Electric Knowledge Needed Please

[dave1949]

I am getting to know my grid-tied solar electric system and have a question.

The inverter sends its output to the buss bars of the service panel through a backfeed breaker, the utility power is applied to the buss bars in the same panel.

My question is, what determines when/if power from the inverter is used versus power from the utility? I don't know the electric theory behind this and I wish I understood it a little bit at least.

The solar system seems to be performing just fine, so this isn't a trouble shooting question. As of 6:30pm yesterday, my total solar generation was 76 kwh in 82 hours of operation and my net meters read 10 kwh in from the utility and 21 kwh out from the solar array. I had a 19kwh solar day yesterday, the best yet.

Thanks.

 

[Marcle]

I am getting to know my grid-tied solar electric system and have a question.

The inverter sends its output to the buss bars of the service panel through a backfeed breaker, the utility power is applied to the buss bars in the same panel.

My question is, what determines when/if power from the inverter is used versus power from the utility? I don't know the electric theory behind this and I wish I understood it a little bit at least.

The solar system seems to be performing just fine, so this isn't a trouble shooting question. As of 6:30pm yesterday, my total solar generation was 76 kwh in 82 hours of operation and my net meters read 10 kwh in from the utility and 21 kwh out from the solar array. I had a 19kwh solar day yesterday, the best yet.

Thanks.

Okay I'm the other side the pond to you, so some bits may be different, so bare with me.

First of all my electric charging meter used (tell you more about this later) to be a disc type where you can watch the disc rotate when you consume power. If its an older type, there is no stop on the disc, so it rotates one way when you consume power from the grid, however if you have PV panels or a wind turbine, when you generate more power than you actually use in the home, spare power flows back in to the grid and makes the disc turn backwards. Think of it as two water supplies, one from the water supply company, and your own spring. If your spring is dry, you consume water from the SC. Then when your spring is gushing, you can push water to the SC that you don't use. In reality, water is not connected this way, but you get the idea.

Now... if you have a newer disc meter, it has a stop on the disc, so it only clocks up (measures) power when being consumed from the grid and ignores any you have surplus that get pushed in to the grid.

You will probably have another meter on your PV or wind turbine generator panel which measures total power generated. Whilst your consuming power in the home, your two meters will never match. You can have a situation where you have max output from your generator and the power companies meter stood still as your consumming so much power in the home!

On the meter point. My power company eventually found out my meter was going backwards (so I was getting credits on my bills) as I was using so little electric, they came along and changed the meter so it only clocked up power consumed from the grid, rather than credits as well

 

[Marcle]

Oh, the other thing is that your system will shut down if there's a power cut. Its a safety feature to stop your system putting power in to the grid when its dead, as some sparky may want to work on the supply, only to get a shock (literally) off the system which he's isolated, but your system is still powering the grid, hence it shutting down when the grid is off. It taken several minutes to reconnect after main power has been restored.

 

[lstinthot]

You can think of electricity as oil pressure in a system. The oil will seek the path of least resistance. When your inverters are supplying more power than you can use, your house could be thought of as being at a higher pressure and not letting the electricity to flow in from the grid and it is forcing it out to the grid.

 

[dave1949]

Thanks Marcle, That wasn't nice of the power company to lock you into forward only mode. Do you not have net-metering laws in the UK?

What we have for metering is two meters; one is the "in" meter or what I take from the utility, the other is the "out" meter or what I push to the utility. They are both digital meters, they swapped out our analog disc meter last winter. They pulled our old digital meter, plugged a dual meter base into that meter socket, then plugged in two digital meters.

They are supposed to be "smart" meters that can be remotely read and allow the utility to sense where power is out, and allow customers to see their usage on-line. They haven't gotten the "smart" part working yet.

I am with you on the usage explanation, from looking at the readings, I would express it this way:
my consumption = solar generation + "in" meter reading - "out" meter reading

What I am missing is an understanding of the electrical principle involved when two in-phase power sources are applied to a common conductor, what determines which power source supplies the electrons? It has to be driven by potential difference and I assume the relative reactance of the two power sources is involved somehow. They must see each other as part of the load. That's is as far as my fuzzy understanding takes me.

 

[dave1949]

Oh, the other thing is that your system will shut down if there's a power cut. Its a safety feature to stop your system putting power in to the grid when its dead, as some sparky may want to work on the supply, only to get a shock (literally) off the system which he's isolated, but your system is still powering the grid, hence it shutting down when the grid is off. It taken several minutes to reconnect after main power has been restored.

