Limit to Solar Power back into grid at residence

[chrisski]

Does a limit exist to the amount of solar power production that can be pushed back into the grid? When I google this, I don’t really see an answer, more of advertisements and a few paragraphs on how solar power ties feeds back into the grid system.

It seems to me its a great idea And a huge cost savings to push excess solar production Back into the grid. However, there has to be some practical limit to this before the transformers get overwhelmed, which here in sunny AZ, I expect would be in winter when my electricity bill is $60 a month and there would be a lot of excess power put back into the gird versus when it is summer and my electric bill is closer to $500 a month, and panels would produce less power than I use.

Every couple of months, another house on the block has solar panels going up. In time nearly every house on my block that’s fed by that one transformer will be pushing power back into the grid, and if this transformer is overwhelmed, I’m not sure it pushes power from 110 VAC back up to larger voltages, so I figure there has to be some limit on power production that the power company would want to tap into inverters and shut them off or limit what is being fed back.

In AZ, lower income houses with solar panels are non-existent, but upper income houses can approach 30%, and I’m in between that. At least that data was a couple years old. As more and more panels go on roofs, I would think at some point where we’re getting close to using this green energy, there’s got to be some limits to it since the grid probably can’t react to a cloud going overhead and the power plant instantly making up for the loss in power production from a cloud going overhead.

My gut tells me, that an electric company would want no more than 30% power total from solar energy to avoid brown outs from cloudy days. For me if this is true, I could see five years down the road with all the houses having solar, if I did get a grid tied system that I might be limited on the 8 months a year my AC is not running full time about how much power to push back into the grid.

[SunEagle]

I believe each POCO may have a specific requirement or limitation on what someone can Co-generate from solar back onto the Grid. Instead of trying to find that on Google I would contact your POCO.

[bob-n]

Yes, you bring up many good questions. For most communities, when the grid was installed, solar was not considered.

When anyone applies for a permit for netmetering, the power company is supposed to assess the questions you raise - transformer rating, wire current capacity, etc. In theory, when the installed solar base gets high enough, they will reconfigure the grid.

There are planned communities where solar is considered and even some cases where solar is mandated. They can budget the wire and transformers more optimally in those cases.

You mention Arizona. I suspect that a lot of energy demand in Arizona is for air conditioning, which is a heavier burden in daylight than at night. Solar is a good mate to that situation. The same is true for many large office buildings, with many rows of fluorescent or LED lights and HVAC running during the day, but no occupancy at night.

Another question you mention is handling the situation of solar sometimes being unavailable, such as in a storm or heavy cloud cover. Either they need excess generation capacity, the ability to draw from neighboring states with excess capacity, or energy storage. Large scale energy storage is feasible in terms of pumping water upstream and later using it for hydro power. I've read that the overall efficiency of that storage system can be 60% to 70%. Arizona has two huge hydro facilities: the Glen Canyon Dam and Hoover Dam. More on Arizona hydro at: https://www.eia.gov/state/analysis.php?sid=AZ

[bob-n]

You made one statement: "...I’m not sure it pushes power from 110 VAC back up to larger voltages..."

Yes, home grid-tied solar will push power back up the grid, all the way to the high voltage lines. Power transformers are completely bi-directional. They may not be optimal in reverse, but they are fully functional in reverse, within their limits.

You may have heard that there are different rules for homes with >10kW solar. When homes install larger production capacity, the power company studies it more carefully and may have to change some of their hardware to enable the installation. A typical home has a 200A load center, some smaller, some larger. 200A * 240V = 48kW. A typical home solar installation is 5kW. It's really rare to hear of a home solar installation >15kW. That gap gives some breathing room for grid design, but the concerns you raise are still valid.

[solarix]

Here in Arizona, the big utilty APS now has a policy of limiting new solar systems to 150% of your existing peak demand. Its a problem - as the rate plans are biased against solar (you buy from them at 25cents plus taxes and fees and sell to them at flat rate of 10.4 cents) so we have to seriously oversize a system to break even which is limited by this system size rule.

APS is currently about 10% solar with about half of that being distributed rooftop. The company is now committed to being 100% renewable (they count nuclear as being 50% of that) by 2050. The general assessment is that when solar gets above 15%, there could start to be stability problems which is why our industry is working so hard on a good storage solution. We've started to install the now available SMA system using 10kWh of BYD batteries but at $16k installed its not exactly cost effective.

In my neighborhood, there are 9 of 18 homes with solar and during the midday I see the line voltage climb - I even had to up my wire size from the inverter located on the opposite side of the house so it would not trip off when the AC voltage got too high....

In Hawaii, the POCO no longer allows you to back feed the grid, as some neighborhoods are 100% solar. Hawaii has become the testing and proving ground for battery storage systems as a result. KWh prices there are so high (virtually all POCO generation is from diesel generation) that solar is a no brainer but you need to store your excess yourself.

[bcroe]

You made one statement: "...I’m not sure it pushes power from 110 VAC back up to larger voltages..."

Yes, home grid-tied solar will push power back up the grid, all the way to the high voltage lines. Power transformers are completely bi-directional. They may not be optimal in reverse, but they are fully functional in reverse, within their limits.

You may have heard that there are different rules for homes with >10kW solar. When homes install larger production capacity, the power company studies it more carefully and may have to change some of their hardware to enable the installation. A typical home has a 200A load center, some smaller, some larger. 200A * 240V = 48kW. A typical home solar installation is 5kW. It's really rare to hear of a home solar installation >15kW. That gap gives some breathing room for grid design, but the concerns you raise are still valid.

