Disabling active grid-tied inverters

[Swing]

Where I live we are beginning to see that we have too much solar power at certain times (and these times will only increase in next years)
So it is becoming more and more useful to disable grid-tied solar inverters. When you don't have any storage or can't consume the power yourself.
This topic is not about discussing why this is necessary, or that there are other options like storage.

This topic is a question/discussion about how you can best disable an active and working grid-tied inverter, to turn it back on later.

First of all, several grid-tied inverter brands are controllable by software. This way you can limit power as much as you want.
Whenever that is possible, that is the best way by far. For example, SolarEdge is commonly used, and you can control them that way.

But, there are a lot of "dumb" grid-tied inverters. Especially for private homes, where there was never thought about the use case of "too much energy".

Every grid-tied inverter here on the market in Wester Europe can always be disabled by cutting of the AC/grid to the inverter. When the inverter doesn't see any grid AC voltage anymore, it immediatly stops working.
This is also used for maintenance.

However, I doubt that it is a good idea to cut the AC everytime the solar power is unwanted. The inverter is for example producing 2000W on the AC side and suddenly you cut the AC. I assume you would seriously decrease the life expectancy of the inverter if you do that a 100 times a year.

To me it would seem safer to cut off the DC side (which would require high voltage DC contactors, but it can be done). This would offer a universal way to disconnect the inverter, regardless of what brand it is.
But the question remains, wouldn't you increase wear this way as well? So it is producing some power, and you cut of the source. It might be better than cutting the AC voltage. Perhaps an easy way to power down the flow coming from the solar panels. Which would require a lot more complicated solution. That would be a soft start/end to the power coming in over DC.

Cutting the AC or DC power will both stop the inverter from working. But what would be a solution that doesn't decrease the lifetime of the inverter?

 

[MajicDiver]

The inverter is for example producing 2000W on the AC side and suddenly you cut the AC. I assume you would seriously decrease the life expectancy of the inverter if you do that a 100 times a year.

I would not expect this to be the case. For example, a 4 ton a/c will draw 3.5 to 4 kW and cycles a couple of times an hour in summer. The normal duty cycle for an inverter is rapid load changes within it’s capacity range - if that kills it then most of the population of that brand will die quickly. Same goes for dc side, think clouds then no clouds on partly cloudy days, inverter must be designed to handle it.

 

[Swing]

Yeah in general I think so too. Just to be clear, when I mentioned AC in the above post, I meant alternating current.
I am not discussing air conditioning.

But I am expecting to cause some peaks and surges when cutting AC or DC. Maybe, when cutting the AC side, those peaks or surges don't reach the rest of your home grid so they don't get hurt.

Also, some generic battery inverters actually can use their generator output as an input to connect grid-tied inverters. When the grid is gone and the battery is full, they simply cut off that port, to stop the grid-tied inverter from working.

Clouds can come and go, so I guess the DC side is used to fluctuations a lot. You can co from 5000W to 1500W in a moment.
However, I don't feel like you can state that you can let all of these inverters go from their peak power to 0 in a single moment, repeatedly over the year, without causing any of them to fail.

 

[zanydroid]

What is the mechanism of damage you are thinking of?

The inverter is completely handled by solid state devices. It actually switches the transistors on and off at a few khz. Switch mode DC power supplies in general both have PWM switching to lock voltage at khz rate (constant switching) and may have loads that surge up and down on the output. For instance LED dimming will do this albeit at low power levels typically.

I would be more worried about voltage surges created by the inverters during disconnect than the inverters...

Now for a disconnect on a grid tie inverter that has no battery, that will likely have a voltage surge until the GTI turn off. but this should be rare and only occur on a power outage. Or it might happen on an AC coupled system if the hybrid cannot sink energy or modulate quickly enough. If only inverters on the cutoff relay only inverters will take the hit. If shares with utilization equipment they will potentially take the hit. As a protective measure maybe some SPD layers can mitigate this if it is desirable to mix inverters with utilization equipment.

For GTI connected to a hybrid via AC coupling a hard disconnect should be rare since presumably frequency watts or other modulation is in effect to prevent a hard cut. And frequency watts is required for modern inverters so a properly designed system should have few hard disconnects in theory

 

[SamDeleted]

To me it would seem safer to cut off the DC side (which would require high voltage DC contactors, but it can be done). This would offer a universal way to disconnect the inverter, regardless of what brand it is.
But the question remains, wouldn't you increase wear this way as well? So it is producing some power, and you cut of the source. It might be better than cutting the AC voltage

Don't all grid tie systems have isolators in the PV lines ? (at least in the western world, mine do in the UK)


Not sure which is better for the inverter, DC disconnect or AC disconnect. I doubt it matters

 

[SamDeleted]

Don't all grid tie systems have isolators in the PV lines ? (at least in the western world, mine do in the UK)


Not sure which is better for the inverter, DC disconnect or AC disconnect. I doubt it matters

Certainly disconnecting the DC will put wear on the isolators, especially if it's a high voltage PV system eg. 500v

 

[Swing]

Certainly disconnecting the DC will put wear on the isolators, especially if it's a high voltage PV system eg. 500v

What do you mean?

