Inverter Sinewave Specifications

I've also posted this question at Solar-Guppy as well to get the widest amount of input.

I am presently in the learning stage of building a "grid-tie" system. At present the inverters of consideration are the (Outback GVFX3524) and (Xantrex SW4024).

My concern for the moment is avoiding "mod-sinewave" issues: front-load washer has variable-speed drive, garage door openers, home theater system components, computer/peripheral components. Just don't want to install a system might cause operational or damage issues with the unknowns.

The Outback appears on the surface to be the closest to true sinewave, right or wrong? When I research the Xantrex site I cannot determine which units are "modified-sinewave", not clearly stated in the docs.

Are the  Xantrex GT series "modified" or regular sinewave units? Sure is hard to tell in the literature for all the models.

Thanks for any input.

Comments

  • BB.BB. Super Moderators, Administrators Posts: 29,002 admin
    Re: Inverter Sinewave Specifications

    The Xantrex GT series is a Sinewave, not modified sinewave (really a modified square wave)... You will have no problem with the unit running any of your appliances.

    The user manual (PDF link below, page A-2, or page 68 of 82) says that it is a sine wave (technically, it is an stop-wise approximation of a sine wave with harmonics that, IIRC, are less than 5% of the fundamental frequency).

    http://www.xantrex.com/web/id/1172/docserve.asp

    I have my Xantrex GT 3.0 installed in my garage right next to my garage door opener switch, about 1' from my AM/FM Intercom, and run a wired/wireless network in my home. One computer is on a UPS. I have Fisher & Paykel Washer/Drier which use DC stepper motors instead of the normal induction motors.

    A question for you... For example, the Xantrex SW family is an inverter/charger unit. Are you thinking about battery backup for your home?

    A Xantrex GT cannot supply any AC power if the utility mains have failed--and I don't believe it would work with a normal home backup generator either. For backup at my home, I currently only use a Honda eu2000i gasoline generator and a manual transfer switch. Near SF CA, I don't have any power failures that last more than a few minutes or an hour. I have the generator prep'ed for earthquake use (fridge/freeze and a few lights). Propane and camping stoves for cooking/heating water.

    A properly configured SW (or other system) can supply backup power from batteries with charging/power also supplied by solar panels/windmills/generators... Owning a battery backed system involves more money and servicing time by the owner. But is nice if you have or expect lots of power failures.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • System2System2 Posts: 6,290 admin
    Re: Inverter Sinewave Specifications

    Hi BB,

    Thanks for your reply and I will check out the Xantrex link for the GT3.0.  I assume the GT3.8 will be of similiar design as well.  Using the worksheet from the Alternative Energy Store my solar array design wattage works out to 3.94KW.  Initially I will probably purchase about 2KW equivalent solar panels.

    My major concern was injecting noise into home entertainment equipment or other problems that could relate from sinewave shaping.

    In answer to your question, I am also considering some battery backup capability. I also have a portable Generac 5KW as well...manual start.  We live in a suburb of Denver, Aurora Colorado.  We average about 3 outages a year, mine have always occured in fine weather...go figure.  But during heavy snows there are outages of several days in the adjoining areas depending on severity, just haven't hit us so far.

    Xcel Energy offers a rebate program through their Solar Rewards which requires a 20 year contract.  If it looks like I don't go down that road I might end up changing things around.

    It would be nice to have some savings via solar, and would also be a priority to have the gas furnace operational in event of grid failure.

    Your response seems to indicate the GT3 does produce a clean signal.

    As far as the SW line my uderstanding at this point is they require a seperate GTI module, some folks have characterized it as possibly problematic.  And I believe the SW's are modified sinewave?


    Regards,
  • nielniel Solar Expert Posts: 10,311 ✭✭✭✭
    Re: Inverter Sinewave Specifications

    cumulus,
    i have to clarify to you that all inverters are technically modified sinewave. it is the number of steps in each cycle that determine its distortion away from a real pure sinewave. yes they at the very beginning of the waveform look like stairs and the more steps in it the closer it is to the true sharp the natural sinewave would have. if the inverter were of a true sinewave it would have had to have been produced by a 60hz oscillator and amplified to the proper point with class a amplifiers. that type of amplifier has extremely low distortion, but would consume vast amounts of power to produce a good ac wave with a fraction of this outputted to usable power. many quality stereo amplifiers are like this. the input power from the utility may be 3-4x as high as the output watts making efficiency very low.
    if memory serves correctly xantrex is as i have last heard 56 steps and outback is 256 steps.
  • System2System2 Posts: 6,290 admin
    Re: Inverter Sinewave Specifications

    Hi Niel,

    Thanks for your response.  Yes, I just got done perusing the Xantrex SW4024 manual and it shows sinewave steps of 34-52 steps per cycle.  I've looked through a Outback GTFX manual but haven't run across the number of steps in their sinewave cycle.  256 would indeed be much smoother but can't confirm as yet.  In any event, if a inverter like the SW4024 does use 34-52 steps in it's waveform, is this smooth enough to eliminate my concerns regarding peripheral devices developing problems?  I.E. cablemodems, printers, network hubs, satellite receivers audio eqpt and such.  Would be a shame to have to shut down the inverter everytime you operate a home theatre system because some transient noises came through.

    Just trying to avoid some nasty incompatibilities.


    It also looks like my local utility only allows CEC listed inverters to participate in their Solar Rewards program.  The list for now only shows the one model for the Outback, the GVFX series in 48V.

    My current load center is of the outdoor rainproof variety, so at least I don't need to penetrate walls during an install.  Maximum distance to fixed ground-mounted panel arrays should work out to about 40-50 feet max.

