Author Topic: How does islanding (in solar inverters) work?  (Read 203 times)

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Offline woody

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How does islanding (in solar inverters) work?
« on: December 04, 2017, 07:41:11 PM »
I have 30 solar panels on my roof, each connected to an Enecsys microinverter.

These inverters only produce power when they are connected to utility power. If that net is down, they cease operation, a feature called anti-islanding. I fully understand why that is. But it leads to difficult questions from the other people in my house whenever the net fails, the sun shines and they still can't have coffee. Tell me again, why did we invest a new car in those panels?

This made me think dangerous thoughts. How can I make a microinverter think there is utility power even when there isn't. How does a microinverter find out if it is connected to the net? Before I crack open one of  my spare inverters it seemed sensible to first ask here.
 

Offline T3sl4co1l

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Re: How does islanding (in solar inverters) work?
« Reply #1 on: December 04, 2017, 07:58:00 PM »
Get a utility disconnect and whole-house inverter (and maybe a PowerWall or something like that).  Then it's safe to become an island, at least within the capacity of that equipment.

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Offline Seekonk

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Re: How does islanding (in solar inverters) work?
« Reply #2 on: December 04, 2017, 08:43:30 PM »
It can be tricky aside from proper grid disconnect. A couple hundred watt pure sine wave inverter powered by a battery can supply the signal necessary. Problem is you have a lot of power and if that isn't used, it goes into the battery operated  inverter and will likely destroy it.  The grid is a big sink of power. Grid tie keep raising the voltage till it sends out all the power available. There are inverters that will feed that power back into the battery.  If the battery is fully charged then it requires an external load. A couple hundred watts is easy to dump, several KW is another problem.  There are youtube videos showing destruction and dump load success. Generally more expense than it is worth.
 

Offline Jeroen_Bezemer

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Re: How does islanding (in solar inverters) work?
« Reply #3 on: Today at 08:28:38 AM »
I have 30 solar panels on my roof, each connected to an Enecsys microinverter.

These inverters only produce power when they are connected to utility power. If that net is down, they cease operation, a feature called anti-islanding. I fully understand why that is. But it leads to difficult questions from the other people in my house whenever the net fails, the sun shines and they still can't have coffee. Tell me again, why did we invest a new car in those panels?

This made me think dangerous thoughts. How can I make a microinverter think there is utility power even when there isn't. How does a microinverter find out if it is connected to the net? Before I crack open one of  my spare inverters it seemed sensible to first ask here.
The problem is not Running as Island, it is returning to the net. The inverter needs a way to synchronize when the net returns. As this is not possible without external switch and sensing line, the only safe way is not to do that. If you have a main feeding power switch with automatic disconnecting on power failure, you may configure an UPC like construction which can take over, but for switching back, you will need to "go through the dark".

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Offline coppice

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Re: How does islanding (in solar inverters) work?
« Reply #4 on: Today at 10:02:01 AM »
The anti-islanding is only to stop your power being feed to the grid, when the grid is down. Without this, the utility's technicians have no way to power down your block when they need to do maintenance work. Its not intended to starve your own site of power, but it will do if your system is simplistic. You could have a setup that will isolate your premises, and continue to power it (or a part of it, if you lack enough generation capacity for the whole house). How unreliable is your local grid that you actually care about this?
 

Offline Kire Pûdsje

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Re: How does islanding (in solar inverters) work?
« Reply #5 on: Today at 10:49:52 AM »
I do not know about these micro-inverters, but normally the grid tied inverters cannot be used stand-alone. They rely on the grid to keep the voltage constant. Essentially these inverters are current sources.

Mentally I also had the same problem. Having a roof full of solar panels and when you really need them they cnnot be used.
I see you also live in the Netherlands, so power outages are not a big issue. Therefore I also did not want to invest a lot in an off-grid setup.

I generated my own solution by using a 24V Meanwell DC power supply directly attached to the panels (a switch and fuse in between). A lot of these can just be fed from directly from DC, no AC needed, this possibility is even stated in the manual, so there should be no safety issues as well. I used a small opamp circuit attached to the sense input to generate a CC/CV mode. At low battery voltages it will limit the current to the batteries. When fully charged, it will remain at a constant voltage.
I specifically avoided an MPPT charger, as I want to be able to have the input in parallel to my normal grid-tied inverter. having two separate MPPT controllers in parallel did not seem like a good idea.
For the batteries I used lead truck batteries. These are replaced in garages at regular intervals. Most of them are good enough for solar panel use. For just coping with power outages, no fancy batteries are needed, as (hopefully) it will not go through a daily charge/discharge cycle. It will only be used incidentally.
I also experienced, that big truck batteries can easily be exchanged at the local car junkyard for about 10 euros. They are only interested in the scrap metal, so no difference to them if you exchange bad ones for newer ones.

After the battery I just use a 600W inverter to generate the mains voltage. I do not need the full output power capability of the solar panels. I need enough to drive the fridges, the central heating pump, etc. The inrush current being the dominant driver for these motors I chose an LF inverter.
This whole setup only costed me 160 euros.
In case of a power outage I could use extension cords to critical applications. However in the Netherlands, there is hardly any need.
« Last Edit: Today at 11:29:20 AM by Kire Pûdsje »
 
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Online mtdoc

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Re: How does islanding (in solar inverters) work?
« Reply #6 on: Today at 12:14:36 PM »
This Wiki article on islanding may answer your questions.

In the US, grid tied inverters must meet the UL 1741 anti-islanding standard. This applies to both standard grid-tie only inverters or so called "grid interactive" inverters that are used with battery back up systems.  More info HEREmand HERE[/ur]/
 


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