How to 'trick' PV into staying on during a grid outage

[Burningislove]

I have a standard, GT 9KW system at my home in MA. As my road "does not exist" on town maps, National Grid has no monitoring equipment of any kind on my road and power outages even on sunny, windless days are frequent even if they only average 30min-2hours. My system automatically shuts down when the grid goes out, in theory to protect line workers who may be equally in theory fixing something on the street which of course makes sense.

Prior to installing PV, I had my trusty Generac generator for long standing outages in winter and the such. Of course, anyone who's used a generator knows the first step is to turn off the main breaker to the street so the generator doesnt try and power the entire street.

Is there something similar I can do with my PV system to keep it turned on to power my home when there is a grid outage? I'd really like to avoid all the cost of batteries for obvious reasons (no religious debates on that topic, please!). My existing system puts out way more power spring-fall than my home uses, most months I'm running a credit with NGrid in the 100-250KWh range. Like with my generator, I'm totally fine with this being a manual process to turn on and then switch back to normal operations once the grid gets back online.

I have an large section of roof ideally suited for additional panels that I could do myself, without GT. But that means in addition to new panels, a new inverter, separate runs to the main breaker panel, and so forth. I'd much rather utilize the system I already have.

Gotta imagine there is already a thread out there on this, just didn't find one that didnt quickly burn down into a war about batteries and EROI.

[Naptown]

Either switch to a bimodal inverter add something along the lines of a sunny island etc
Oh and did i mention a lot of batteries?

[inetdog]

Or add a new additional off-grid-only inverter and its associated batteries and a Charge Controller and just switch the connection on your solar panels to the CC when the grid is down and you are using battery power.
Essentially you will end up with a very large and expensive UPS whose batteries are kept charged from the grid 99% of the time.

Or you can get a new replacement inverter from SMA which can provide up to 1500 watts of power through a dedicated outlet when the grid is down and the sun is shining strongly. No batteries needed, but no power when a cloud goes by either.

Or, there may be some interesting options involving the Tesla PowerWall battery pack and a new inverter from Solar Edge. Not actually available yet AFAIK.

There are lots of options, but none of them simple or cheap.

If you try to trick the GTI into thinking that your generator (for example) is the grid, you are likely to destroy your generator at some point.

[Burningislove]

Either switch to a bimodal inverter add something along the lines of a sunny island etc
Oh and did i mention a lot of batteries?

LOL. It's a new system and under warranty for 20 years, so I know any mucking with it will void the warranty, especially replacing the inverter with something like SMA. I also read some articles & materials on inverters with a dedicated, non-house tied plug for direct plug in.

Was very much joking with someone about turning on the generator and then having the inverter try and match the phase, so the generator would have little to no load on it while the PV would do most of the work. But my EE background says that something would go "boom" in technical speak.

The real short outages arent a big enough hassle to warrant the cost of a new inverter (or batteries). In winter though when outages can last days after a storm, it would be nice for the fuel savings on the generator. I do have several UPS in the house for my home office and such, and my wood stove provides more than ample heat for the entire house as well as hot water and a nice cooking service (gotta have my coffee!!!)

[Burningislove]

Or add a new additional off-grid-only inverter and its associated batteries and a Charge Controller and just switch the connection on your solar panels to the CC when the grid is down and you are using battery power.
Essentially you will end up with a very large and expensive UPS whose batteries are kept charged from the grid 99% of the time.

Or you can get a new replacement inverter from SMA which can provide up to 1500 watts of power through a dedicated outlet when the grid is down and the sun is shining strongly. No batteries needed, but no power when a cloud goes by either.

Or, there may be some interesting options involving the Tesla PowerWall battery pack and a new inverter from Solar Edge. Not actually available yet AFAIK.

There are lots of options, but none of them simple or cheap.

If you try to trick the GTI into thinking that your generator (for example) is the grid, you are likely to destroy your generator at some point.

This is along the lines of what I was thinking (Change Controller or SMA). 1500W is more than what I need during an outage, just charging the dridge and some electronics is all I need. For now, Tesla Powerwall is something that looks & sounds wonderful if you're Ed Begley Jr. But who knows, 20 years ago rooftop PV was out of reach economically for most, but the cost did eventually come down as adoption rose.

And yeah, I love my generator too much to fry it in that manner.

[inetdog]

Was very much joking with someone about turning on the generator and then having the inverter try and match the phase, so the generator would have little to no load on it while the PV would do most of the work. But my EE background says that something would go "boom" in technical speak.

