"Starting" a PVP Inverter during Power Outage

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rocmor
rocmor Registered Users Posts: 11
I have a 5kW PV installation using a PVP 4800 to tie into the grid. I do not have (or want) battery backup. As I understand it, the PVP control circuits detect and use grid voltage to, in turn, control the DC to AC inversion/regulation/synchronization process. Thus, when the grid goes down due to a power outage, the inverter shuts down. This makes sense from a safety standpoint as you wouldn't want the PV/inverter system to dump 5 kW at 240V into a grid that is under repair. So, if the line repairs are extensive, then I could be totally out of power even on a bright sunny day. A bit ironic.

I'm thinking about buying a 120V/240V backup generator so we can have some power during the nighttime hours for the essentials. With this generator running during the day, will the PV inverter "start" up properly and function normally? This way, I could have enough power for the entire house. I can't see why not, but I was hoping there might be some out there who have actually done this. Thanks

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  • BB.
    BB. Super Moderators, Administrators Posts: 33,479 admin
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    Re: "Starting" a PVP Inverter during Power Outage

    The answer is a bit complex...

    First, if you have a standard generator (Alternator based), then the generator is usually not able to hold 60 +/- 0.5 Hz for 5 minutes--So the GT inverter determines the line voltage is not stable and will never "turn on".

    If the generator is a very good one and can meet the voltage/frequency requirements, then it is possible for the GT inverter to turn on. What will happen is a bit of a guess--But if the GT Inverter output is larger than your home's consumption, the GT inverter will attempt to push power back down the generator's output leads... And will usually cause the voltage to rise or upset the frequency--It is possible it could damage the genset--I just do not know.

    If you have an Inverter / Generator (Honda, Yamaha and some other families), the frequency and voltage can be good enough to sync a GT inverter--And same problem, the GT inverter will try to feed power back into the genset and either cause a voltage/frequency fault or even damage your genset.

    The correct way to do this--You create a sub panel (with transfer switch) that will be powered only by your generator and wire all the home circuits you will need backup power to that sub panel. And the GT inverter should remain connected to your main panel and not be subjected to generator power.

    The whole GT Inverter "back driving" an Off Grid TSW AC inverter has been done and is actually a product in some cases.

    The GT Inverter pushes power back through the off grid inverter which then charges the battery bank (some place for that extra power to go). When the battery bank is full, the Off Grid inverter shifts frequency to 61 or 59 Hz which then "turns off" the GT Inverter...

    It is an interesting way of getting backup power--But is a bit complex and can be pricey to do it this way. And can cause fires if done wrong/with equipment not designed to do this function.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • rocmor
    rocmor Registered Users Posts: 11
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    Re: "Starting" a PVP Inverter during Power Outage

    Yeah, I was wondering about frequency stability. I haven't seen it speced on the smaller (<= 10kW) generators. I did get something from PV, an application note or sorts. I'll try to attach it. It still doesn't, in my mind, fully address the interfacing of GT inverter and generator, unless, of course, there is none.

    Appreciate the feedback
  • inetdog
    inetdog Solar Expert Posts: 3,123 ✭✭✭✭
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    Re: "Starting" a PVP Inverter during Power Outage
    rocmor wrote: »
    Yeah, I was wondering about frequency stability. I haven't seen it speced on the smaller (<= 10kW) generators. I did get something from PV, an application note or sorts. I'll try to attach it. It still doesn't, in my mind, fully address the interfacing of GT inverter and generator, unless, of course, there is none.

    Appreciate the feedback

    To simplify the answer: you have no issue because it is not possible to interface a standard GT inverter to a generator. Period.

    The control system of the GT inverter is not designed to work in this situation. BB has described some of the bad things that can happen if the GT inverter turns on, as well as some reasons why it might not turn on. But these are not problems to be solved, they are immovable obstacles.

