Grid tie inverter shut down

boisblancboyboisblancboy Solar Expert Posts: 131 ✭✭✭✭✭
Just a curious questions, why do grid tie inverters shut down when the grid goes down?

Comments

  • nielniel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: Grid tie inverter shut down

    the primary reason is safety. you don't want power to be sent up the lines to unsuspecting linemen that are trying to fix the problem on the grid that caused the outage in the first place. even if it were possible to do, it wouldn't take too much to overload the draw upon the inverter as it would try to power many homes in the grid's absence and pop the fuse/circuit breaker you are supposed to have inline.
  • BB.BB. Super Moderators, Administrators Posts: 32,612 admin
    Re: Grid tie inverter shut down

    A grid tied inverter is hard wired to the house AC mains... If the utility fails somewhere, then the GT inverter would try to power your home and the rest of the neighborhood.

    Also, from a practical point of view--the GT inverter is like the alternator on the car... The GT inverter simply generates as much power/current as it can, and pumps the energy back into the utility grid--just like a car alternator charges the battery. The battery sets the voltage in the car, the utility grid sets the voltage and frequency for the GT inverter.

    And from a safety point of few---the utility cannot have inverters (and house hold emergency generators) electrify the grid--a shock hazard to their line workers, and a problem when trying to reconnect the grid (local generator power out of phase with AC utility power during connection--can literally tear a home generator right off of its mounts). That is why if the GT inverter detects a problem (line voltage or frequency out of spec)--it must shut down within a couple cycles, and wait for at least 5 minutes before it starts up again.

    There are hybrid GT/Off-Grid inverters that can supply power when the grid is down... But they work like a UPS with an AC Transfer switch. When the AC fails, the transfer switch opens and the inverter can now power just the local loads (in Off-Grid mode, not in GT mode).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • boisblancboyboisblancboy Solar Expert Posts: 131 ✭✭✭✭✭
    Re: Grid tie inverter shut down

    Makes sense! Thanks guys.
  • boisblancboyboisblancboy Solar Expert Posts: 131 ✭✭✭✭✭
    Re: Grid tie inverter shut down

    Along the same lines i think, but is there a setup that when the grid is down and your inverter shuts down to have your power engerize your house but make sure it wasnt feeding the grid?
  • BB.BB. Super Moderators, Administrators Posts: 32,612 admin
    Re: Grid tie inverter shut down

    Yes, it is called a "Transfer Switch"...

    They can be very simple--just a double throw, single pole switch (usually with 1 breaker/fuse for safety).

    Or there are automatic devices that connect inverters or generators to your "protected" loads...

    Think of the switch as the common (center) point connected to the protected load. The "A" side to the Mains, and the "B" side to the alternate source (generator, inverter, etc.). Common to A when mains are up. Common to B when you want backup power.

    Some inverters include the automatic transfer switch inside (and internal AC to DC battery charger). This makes them, for all intents and purposes, a UPS (uninterpretable power supply).

    Some of the more fancy units even include several input options such as AC1 Mains and AC2 Generator.

    Here is an example of some options out there.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • boisblancboyboisblancboy Solar Expert Posts: 131 ✭✭✭✭✭
    Re: Grid tie inverter shut down

    Thanks Bill! Just neat gadgets everywhere for different uses.
  • crewzercrewzer Registered Users, Solar Expert Posts: 1,832 ✭✭✭✭
    Re: Grid tie inverter shut down
    Along the same lines i think, but is there a setup that when the grid is down and your inverter shuts down to have your power engerize your house but make sure it wasnt feeding the grid?
    Brandon,

    You may need to clarify your question. A battery-less grid tie inverter generally won't operate if the grid isn't operating. So, separating it from the grid won't help.

    There are a couple of exceptions. Grid-tie inverters with battery backup will disconnect from the grid but still provide power to designated loads. This application brochure is an example of the OutBack solution.

    Another approach is called "AC coupling". But, that's another topic for another day. ;)

    Regards,
    Jim / crewzer
  • boisblancboyboisblancboy Solar Expert Posts: 131 ✭✭✭✭✭
    Re: Grid tie inverter shut down

    Ah ha the Grid-tie inverters with battery backup is what I was talking about. But of course to have an expensive battery bank for a once in awhile short term power outage would be kinda silly. A small generator would do the trick for those time.

