Sending power back onto grid...?

Could someone explain to me how exactly power is sent back into the grid as happens in a grid tied PV system. Specifically how is power "stepped-up" to the high voltages that the grid uses? As I understand it- power that comes off the line is stepped down via a transformer to typical household voltage (240v). Thus how does power flow in the other direction (back onto the grid) without first being stepped up to the higher voltages to be distributed back onto the grid? Does the transformer do this?

I was talking about this with an electrician the other day but when I pressed him for details- he didn't seem to really know. Could someone explain to me how this works at least on a conceptual level. I am curious... :confused:
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Comments

  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    Re: Sending power back onto grid...?

    Welcome to the forum.

    Alternating Current fed to a transformer can go up or down; transformers are bi-directional.

    If you have a transformer with a 2:1 ratio, for example, it can either take 240 Volts down to 120, or vice versa. When you step the Voltage up the current available goes down, and when you step the Voltage down the current available goes up. (This is excluding losses and the other fine details.)
  • runningduck
    runningduck Registered Users Posts: 6
    Re: Sending power back onto grid...?

    Well that was simpler then I thought... thank you Cariboocoot. Thinking about it some- Given that transformers are bi-directional; Does power flow in both directions (stepped up or down via the transformer) in all homes or is the ability of the transformer to go in the 'other direction' only utilized in homes with generation capability (PV... etc)? Any suggestions on what I could read to get a better grasp of how this works? Wikipedia?
  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    Re: Sending power back onto grid...?

    Both directions; that's how GT systems are able to "sell back" to the utility. Between the house and the generating plant there can be quite a few transformers, in fact. Then you get into the more complex issues of impedance, Voltage differential, et cetera. Really power transmission is a whole sub-set specialty of electrical engineering.

    You can certainly look things up in Wiki, but you're going to find there's a lot to look up and everything you read will lead you to even more things to look up. It's impossible to take something so complex and boil it down into a simple explanation without leaving something out. Like about eight years of college. ;)

    I prefer to dwell in the simpler world of just making it work. No matter how big a hammer is needed to do so. :p
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Re: Sending power back onto grid...?

    And regarding the "details" of how this works without letting the magic smoke out of your home electrical system...

    There are two ways to get AC power from a device (generator--really called an Alternator, AC inverter) into your homes wiring.

    The first way is "synchronous" generation. The line is exactly at 60 Hz (60 times a second) and the alternator is turning at 60 times a second... They alternator needs to be "phase sync'ed" with the AC line frequency. One way to do this is put a couple light bulbs across the open switch contacts. When the lights are "bright", the two sources are out of phase. When the lights go out, they are in phase and the switches can be closed.

    Now the alternator is spinning at exactly 60 Hz locked to the 60 Hz of the grid power. If you want to push energy out to the grid, you open the throttle on the diesel motor and apply torque to the alternator which "pushes" current (and a little bit higher voltage) out to the local wiring and spins the utility meter backwards (if the alternator output current/power is higher than the local loads). This placing two "AC voltage sources" in parallel.

    Another way to do this with GT inverters is to make the GT inverter a "current source". And the electronics simply follow the AC voltage wave from the grid and push out current that is related to the Varray * Iarray = Power-array. The GT inverter checks the frequency and the voltage of the grid to make sure that it is OK--Then simply pushes out current in relationship to the sine wave voltage. There is no "sync'ing" of the GT inverter to the utility frequency. It simply just pushes out the current/power available for that part of the AC cycle. This is connecting a "battery or voltage source" (the "grid") with a "current source" (the solar GT inverter).

    Notice that if you connect a mechanical alternator (i.e., motor/genset) to the AC wiring out of phase, all sorts of bad things can happen... This is like you having to choose to connect to car batteries in parallel--And you have the choice of connecting them + to + and everything is fine. Or if you connect them + to - and get a dead short between the two "AC" voltage sources. When this happens to large motor/generator or utility scale generators, they can actually rip themselves loose from their mounts.

    Lastly--There is another way of generating AC power "on the side". Take your standard induction motor. Plug it into the grid and it turns up to speed (around 1,725 RPM or so)... Now connect that to a gasoline motor and crank up the throttle to > 1,800 RPM, and you will be dumping power into your wiring/utility grid. Induction motors have "slip" that generates the magnetic field in the rotor. As you spin the rotor faster and faster, it "gets ahead" of the rotating field supplied by the utility and you start pumping out energy to the grid. Of course this is not legal to do for your home (and not safe)--But it does work.

    There are a lot more details--and how it affects the utility--But from a 60,000 foot view--That is pretty much it.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • etcm
    etcm Registered Users Posts: 11
    Re: Sending power back onto grid...?