    An inverter which is specifically designed for this kind of use (a hybrid inverter with generator connection) might be able to do what you want, but will need a battery bank, etc. just like an off-grid PV system would.
    SMA SB 3000, old BP panels.
  • BB.
    BB. Super Moderators, Administrators Posts: 33,479 admin
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    Re: "Starting" a PVP Inverter during Power Outage

    I would not recommend it unless you are very much into DIY with 120/240 VAC power.

    It is one of those things (trying to balance loads with GT output and a frequency stable generator and not back feed the genset) that is going to be difficult to do automatically and reliably.

    Replacing the generator with an off grid inverter and battery bank--Much easier, but still need to "do it right" so you don't fry a battery bank and/or set it on fire (basically dump load DC controller and/or frequency control with a an off grid inverter).

    You can also install a voltage sensitive switch that "turns off" the GT inverter at ~14.8/29.6/59.2 volts and limit charging that way.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • ggunn
    ggunn Solar Expert Posts: 1,973 ✭✭✭
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    Re: "Starting" a PVP Inverter during Power Outage
    rocmor wrote: »
    I have a 5kW PV installation using a PVP 4800 to tie into the grid. I do not have (or want) battery backup. As I understand it, the PVP control circuits detect and use grid voltage to, in turn, control the DC to AC inversion/regulation/synchronization process. Thus, when the grid goes down due to a power outage, the inverter shuts down. This makes sense from a safety standpoint as you wouldn't want the PV/inverter system to dump 5 kW at 240V into a grid that is under repair. So, if the line repairs are extensive, then I could be totally out of power even on a bright sunny day. A bit ironic.

    I'm thinking about buying a 120V/240V backup generator so we can have some power during the nighttime hours for the essentials. With this generator running during the day, will the PV inverter "start" up properly and function normally? This way, I could have enough power for the entire house. I can't see why not, but I was hoping there might be some out there who have actually done this. Thanks
    The safest and easiest to implement solution is to just run your critical loads from the generator and tie the PV to the grid outside the transfer switch. Yes, the PV will shut down when the grid is down, but unless you have very long and/or very frequent power outages, the contribution to your bottom line that the PV would give you while the grid is down won't be worth the cost of finding a way to get it to keep running while preventing it from backfeeding your generator.
  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
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    Re: "Starting" a PVP Inverter during Power Outage

    ggunn is making sense here as the conditions that cause you to run the generator in the first place is the lack of grid power. you don't want to feed power into the grid and it makes no sense to backfeed the generator either even if you could. let the solar handle feeding the grid when it becomes present and let the generator feed your loads in the house which should be isolated from the grid at this point. when the grid is down the solar is pretty much useless to you with a straight gt inverter.
  • boB
    boB Solar Expert Posts: 1,030 ✭✭✭✭
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    Re: "Starting" a PVP Inverter during Power Outage

    Of course doing what you are asking is definitely "possible".... Just not very "probable". It depends on the generator and I don't know
    of any consumer generators < 10kW that are capable of this. But, it is most likely that when your grid is up and your GT inverter is
    selling back, it is, in fact, selling back to a generator. It's just the right type, like a dam (Hoover, Grand Coulee, etc), or a coal or
    gas fired or nuclear power plant generator. So it is done every day, technically.

    You can also keep the right type of grid tie inverter from selling back into the output of a generator too, but I'm not sure I know
    of one that would handle that either.

    In reality though, it probably wouldn't gain you much if you could sell back to a generator excited grid tie unless you had enough
    loads to run the GT inverter at very high levels because gas powered generators are very inefficient at light to medium loads and
    you would be wasting fuel either way.

    AC coupling with a battery based inverter is the best solution to do what you want to do in my opinion.

    boB
  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
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    Re: "Starting" a PVP Inverter during Power Outage

    sorry boB as gt inverters are really feeding the loads that are on the grid.
  • ggunn
    ggunn Solar Expert Posts: 1,973 ✭✭✭
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    Re: "Starting" a PVP Inverter during Power Outage
    boB wrote: »
    But, it is most likely that when your grid is up and your GT inverter is
    selling back, it is, in fact, selling back to a generator. It's just the right type, like a dam (Hoover, Grand Coulee, etc), or a coal or
    gas fired or nuclear power plant generator. So it is done every day, technically.

