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Connect grid tied solar inverter to DC+ Bus of VFD?

  1. Feb 7, 2019 #1
    What solutions are there to feeding a high DC voltage rated (eg 1000VDC) certified grid tied solar inverter such as a Yaskawa Solectria PVI23-480; from an alternate DC+ bus supply. For instance the 670 VDC input from a solar string; might be replaced by a a 6 pulse rectifier; (possibly from a multi kilowatt VFD fed from a 480VAC synchronous gen set) which would in effect supply the DC+ requirement of the grid tied solar inverter.
    Whilst extremely straight forward... how would the lack of current control into the grid tied converter be handled. Suggestions and comments deeply appreciated.
  2. jcsd
  3. Feb 7, 2019 #2


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    Welcome to PF.

    It might seem simple at first, but high DC voltages are very difficult to control. Simply switching on/off takes special techniques, as do circuit breakers. DC voltages above 600V are expensive to control. 1kVDC will present you with real challenges. You are entering a very dangerous world.

    The MPP controller in the grid-tie inverter will be confused by the supply impedance of rectified AC from an alternator. The inverter is expecting a constant current input but will receive a low impedance rectified voltage with a high ripple current. The impedance inversion also precludes placing a battery bank between solar panels and a grid-tie inverter. I have considered similar modifications, but the risks significantly outweigh the benefits.

    It is better to install separate independent systems.
    The danger of lethal shock and of fire are just too high.
  4. Feb 9, 2019 #3
    Thankyou for your wise and sober summary. Every word is duly noted

    BUT grid tieing synchronously generated AC (of say 20-100 KVA) in an acceptable manner to the grid utility poses several concerns beyond the tens of thousands of dollars required. Could grid tieing be accomplished in any innovative way if cost is the least of any constraints

    The fact is that there are no such installations on the province wide grid of 3542MW generating capacity. I can safely assume that applies both country or continent wide. And venting and flaring waste flare gas has to be one of the greatest wastes of resources that goes back 70 plus years and has not yet ended.

    For anyone willing to pay whatever the cost of an innovative solution ......

    The question is: Are there any "standards approved" devices (eg UL or CSA with the trademarked C accompanying) that could safely and reliably connect a properly rated solar grid tied inverter to a "common" DC+ bus system provided by a 6 pulse rectifier built into an CSA approved and properly sized variable frequency drive.

    No batteries involved please; just a standalone AC genset running like a top producing water and CO2; and grid tied through a usable and acceptable converter/"black box"/inverter system.
    Last edited by a moderator: Feb 9, 2019
  5. Feb 9, 2019 #4


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    You do not need the inverter if you run an engine on waste gas with a synchronous generator connected to the grid.
  6. Feb 9, 2019 #5


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    Bingo; that's exactly right. @looking, do you undersatnd @Baluncore 's point? An inverter converts DC to AC. If you have an AC generator, and a AC power grid, no inverter is needed.

    But you will need auxiliary equipment to protect the generator and the grid from malfunctions, and to synchronize, and to meter, and potentially to respond to the commands of the grid operator. You would need analogous auxiliary equipment using an inverter too.

    There are numerous commercial solutions meant to tie homes with rooftop solar to the grid. They integrate the inverter and auxiliary functions into the same box and call it "interface". Homeowners typically buy or rent the interface from the power company.

    But I don't know of any similar commercial packages to interface AC generators to the grid. That is probably because AC generators come in all sizes from kW to GW.

    Your idea of capturing waste heat to make electricity may be welcomed by the local power company. But you need to talk with their engineers about their specific requirements before you connect. If you want to spread the idea across the province and the country, you must comply with the specific requirements of each local power company. If there is an approved interface available, those utility engineers can tell yuou about it.

    So, my answer is to ask your questions to the local power company, not to PF.
  7. Feb 9, 2019 #6
    Of course the above observation is 100% correct

    But; from my first hand experience; I can say that for smaller scale synchronous generation (approx. 5KW to <100KW) there are no examples of "synchronous generators connected to the grid."

    That would change if some way were found to use certified existing products to do things differently. It apparently won't happen by using conventional relay controls and off site direct control somewhat equivalent of commissioning much larger synchronous utility owned units. That is a non starter and would only be proven untrue if we could be pointed to at least a single case where anyone has actually overcome the dilema of no small scale synchronous generation connected to the grid.

    Maybe there are no solutions; but also how can it hurt to identify any currently available alternative possibilities (if any); and give it a fair consideration.
  8. Feb 9, 2019 #7
    Our messages have crossed whilst I was preparing a reply

    QUOTE There are numerous commercial solutions meant to tie homes with rooftop solar to the grid. They integrate the inverter and auxiliary functions into the same box and call it "interface". Homeowners typically buy or rent the interface from the power company.