    How about this take, I understood BB's post but it took a lot of discernment. I caught the there was more what you do than how it does it. With electric/electronic that is usually the best way to go. Military electronics/training can easily take the place of that 8+ years of school. You can mix the why and what to do very effectively.
    If your (120v) phase has even a little higher voltage than the incoming line phase voltage, you can cause AC current to flow from your wires to the line side. The (line side) transformer (abbreviated XFMR) has less loading that is felt on the line wires, or if you are selling, the line wires receive a current (induced - because it is via XFMR action), XFMR's are very interesting.
    Good luck in your quest for knowledge
  • NorthGuy
    NorthGuy Solar Expert Posts: 1,913 ✭✭
    Re: Sending power back onto grid...?
    etcm wrote: »
    you can cause AC current to flow from your wires to the line side.

    That's alternating current. For half of the time it flows in one direction, and for the other half it flows in opposite direction. That's why they call it Alternating Current (AC).
  • solarix
    solarix Solar Expert Posts: 713 ✭✭
    Re: Sending power back onto grid...?

    The wire resistance coming into you service panel is very low. Big wire and short distance to your local grid transformer, probably less than 1/10 ohm. That means very little voltage difference to make a lot of current flow. If your inverter puts out 30 amps for example, then the inverter would only have to bump up the grid voltage 3 volts from say 240Vac to 243Vac to push its power into the grid. No relays, no problems.
  • ggunn
    ggunn Solar Expert Posts: 1,973 ✭✭✭
    Re: Sending power back onto grid...?
    etcm wrote: »
    If your (120v) phase has even a little higher voltage than the incoming line phase voltage, you can cause AC current to flow from your wires to the line side.
    Not really. The reason that the voltage is a little higher at the terminals of the inverter than they are at the interconnect is because the current in the wires causes a voltage drop in the direction of current flow due to the resistance of the wire. A grid tied inverter is a current source and would still push current onto the grid when your household loads are demanding less power than the inverter is putting out, even if its conductors were superconducting and the voltages were exactly the same.
  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    Re: Sending power back onto grid...?

    Technical point: GTI's don't actually raise the grid Voltage. They have a slightly higher Voltage potential which is the impetus for the current flow. This allows it to overcome the resistance of wiring until the point where it matches the grid Voltage. If there is too much resistance (as we have seen in some installs) the GTI Voltage goes too high and it shuts down (thinking there is no grid connected).
  • ggunn
    ggunn Solar Expert Posts: 1,973 ✭✭✭
    Re: Sending power back onto grid...?
    Technical point: GTI's don't actually raise the grid Voltage. They have a slightly higher Voltage potential which is the impetus for the current flow.
    The voltage rise is the result, not the cause, of the current flow. If the resistance in the conductors were zero ohms, the inverter would be perfectly happy exporting power with exactly the same voltage at its terminals as is the line voltage at the interconnect. It's a current source, and it's not pushing against the grid. It's pushing with the grid generators as part of the power supply to a massively parallel load.
  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    Re: Sending power back onto grid...?

    When you have two power sources connected together current flows from the one with the higher Voltage potential to the one with the lower Voltage potential. It does not matter if one is a current source and the other a Voltage source.

    Connect a 12 Volt solar panel directly to a 24 Volt battery bank. How much current flows from the panel to the battery bank? None.

    A solar panel can flow Isc to a dead short because the Voltage potential of the load (or any non-power source load) is always zero.

    GTI's in function are a current source, as their Voltage output is stable to the grid and all the power available from the array is transferred as current. But if their Voltage potential was lower (as it is when illumination is poor) the current flow is zero.

    Realize that this is too complicated to explain in a few paragraphs.
  • inetdog
    inetdog Solar Expert Posts: 3,123 ✭✭✭✭
    Re: Sending power back onto grid...?
    ggunn wrote: »
    ... it's not pushing against the grid. It's pushing with the grid generators as part of the power supply to a massively parallel load.
    Another way of looking at this is that for the case of zero ohm interconnections, it does not matter, and you cannot tell from the GTI end where the power is actually going. With zero ohm interconnects, the only way to tell whether power is flowing into a terminal or out of it is to compare the direction of the current to the polarity of the voltage.
    SMA SB 3000, old BP panels.
  • solarix
    solarix Solar Expert Posts: 713 ✭✭
    Re: Sending power back onto grid...?