    Not really. What your coal or gas fired or nuke generator sees as an effect of your GT inverter feeding the grid is reduced demand, not power flowing back into the generator. A small closed system where the output of the PV may exceed the demand of the loads is a different scenario.

    That's not to minimize the heartburn that high capacity renewables can give utility generators when they turn on and off abruptly. I heard a speaker from ERCOT (the Texas grid) speak about all the computer controls they have to use to counterbalance the effect of wind farms in west Texas switching on and off when the wind changes, and solar has similar behavior. Gas fired power plants have to be pretty nimble to respond to rapid changes in demand because coal fired plants and nukes cannot.
  • inetdog
    inetdog Solar Expert Posts: 3,123 ✭✭✭✭
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    Re: "Starting" a PVP Inverter during Power Outage
    ggunn wrote: »
    Not really. What your coal or gas fired or nuke generator sees as an effect of your GT inverter feeding the grid is reduced demand, not power flowing back into the generator. A small closed system where the output of the PV may exceed the demand of the loads is a different scenario.

    That's not to minimize the heartburn that high capacity renewables can give utility generators when they turn on and off abruptly. I heard a speaker from ERCOT (the Texas grid) speak about all the computer controls they have to use to counterbalance the effect of wind farms in west Texas switching on and off when the wind changes, and solar has similar behavior. Gas fired power plants have to be pretty nimble to respond to rapid changes in demand because coal fired plants and nukes cannot.

    A PV user in, I believe, Puerto Rico, reported that since the nearby Coast Guard station put in a giant PV array, the POCO line voltage at his home fluctuates by 20 volts or more whenever clouds pass over the base. I think that the problem there was more in the poor infrastructure (transmission grid with boost transformers to make up for losses) than feedback to the generating plant though.
    SMA SB 3000, old BP panels.
  • boB
    boB Solar Expert Posts: 1,030 ✭✭✭✭
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    Re: "Starting" a PVP Inverter during Power Outage
    niel wrote: »
    sorry boB as gt inverters are really feeding the loads that are on the grid.

    Maybe I didn't state that quite right... That it is, a generator is the utilities' reference AC source for a GT inverter in the typical on grid installation.

    Consumer home generators are not nearly as good as the ones that GT inverters synch to. If they were
    as stable in voltage, frequency and waveform, then the GT inverter would synch up and sell back to the loads
    that generator is running. (except for some crummy utilities)

    Is that worded better ?

    boB

    PS, Hawaii has looser frequency standards for GT inverters than the mainland USA.
  • ggunn
    ggunn Solar Expert Posts: 1,973 ✭✭✭
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    Re: "Starting" a PVP Inverter during Power Outage
    inetdog wrote: »
    A PV user in, I believe, Puerto Rico, reported that since the nearby Coast Guard station put in a giant PV array, the POCO line voltage at his home fluctuates by 20 volts or more whenever clouds pass over the base. I think that the problem there was more in the poor infrastructure (transmission grid with boost transformers to make up for losses) than feedback to the generating plant though.
    IMO, this is a function of the relative sizes of the PV and the rest of the infrastructure, though of course not necessarily exclusively so. The larger a monolithic PV array and the smaller the conventional power infrastructure is, the harder it is going to be for the grid to respond to rapid changes in power contribution from the PV. Grid tied PV inverters are essentially clamped voltage current sources, which forces the grid as a whole to be a variable current constant voltage source. It's a nontrivial problem even for a large relatively advanced grid structure like ERCOT; it's not surprising that Puerto Rico would have problems.
  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
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    Re: "Starting" a PVP Inverter during Power Outage
    boB wrote: »
    Maybe I didn't state that quite right... That it is, a generator is the utilities' reference AC source for a GT inverter in the typical on grid installation.

