both inverters work fine. But as soon as the 5kw system gets above 3600w coming in , Inverter calls a Grid Volt Fault and resets. This is not happening if the other 4kw system is turned off.
electrician, who does the mains connections for me, reckons the inverters are fighting and the one shorter distance to the mains makes the new one (further away) to go over volt limit. Any idea what could be happening here? Any comments Much appreciated .

Need to describe or sketch your layout and identify what inverters.

Without more details it could be many things. Are the volt limits and time delays the same on both inverters? Are you using power meters to limit export? Have you been able to see the voltage increase as power from inverter increases?

Are the conductors from the inverter sized such that there is little or no voltage drop in the path to grid for the inverter you have the problem with?

 

[GXMnow]

Hello, new to the forum and found your comment while searching for my issue. I have recently connected my new 5kw system to our electric connection. I already had 4kw on a different inverter in different position. Now when its not clear sky, both inverters work fine. But as soon as the 5kw system gets above 3600w coming in , Inverter calls a Grid Volt Fault and resets. This is not happening if the other 4kw system is turned off.
electrician, who does the mains connections for me, reckons the inverters are fighting and the one shorter distance to the mains makes the new one (further away) to go over volt limit. Any idea what could be happening here? Any comments Much appreciated .

You need to measure the voltage back at your main panel both with and without the full sun hitting the solar panels. What is your proper grid voltage there? Most inverters will shut down if it is more than 10% high. With just the one inverter running, you don't have a problem, but when the second inverter is also pulling the voltage up at the main panel, it looks like it is then going high enough to cause a problem.

Here in USA, I commonly see 245 volts from L1 to L2 and the spec is 240. That is only 2% high. At my roof mounted microinverters, they do report as high as 251 volts. That is getting close to 5% over with the long wire run up onto my roof. In most cases, that is ok, but on my system, it does occasionally cause the microinverters to reduce their power a little. It is programmed into the microinverters to help stabilize the grid. As the voltage rises, it should slowly reduce current to let it drop back down. Getting 80% of your possible solar power is better than having it shut down, or have the grid power shut off due to over voltage if there are many megawatts of solar panels all pushing into the local grid.

On a 230 volts system, 10% high would be 253 volts, 250 is probably a safe limit. Are you on 120/240 like the USA, on 230 like most of the rest of the world, or something different?

Keep in mind the inverter is measuring the "grid" voltage at it's end of the wire. You should measure the voltage at the main breaker panel as well as at the inverter's grid terminals to see how much voltage rise you have in the wire. On longer wire runs, the wire may be able to easily handle the amount of current without getting hot, but over the long distance, it can still allow too much voltage rise. The wire from the main panel to the inverter should be sized to only allow a less than 3% voltage rise from the main panel. If the main panel is already high, there is not much you can do, but if you see just 235 at the panel, but it is up over 250 at the inverter, you will need to check all of the connections and also the wire size to make sure it is large enough to handle the current over that distance. In most cases, electricians are only concerned about "Voltage Drop" when running a load. A grid tied PV solar inverter is an odd case. The power is flowing backwards. This is why the voltage climbs over the distance.