    But I don't know of any similar commercial packages to interface AC generators to the grid. That is probably because AC generators come in all sizes from kW to GW. UNQUOTE

    I have carefully tried to say "grid tied solar inverter" at all times. The "Interface" is already included in the grid tied package AND other than the metering is not in any way the responsibility of a grid operators liability. We both agree that such an interface isn't probably available as a commercial package for small scale grid tied synchronous AC generation .

    But the facts all point to the conclusion that both my problem (and everyone else's similar predicament) is not being advanced; by saying you don't need an inverter for AC generation. Maybe nothing is further from the truth.
  9. Feb 9, 2019 #8
    Last sentence should be "by saying you don't need an inverter for AC grid tied generation. Maybe nothing is further from the truth"
  10. Feb 9, 2019 #9


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    My first hand experience is the opposite. I once lived in Potsdam NY. We had 22 hydro plants (with 3-4 generators each) on the Raquette River alone. (We used to joke that it was the damdest river in the country.) In New York state alone, there are more than 300 grid connected small hydro plants.

    Diesel generators on islands. Wind farms. There are tens of thousands of small synchronous generators connected to the grid in North America. Where does your experience come from?

    Edit: not wind farms.
    Last edited: Feb 9, 2019
  11. Feb 9, 2019 #10
    I'll conclude by responding to this quote from above

    QUOTE But you will need auxiliary equipment to protect the generator and the grid from malfunctions, and to synchronize, and to meter, and potentially to respond to the commands of the grid operator. You would need analogous auxiliary equipment using an inverter too. UNQUOTE

    I think that the strong points of a "solar grid tied inverter" include automatically synchronizing connection to the grid; lack of additional requirements of auxilliary equipment for grid connection; and no necessity of responding to commands of the grid operator (which is currently typical with less than 100 KW generating capacity) are all good reasons for wanting to feed an identical "solar grid tied inverter; with the energy that initially comes from synchronous generation... but through some approved black box creates a close enough DC bus output that would fool any "Solar grid tied inverter" into thinking it was connected directly to a solar panel array.

    Ergo the "solar grid tied inverter" can't tell the difference and I'm sure the electrical utility would have no legitimate concern about whether the sun or flare gas was the energy source. Both sources are considered in same light for environmentally appropriate special generation programs.

    Again small scale grid tied AC synchronous generation is a non starter; got to try to get to point of actually looking for alternative ways such as what apparently solar energy has accomplished in regards to grid connection

  12. Feb 9, 2019 #11
    My experience comes from being told by the electrical utility some two years ago (and I have asked since if anything has changed) is that " if approved under the special programs for <100Kw flare gas generation; the application would be the first one in the utilities history"

    Proof by not submitting the application (for various reasons including "carbon taxes"; contract details; "demand charge potential; upgrades as demanded etc). Apparently; other potential generators agree; as not much of a rush by potential similar other recruits (to my knowledge).

    Sure there are a handful of 5 MW Rankin cycle waste heat recovery generation projects nearby. Sure there are windfarms (but not many grid tied working connections make sense less; and I'm talking tens of KW generating capacity ) and even smaller ones may utilize equipment like Aurora "grid tied wind inverters" similar to their "grid tied solar inverters". And there is hydroelectric generation; but most of those are orders of magnitude away from what I'm talking about.

    What is needed is something like the convenience and ease of connecting the numerous common "grid tied solar inverters" for anyone who wants to use internal combustion engines that commonly happen to have a synchronous generator attached.

    The solar panel craze isn't 20 years old (more like 5 years or less here)... so maybe my inquiry is just a few years premature.

    And Geez I never saw such prompt responses on any other forum. I do appreciate it all.
  13. Feb 9, 2019 #12


    Staff: Mentor

    To do maintenance on the power lines leading to the neighborhood, the utility needs the ability to shut off the power. But a household grid connected power source could energize, or re-energize the local wires, causing electrocution hazard for the utility work crew. That is just one of several reasons why the utility can and does insist on those auxiliary functions.
  14. Feb 9, 2019 #13
    And that protection comes from the last piece of consumer supplied equipment just before the utility owned metering equipment. In a solar photovoltaic system it is called a "grid tied solar inverter" CSA approved of course.