    While not privy to actual inverter details (although I once-upon-a-time designed my own), an inverter's output controls both it's voltage and current. The voltage is regulated to synchronize to the grid's voltage waveform. As a result that control rate is significantly higher than at least double the 60hz grid frequency - probably more like 10kHz - maybe higher, even above the audio spectrum. At a much slower rate, the inverter will control its output current (by setting its output voltage) in order to set its power level. You can say it is current sourced as a result, but really, the voltage is the primary driver, forcing current out to the grid.
  • inetdog
    inetdog Solar Expert Posts: 3,123 ✭✭✭✭
    Re: Sending power back onto grid...?
    solarix wrote: »
    While not privy to actual inverter details (although I once-upon-a-time designed my own), an inverter's output controls both it's voltage and current. The voltage is regulated to synchronize to the grid's voltage waveform. As a result that control rate is significantly higher than at least double the 60hz grid frequency - probably more like 10kHz - maybe higher, even above the audio spectrum. At a much slower rate, the inverter will control its output current (by setting its output voltage) in order to set its power level. You can say it is current sourced as a result, but really, the voltage is the primary driver, forcing current out to the grid.
    I am not privy to the details either, but if I were called upon to design a GTI, I would instead build a current source whose amplitude was set by the available PV power and whose instantaneous value was set to be proportional either to the instantaneous value of the grid voltage or of a phase locked reference sine wave generated internally.

    Maybe one of the actual GTI designers who visit the forum will enlighten us?
    SMA SB 3000, old BP panels.
  • ggunn
    ggunn Solar Expert Posts: 1,973 ✭✭✭
    Re: Sending power back onto grid...?
    When you have two power sources connected together current flows from the one with the higher Voltage potential to the one with the lower Voltage potential. It does not matter if one is a current source and the other a Voltage source.

    [...]

    Realize that this is too complicated to explain in a few paragraphs.
    But with a current source the power flows just fine even if there is no voltage differential. The voltage rise at the terminals of a GT inverter is a result of the current flow through conductors of finite resistance, not the cause. If there were no resistance there would be no voltage differential, but the current flow would persist. It's really not complicated at all.
  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    Re: Sending power back onto grid...?
    ggunn wrote: »
    But with a current source the power flows just fine even if there is no voltage differential. The voltage rise at the terminals of a GT inverter is a result of the current flow through conductors of finite resistance, not the cause. If there were no resistance there would be no voltage differential, but the current flow would persist. It's really not complicated at all.

    Nope. There is always a Voltage differential. There is always resistance. (We are not Borg; resistance is not futile.)

    A shorted PV has a Voltage differential of Voc:zero, as a piece of wire always has zero Voltage potential.

    If you remove the regulator circuitry from a GTI you have a device that basically turns DC to AC. After that the function follows the same as for applying PV directly to batteries. It has an I*V curve, and when that curve exceeds Vmp then current falls off. The difference is because it is feeding another power source (batteries or grid) the output is set at a fixed minimum Voltage. If the PV output power is pulled up by that fixed Voltage to a point that is above the Vmp * Imp point for the illumination at that time the current falls. You can see this affect with mismatched panels (12 Volt & 24 Volt) connected in parallel. The high Voltage panel will pull the Voltage of the circuit above Vmp for the low Voltage panel and the latter's current drops.
  • ggunn
    ggunn Solar Expert Posts: 1,973 ✭✭✭
    Re: Sending power back onto grid...?
    Nope. There is always a Voltage differential. There is always resistance. (We are not Borg; resistance is not futile.)
    Nope right back atcha. Did you miss the subjunctive in my statement? I said if there were zero resistance there would be no voltage rise, but the current would still flow. Heck, connect the inverter directly onto the output lugs of the medium to low voltage transformer, the current will still flow into the grid (many feed in tariff systems are interconnected that way). In dealing with a voltage source, the magnitude of the current flow is dependent on the magnitude of the voltage differential, but with a current source, the magnitude of the voltage differential (if there is any) has virtually no effect on the amount of current. The water analogy is just an analogy. It's water, not electricity, and the analogy does not hold up in every way of looking at it.

    We have had this argument before, and we are just going to have to agree to disagree.
  • NorthGuy
    NorthGuy Solar Expert Posts: 1,913 ✭✭
    Re: Sending power back onto grid...?

    There's a huge difference between the output of the GT inverter and the output of solar panel.

    With solar panel, the current that can be produced is limited to ISC and stay about the same over a wide range of voltages from 0 to close to VMP.

    With GT inverter there's no such limitation, because this is a man-made device, which can output any voltage or current that the designer wishes. Instead, it is limited by the power that it can get from the panel - VMP x IMP x Efficiency.

    So, the behaviour of them will be drastically different from each other.
  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    Re: Sending power back onto grid...?

    Actually the GTI's output is limited by the power available from the PV array on its input, so their behaviour is not drastically different. They both follow the I*V curve of the panels because that is the only power source.