Here is the technical details, since you have the background for it:

A GTI (at least the current crop of consumer models) will try to deliver into the grid all of the incoming power from the solar panels. It has no sliding control, it is on or it is off.
The amount of power it will deliver will vary based on weather, time of year (tilt) and time of day.
As long as your local loads are demanding all that the GTI can deliver, plus a little more to keep the generator happy, all will be well. The GTI will match the frequency and voltage of the grid as long as they are stable and within the programmed limits.

The crunch comes when your local load drops below the current GTI output. The GTI will happily try to feed energy into the generator. The generator will not like that, and depending on its circuitry, the design of its voltage regulation, etc. it will either blow up the regulator or try to get the generator to spin faster than the governed engine speed.

No good will come of it.
Same if you try to simulate the grid with a small off-grid inverter or UPS.

There are some systems designed for use outside the US which walk the narrow line of feeding power back into the "grid", but they are also expensive and not available here.


And finally a true hybrid system with all of the bells and whistles has all of the controls needed to coexist with grid, no grid, and generator taking the place of a grid. But it will not use any of the components you already have except the panels themselves.

[Burningislove]

Here is the technical details, since you have the background for it:

A GTI (at least the current crop of consumer models) will try to deliver into the grid all of the incoming power from the solar panels. It has no sliding control, it is on or it is off.
The amount of power it will deliver will vary based on weather, time of year (tilt) and time of day.
As long as your local loads are demanding all that the GTI can deliver, plus a little more to keep the generator happy, all will be well. The GTI will match the frequency and voltage of the grid as long as they are stable and within the programmed limits.

The crunch comes when your local load drops below the current GTI output. The GTI will happily try to feed energy into the generator. The generator will not like that, and depending on its circuitry, the design of its voltage regulation, etc. it will either blow up the regulator or try to get the generator to spin faster than the governed engine speed.

No good will come of it.
Same if you try to simulate the grid with a small off-grid inverter or UPS.

There are some systems designed for use outside the US which walk the narrow line of feeding power back into the "grid", but they are also expensive and not available here.


And finally a true hybrid system with all of the bells and whistles has all of the controls needed to coexist with grid, no grid, and generator taking the place of a grid. But it will not use any of the components you already have except the panels themselves.

Thanks! The nerd in me appreciates the detail.

[SunEagle]

This is along the lines of what I was thinking (Change Controller or SMA). 1500W is more than what I need during an outage, just charging the dridge and some electronics is all I need. For now, Tesla Powerwall is something that looks & sounds wonderful if you're Ed Begley Jr. But who knows, 20 years ago rooftop PV was out of reach economically for most, but the cost did eventually come down as adoption rose.

And yeah, I love my generator too much to fry it in that manner.

While the SMA GTI has the ability to utilize up to 1500 watts of your pv array if the grid goes down the bad news is that without continuous sunshine even that power supply will fluctuate. That is why utilizing a battery system or generator is the only way to provide your loads a stable output.

While you and others might understand the need and actions required to isolate a house panel from the grid by opening the main CB, the vast majority would either not know or forget to perform the required isolation. That is why all GTI's have the isolation safety feature built in without a way to go around it expect to power an isolated receptacle the way that SMA provides. .

[lkruper]

Here is the technical details, since you have the background for it:

A GTI (at least the current crop of consumer models) will try to deliver into the grid all of the incoming power from the solar panels. It has no sliding control, it is on or it is off.
The amount of power it will deliver will vary based on weather, time of year (tilt) and time of day.
As long as your local loads are demanding all that the GTI can deliver, plus a little more to keep the generator happy, all will be well. The GTI will match the frequency and voltage of the grid as long as they are stable and within the programmed limits.

The crunch comes when your local load drops below the current GTI output. The GTI will happily try to feed energy into the generator. The generator will not like that, and depending on its circuitry, the design of its voltage regulation, etc. it will either blow up the regulator or try to get the generator to spin faster than the governed engine speed.

No good will come of it.
Same if you try to simulate the grid with a small off-grid inverter or UPS.

There are some systems designed for use outside the US which walk the narrow line of feeding power back into the "grid", but they are also expensive and not available here.


And finally a true hybrid system with all of the bells and whistles has all of the controls needed to coexist with grid, no grid, and generator taking the place of a grid. But it will not use any of the components you already have except the panels themselves.

I feel stupid for asking this question, but how does the grid absorb power from residential solar? Does the voltage increase a little bit? When I hear the grid spoken of, it almost takes on the characteristic of a personal entity

[inetdog]

I feel stupid for asking this question, but how does the grid absorb power from residential solar? Does the voltage increase a little bit? When I hear the grid spoken of, it almost takes on the characteristic of a personal entity

That is the Grid, not the grid.

