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I'd also like to understand the reason for this restriction on whole house power when the grid is down but the sun is shining. Does anyone know why that might

I'd also like to understand the reason for this restriction on whole house power when the grid is down but the sun is shining. Does anyone know why that might be the case?

zkms 8y ago
Look up "anti-islanding". If the grid is down, you can do whatever the heck you want with your own wiring, but back-feeding power into the grid is a very firm no-no. > Islanding can impact utility asset integrity. For example, these conditions can interfere with manual or automatic reclosing, or loop feed automatic switching on the radial distribution system. Utility assets incorporated into or reconnecting to an island with abnormal voltage or frequency conditions may result in extensive equipment damage, for example, a line recloser damaged by reclosing out of phase or lightning arrestor damaged due to abnormally high voltages. > Public safety risk may increase on delta connected systems. For example, a downed wire can remain energized after a device opens to isolate a fault. Depending on the location of the DG installation and the impedance of the downed wire, there may not be sufficient fault current or voltage deviation to trip a generator offline resulting in power being provided to the downed wire.
sathackr 8y ago
Most solar systems are grid-tie, in that they synchronize their output to your local electric grid, and feed any excess power into that grid. They do this by increasing their voltage output until all power generated by the panels is consumed, either by your house or by the attached grid. Because of the way these systems are designed, they do not work well when the grid is not present. If grid power is down, and your solar system feeds power into your home electrical system, you will also be 'backfeeding' power into the grid. Since you likely cannot possibly generate enough power to satisfy your entire neighborhood's power usage, this will in effect overload your system. There is a second safety issue of feeding voltage into a grid system that the power workers are expecting to be dead. Electrical code requires these grid-tie systems to cease feeding power into the grid if the grid power is removed. Most of these systems do this by just shutting down -- unlike backup generators with auto-transfer switches -- they usually do not have an automated method to disconnect the grid power while still powering your house. If you were to provide some sort of disconnect, due to the nature of these systems, they would try to 'push' all available power into your house, because they're designed to force all available solar power into the electrical system. The grid can absorb this, but generally your home cannot. They do have models that can function off-grid, and even models with attached batteries to handle surge loads and night-time operation, but these models are not as widely installed. Likely due to cost and/or owner education.
- deleted 8y ago
  [deleted]
- ballenf 8y ago
  Would a generator be able to supplant the grid? In effect have an auto-transfer but to the diesel/natural gas/propane generator? The generator would be idling except when power needs exceeded solar panels' capacity. Might feel like a step backwards in terms of environmental impact, but presumable the fuel used would be minimal while the sun was shining or batteries not empty.
  - deleted 8y ago
    [deleted]
  - sathackr 8y ago
    This is something I struggled to understand when my system was installed. With the normal grid-tie systems, all of the energy produced by the panels must be consumed. If you are producing more solar energy than you are consuming, while running on generator, the inverter will still attempt to force that energy into your home. The generator will take the brunt of it and will likely be damaged or destroyed. They do have systems that can take inputs from Solar, Generator, Grid, and Battery and use the best available option. But they are significantly more expensive than the typical grid-tie systems.
  - gwbas1c 8y ago
    At this point, it's probably cheaper or simpler just to use a bigger generator or a battery.
  - stephen_g 8y ago
    Unless you undersize your solar so it can never put out more power than your house uses, you need something to absorb or dissipate excess energy - either a battery and/or a STATCOM or similar device.
  - ridgeguy 8y ago
    > Would a generator be able to supplant the grid? Yes. Our neighbor across the street depends on a respirator. We live in a rural Bay area location that has grid dropouts several times a year. Some last a few seconds, some more than a day. His setup: A transfer switch operates ~ 3 seconds after grid failure, followed ~ 2 seconds later by autostart of a propane generator. His house stays on generator power until the grid comes back up for at least 5 continuous minutes. Then the transfer switch flips back to the grid and the generator shuts off. Why propane? Because gasoline slowly degrades (polymerization) and needs to be drained, disposed of, and replaced with new (typically every year or so). Propane doesn't degrade over time.

