How exactly does grid-tied hybrid inverter detect loss of grid?

[SeaGal]

So, I've just got a 2nd inverter going and was pondering how an inverter knows the grid is disconnected (within a few hundred milliseconds)?

And, specifically, how does that work when you have more than one inverter connected to the grid, each providing output. I guess it would be the same mechanism if you and neighbours had inverters on the same phase, but I'm thinking specifically if you are generating more PV than the house load and exporting, with 2 or more inverters.

Can't quite get my brain around how that works

 

[hwy17]

I think @RCinFLA wrote a comment recently about this, or maybe it was older and I just read it recently. Describing that this is one of the hardest control logics to design because it's hard exactly like you're imagining. Can't remember the explanation, observation of small power flow changes or something.

Edit: Found it: https://diysolarforum.com/threads/how-do-hybrid-inverters-resync-to-grid.63921/post-797926

 

[SeaGal]

Thanks, read that but I still feel I'm missing something... in that description it says..

Disconnect from AC input is a bit more complicated. If grid drops out, there is a momentary overload on inverter as inverter tries to power the collapsed grid, up to possibly the surge current limit of inverter, before it releases AC input pass-through relay...

but that doesn't apply if inverters are outputting to grid...

Toughest thing for inverter to do is detecting when AC input goes open circuit, like when you open AC input breaker. Zero AC input current is a legit condition and voltage on AC input does no good since inverter is feeding through pass-through relay to AC input.

Most inverters normally use the up/down pulses from phase detector, to determine when to release pass-through relay. When a legit AC input is present there should be a repetitive average of up and down phase correction pulses. Some inverters, when AC input is opened, just wander off in frequency until they hit max inverter run AC frequency limit before disconnecting pass-through relay...

But if there are 2 or more inverters, won't they keep each other in sync - or at least for quite a while?

Recognition of loss of AC input can take from a couple of seconds to a minute before it releases pass-through relay...

But for UK ENA Type-tested inverters, disconnection has to take place within 500ms, and I'm guessing the same for other jurisdictions?

 

[HighDesertOffgrid]

But if there are 2 or more inverters, won't they keep each other in sync - or at least for quite a while?

I would think the inverters will have a massive current draw and voltage drop as they try to power a dead grid.

 

[Shinzul]

I think it depends on the utility, but one way is by shifting the frequency in a way the inverter is programmed to understand. The frequency shift means it should switch to net zero export. I'm completely guessing based on what I've read of the UL -SB addendum and the new settings for SB certified inverters.

 

[SeaGal]

I would think the inverters will have a massive current draw and voltage drop as they try to power a dead grid.

Sure, if grid is connected, but if just the house got disconnected, or AC-isolator to where the inverters are?

 

[HighDesertOffgrid]

Sure, if grid is connected, but if just the house got disconnected, or AC-isolator to where the inverters are?

I am interested to know the answer as well. And if your neighborhood is highly solarized why wouldn't 50 or 100 microinverters tend to go into island mode?
I suspect it has something to do with a necessary higher voltage for export.

 

[kommando]

The requirements for the UK are set by G98, go to the ENA site and get one of the test PDF's and the tests will show the criteria the inverter needs to detect and the speed of the disconnect when that criteria is not met.

 

[RCinFLA]

Thanks, read that but I still feel I'm missing something... in that description it says..

but that doesn't apply if inverters are outputting to grid...

Yes it does. It periodically pushes a phase wiggle against grid to test that grid is still present.

But if there are 2 or more inverters, won't they keep each other in sync - or at least for quite a while?

When hybrid grid interactive inverter is connected to AC input source it is slave to AC input freq/phase/voltage. Once pass-through relay releases the inverter goes back to being its own master.

If you have inverters running in parallel, or series connected 240/120vac for split phase, there is usually an interconnect cable to keep units in sync when there is no AC input.

In addition to sync'g the two hybrid units, there is usually other interconnect control lines, like if one unit shuts down the other unit also shuts down, or if one is charging battery, the other is also charging battery and they both switch charging states (bulk, absorb, float) at the same time based on first unit jumping states. Again, if AC is present, cross sync'g control is overridden and they obey the phase of their respective AC input as slaves to AC input.

Inverter-generators that can run in parallel require AC output be connected first before one generator is started first and it becomes master for phasing. Second inverter-generator, seeing AC voltage present on its input when started, becomes slave to first generator started.

But for UK ENA Type-tested inverters, disconnection has to take place within 500ms, and I'm guessing the same for other jurisdictions?

Only way to detect an open circuit disconnect on AC input is by inverter pushing against phase lock to AC input. Pass-through relay allows inverter output to be present on open circuit AC input so voltage detection is of no use. There will be a current deviation push back when a legitimate grid is present. Grid is an 'immovable' object when inverter tests for grid presence by attempting to wiggle it phase lock to AC input.