Yep, the inverters have to pass tests showing they disconnect from the utility when utility power is off. The inverter is internally programmed to wait 5 mins. then look for utility power presence. It loops like that until utility power is restored. It's a shame a grid-tied system is worthless when the power is out, but I suppose it doesn't happen often enough to warrant a fail-safe engineering solution.

The installers also warned me to turn off the backfeed breaker if I use my generator, since many generators can be damaged by sending power TO them.

 

[Ken]

Thanks Marcle, That wasn't nice of the power company to lock you into forward only mode. Do you not have net-metering laws in the UK?

What we have for metering is two meters; one is the "in" meter or what I take from the utility, the other is the "out" meter or what I push to the utility. They are both digital meters, they swapped out our analog disc meter last winter. They pulled our old digital meter, plugged a dual meter base into that meter socket, then plugged in two digital meters.

They are supposed to be "smart" meters that can be remotely read and allow the utility to sense where power is out, and allow customers to see their usage on-line. They haven't gotten the "smart" part working yet.

I am with you on the usage explanation, from looking at the readings, I would express it this way:
my consumption = solar generation + "in" meter reading - "out" meter reading

What I am missing is an understanding of the electrical principle involved when two in-phase power sources are applied to a common conductor, what determines which power source supplies the electrons? It has to be driven by potential difference and I assume the relative reactance of the two power sources is involved somehow. They must see each other as part of the load. That's is as far as my fuzzy understanding takes me.

Would it help to look at as 2 generators running both are providing the power the largest is the sync.(Cycles) controller of the 2 tied together.

ken

 

[tcartwri]

Dave,

The meter on the house will always count the power you are consuming in the house, the meter for the array will count the output from the solar. If you're like me, you will have two seperate accounts. One for the house, and one for the solar. So your household bill should not change, but your solar account will start to accumulate money based on your contracted amount.

 

[dave1949]

Dave,

The meter on the house will always count the power you are consuming in the house, the meter for the array will count the output from the solar. If you're like me, you will have two seperate accounts. One for the house, and one for the solar. So your household bill should not change, but your solar account will start to accumulate money based on your contracted amount.

The solar system was installed a week ago today, the meters were installed Monday, so I haven't seen a bill yet.

My understanding is they apply credits, the reading of the "out" meter, to my bill each month. If I have more credits than needed to cover my monthly usage, they are "banked". Any unused banked credit that is over 12 months old gets tossed out. So, I will never get "paid" other than by subtracting current and banked credits from usage. I will always be charged the monthly minimum connection fee which is currently $8.91, no matter how many credits I have banked.

 

[tcartwri]

Different system than us then! Carry on.

 

[BobRip]

The solar system was installed a week ago today, the meters were installed Monday, so I haven't seen a bill yet.

My understanding is they apply credits, the reading of the "out" meter, to my bill each month. If I have more credits than needed to cover my monthly usage, they are "banked". Any unused banked credit that is over 12 months old gets tossed out. So, I will never get "paid" other than by subtracting current and banked credits from usage. I will always be charged the monthly minimum connection fee which is currently $8.91, no matter how many credits I have banked.

You are using the grid as a battery. $8.91 a month is pretty cheap for an infinite battery. You can get your own battery and be able to operate when the grid is down. When you start paying for this you will really appreciate the grid.

 

[dave1949]

You are using the grid as a battery. $8.91 a month is pretty cheap for an infinite battery. You can get your own battery and be able to operate when the grid is down. When you start paying for this you will really appreciate the grid.

If I understand your comment, yes, batteries for off-grid are a game stopper. They are expensive, they need to be maintained, they need a 70* environment for optimal life and output, and last I calculated, about the time you break even on them, you need a new set.

What I meant about grid-tied systems not working when the power is off, is the reason they will not allow them to be connected is for lineman safety. There are ways to deal with that but the inexpensive methods all allow human error. Other than using the line frequency to clock/sync to the 60 cycles, I think the inverters are capable of working in stand-alone mode.

Having to Start up the generator on a sunny day while looking at solar panels just seems a bit goofy, but how often do you lose power on a sunny day? The storms that took out power across the mid-atlantic area this summer for extended time periods are a once in many years occurrence for any given location. It just isn't worth worrying about or cost-effective to solve unless we go over to the doomsday thread

 

[madmax12]

I am getting to know my grid-tied solar electric system and have a question.

The inverter sends its output to the buss bars of the service panel through a backfeed breaker, the utility power is applied to the buss bars in the same panel.

My question is, what determines when/if power from the inverter is used versus power from the utility? I don't know the electric theory behind this and I wish I understood it a little bit at least.