    Consumer home generators are not nearly as good as the ones that GT inverters synch to. If they were
    as stable in voltage, frequency and waveform, then the GT inverter would synch up and sell back to the loads
    that generator is running. (except for some crummy utilities)

    Is that worded better ?

    boB

    PS, Hawaii has looser frequency standards for GT inverters than the mainland USA.

    boB,
    yes, that is better.

    they may have looser standards, but don't they have the same choices in gt inverters as we on the mainland have? it is not cost effective to do special for them, is it?
  • boB
    boB Solar Expert Posts: 1,030 ✭✭✭✭
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    Re: "Starting" a PVP Inverter during Power Outage
    niel wrote: »
    boB,
    yes, that is better.

    they may have looser standards, but don't they have the same choices in gt inverters as we on the mainland have? it is not cost effective to do special for them, is it?

    Hawaii's grid is so small and they have so many renewables there that they have relaxed the grid frequency (and voltage ?) for GT inverters.

    I'm not sure exactly which inverters comply looser but I do know that they use a LOT of Enphase inverters.

    boB
  • inetdog
    inetdog Solar Expert Posts: 3,123 ✭✭✭✭
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    Re: "Starting" a PVP Inverter during Power Outage
    boB wrote: »
    Hawaii's grid is so small and they have so many renewables there that they have relaxed the grid frequency (and voltage ?) for GT inverters.
    boB

    Something that mainland POCOS cannot do since they have long distance transmission lines and regional interties to worry about and therefore have to maintain a tight synchronization. (Except for Texas which only allows DC to cross the state borders!)
    SMA SB 3000, old BP panels.
  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
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    Re: "Starting" a PVP Inverter during Power Outage
    inetdog wrote: »
    Something that mainland POCOS cannot do since they have long distance transmission lines and regional interties to worry about and therefore have to maintain a tight synchronization. (Except for Texas which only allows DC to cross the state borders!)

    that's because they are on their own little grid. maybe that's just in case mexico gets that area back again.:p
  • rocmor
    rocmor Registered Users Posts: 11
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    Re: "Starting" a PVP Inverter during Power Outage

    As I said, once upon a time, it's just a bit ironic having 5 kW of solar power capacity unavailable during a power outage (without proper battery backup, etc.). This all came about with the approach of "Superstorm" Sandy and the worry that, with no power and, at this time, no generator, significant rain = flooded basement (it's hard to keep the water out when you have fieldstone walls, a dirt floor and a healthy layer of clay underneath). As it stands, even with power, I have an old pool pump on standby downstairs as my 1/3 hp sump pump cannot keep up in heavy deluges. I guess it's off to the generator store... as soon as there's any in stock.

    Interesting discussion. I appreciate the knowledge and ideas.

    Speaking of sync-ing, I was just pondering how they sync distant generation sites on a same grid. For example, for every 1000 miles transmission, you get a phase shift (relative to source) of 116 degrees. A little off-thread but fun to think about.

    Cheers
  • boB
    boB Solar Expert Posts: 1,030 ✭✭✭✭
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    Re: "Starting" a PVP Inverter during Power Outage
    rocmor wrote: »
    Speaking of sync-ing, I was just pondering how they sync distant generation sites on a same grid. For example, for every 1000 miles transmission, you get a phase shift (relative to source) of 116 degrees. A little off-thread but fun to think about.

    Cheers

    The generators at each end are synchronized with each other like your PVP inverter so delay would not matter.

    1000 miles is usually done by DC transmission these days I think. Is it really that much shift ? Hmmm...

