Why can't I use my solar system during a grid failure?

[JJR]

My Sunrun system in CT must shut down during a grid power failure to protect utility workers that may be working on the power lines, even though a gas-powered generator would be isolated from the grid via an automatic transfer switch. Is this due to the design of the individual solar system, or to state and/or power company requirements?

[PVAndy]

My Sunrun system in CT must shut down during a grid power failure to protect utility workers that may be working on the power lines, even though a gas-powered generator would be isolated from the grid via an automatic transfer switch. Is this due to the design of the individual solar system, or to state and/or power company requirements?

Your inverter is grid interactive meaning it requires the grid to set it's output voltage and frequency. You can created a micro grid using batteries and an inverter to create a simulated grid which will allow your system to operate during a grid outage. In Conn it would required a revised interconnect application showing the batteries & inverter set to a non export configuration

Andy

[SunEagle]

A grid tied pv system will shut down anytime the grid is down. The reason is too keep someone from feeding power back onto the grid and not electrocute them.

All standard grid tie inverters are designed this way. I hybrid inverter can switch over to charging a battery system but they cost a lot more than the standard inverter.

[solardreamer]

My Sunrun system in CT must shut down during a grid power failure to protect utility workers that may be working on the power lines, even though a gas-powered generator would be isolated from the grid via an automatic transfer switch. Is this due to the design of the individual solar system, or to state and/or power company requirements?

Are you are asking why your grid-tied solar system needs to be isolated and not just work with your generator EVEN after the automatic transfer switch has disconnected your home from the grid? If so, read on.

Conventional grid-tied solar systems must remain isolated even after the automatic transfer switch has disconnected your home from the grid so your generator won't get damaged. Generators, unlike the grid, cannot absorb excess power from the grid-tied solar systems when there is not enough loads to consume all grid-tied solar power. So, if a generator is connected to a conventional grid-tied solar system then it will likely get damaged unless there is some other mechanism to absorb excess grid-tied solar power which can vary frequently with changing loads and solar energy. I have not seen any solution that could do this until recently in this video https://youtu.be/jzSpzCkQl6k.

[JJR]

Thanks all.

To Solar Dreamer: No that's not what I was asking. I should have bee more specific but was trying to be concise. I should have asked: If an automatic transfer switch can isolate the grid for a generator, why can't a similar system be used for a solar system.

To Andy: Sunrun told me that they can't presently retrofit a battery system, as that system is designed differently than mine, but that they are working on a solution. Your answer seems to imply that a 3rd party could do it. That would probably void my Sunrun warranty.

[solarix]

1) A puny little solar system cannot backfeed the grid during an outage. The grid is such a monumental load, a grid-tied solar system will be just totally overloaded with its outputs essentially shorted and will just shutdown. There is NO chance of it electrocuting anyone. The practice of having an AC disconnect is just an OSHA safety concept with no basis in reality. Of course, the utilities are only too happy to require OSHA mandated stuff which drives up the costs on their competition.

2) A grid-tied solar system is a voltage following, current source circuit that simply generates as much current as it possibly can at whatever voltage the grid is at any instant. It can't work without a grid and it can't function on its own and it can't function without a low impedance load which can absorb all that current.

Grid-tie solar is what kicked off the solar revolution because it is highly efficient at harvesting solar PV power, is highly reliable, and low cost - all a result of eliminating the need for batteries. The down side is grid-tied systems won't function without a grid. The only exception is the SMA Sunnyboy inverter that can change its mode of operation and power a very limited load by converting the DC array power directly to AC for driving a single 120Vac receptacle. Providing more off-grid power than that requires batteries and a lot more supporting equipment and will generally double the cost of your grid-tied system.

[SunEagle]

1) A puny little solar system cannot backfeed the grid during an outage. The grid is such a monumental load, a grid-tied solar system will be just totally overloaded with its outputs essentially shorted and will just shutdown. There is NO chance of it electrocuting anyone. The practice of having an AC disconnect is just an OSHA safety concept with no basis in reality. Of course, the utilities are only too happy to require OSHA mandated stuff which drives up the costs on their competition.

.....

So I guess you wouldn't care if you were holding a "dead" grid wire when I turn on my solar pv system?

Remember it isn't just one pv system tied to the grid it can be many which will pump back electricity above the AC voltage that can kill someone.

