How does an Inverter know?

Hi all you solar brainiacs,
I need a quick course in Inverter 101.
I'm installing a ground mount 7.7Kw pv system, and my electrician and I keep wondering, why does a grid-tie inverter supply electrical demand to loads, rather than the grid? The inverter is tied into the main panel, and so is the grid. If a refrigerator starts up, and the PV system is producing, why does it use the current supplied by the inverter instead of what is supplied by the grid? They are both suppliers of AC power, so why one over the other?


  • mike95490mike95490 Solar Expert Posts: 9,572 ✭✭✭✭✭
    Re: How does an Inverter know?

    Your fridge is closer to the GT, than your neighbors. The miniscule amount of wire resistance causes that, local loads get first shot at the power.
    Powerfab top of pole PV mount | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
    || Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
    || VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

    gen: ,

  • autoxsteveautoxsteve Solar Expert Posts: 114 ✭✭✭✭
    Re: How does an Inverter know?

    ditto on Mike's reply.

    Due to the wire runs between your internal loads (shorter) and the loads elsewhere (longer) there is a small resistive difference. Electrons take path of least resistance. Voltage limits from home to home in that situation are going to be a little different (inverter will push current limit up while maintaining voltage to within a narrow window).

    I apologize if I spewed a little to much here...
  • System2System2 Posts: 6,290 admin
    Re: How does an Inverter know?

    Hi, Guys:
    I'm seeing a glimmer of light at the end of the tunnel, but I'm not quite there yet. Are you saying that the electrons in the grid find it easier to go to neighbors than to supply my running fridge when the inverter is operating? And this is because there's a supply from the inverter that's closer to the load? Is EMF coming into play here?
  • BB.BB. Super Moderators, Administrators Posts: 32,612 admin
    Re: How does an Inverter know?

    Don't even go down the electron motion track... In real life there are so many electrons in a inch of wire (remember Avogadro's number?) that actual electron movement at 60 Hz is something like a fraction of an inch per cycle (I don't remember the exact number right now--but the electron motion itself a massive number of electrons all "shifting" right and left (AC power) just a tiny bit).

    At higher level, this is the same as paralleling batteries and power supplies. For the sake of argument, assume that the "Grid" is a giant 120 Volt battery. And the Grid tied inverter is a variable power supply. And your refrigerator is a load on the house wiring.

    With no sun, the grid tie inverter outputs zero voltage (high resistance) and all of the current (power) flows from the Grid Battery through the electric meter to any loads (lower resistance devices).

    As the sun comes up, the grid tie inverter shoves current (power) into the local house wiring. If the current supplied by the inverter is less than the total current draw of the home, the voltage in the home wiring is slightly lower than the Grid "Battery" and the net difference in current through the meter is supplied by the Grid "Battery". So both the Grid and the solar inverter are supplying power into the house circuits.

    When the solar power is high, the inverter current (power) is high and its voltage is slightly higher than the Grid voltage. So, now the current (power) is supplied by the inverter to the local load and any excess current is pushed backwards through the meter and "effectively is charging the Grid".

    Now, the difference with reality is that this is AC voltage and current, so there is inductance and capacitances involved, phase angles between voltage and current and between phases (120, 240 phases, 3 phase at the street, etc.) that this all becomes a highly complex math problem for utility engineers (I took some of the math 30 years ago--and I am not going to claim to remember any of it now).

    But, basically, the grid is a giant AC battery (~fixed voltage and almost unlimited ability to source and sink current with respect to home wiring) and the loads (fridge) and other sources like the Grid Tie inverter (whose job is to pump as much current back into the grid based on how much DC power is available from the DC solar array) are just sharing power between all of the sources and sinks.

    Personally, I always like water analogies. The Grid is the storage lake of water, wiring is the pipes. And the Grid Tied inverter is a variable pressure/volume pump that can drive water back into the local pipes and eventually back into the lake (for storage). The Fridge and lights are just little faucets that are opened and closed to drain water as needed.

    Of course the grid cannot convert electricity back into stored power (electricity->unburned coal, electricity->natural gas, etc.)--we are just taking our "excess power" and adding to the overall generating capacity of the grid and our excess power is just used by other homes down the street and our local grid is using slightly less power from the local coal/natural gas/nuclear power plant tens or hundreds of miles away.

    (There is one grid device that can store power--there are pump stations that can generate power with water flowing down from the dam, and can be reversed and pump water back up to the dam for energy storage--but those are relatively rare. And there are other utility sized storage schemes out there being worked on too).

    Does that help?

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • System2System2 Posts: 6,290 admin
    Re: How does an Inverter know?