    Thanks Jim!
  • crewzercrewzer Registered Users, Solar Expert Posts: 1,832 ✭✭✭✭
    Re: Grid tie inverter shut down

    Brandon,

    I agree that a relatively large battery bank could be an expensive backup power solution, especially when oversized. But, the bank need not be large, and generators aren't free. If the goal were to to power key loads for anywhere from a short "spike" to a few hours, then the bank could be relatively small.

    For example, a backup battery bank of four size 4D batteries could supply ~4.8 kWh (gross) down to a 50% SOC. This would provide ~500 W (net) for eight hours -- enough to operate a modern fridge, a computer and home network, a TV, some CFL lights, etc.

    This is how I ran my house and home office one day last September when a sewer contractor severed the buried grid cable in a neighbor's yard.

    We have many customers who have chosen this approach rather than deal with the cost, maintenance, and operation of a generator. Ironically, "auto-start" generators contain a starting battery.

    Regards,
    Jim / crewzer
  • boisblancboyboisblancboy Solar Expert Posts: 131 ✭✭✭✭✭
    Re: Grid tie inverter shut down

    Yeah I can understand that. In my case I dont really have a reason to ever attempt that, or I should say in my current case. I was more interested in how it would be done and if it were possible, which I see it very much is. I do have a back up generator that can run my entire house cause its common here to have the power out for 3 days at a time, a few times a year.
  • crewzercrewzer Registered Users, Solar Expert Posts: 1,832 ✭✭✭✭
    Re: Grid tie inverter shut down

    Understood. :cool:

    Regards,
    Jim / crewzer
  • System2System2 Posts: 6,290 admin
    Re: Grid tie inverter shut down
    BB. wrote: »
    Yes, it is called a "Transfer Switch"...

    They can be very simple--just a double throw, single pole switch (usually with 1 breaker/fuse for safety).

    Or there are automatic devices that connect inverters or generators to your "protected" loads...

    Think of the switch as the common (center) point connected to the protected load. The "A" side to the Mains, and the "B" side to the alternate source (generator, inverter, etc.). Common to A when mains are up. Common to B when you want backup power.

    Some inverters include the automatic transfer switch inside (and internal AC to DC battery charger). This makes them, for all intents and purposes, a UPS (uninterpretable power supply).

    Hi, Bill,
    With a transfer switch an inverter would not be a grid-tie (in other words, the loads will be connected either to the grid or to the inverter). Unless I am missing something, a grid tie inverter with a backup power capability should have an auto disconnect switch that cuts off the grid connection when grid fails, but always keep the connection to the loads. I did notice some manuals refer to a built-in transfer switch- I guess that's just how they call a disconnect switch.
    Lzr
  • BB.BB. Super Moderators, Administrators Posts: 32,612 admin
    Re: Grid tie inverter shut down
    lzr wrote: »
    Hi, Bill,
    With a transfer switch an inverter would not be a grid-tie (in other words, the loads will be connected either to the grid or to the inverter). Unless I am missing something, a grid tie inverter with a backup power capability should have an auto disconnect switch that cuts off the grid connection when grid fails, but always keep the connection to the loads. I did notice some manuals refer to a built-in transfer switch- I guess that's just how they call a disconnect switch.
    Lzr

    I was replying to this question:
    Along the same lines i think, but is there a setup that when the grid is down and your inverter shuts down to have your power engerize your house but make sure it wasnt feeding the grid?

    But--the whole set of questions was sort of mixed up... The transfer switch would be for a normal battery powered Inverter to provide backup power--and a manual or automatic transfer switch could be used to make your own UPS (disconnect from failed AC Mains and connect to standby battery backed UPS).

    And, you are correct, that this would not be a "grid tied" inverter--but an off grid inverter setup.

    To make things even more complex--there are hybrid Grid Tied--Off Grid capable inverters that can do both (such as the Xantrex XW system)... The transfer switch would be internal to such a system and not something added on after the fact.