    Voltage is electrical pressure. No Voltage potential, no electrons move. Otherwise we would have 200 Watt solar panels with a Voc of zero and an Isc of 200. This does not happen.
  • NorthGuy
    NorthGuy Solar Expert Posts: 1,913 ✭✭
    Re: Sending power back onto grid...?
    Actually the GTI's output is limited by the power available from the PV array on its input ...

    That's exactly what I tried to say, but I guess what I have in my mind doesn't get through to the paper correctly :blush:
    They both follow the I*V curve of the panels because that is the only power source.

    I would not agree with that. GTI (as any controller) is an active element and it doesn't follow the input curve. Instead it sets its voltage to match the grid, the same as MPPT charge controller sets the voltage to match the battery needs. That is the difference between panel ouptput and controller output.

    I brought that up because in the ongoing discussion the participants didn't seem to make any distinction. Otherwise, I didn't mean to take any sides.
  • boB
    boB Solar Expert Posts: 1,030 ✭✭✭✭
    Re: Sending power back onto grid...?
    ggunn wrote: »

    We have had this argument before, and we are just going to have to agree to disagree.


    Wait ! You're BOTH right ! It's a dessert topping ~AND~ a floor wax !!!
  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    Re: Sending power back onto grid...?
    boB wrote: »
    Wait ! You're BOTH right ! It's a dessert topping ~AND~ a floor wax !!!

    You know we're probably the only two on this forum that understand that. ;)
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Re: Sending power back onto grid...?

    I remember~!

    [video]http://www.hulu.com/watch/61320[/video]

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • boB
    boB Solar Expert Posts: 1,030 ✭✭✭✭
    Re: Sending power back onto grid...?
    BB. wrote: »
    I remember~!

    [video]http://www.hulu.com/watch/61320[/video]

    -Bill


    Thanks Bill !! Great ! I hadn't seen that one in YEARS !

    That's one I could never forget.
  • Photowhit
    Photowhit Solar Expert Posts: 6,002 ✭✭✭✭✭
    Re: Sending power back onto grid...?
    BB. wrote: »

    Thanks Bill, I missed that one, I thought it was real, like the cattle food supplement/Road treatment...
    Home system 4000 watt (Evergreen) array standing, with 2 Midnite Classic Lites,  Midnite E-panel, Magnum MS4024, Prosine 1800(now backup) and Exeltech 1100(former backup...lol), 660 ah 24v Forklift battery(now 10 years old). Off grid for 20 years (if I include 8 months on a bicycle).
    - Assorted other systems, pieces and to many panels in the closet to not do more projects.
  • westbranch
    westbranch Solar Expert Posts: 5,183 ✭✭✭✭
    Re: Sending power back onto grid...?

    Rats, WHATEVER it is is blocked from Canadians watching....must be bad.:-)
     
    KID #51B  4s 140W to 24V 900Ah C&D AGM
    CL#29032 FW 2126/ 2073/ 2133 175A E-Panel WBjr, 3 x 4s 140W to 24V 900Ah C&D AGM 
    Cotek ST1500W 24V Inverter,OmniCharge 3024,
    2 x Cisco WRT54GL i/c DD-WRT Rtr & Bridge,
    Eu3/2/1000i Gens, 1680W & E-Panel/WBjr to come, CL #647 asleep
    West Chilcotin, BC, Canada
  • boB
    boB Solar Expert Posts: 1,030 ✭✭✭✭
    Re: Sending power back onto grid...?
    westbranch wrote: »
    Rats, WHATEVER it is is blocked from Canadians watching....must be bad.:-)


    You're kidding, right ?? Maybe you're not ?
  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    Re: Sending power back onto grid...?
    boB wrote: »
    You're kidding, right ?? Maybe you're not ?

    Nope: Canadians not allowed to view hulu. Probably didn't pay their GST. :roll:
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • westbranch
    westbranch Solar Expert Posts: 5,183 ✭✭✭✭
    Re: Sending power back onto grid...?

    BB, all 3 are only US viewable??? the last one took me back to HULU???
    add Dailymotion to the list too
     
    KID #51B  4s 140W to 24V 900Ah C&D AGM
    CL#29032 FW 2126/ 2073/ 2133 175A E-Panel WBjr, 3 x 4s 140W to 24V 900Ah C&D AGM 
    Cotek ST1500W 24V Inverter,OmniCharge 3024,
    2 x Cisco WRT54GL i/c DD-WRT Rtr & Bridge,
    Eu3/2/1000i Gens, 1680W & E-Panel/WBjr to come, CL #647 asleep
    West Chilcotin, BC, Canada