    Alright Bill!
    Now I get it. I think I'll call you Professor Bill from now on. Thank you for the time you put into your delightfully verbose answer.:-)
    Just to paraphrase, I'm thinking that the inverter's bottom line purpose in life is to add to the grid. Further, using your analogy, that fridge turning on is just a little faucet in the plan that, when opened, uses the excess PV output simply because it's there and it's easy, compared to the inverter's same amount of current having to go into the grid and all the way to a neighbors house. The house loads are easier to supply, so there isn't any competition between the inverter and the grid to supply the local loads. Sounds reasonable to me.
  • BB.BB. Super Moderators, Administrators Posts: 32,612 admin
    Re: How does an Inverter know?


    Sort of right... The details are a bit more complex. But, in the end think of the Grid as "voltage controlled". No matter what happens in the house, the grid will either accept the current or supply the current to keep the voltage fixed.

    The Grid Tied inverter is, more or less, operates at "right angles" to the grid. It can be thought of as a constant power device... But for the first explanation, I will call it a constant current device.

    A constant current device pumps out exactly xx amps (based on what the solar panel has available). If the House/grid voltage goes up or down, it does not matter, it will still output a constant XX amps into the wiring. And the Grid Voltage will stay exactly the same and it will supply (or sink) the exact amount of current required to keep the voltage exactly fixed. Of course, there is resistance in the wiring, so as the inverter tries to output more current, its output voltage will rise in order to keep the current at exactly XX amps. So the inverter does not regulate voltage but only the current it outputs. And the Grid does not regulate current, only the voltage it outputs. Loads are just drawing power as required. Some are resistive (lights, heaters), some are more inductive (like motors). And some are complex current sinks (like power supplies, battery chargers, etc.).

    Once you get the above fixed in your mind, now we can talk about what the Grid Tied inverter really is... A constant power device. Remember power is P=I*V=I^2 * R=V^2 / R etc... We will just replace where I said that Current is held exactly XX Amps to Power is held exactly to X,XXX Watts where Watts=V*I. The Watts is what is available for conversion from the solar array. If the Grid voltage drops, the inverter will output more current. If the house wiring has resistance, the V=I*R will rise, and the voltage seen by the inverter will rise, so the current output by the inverter will have to drop a bit in order to keep the power a constant X,XXX watts.

    Of course the inverter has a maximum allowed range for output voltage and current (typically around 200-264 VAC as well as other requirements). If the output voltage goes outside the "safe range" (as defined by NRTL's/UL/Utilities/etc.), the inverter will shut down for 5 minutes under the assumption that there is something wrong with the grid or local home wiring. The inverter does not have the ability to regulate its output voltage--and it would be impossible for it to regulate the voltage since the VOLTAGE is regulated by an almost infinite source (THE GRID). All the inverter can control is the amount of current (aka POWER) that it can output (where P=I*V -- The GRID controls V and the Inverter controls I).

    Back to the "water analogy", The GRID is a 10 square mile lake where the lake level cannot be changed by a 1" pipe drawing or supplying water. And the Inverter is a piston type water pump that just moves GPH of water, regardless of the water pressure (with the realization that as the pressure rises the RPM of the pump has to drop, or more electrical power supplied to the motor to keep the GPH fixed--vs GPH*Head fixed)...

    And it gets more complex in that you don't want the Inverter to Oscillate... For example, connect a motor and a Grid Tied inverter together. Remember the Inverter output current drops as the voltage rises. Turn on the electric motor, it draws more current because it started from a dead stop, draws lots of current and drops the voltage down. So the GT Inverter sees lower voltage and supplies more current--and the motor sees more voltage and draws less current (a motor is, roughly, also a constant power device). So the voltage shoots up, the GT Inverter now needs to cut current (as V went up), so the line voltage drops, the motor now needs more current and drops voltage, then the inverter drives more current and raises the current, etc.... And the motor also has a mechanical load with "spring-y-ness" (and rotational momentum, etc.)--and those also affect the motor's electrical profile.

    So--the Inverter Designer has to design the control circuit such that it will not get into problematic oscillations (cause damage or electric noise in the home wiring). It is not easy--but a very interesting part of engineering design (control theory).

    Think of what happens to your car in first gear and you hold the gas peddle with your leg in the air (instead of pivoting your foot on the floor). You can get into a cycle where your car surges ahead, your foot/leg lifts of the gas and the car quickly slows, your leg's momentum shoves your foot back on the peddle and the car surges ahead again... Same sort of problem.

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • System2System2 Posts: 6,290 admin
    Re: How does an Inverter know?

    Thanks, Prof. Bill.
    I like your use of analogies.
    The question I asked has actually been asked of me more than once, by neighbors, and friends. I feel better equipped to answer now. Thanks.

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