    And to even blow you mind further... One can setup a standard Off-Grid True Sine Wave inverter -- start it up and run your off-grid loads from the battery bank--and connect a standard Grid Tied Inverter to the AC load connections--fire up the GT Inverter and actually share the AC loads with the Off-Grid Inverter and--if the GT Inverter exceeds the AC loads--back drive the Off-Grid TSW inverter and charge the battery bank...

    However--there are lots of issues and limitations (such as no battery charge regulator--can destroy battery bank, GT inverter + solar panels must be smaller than Off-Grid Inverter+minimum load) with such a setup (GT+Off-Grid Inverter in parallel on AC line)--that I would not suggest anyone do this without a lot of thought and discussion.

    There are some discussions around here about the various setups that we may be able to find if anyone is interested.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • EcnerwalEcnerwal Solar Expert Posts: 101 ✭✭
    Re: Grid tie inverter shut down
    Yeah I can understand that. In my case I dont really have a reason to ever attempt that, or I should say in my current case. I was more interested in how it would be done and if it were possible, which I see it very much is. I do have a back up generator that can run my entire house cause its common here to have the power out for 3 days at a time, a few times a year.

    ...in which case you MIGHT want a battery/grid-tie inverter setup (but yes, it's more money) so that you can have power (at some level) for 3 days without having the generator running for 3 days - running the generator occasionally to take heavier loads and recharge batteries if the sun isn't shining enough. One of the more annoying things about "generator or nothing" is those times when a little power would be nice, but listening to the generator (as it gobble$ fuel inefficiently at low load) is tiresome.
  • RCinFLARCinFLA Solar Expert Posts: 1,453 ✭✭✭✭
    Re: Grid tie inverter shut down

    A PV grid tie system works by using the low impedance power lines as the reference. PV panels are a relatively high impedance current source device and do not act at all like a low impedance voltage source battery. PV grid-tie inverter is a current source generator, not a voltage source generator.

    The grid tie PV system simply pushes against the grid, injecting current into the grid. The amount of current is based on availablity of the PV panel source, not the demand of your house A.C. load. If a PV is blocked by a cloud all that happens is less current is pushed at the grid. Nothing in PV grid tie inverter is based on your house load demand.

    If grid-tie inverter was allowed to run only from the PV panels and load exceeded PV supply capability at the moment something has got to give, AC output voltage would have to slump. At the other end, if PV exceeded output A.C. load demand, something would have to shunt away the excess power to keep AC output voltage from rising.

    In order to work without the grid, a battery, or a low impedance reservor tank, like a super cap, is needed to provide a voltage anchor source and cover surge currents of the a.c. load.
  • System2System2 Posts: 6,290 admin
    Re: Grid tie inverter shut down

    RC in FL
    I was doing research about this and find your answer most helpful. Looks like I will need to invest in a hybrid GTI with some batteries to to be the tank. In other words, the inverters are made to output very differently.

    Sure wish these new enphase inverters could be set up to work with a battery bank somehow. The connection process is very attractive to me.
  • BB.BB. Super Moderators, Administrators Posts: 32,612 admin
    Re: Grid tie inverter shut down

    They can be... You setup a regular Off-Grid True/Pure Sine Wave Inverter + battery bank that is larger than your total Enphase maximum AC output.

    Place the Enphase on the Off Grid inverter's output. And turn them on.

    The Off Grid inverter and the Grid Tied Inverters will share the load, and the GT inverters will back-drive the OG inverter and charge the battery bank.

    HOWEVER--that has only be tested with a few combination of GT and OG Inverters (probably not including the Enphase products). Is an "off data sheet" use of an Off Grid Inverter (i.e., safety of the product has not been evaluated and could be a fire hazard). And, there is no battery charging control in the setup--you would need to configure/install you own battery charger control system (diversion loads, interrupt AC power from GT inverters, backup controller, etc.).

    Possible--yes... Something practical, perhaps. Something that should be done by somebody without extensive Solar RE / Electrical / Safety experience and a bank account to pay for the experiments--probably not.

    There are a few threads here I can probably find if you want to read about the details (including at least one person that did limited testing of such a setup--without the charge controller, just manually watched the battery charge--at his home and found it to work fine).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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