Electric Knowledge Needed Please

dave1949 · Aug 1, 2012

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 · Aug 1, 2012

dave1949 said:
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 · Aug 1, 2012

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 · Aug 1, 2012

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 · Aug 1, 2012

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 · Aug 1, 2012

Marcle said:
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 · Aug 1, 2012

dave1949 said:
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 · Aug 1, 2012

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 · Aug 1, 2012

tcartwri said:
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.