Most cheaper, non-UL1741 compliant inverter just bias their phase locking circuitry so if AC input goes open circuit there is no phase locking reference and the inverter frequency will wander higher in frequency until it hits maximum allowed frequency spread limit of inverter before it releases pass-through relay. Some count how many zero crossings occur in AC cycle without any phase correction generated. If too long a period without phase corrections then it assumes AC input has been lost and releases pass-through relay.

 

[timselectric]

Short simple answer.
The grid or grid forming sources are both voltage and amperage producers.
Grid-tied inverters are only amperage producers.
They follow the sources voltage, but don't create it.

 

[Lt.Dan]

I mean it makes sense to me that once the grid drops, it becomes this vast bottomless pit and your inverter cannot supply enough energy to support it, so it goes into overload and turns off. All in the blink of an instant.

 

[Mattb4]

I mean it makes sense to me that once the grid drops, it becomes this vast bottomless pit and your inverter cannot supply enough energy to support it, so it goes into overload and turns off. All in the blink of an instant.

While that is the most likely situation There could be instances of grid loss where the only loads are near your home and within the capacity of your system. Thus a danger to someone not prepared to find a voltage on the grid. I find RCinFla explanation more comprehensive since the anti-islanding protection is not reliant on simply overload.

How it accomplishes this is in the Voodoo electronics land to me though.

 

[timselectric]

I mean it makes sense to me that once the grid drops, it becomes this vast bottomless pit and your inverter cannot supply enough energy to support it, so it goes into overload and turns off. All in the blink of an instant.

It has nothing to do with loads.
It's the loss of voltage.

 

[HighDesertOffgrid]

How it accomplishes this is in the Voodoo electronics land to me though

it does. It periodically pushes a phase wiggle against grid to test that grid is still present.

Apparently grid tie inverter know how to twerk.

 

[Lt.Dan]

It has nothing to do with loads.
It's the loss of voltage.

I was just incredibly oversimplifying to make my feeble mind understand

I actually think its pixy dust and fairies that are working their magic. Nobody can tell me otherwise.

 

[zanydroid]

There is a list of about 20 different anti-islanding modes out there. It’s required by some POCOs in California to declare what your inverter uses, out of the ones on this list. Enphase has a letter declaring what they use, to support this requirement

Maybe find the list and cogitate on the name of a couple of the options until one of them makes sense

 

[MajicDiver]

Yes it does. It periodically pushes a phase wiggle against grid to test that grid is still present.

I think many here don’t understand what this means or misunderstand it. I will try to explain further what you are saying, hopefully I don’t muddy it more:

1) the “grid” is huge. If you want to speed the grid up (increase the frequency) you must supply enough power to the grid to measurably increase the speed of every single motor on the grid. Not just your neighbors, but everyone in your county, state and neighboring states. In the US there are several large regions that form their own “grid” I forget exactly how many.

2) Their is no way your inverter can do anything to impact grid frequency. A large 1,200 Megawatt nuclear unit cannot impact grid frequency on its own in normal circumstances except a very fast transient if it trips for example.

3) when he says “wiggle” what he means (I believe) is that, for example, the inverter is programmed to attempt to increase frequency periodically. If it is actually attached to the grid, the frequency will not budge but the inverter power output will increase sharply. If frequency does increase, it knows it’s no longer connected to a grid.

4) I’m sure the electronics to accomplish this as fast as it is required are complex.

 

[zanydroid]

1) the “grid” is huge. If you want to speed the grid up (increase the frequency) you must supply enough power to the grid to measurably increase the speed of every single motor on the grid. Not just your neighbors, but everyone in your county, state and neighboring states. In the US there are several large regions that form their own “grid” I forget exactly how many.

Anti-islanding is intended to address "islands" though, like suppose a tree chops off your block from the grid. Then there's only 20 houses on this grid, and during summer solar peak the grid tie inverters can easily power every home (setting aside considerations about power factor and power quality of the loads).

 

[MajicDiver]

Anti-islanding is intended to address "islands" though, like suppose a tree chops off your block from the grid. Then there's only 20 houses on this grid, and during summer solar peak the grid tie inverters can easily power every home (setting aside considerations about power factor and power quality of the loads).

In that situation, if as you say the inverter can actually power it’s neighbors, it will see it can increase frequency and trip it’s output as it is supposed to.

 

[MajicDiver]

In that situation, if as you say the inverter can actually power it’s neighbors, it will see it can increase frequency and trip it’s output as it is supposed to.

And of course it’s even more complex, I just explained the increase part of the wiggle