The GTI is a current source, so it does not inherently need to raise the voltage above what it sees coming in. And the grid can be considered for this purpose to be an infinite power source and sink with near zero series impedance in the equivalent circuit.

As a practical matter, just as the voltage at a piece of equipment drawing 20A will be lower at its terminals than the voltage at the main disconnect (simple IR voltage drop in the wiring) the voltage at the inverter terminals when it is delivering 20A will be that same amount higher than at the main disconnect because the current is flowing in the opposite direction relative to the voltage.
The voltage at the main disconnect will rise too, based on the service wire and transformer impedance.

The grid will mix that power with the power coming from its generators and feeding all of the network loads. To the extent that the GTI is supplying power the POCO generator will produce that much less. (Or the grid voltage will rise and some loads will take more power to take up the slack.)

[Naptown]

I feel stupid for asking this question, but how does the grid absorb power from residential solar? Does the voltage increase a little bit? When I hear the grid spoken of, it almost takes on the characteristic of a personal entity

it is a personal entity and to some an evil one.What needs to be balanced is the need for clean carbon free energy and the reality of the fact that it ain't gonna happen with solar or wnd.
Until storage becomes inexpensive and plentiful we are stuck with burning coal or other fuels for electricity.
Now if you want a carbon free fuel that is plentiful think nuclear. But the problem is nobody wants to build them any where near where they live.
And if you think about it solar is actually thermonuclear energy. only difference is the reactor is 93 million miles away.

[lkruper]

That is the Grid, not the grid.

The GTI is a current source, so it does not inherently need to raise the voltage above what it sees coming in. And the grid can be considered for this purpose to be an infinite power source and sink with near zero series impedance in the equivalent circuit.

As a practical matter, just as the voltage at a piece of equipment drawing 20A will be lower at its terminals than the voltage at the main disconnect (simple IR voltage drop in the wiring) the voltage at the inverter terminals when it is delivering 20A will be that same amount higher than at the main disconnect because the current is flowing in the opposite direction relative to the voltage.
The voltage at the main disconnect will rise too, based on the service wire and transformer impedance.

The grid will mix that power with the power coming from its generators and feeding all of the network loads. To the extent that the GTI is supplying power the POCO generator will produce that much less. (Or the grid voltage will rise and some loads will take more power to take up the slack.)

Thanks, way over my head but I recognize that impedance is like resistance in an AC system. So if there is very low impedance and infinite power in the grid, why are they worried that in placed like Hawaii where there is lots of residential PV that it can potentially destabilize the grid?

[inetdog]

Thanks, way over my head but I recognize that impedance is like resistance in an AC system. So if there is very low impedance and infinite power in the grid, why are they worried that in placed like Hawaii where there is lots of residential PV that it can potentially destabilize the grid?

Good question.
1. The grid in Hawaii is smaller than any US grid. Much smaller because of the existence of regional interconnects.
2. 10kW from a PV system is small compared to the overall grid. But in HI the economics have encouraged PV, and in particular grid tie, installation to the point that at solar noon the peak power of all of the solar PV is close to 50% of the consumed power. That can be destabilizing when they all go dark at the same time.

[Amy@altE]

This is along the lines of what I was thinking (Change Controller or SMA). 1500W is more than what I need during an outage, just charging the dridge and some electronics is all I need.

What you can do is called DC Coupling. Morningstar Corp has a new 600v charge controller that you can install between your inverter and your solar panels. When the grid is up, it just passes the power through to your inverter, same as usual. When the grid goes down, you can flip a switch and the panels will charge a battery bank. So you can use your some of your existing panels to charge the batteries. If you only have small loads, you can have a small battery bank and small off grid inverter, controlling the price. Go to a loads list calculator and off grid calculator to figure how big of a battery bank you need.

[Burningislove]

it is a personal entity and to some an evil one.What needs to be balanced is the need for clean carbon free energy and the reality of the fact that it ain't gonna happen with solar or wnd.
Until storage becomes inexpensive and plentiful we are stuck with burning coal or other fuels for electricity.
Now if you want a carbon free fuel that is plentiful think nuclear. But the problem is nobody wants to build them any where near where they live.
And if you think about it solar is actually thermonuclear energy. only difference is the reactor is 93 million miles away.

I would welcome (another) nuclear facility near me. Already have two, Pilgrim & Seabrook, and they reliably produce about 1/3 of the regions power. While I love rooftop solar for what it's designed to do, I'd love to see more coal plants like Salem (one of the Filthy Five in the US) go offline and be replaced with nuclear.