grid-tied system design questions

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  • solar_davesolar_dave Solar Expert Posts: 2,338 ✭✭✭✭
    Re: grid-tied system design questions
    I'm from egypt and we don't have a net meter agreement to sell power to the government.
    actually I don't want to sell power to the government, I only want to reduce my consumption.
    However, reduction in consumption may result from feeding power to the grid (even if I will not be paid for).
    For example, if I consumed 500 kWh in a month and I produced 100 kWh in the same month so I will be billed for only 400 kWh.
    Let's make it more clear, assume that I'm not at home in the morning and the system is generating energy with no consumption. assume that morning the reading of the meter was 2,500 kWh and the system generated 20 kWh, this means the reading will go down to 2,480 kWh or not??

    I hope I made things clear!

    Thanks

    It depends, if the meter will spin backwards you will see the reduction. That is a big if. Most meters here need to be switched to get that capability. As many have said on this forum later model meters have a feature that prevents the reverse spin of the meter, this prevents flipping the meter upside down to offset consumption. In fact you may be charged for the power returned to the utility.
  • BB.BB. Super Moderators, Administrators Posts: 29,002 admin
    Re: grid-tied system design questions
    I'm from egypt and we don't have a net meter agreement to sell power to the government.
    actually I don't want to sell power to the government, I only want to reduce my consumption.
    However, reduction in consumption may result from feeding power to the grid (even if I will not be paid for).
    For example, if I consumed 500 kWh in a month and I produced 100 kWh in the same month so I will be billed for only 400 kWh.
    Let's make it more clear, assume that I'm not at home in the morning and the system is generating energy with no consumption. assume that morning the reading of the meter was 2,500 kWh and the system generated 20 kWh, this means the reading will go down to 2,480 kWh or not??
    The short answer is "we do not know".

    It is based on the custom/requirements of your local utility and how they program their utility meters.

    In the US, the utilities have addressed power theft by making newer meters that only go forwards. So in your example, you generate 20 kWH, then the meter would read 2,520 kWH and you would be charged for this usage.

    In general, for utility grids, you can feed back (limited) amount of power back into the utility without issue. However, utilities will use "safety" as a reason to now allow GT inverters to be used. In reality, they do not like people to generate their own power--It reduces the amount of money they collect at the end of the month.

    In some cases, utilities that do not allow GT inverters will still allow large facilities with backup generators (such as hospitals) to feed power back into the network for their monthly testing (instead of feeding a large electric heater and wasting the fuel/energy).

    In the US/North America... The typical method to connect a GT inverter is to do the following.

    Assume that a 200 amp main panel can take 20% extra current as solar power from a GT inverter... Or an extra 40 amps.

    Reduce the main breaker to 175 amps, and that gives you 40+25=65 Amps for "legal" GT generated power (branch circuit).

    At the bottom of the bus bars (oposite end of bus bar from main breaker), install your GT branch circuit(s). This ensures that you never have more than the 200 amps flowing any where in the main bus bars (do not install GT branch circuits next to main breaker, then you get main breaker+GT circuits worth of current in the bus bars).

    Then run the branch circuits to the GT solar inverter.

    That is pretty much it. For 3 phase power, your utility may have load balancing rules... For example, if you want 9 kWatts of solar power, you have to put in 3x 3kWatt AC inverters (one on each phase pair of wires).

    Or, you may be allowed a maximum of 10 kWatt per phase, and you can put one 10 kWatt inverter on one phase pair only.

    If you have open Wye service (two transformers feeding your three phase panel)--You may be required to put the 10 kWatt inverter on one of the two "real" phases.

    If you have a requirement to not generate GT solar power if one of the phases fail, you may need a special relay or AC inverters that are three phase aware to shut down if there is a loss of a single phase.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • jonrjonr Solar Expert Posts: 1,266 ✭✭✭✭
    Re: grid-tied system design questions

    > In the US, the utilities have addressed power theft by making newer meters that only go forwards. So in your example, you generate 20 kWH, then the meter would read 2,520 kWH and you would be charged for this usage.

    Not going backwards and charging for backwards current are different things. My understanding is that the latter is very rare.

    I am available for custom hardware/firmware development

  • South AfricaSouth Africa Solar Expert Posts: 295 ✭✭✭
    Re: grid-tied system design questions

    Where I live, if you have the old analogue meters, they have a wheel that turns, they can reverse direction if you generate more than you consume. Works well till you are caught.

    All new meters, and the new favoured pre-paid meters, will charge you for what you use and what you push back onto the grid, as various local solar installation people have discovered over the last few years, at a cost to the client.