    In a smaller wind generation project that piece of equipment would commonly be a "grid tied wind inverter" CSA approved of course

    And in a few tens of kilowatts of synchronous generating capacity; in theory; the power could be rectified and with the proper "interface" (black box or any name that anyone comes up with for a CSA approved electrical apparatus) could feed that same "grid tied solar inverter" mentioned above. There ain't nothing completely magical about creating a voltage and current similar enough to what a "grid tied solar inverter" expects as input from a solar array. All having to be CSA approved before applying to connect to the electrical grid.

    Anyone else who has any ideas about a suitable "black box" Please weigh in. Thanks.
  15. Feb 9, 2019 #14

    jim hardy

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    A slight;y oversped induction motor becomes a generator and will return power to the mains so long as it can draw its magnetizing current from them.
    Read up on "Induction Generator". It's dirt simple..
    But the thought needs careful consideration - it'll hurt their power factor and might violate the business agreement with their utility.

    It could interact with power factor correction equipment(capacitors) and backfeed the utility as mentioned by @anorlunda, so look before you leap.

    Down in South Florida the sugar mills used to have small inhouse steam power plants to burn the cane residue and save on their electric bill. They had agreements to sell excess power to the utility.
  16. Feb 9, 2019 #15


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    If you take an off-the-shelf grid-tie inverter and supply it with energy, then it will require an intelligent switching converter that produces an output voltage somewhere between 300 and 600 volts. That output must appear to be a quiet constant current of between zero and about 20 amps. Changes in available power must appear as a gradually changing current. Both maximum output voltage and maximum output current need to be programmed to suit the inverter. Control signals from that switching converter need to feed back to regulate the engine power, also with at least an emergency stop. The alternator on the engine will need field control to regulate voltage, while fuel input will need to be controlled by the switching converter current source.
    With feedback within feedback, and many different engine and alternator possibilities the design will need to be adaptable to many possible configurations.
  17. Feb 10, 2019 #16

    Excellent and to the point comments.
    Until now I was always thinking of feeding the grid tie inverter component from a "15 amp" 3 phase breaker source off the existing standalone fixed speed rocksolid 480VAC genset.

    But having a different separate dedicated smaller genset to supply the necessary power to an isolated "solar grid tied inverter" will be investigated with all your comments above taken into account. Just maybe the front end (DC+ bus "output" of an Allen Bradley VFD.) could be one of the components to feed the grid tie converter.

    I'll maybe begin with a 1HP VFD and experiment to prove that the DC + voltage can be suitably controlled through feed back signals that regulate field voltage on alternator end; as well as determining if current control is maintained within inverter parameters by other feedback signals. Thanks

    How does that sound??
  18. Feb 10, 2019 #17

    That was what I initially thought was the first choice.

    So two years ago or so I built a 480V 20 HP electric motor drive coupled directly to an identical 480V 20 HP induction motor to function as the asynchronous generator when driven at slightly over its 60 HZ synchronous speed. The drive motor was switched in after a time delay for the asynchronous end to come up to "running" speed through the live utility feed. And in a short test it did work and produced some 16,000 watts of power as I recall. It does indeed solve the synchronization to "line" utility problem.

    However; because of a very poor power factor which no doubt was contributed by the "asynchronous induction generator" power facor of 0.8; as ll as a large transformer needed to reduce down to a 208V AC 3ph utility service voltage....this solution was abandoned.. Another reason was that although such an asynchronous AC generator is "inherently" quite safe for utility islanding concerns; there are possible scenarios where capacitance can initiate self-generation even though the utility supply is down. Thus there were going to be requirements of various controls and power quality issues to be resolved to whatever the utility deemed satisfactory.

    Maybe readers can see why I am looking for an off the shelf solution that is reported to meet all the possible concerns for grid connection...as it does appear to be the case for solar grid tie; and even smaller wind grid tie inverter apparatus.

    The utility is obliged to allow tie ins. But they can and do make all the rules; including a "blank cheque"; and a contract that they get to change at any time. Nothing guarantees that you will ever have any thing but expenses at the "end of the day".
  19. Feb 10, 2019 #18

    jim hardy

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    Thanks - i wasn't sure if you were aware.

    And you were a few year ahead of me with the idea.

    Thanks for the feedback !!

    good luck

    old jim
  20. Feb 10, 2019 #19


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    I don't think you have understood that the PV grid-tie inverter needs to be fed from a steady current source that behaves just like a PV panel.

    The rectified DC bus from a VFD is simply the input reservoir capacitor voltage, which has very low impedance, but you need very high impedance.
    You might be able to program a VFD to generate a constant output current, then rectify that current for input to the grid-tie inverter.
  21. Feb 10, 2019 #20

    jim hardy

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    Is that because of the inverter's MPPT characteristic ?
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