The solar system seems to be performing just fine, so this isn't a trouble shooting question. As of 6:30pm yesterday, my total solar generation was 76 kwh in 82 hours of operation and my net meters read 10 kwh in from the utility and 21 kwh out from the solar array. I had a 19kwh solar day yesterday, the best yet.

Thanks.

If I understand what you wrote, then my question is this:

When the grid goes out, can you turn off your service panel's MAIN breaker (that would be the power cable from the grid entering your panel)... and use the "backfeed breaker" to power the panel?

The way I read your description, sounds like the cheap (aka: dangerous) way many people create a generator "hookup" for their panel.

 

[dave1949]

If I understand what you wrote, then my question is this:

When the grid goes out, can you turn off your service panel's MAIN breaker (that would be the power cable from the grid entering your panel)... and use the "backfeed breaker" to power the panel?

The way I read your description, sounds like the cheap (aka: dangerous) way many people create a generator "hookup" for their panel.

Nope, the inverter will not meet the UL requirements if it produces power without sensing grid power presence.

For a generator, you can use a backfeed breaker if you have a mechanical lockout slide that prevents the main and generator backfeed breakers from being on at the same time. That is within code, or at least it was 6 years ago.

 

[madmax12]

dave1949, I want to hook up solar panels soon, as well.

I was not aware that if the grid went out, I would not have access to my solar power. I, like u, don't think I'm all that interested in batteries.

I hope u keep this thread alive as you discover your answers... I hope to learn much here too.

Thanks for posting.

 

[dave1949]

dave1949, I want to hook up solar panels soon, as well.

I was not aware that if the grid went out, I would not have access to my solar power. I, like u, don't think I'm all that interested in batteries.

I hope u keep this thread alive as you discover your answers... I hope to learn much here too.

Thanks for posting.

I started with this thread: http://www.tractorbynet.com/forums/related-topics/252839-grid-tied-solar.html

That is where I will post some performance results when enough time has passed to gather some. I know there are several EE's here on TBN and I thought one might explain the question I asked in this thread about the principle of how it works in service panel.

I don't think it is possible to justify batteries or being off the grid unless it is really expensive to get utility power to your home. I like the idea, but I wouldn't want to saddle my wife with battery maintenance and troubleshooting if kicked the bucket, plus it is expensive. Grid-tied is expensive enough as it is and I may not live to see the break-even point 17 or 18 years from now.

 

[madmax12]

Thanks for that link. 17-18 years to recoup your cost... If rates stay the same... How long do the panels last?

I've worked with electrical, but do not have the knowledge u seek. Hopefully someone will come forth with some education. I could be wrong, but I think soundguy chimes in with a lot of electrical knowledge. Maybe if u search for his posts, u could see if it would merit PMing him.

Just ask him to give his responses in a thread, so we can all learn together.

 

[madmax12]

(I know this question is simplistic and not a new idea to u)...

Is there a way to install a transfer switch, like a whole house backup generator has, to kick in when the grid goes out, allowing u to utilize solar?

 

[dave1949]

(I know this question is simplistic and not a new idea to u)...

Is there a way to install a transfer switch, like a whole house backup generator has, to kick in when the grid goes out, allowing u to utilize solar?

I am not aware of any. They could be built though. If a battery operated circuit provided the 60 cycle clocking to the inverter and produced a AC voltage within the "grid voltage present" limits (a small range is allowed to account for variances), then you could "fool" the inverter in thinking there is grid power present. The inverter would have to recharge the battery and of course, it won't do you any good unless there is sunshine.

I think the challenge would include load management while getting the inverter fooled and "booted" into operation. You would need to disconnect all other normal loads or they would be well above what the battery operated circuit could handle, pulling its output voltage down below the inverters range of okay grid voltage levels.

All in all, a generator which works in the dark too is probably the better choice, you would need one anyways to have power overnight. In our winters here "overnight" could be from 3pm to 10am the next morning. You know, almost all people with off-grid systems have a generator around too.

 

[madmax12]

I am not aware of any. They could be built though. If a battery operated circuit provided the 60 cycle clocking to the inverter and produced a AC voltage within the "grid voltage present" limits (a small range is allowed to account for variances), then you could "fool" the inverter in thinking there is grid power present. The inverter would have to recharge the battery and of course, it won't do you any good unless there is sunshine.

I think the challenge would include load management while getting the inverter fooled and "booted" into operation. You would need to disconnect all other normal loads or they would be well above what the battery operated circuit could handle, pulling its output voltage down below the inverters range of okay grid voltage levels.

All in all, a generator which works in the dark too is probably the better choice, you would need one anyways to have power overnight. In our winters here "overnight" could be from 3pm to 10am the next morning. You know, almost all people with off-grid systems have a generator around too.

Excellent points. Thank you.