    Only way to solve the matter, if you are approved to go grid tie, is to have a new bi-directional meter installed at a additional installation fee plus a monthly connection fee and if you are feeding back to the grid for a fee, an additional fee is charged for reading the meter more than once a month.
  • inetdoginetdog Solar Expert Posts: 3,123 ✭✭✭✭
    Re: grid-tied system design questions
    jonr wrote: »
    > In the US, the utilities have addressed power theft by making newer meters that only go forwards. So in your example, you generate 20 kWH, then the meter would read 2,520 kWH and you would be charged for this usage.

    Not going backwards and charging for backwards current are different things. My understanding is that the latter is very rare.
    Based on the information posted by forum members in that position over the last two years, I would have to say that it is not rare at all but rather more common than the first behavior. For both digital and analog meters.
    SMA SB 3000, old BP panels.
  • minabenyaminminabenyamin Solar Expert Posts: 45
    Re: grid-tied system design questions

    Thanks for you all my dear friends,

    For the problem of the meter, I will contact the factory that manufactures the meters and I will talk with them about that point.

    The point that I'm searching for nowadays is how to connect the inverter to the grid ( grid and inverter have same voltage and frequency). I asked an expert of networks here (but he is not specialized in solar energy), he said that will be something like parallel operation of two generators and they must be typical of eachother and bla bla bla. for now i want a diagram showing how i can connect the inverter to the grid if possible.

    Also, is it possible that the grid can make an overloading on the inverter (sinking very high current from the inverter) leading it to be damaged?

    Thanks
  • inetdoginetdog Solar Expert Posts: 3,123 ✭✭✭✭
    Re: grid-tied system design questions

    It is very simple when you start with a grid tie inverter (GTI); they are designed to have their output connected directly to the mains (through a breaker for safety) and their input connected directly to the PV array. All of the good electronics to synchronize is within the inverter. You cannot do it with an ordinary off grid inverter, although a hybrid inverter can do both.
    SMA SB 3000, old BP panels.
  • minabenyaminminabenyamin Solar Expert Posts: 45
    Re: grid-tied system design questions

    So you mean that I will need only a connection (junction) box with 2 inputs and 1 output. I will connect both Grid and inverter output to the 2 inputs of the box then I will take the output of the box to feed the meter ??
  • inetdoginetdog Solar Expert Posts: 3,123 ✭✭✭✭
    Re: grid-tied system design questions
    So you mean that I will need only a connection (junction) box with 2 inputs and 1 output. I will connect both Grid and inverter output to the 2 inputs of the box then I will take the output of the box to feed the meter ??

    Not really two inputs and one output. Just three connections in parallel. Inverter to breaker to house wiring to breaker to meter to grid.
    But if you do not see that clearly you should not be the one making the connection!
    SMA SB 3000, old BP panels.
  • minabenyaminminabenyamin Solar Expert Posts: 45
    Re: grid-tied system design questions

    The short answer is "we do not know".

    It is based on the custom/requirements of your local utility and how they program their utility meters.

    In the US, the utilities have addressed power theft by making newer meters that only go forwards. So in your example, you generate 20 kWH, then the meter would read 2,520 kWH and you would be charged for this usage.

    In general, for utility grids, you can feed back (limited) amount of power back into the utility without issue. However, utilities will use "safety" as a reason to now allow GT inverters to be used. In reality, they do not like people to generate their own power--It reduces the amount of money they collect at the end of the month.

    In some cases, utilities that do not allow GT inverters will still allow large facilities with backup generators (such as hospitals) to feed power back into the network for their monthly testing (instead of feeding a large electric heater and wasting the fuel/energy).
    BB. wrote: »
    In the US/North America... The typical method to connect a GT inverter is to do the following.

    Assume that a 200 amp main panel can take 20% extra current as solar power from a GT inverter... Or an extra 40 amps.

    Reduce the main breaker to 175 amps, and that gives you 40+25=65 Amps for "legal" GT generated power (branch circuit).

    At the bottom of the bus bars (oposite end of bus bar from main breaker), install your GT branch circuit(s). This ensures that you never have more than the 200 amps flowing any where in the main bus bars (do not install GT branch circuits next to main breaker, then you get main breaker+GT circuits worth of current in the bus bars).

    Then run the branch circuits to the GT solar inverter.

    can you explain this point more clear pls?

    this method can be used with meters that are NOT "net-meter"

    Thanks

    That is pretty much it. For 3 phase power, your utility may have load balancing rules... For example, if you want 9 kWatts of solar power, you have to put in 3x 3kWatt AC inverters (one on each phase pair of wires).

    Or, you may be allowed a maximum of 10 kWatt per phase, and you can put one 10 kWatt inverter on one phase pair only.

    If you have open Wye service (two transformers feeding your three phase panel)--You may be required to put the 10 kWatt inverter on one of the two "real" phases.

    If you have a requirement to not generate GT solar power if one of the phases fail, you may need a special relay or AC inverters that are three phase aware to shut down if there is a loss of a single phase.