[solarix]

The trouble with handling a "grid wire" is you never know when the utility power may come back on.
A grid tie inverter is required to have 5 minutes of stable grid voltage before back-feeding power. You can have a whole neighborhood of solar systems and they will all be off within a few AC cycles of the grid going down. Grid-tied inverters can not, can not, can not - energize the grid during an outage.

[SunEagle]

The trouble with handling a "grid wire" is you never know when the utility power may come back on.
A grid tie inverter is required to have 5 minutes of stable grid voltage before back-feeding power. You can have a whole neighborhood of solar systems and they will all be off within a few AC cycles of the grid going down. Grid-tied inverters can not, can not, can not - energize the grid during an outage.

If you say so. I will go with the OSHA rules and just be safer.

[solardreamer]

I should have asked: If an automatic transfer switch can isolate the grid for a generator, why can't a similar system be used for a solar system.

An automatic transfer switch or relay/contact could be used to isolate grid-tied solar system but unnecessary since grid-tied systems already shutdown on their own for grid outages. A transfer switch or relay/contactor with manual reset is often used to isolate grid-tied solar systems in homes that have a transfer switch for generator but doesn't/can't do a line side tap connection for the solar system. It's to ensure isolation between the generator and solar system.

[solardreamer]

1) A puny little solar system cannot backfeed the grid during an outage. The grid is such a monumental load, a grid-tied solar system will be just totally overloaded with its outputs essentially shorted and will just shutdown. There is NO chance of it electrocuting anyone. The practice of having an AC disconnect is just an OSHA safety concept with no basis in reality. Of course, the utilities are only too happy to require OSHA mandated stuff which drives up the costs on their competition.

I agree, the risk of lineman getting electrocuted from grid-tied solar is really low. However, is backfeed prevention the main purpose of the AC disconnect? I always thought the main purpose of the AC disconnect is for maintenance/emergency situations like when you need to work on the solar system or fires while the grid is up.

[solarix]

All grid tied solar systems have Overcurrent Protection breakers (ocp) that can be used for maintenance purposes as well. OSHA however requires a device that is "lock-out/tag-out" so any personnel can be assured there is no chance of being energized. All very well and good, but in the case of grid-tied solar (where energizing the up-stream load is not possible) - an AC disconnecting means is superfluous and in many jurisdictions not required. (which is why most inverters do not include an AC disconnect switch as a standard feature) In our area, I can tell you for sure an AC disconnect is only a utility requirement prompted by OSHA rules. And it causes a lot of grief sometimes. We've had to run wiring from the back of the building where the solar and inverter are - to the front of the building where the disconnect is "accessible in an emergency" by the utility - and then back to the rear of the building where the breaker panel is located.
An automatic transfer switch could definitely serve to disconnect a solar system, but is way more expensive and still doesn't provide the visibly locked out capability.

[sdold]

2) A grid-tied solar system is a voltage following, current source circuit that simply generates as much current as it possibly can at whatever voltage the grid is at any instant. It can't work without a grid and it can't function on its own and it can't function without a low impedance load which can absorb all that current.

Does this mean that there is nothing in a grid tie inverter that can generate a sine wave on its own? That it can only source current and simply follows the input voltage? I've always wondered if they worked this way, instead of generating its own sine wave and somehow matching its phase to the incoming power.

[solardreamer]

Does this mean that there is nothing in a grid tie inverter that can generate a sine wave on its own? That it can only source current and simply follows the input voltage? I've always wondered if they worked this way, instead of generating its own sine wave and somehow matching its phase to the incoming power.

No, a grid-tied inverter generates its own sine wave but it will only do so if it senses proper grid sine wave on the output. In general, a grid-tie inverter will synchronize its sine wave to the grid sine wave. However, a grid-tied inverter will periodically generate sine wave that leads or lags the grid sine wave for anti-islanding detection purposes.

[appliedthinking]

No, a grid-tied inverter generates its own sine wave but it will only do so if it senses proper grid sine wave on the output. In general, a grid-tie inverter will synchronize its sine wave to the grid sine wave. However, a grid-tied inverter will periodically generate sine wave that leads or lags the grid sine wave for anti-islanding detection purposes.

I would like to know a little more about "anti-islanding". I have two SunnyBoy inverters that I have set up for SPS power. SunSpecs module will not support this.
Reading Grid Emulation documents, leads one to believe if there is a stable (.5-1hz) frequency for 5 min, the grid tie inverter will operate in a non-grid tied mode. I have not tested this.
This thread is good reading material.
Best Regards