This was a 70s All Electric home, and has its own transformer apparently original
(judging from the rust color). It has been converted back to all electric with a solar
system of 15KW peak power. To the benefit of the PoCo, the peak power is pretty
much flat the entire time sun hits the panels. It generates in summer, consumes in
winter, and manages to have some net metering surplus energy.

For places like AZ the direct solar power heat pumps should be a good upgrade. However
they usually have line backup, so clouds will still throw a load onto the PoCo. Perhaps
not nearly as much a load, because clouds will cause them to throttle down, and not all
in unison. That does not work here, because the biggest energy consumption is heating
with heat pumps. Bruce Roe

[Ampster]

.........
Yes, home grid-tied solar will push power back up the grid, all the way to the high voltage lines. Power transformers are completely bi-directional. They may not be optimal in reverse, but they are fully functional in reverse, within their limits.
........

The biggest issue is the measurement and control systems for balancing energy among subgrids. The transformers can handle bidirectional flow efficiently but the measurement and control systems were only design for one way flow. The utilities have a term for this and it is one of the important upgrades that needs to take place to address the reality of more rooftop solar penetration.

[inetdog]

The National Electrical Code allows you to push back as much power as the service is rated to deliver. On a 200A service you could, by Code, deliver as much as 200A at 240V.
But, for all the reasons of network stability, financial incentive and state law mentioned, each power company will set, in its tariffs, the maximum power your system can feed back into the grid. You need to find the right person there to answer your question.

[peakbagger]

I have been involved with various cogeneration systems installations that normally operate in parallel with the grid. These are for businesses and healthcare facilities that can buy natural gas and generate power plus supply their heating and cooling demands for less cost than they buy power from the utility. for various reasons these plants have the technical capability to sell power to the grid. In some cases 6 MWs (6000 KWs). Before these plants can be connected to the grid, they have to go through an interconnect process. In the state we do our work in, the customer has to pay the utility to study the potential problems with selling power to the grid and then pay if the grid needs upgrade. This is an expensive time consuming process. The utility is required to complete the process in 18 months from the start of application. They suspend the "time clock" every time they ask a question and are waiting for a reply so it can and does go longer. The customer trying to sell power to the grid has to pay for any impacts the electric power system. In some cases the impacts to the system are significant to the point that the economics do not pencil out. This happens frequently in a rural area where wind farms get built. This is currently the issue in Maine where there is a fairly good resource (for the east coast) and there is huge demand for renewable power. There are numerous projects ready to go but the regional transmission lines are too small or too far away. The cost to put in this major infrastructure is in the 100s of millions, thus the plants do not get built. The wind turbine firms hire lobbyists and are always trying to get some government to pay to upgrade the grid and on occasion they are successful.

The statement was made that the grid and its equipment is bi-directional. Sadly that is not true, many of the electrical utility substations were built long ago when power only flowed from the grid to the customer. There are numerous protective systems hard wired into these substations for one way flow of power. The way the customers are connected to the substation also vary. A large user may have their own circuit fed from the substation but the common approach is many users are fed from one circuit. That means that what one customer does can impact others on the circuit and unintended consequences can occur. The utility interconnection study has to model the system and then investigate for every possibility..

Prior to the solar era, the utilities would force this process on every small producer. The utility did not want the competition and rarely did the cost for the study and the resultant upgrades to be paid for by the customer offset the potential revenue so the project did not get built or was built off grid. In order to encourage small scale solar various legislation had to be passed. Net metering gets all the press but inevitably along with that high profile legislation is a waiver or vast simplification of the interconnect process for small producers. The threshold is frequently 10 KW. Anything less than 10 KW, the utility is forced to accept solar, anything over they can require an interconnect process.

Hawaii is the "guinea pig" for large penetration of solar, they have a small grid designed for one way power, the power is quite expensive as much of is produced by diesel generators. They have had numerous brown and black outs tied to the interaction of solar and their generators. Typical grid tied inverters are designed to disconnect from the grid if the grid power quality falls out of tight range of specs, With a lot of solar on the gird on sunny day the big generators are idling. Everything is fine until a cloud come over the island. The amount of solar going into the grid drops quickly forcing the diesels to drastically increase their power output. This doesn't happen instantly so the voltage and frequency can droop. The older style inverters see this droop as a problem with the grid and all the inverters that see the droop drop off the grid for a minimum of five minutes which puts even more load on the system. After some fits and starts the utility finally required the installation of inverters that dont need to drop off the grid when the power droops. There are new UL rating for inverters with this capability called 1741 SA. Many states now require this capability but there needs to be upgrades and management protocols on the utility side of the grid so many of these 1741 SA units despite having the capability are still acting like the old "dumb" units.

[Ampster]

.......

The statement was made that the grid and its equipment is bi-directional. ........

I may have been one of the ones making that statement but I said, "measurement and control systems are not bidirectional. The transformers can handle bidirectional flow efficiently but the measurement and control systems were only design for one way flow."

I think we are saying the same thing. The industry term is "command and control" (C2) and that is one of the most important upgrades that needs to be prioritized in states like California and Hawaii. Those two states have taken UL1741 and added a few things in the past 3 years.

The big issue is who is going to pay for it. To me in California solar customers should pay their fair share. Perhaps part of it can be paid for by DER resources. There are actually times in California that the grid or segments of it need load. I know a guy in Vermont who has a free Tesla Powerwall which is controlled by the grid operator in aggregate with many others. Those kind of systems can help make the grid more resilient. California SGIP program caused a lot of Tesla Powerwals and one other systems to be installed. If those ever get aggregated they could have a big impact on the resiliency of the grid.