    -Bill
  • minabenyaminminabenyamin Solar Expert Posts: 45
    Re: grid-tied system design questions

    can you send me a diagram pls

    Actually I will not make it myself but I must understand it

    Thanks
  • BB.BB. Super Moderators, Administrators Posts: 29,002 admin
    Re: grid-tied system design questions

    I am chatting with Mina Benyamin to get the details addressed.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • hill5673hill5673 Registered Users Posts: 2
    Re: grid-tied system design questions

    Seems to me the best (although expensive) solution here would be a battery based system with an inverter/charger.
    During sunlight you would draw power from the panels and they would charge the batteries. At higher demand or at night the grid would take over the charging of the batteries. This is the only way I can think of to use the panels' full potential and still have reliable power (without grid tie).
  • South AfricaSouth Africa Solar Expert Posts: 295 ✭✭✭
    Re: grid-tied system design questions
    hill5673 wrote: »
    Seems to me the best (although expensive) solution here would be a battery based system with an inverter/charger.
    During sunlight you would draw power from the panels and they would charge the batteries. At higher demand or at night the grid would take over the charging of the batteries. This is the only way I can think of to use the panels' full potential and still have reliable power (without grid tie).

    Or, panels charge batteries, inverter runs on batteries and if batteries are 25%DOD, measured by a battery monitor, using its relay to switch a quality AC relay to switch power source back to the grid. Next morning, panels charge batteries first and once charged, battery monitor's relay switches the AC relay back to use inverter power.

    Break between power sources, to ensure no sine wave conflict, can be set to 8ms or less.

    And if you do the wiring right, set the DOD to say 10% before relay switches back to grid so that lights can be powered from the inverter at night, whilst equipment is powered from the grid.
  • hill5673hill5673 Registered Users Posts: 2
    Re: grid-tied system design questions

    Wouldn't that decrease the life of the battery bank running them down before charging versus keeping them charged up with the Inverter/charger? It also seems more complicated. I would think either scenario would use the same amount of grid power in the end, or at least close.
  • South AfricaSouth Africa Solar Expert Posts: 295 ✭✭✭
    Re: grid-tied system design questions
    hill5673 wrote: »
    Wouldn't that decrease the life of the battery bank running them down before charging versus keeping them charged up with the Inverter/charger? It also seems more complicated. I would think either scenario would use the same amount of grid power in the end, or at least close.

    To keep batteries on float their entire life, is not economical, you must use them.

    so, if you cannot go grid tie, want to use solar power with grid as backup, you have to use batteries.

    And in using the grid as backup, you can have less batteries than a completely off grid system that has to have storage for 2-3 days, but in using less batteries, as I have learned here, you need to ensure the array and the batteries used are balanced.

    Then you can have the savings from solar power, limited expenses ito batteries and lights on solar at night.

    Complicated, not at all. Had a box built for me that is plug and play. And it is cheaper than most off the shelf inverters that does the same function.
  • BB.BB. Super Moderators, Administrators Posts: 29,002 admin
    Re: grid-tied system design questions

    Mina is looking to save money/power costs... Not in backup/emergency power. Grid Tied would be much less expensive and maintenance. But, at this point, his utility (government owned?) does not allow grid tied/feeding power back to the utility.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • South AfricaSouth Africa Solar Expert Posts: 295 ✭✭✭
    Re: grid-tied system design questions
    BB. wrote: »
    Mina is looking to save money/power costs... Not in backup/emergency power. Grid Tied would be much less expensive and maintenance. But, at this point, his utility (government owned?) does not allow grid tied/feeding power back to the utility.

    -Bill

    That was / is my situation, there my design below.
  • solar_davesolar_dave Solar Expert Posts: 2,338 ✭✭✭✭
    Re: grid-tied system design questions

    Here is some commercial sized equipment:
    http://pvpowered.com.phtemp.com/index.php
  • ggunnggunn Solar Expert Posts: 1,973 ✭✭✭
    Re: grid-tied system design questions
    For the issue of feedback energy which will run the meter up, this can be solved easily in my opinion. I can install power diode on the output of the meter which will act as a buffer permitting the energy (current) to flow in only one direction and prevents it from flowing in the other direction!

    Can that solution be considered??
    No. Diodes only control the direction of current flow in DC.
  • inetdoginetdog Solar Expert Posts: 3,123 ✭✭✭✭
    Re: grid-tied system design questions
    ggunn wrote: »
    No. Diodes only control the direction of current flow in DC.

    minabenyamin:
    The common term "power diode" refers to a diode that allows current to flow in only one direction, in quantities suitable for a high power circuit.

    Much as we would like to have one, there is not currently a diode which is sensitive to the flow of energy (power). :(
    SMA SB 3000, old BP panels.
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