I'd also like to understand the reason for this restriction on whole house power when the grid is down but the sun is shining. Does anyone know why that might be the case?

zkms 8y ago

Look up "anti-islanding". If the grid is down, you can do whatever the heck you want with your own wiring, but back-feeding power into the grid is a very firm no-no. > Islanding can impact utility asset integrity. For example, these conditions can interfere with manual or automatic reclosing, or loop feed automatic switching on the radial distribution system. Utility assets incorporated into or reconnecting to an island with abnormal voltage or frequency conditions may result in extensive equipment damage, for example, a line recloser damaged by reclosing out of phase or lightning arrestor damaged due to abnormally high voltages. > Public safety risk may increase on delta connected systems. For example, a downed wire can remain energized after a device opens to isolate a fault. Depending on the location of the DG installation and the impedance of the downed wire, there may not be sufficient fault current or voltage deviation to trip a generator offline resulting in power being provided to the downed wire.

sathackr 8y ago

Most solar systems are grid-tie, in that they synchronize their output to your local electric grid, and feed any excess power into that grid. They do this by increasing their voltage output until all power generated by the panels is consumed, either by your house or by the attached grid. Because of the way these systems are designed, they do not work well when the grid is not present. If grid power is down, and your solar system feeds power into your home electrical system, you will also be 'backfeeding' power into the grid. Since you likely cannot possibly generate enough power to satisfy your entire neighborhood's power usage, this will in effect overload your system. There is a second safety issue of feeding voltage into a grid system that the power workers are expecting to be dead. Electrical code requires these grid-tie systems to cease feeding power into the grid if the grid power is removed. Most of these systems do this by just shutting down -- unlike backup generators with auto-transfer switches -- they usually do not have an automated method to disconnect the grid power while still powering your house. If you were to provide some sort of disconnect, due to the nature of these systems, they would try to 'push' all available power into your house, because they're designed to force all available solar power into the electrical system. The grid can absorb this, but generally your home cannot. They do have models that can function off-grid, and even models with attached batteries to handle surge loads and night-time operation, but these models are not as widely installed. Likely due to cost and/or owner education.

deleted 8y ago
[deleted]
ballenf 8y ago
Would a generator be able to supplant the grid? In effect have an auto-transfer but to the diesel/natural gas/propane generator? The generator would be idling except when power needs exceeded solar panels' capacity. Might feel like a step backwards in terms of environmental impact, but presumable the fuel used would be minimal while the sun was shining or batteries not empty.
- deleted 8y ago
  [deleted]
- sathackr 8y ago
  This is something I struggled to understand when my system was installed. With the normal grid-tie systems, all of the energy produced by the panels must be consumed. If you are producing more solar energy than you are consuming, while running on generator, the inverter will still attempt to force that energy into your home. The generator will take the brunt of it and will likely be damaged or destroyed. They do have systems that can take inputs from Solar, Generator, Grid, and Battery and use the best available option. But they are significantly more expensive than the typical grid-tie systems.
- gwbas1c 8y ago
  At this point, it's probably cheaper or simpler just to use a bigger generator or a battery.
- stephen_g 8y ago
  Unless you undersize your solar so it can never put out more power than your house uses, you need something to absorb or dissipate excess energy - either a battery and/or a STATCOM or similar device.
- ridgeguy 8y ago
  > Would a generator be able to supplant the grid? Yes. Our neighbor across the street depends on a respirator. We live in a rural Bay area location that has grid dropouts several times a year. Some last a few seconds, some more than a day. His setup: A transfer switch operates ~ 3 seconds after grid failure, followed ~ 2 seconds later by autostart of a propane generator. His house stays on generator power until the grid comes back up for at least 5 continuous minutes. Then the transfer switch flips back to the grid and the generator shuts off. Why propane? Because gasoline slowly degrades (polymerization) and needs to be drained, disposed of, and replaced with new (typically every year or so). Propane doesn't degrade over time.