Do you mean the type of isolators that you manually switch off? Yeah those are nowadays a part of most installations.
But I don't see how they would play any rope in this since they are just normally closed/connected is if they aren't there.

 

[Swing]

@zanydroid
Inverters like Victron can do some modulation to turn down the power. Any European inverter should comply with this.
If you take the bigger Chinese brands (Deye and copies for example) they use the generator port and just turn it off. They have to work globally with anything.

But yeah the AC frequency modulation is, at least from a European perspective, probably the best generic way to safely turn off any grid tied inverter.

It is a lot different than a DC contractor retrofit solution would be, that would be cheaper and simpler to add to an existing situation. However no point in doing something cheap and easy if it will break the inverter over time.

 

[SamDeleted]

This topic is a question/discussion about how you can best disable an active and working grid-tied inverter, to turn it back on later.

But I don't see how they would play any rope in this

Well you're trying to turn your grid tie inverter on and off , the DC isolators are one of your two main option , so that's how it's relevance ....

Unless you're trying to automate it ? But it doesn't say that in your post

 

[Swing]

Ah sorry, you are right. That was perhaps not clear. The goal is to automate it. Based on hourly pricing or grid operator incentive. Thats why I mentioned the repeated times.

So, whenever that is possible through software (hence the SolarEdge example) then that is the way to do it.

But, a more direct universal approach could be a DC contractor. However I can't judge if that will decrease lifetime.

I prefer a solution on the DC side, because it will be easier to retrofit through the MC4 connectors.

 

[zanydroid]

@zanydroid
Inverters like Victron can do some modulation to turn down the power. Any European inverter should comply with this.
If you take the bigger Chinese brands (Deye and copies for example) they use the generator port and just turn it off. They have to work globally with anything.

Do they not have AC coupling implementation? SolArk has it but some folks say it has to fall back to using the relay sometimes. IMO the relay is a good fallback for cross vendor AC coupling because most combinations of hardware are likely only lightly tested.

If an ancient string inverter blows up from being retrofitted into a storage system, IMO it's not a great loss.

If buying a new inverter the money could go into getting one with native hybrid design.

But yeah the AC frequency modulation is, at least from a European perspective, probably the best generic way to safely turn off any grid tied inverter.

It is a lot different than a DC contractor retrofit solution would be, that would be cheaper and simpler to add to an existing situation. However no point in doing something cheap and easy if it will break the inverter over time.

In one of the AC coupling threads someone mentioned that there are some newer AC coupled batteries that also use relay to control inverters that do not support frequency watts

A safety listed DC contactor or SSR might not be that cheap, though I guess we are taking about PV not batteries here so it is not as intense of a performance spec as it could be. That said the Rapid shutdown devices used in US PV can "interrupt" up to 60V and 24A of input (installed with one per panel). They are solid state and supposedly last 25 years on a roof. They basically interrupt the DC and switch to a 1V or so standby mode (low power buck converter). But not intended/tested for cycling multiple times per day, and if they blow up they need to be serviced on the roof.

 

[Swing]

Well, it is not that hard to find DC contractors for 1000v and way more amps than needed.
I think whatever is needed, you need to make it and have it tested/certified.

You can find a lot of things, just like these rapid shutdown devices. But they are still intended, just like an AC cutoff, for exceptional situations, like when there is fire, or when maintenance is being done.
Switching this on and off a hundred times each year, might break some inverters. And it is not acceptable if they break.
Even though it would be a good moment to switch to a battery hybrid inverter. But that doesn't solve everything either if the battery isn't used in an efficient way.
The problem is not in the home, the problem is in the grid, as in, too much power available, not enough consumers.

 

[SamDeleted]

Ah sorry, you are right. That was perhaps not clear. The goal is to automate it. Based on hourly pricing or grid operator incentive. Thats why I mentioned the repeated times.

So, whenever that is possible through software (hence the SolarEdge example) then that is the way to do it.

But, a more direct universal approach could be a DC contractor. However I can't judge if that will decrease lifetime.

I prefer a solution on the DC side, because it will be easier to retrofit through the MC4 connectors.

https://www.mouser.co.uk/new/omron-electronics/omron-g9kb-power-relays/

If you can't find another solution, this 600v DC relay should do it , but you'd have to find a way to fire it, timer perhaps?

 

[zanydroid]

The problem is not in the home, the problem is in the grid, as in, too much power available, not enough consumers.