    boB

    PS... I think it's really quite a bit more than 116 degrees.
  • ggunn
    ggunn Solar Expert Posts: 1,973 ✭✭✭
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    Re: "Starting" a PVP Inverter during Power Outage
    rocmor wrote: »
    As I said, once upon a time, it's just a bit ironic having 5 kW of solar power capacity unavailable during a power outage (without proper battery backup, etc.). This all came about with the approach of "Superstorm" Sandy and the worry that, with no power and, at this time, no generator, significant rain = flooded basement (it's hard to keep the water out when you have fieldstone walls, a dirt floor and a healthy layer of clay underneath). As it stands, even with power, I have an old pool pump on standby downstairs as my 1/3 hp sump pump cannot keep up in heavy deluges. I guess it's off to the generator store... as soon as there's any in stock.
    Ironic, maybe, but that's the way it has to be and it's not just the law. You can't pull current on demand from a PV inverter without batteries.
  • inetdog
    inetdog Solar Expert Posts: 3,123 ✭✭✭✭
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    Re: "Starting" a PVP Inverter during Power Outage
    boB wrote: »
    The generators at each end are synchronized with each other like your PVP inverter so delay would not matter.

    1000 miles is usually done by DC transmission these days I think. Is it really that much shift ? Hmmm...

    boB

    PS... I think it's really quite a bit more than 116 degrees.

    AFAIK, long haul transmission, especially open wire lines, is still mostly done over AC, and the cumulative effect of many short haul interconnections would still cause troublesome phase shifts.

    I believe that phase shifts of that order are currently dealt with by expensive phase shifting devices or AC to DC to AC conversion, with corresponding large energy losses.



    Well, if one foot of transmission line corresponds to about 1.3 nanoseconds, then 1000 miles would be 6.86 milliseconds. And the period of a 60 Hz waveform is 16.66.... milliseconds, so 116 degrees seems to be in the ball park.
    ...delay would not matter

    It would not matter with only two power generators and one interconnect AND the power is flowing only in one direction. But when you have alternate paths and many nodes in a network and when you allow the direction of power flow in the interconnect to reverse, you will have issues that have to be addressed.
    SMA SB 3000, old BP panels.
  • BB.
    BB. Super Moderators, Administrators Posts: 33,479 admin
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    Re: "Starting" a PVP Inverter during Power Outage

    And it gets even more complex... Once your transmission line reaches about 1/4 wave length, if you pop a main line breaker at the destination (load) side of the line (call it ~800 miles away), then the open end of the transmission line reflects the energy back to the source--A 1/2 wave out of phase--Or basically looks like a dead short to the source (generator).

    There you go--popping a circuit breaker puts a dead short on the line as far as the generator is concerned. Very handy. :p

    From ~30 year ago AC Power Engineering Class (from what I remember)--That exact thing happened in Germany (probably long before that 30 year old class)--Far end transmission line breaker opening caused a "dead short" on the generator plant.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • inetdog
    inetdog Solar Expert Posts: 3,123 ✭✭✭✭
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    Re: "Starting" a PVP Inverter during Power Outage
    BB. wrote: »
    From ~30 year ago AC Power Engineering Class (from what I remember)--That exact thing happened in Germany (probably long before that 30 year old class)--Far end transmission line breaker opening caused a "dead short" on the generator plant.

    -Bill

    I bet that smarts. :-)

    Simple solution is to send a faster-than-light message to the other end so that they can open their breaker too before the reflection gets there. And then watch the sparks fly. :-)

    More seriously, what is the solution for that case? For the receiving end to short the line after disconnecting from it? Or just to dissipate the energy somehow for the next few cycles?
    SMA SB 3000, old BP panels.
  • BB.
    BB. Super Moderators, Administrators Posts: 33,479 admin
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    Re: "Starting" a PVP Inverter during Power Outage

    Dissipating a few 100 Megawatts in your generator--One way to do it. :cry:

    I honestly do not know--One obvious method would be to turn off the power "at the source" (remote trip breaker, keep power connected at the destination for that 1/2 cycle or so--no reflection back to the source)--But that is probably not aways possible or even desirable.

    I never had a chance to walk with a real power engineer around a power plant (boy, I would love that)... But it is amazing all of the issues that there are when you work with a large/distributed power system (or any large system).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • peakbagger
    peakbagger Solar Expert Posts: 341 ✭✭✭
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    Re: "Starting" a PVP Inverter during Power Outage

    I have a friend who does it as a consultant, the math gets real deep. To add some more complication throw in higher level harmonics and power factor.