In the US the first generation of smart inverters added carry over and distributed feedback (IE including frequency watts)

The second generation of smart inverters added API based explicit throttling that the grid will eventually use.

And many places put in aggressive export control much sooner than the US did.

So the ground is prepared to mitigate excess production from a grid angle.

 

[DIYrich]

Every grid-tied inverter here on the market in Wester Europe can always be disabled by cutting of the AC/grid to the inverter. When the inverter doesn't see any grid AC voltage anymore, it immediatly stops working.
This is also used for maintenance.

Usually the inverter is between the grid and load. Cutting off the inverter from the grid also disconnects load. Not optimal.

Are you trying to keep inverters from feeding the grid, or producing entirely? If the former, then all you need is to activate grid zero function on many inverters.

If the latter, then you have the question about charging for electricity that would otherwise be free. You are basically talking about free electricity time of use periods.

Frequency shifting can get many inverters to stop producing. Some may take their load with them. You would then have to frequency shift the other way to average to the right number of cycles per day, mainly for clocks.

 

[zanydroid]

So I think the idea of being more critical about operating inverters outside of their standard/tested operating envelope is a really interesting one.

However it's not clear which common cases (large potential base of users using a given config/power architecture) would have a problem.

It feels like some off grid use case with the following configs
1. using crappy storage inverters that lack AC coupling, which would not be able to modulate even post F-W era inverters
2. Using modern storage inverters that have AC coupling with pre F-W era inverters. Which are over 6+ years old in my market (California). Probably not that much warranty lifespan left on string inverters. On microinverters there is plenty of remaining lifespan and it would be annoying to replace, so maybe this is a good concern.

On the other hand your DC idea is not viable for microinverters. Harsh environment for a niche boutique product and might as well swap microinverters in a labor dominant cost environment like Europe or US.

For all-on grid use cases cutting AC is kind of weird. You would already have an export limiter if your grid cared about that originally, and if you were trying to shift into a battery the charger can use CTs to modulate the charger power without having to cut AC. If it under or overshoots the consequence is similar to export limiter under or overshooting. Nothing will explode or disconnect.

 

[zanydroid]

For hybrid ac coupled systems that are primarily grid tied but sometimes work off grid, I doubt the number of disconnect cycles that happen when grid is down is going to be enough to worry about. Since the grid down is an exceptional condition.

 

[Externet]

A tiny bit of planning/prevention for next day can shut off the inverter or its circuit breaker from mains by midnight. If you must disconnect PV panels for whatever, do it at night time. I think...

 

[Hedges]

Using frequency-watts to ramp down PV inverter output is primarily useful for AC coupled systems when grid is disconnected.
It is a requirement for grid-tied inverters now, allowing utility to increase frequency and curtail PV production, but would do so across the entire West or East half of the United States at once. A local area might have excess supply which couldn't be addressed well this way.

Zero or limited export may be what you want. Hawaii reached to high a level of GT PV, so additional installations used current transformers to measure grid current and inverters throttled back production to only supply house loads, not export.

Many AIO support CT this way.


As for switching off inverters, it is usually recommended to turn of AC because interrupting the arc is easy. DC is more difficult but there are switches rated for it.

Seems to me that switching off PV DC causes no stress at all to inverter, it just discovers that capacitor voltage declines. Switching off AC while current is being driven through inductor into grid, I think would cause voltage spikes. Those might deteriorate components.

Preferable to command inverter to adjust its PWM and reduce power output.


Existing "dumb" inverters? Just let them be. They were allowed when their numbers weren't excessive. Inverters with controllable output were required as penetration increased.

 

[Swing]

For hybrid ac coupled systems that are primarily grid tied but sometimes work off grid, I doubt the number of disconnect cycles that happen when grid is down is going to be enough to worry about. Since the grid down is an exceptional condition.

Grid going down is not the use case I am talking about.

Also @Hedges:

Current situation has been there for several years. Whatever your solar array produces throughout the year can be deducted from what you consume in a year. Even though you produce most in the summer and consume most in winter.
It has been so popular that we have now, at certain times, too much solar power.

I guess grid operators could use the frequency shift, but I haven't seen them do that.
What they do is they have balancing partners, and pay them good amounts of money to stabalize the production and the demand.
This is getting worse. There are moments when a solar park can make more money with turning off their solar, than on a sunny day. (also because the value on a sunny day goes down)
Next to those problems, there are the new hourly tarrifs which are gaining traction.
Market hourly tarrifs, there are more and more hours where the price is negative. In other words, it is costing money to put solar power on the grid.
The amount of hours is set to increase drastically in the next few years.

So the way I see it, it will become more and more interesting to disable the grid-tied solar inverter from time to time.

And, it looks like the dumb inverters are still being sold. Yes, they do support frequency shifting, otherwise they are not allowed.
But that is hardly used, there are more reasons to disable it, before you get to the stage of national frequency shifting.