    Regards,
  • halfcrazyhalfcrazy Solar Expert Posts: 719 ✭✭✭
    Re: Inverter Sinewave Specifications

    i have a pair of sw plusses and have no problem with interference the only time i get anything is when i have generator running and the fans are on i get a little but that is the same with outback.
  • BB.BB. Super Moderators, Administrators Posts: 29,002 admin
    Re: Inverter Sinewave Specifications

    From what I have read from Solar Guppy, the GT 2.8, 3.0, and 3.something are all the exact same unit. The only difference is that the firmware has been modified to limit max power to other levels (basically, the 3.0 had lots of headroom that allowed them to crank up the power and still keep everything in spec... Xantrex marketing decided that they wanted address the market with one piece of hardware at different price points). Regarding the 3.8--Solar Guppy will have to tell you if that one is part of the same marketing/hardware plan.

    Regarding Modified Sinewave, the newer UL/Government requirements would probably prevent a currently manufactured high powered inverter from being connected to the public mains now-a-days. Power Factor corrected power supplies (larger than a few tens of watts--maybe a hundred watts or so), were mandated a few years ago (forgot the exact year). The old, cheap, switching power supplies would just use a diode and capacitor bridge to convert AC to high voltage DC which would then be converted to the needed voltages. A diode/cap rectifier would only take current at the peak of the voltage wave form--causing high peaks, high I^2R losses for the utilities, and can even lead to wiring failures and fires in buildings with lots of old PC. Having proper power factor for a modern GT inverter should be a requirement too--I cannot imagine it no being required.

    So, I would not worry at all about anyones large Grid Tied unit. And, in any case, you would still have the utility supplying "clean power" to your devices. Only in an off-grid application would a modified square wave inverter cause any problems (and, usually, it is motors that are the worst affected by modified square wave supplies).

    I have a cable modem plugged in less than 3 feet from where my Xantrex GT 3.0 is connected. No problems since it was installed last September and a 108 MB Belkin Pre-N wireless network 50' away. I can run the laptop 10' from the Xantrex and see any problems at all. The Stereos, digital over the air receiver, two laser printers all work fine too.

    The only noise you may notice is interference (hum) on the AM radio band. I can hear it when I back my car next to the garage/AC Power Meter during the day (but I cannot hear the AM band hum on the intercom radio install 1 foot away from the Xantrex). But, I can even hear AC hum from my dimmers too. I would not expect much problem with any other RF appliances.
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • nielniel Solar Expert Posts: 10,311 ✭✭✭✭
    Re: Inverter Sinewave Specifications

    either will work. you seem more interested in the xantrex so i took the liberty to go straight to the specs of the gt 3.8 and it also lists others there. of interest is the thd percentage. this is short for total harmonic distortion and you can see it is under the utility standards of being less than 5%.
    http://www.xantrex.com/web/id/172/p/968/pt/23/product.asp
    the xantrex is a basic grid tie no battery backup style inverter. if you desire battery backup you don't want the gt 3.8.
  • System2System2 Posts: 6,290 admin
    Re: Inverter Sinewave Specifications

    Thanks guys, you've definitely eased my concerns on the aforementioned.

    Is it also typical to have a 20-year contract with the utility for NetMetering?

    I feel confident in installing a system once I am clear on all the technical details.

  • System2System2 Posts: 6,290 admin
    Re: Inverter Sinewave Specifications

    Neil:

    Your right, I'll probably lean in the direction of the SW4024, haven't worked out the wire sizes yet for an array of 3.9KW at max distance of 50 feet. Depending on how things budget out will probably start out with 50% of the panels to get things up and running and then addon at appropriate fund point.

    I didn't notice in your post if you are participating in NetMetering, any opinions on this?

    Regards,
  • BB.BB. Super Moderators, Administrators Posts: 29,002 admin
    Re: Inverter Sinewave Specifications

    Cumulus,

    Here in California, I am on a 1-year contract with Net Metering and Time Of Use metering (E7). You can see the rates on this link.

    http://www.pge.com/tariffs/ERS.SHTML

    E1 is standard residential. E7 is Time of Use between Noon and 6 PM Mon-Fri. Those two will give you the basic spread in price out here.

    For California, Net Metering basically adds up your bill every month and puts the money into a paper account (positive and negative/used and generated power). You have to pay about $6 / month service fee and minim electric charges. You can pay any net amount used every month, or wait until the end of the year and pay (if negative balance) it all at once. If you you have a positive credit, it is wiped out and you start from zero again. I believe that the first year is a one year minimum contract--you are month to month after than (although, the one year accounting is still there).

    Using Time of Use metering, I can generate more power than I use Noon to 6PM Mon-Fri. So, during the summer, I can get a net factor of 3x credit (like having 3x the size of solar panels) for my bill. Summer Peak / off peak is $0.29 / $0.09 per kWhr. Winter is $0.12 / $0.09 per kWhr.

    If you expect your power to be constant 24/7, TOU for E7 would break about even. If you use power more during peak time (like a home business and/or heavy summer air conditioning), you should probably not use this Time of Use.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • RCinFLARCinFLA Solar Expert Posts: 1,280 ✭✭✭
    Re: Inverter Sinewave Specifications
    Cumulus wrote:
    the SW4024 does use 34-52 steps in it's waveform 

    Although, as the manual states, the SW series can make up to 52 changes during the 16.667 msec, 60hz period, it is actually only capable of 27 discrete voltage levels over the entire sinewave voltage swing during this period.   The approximate worse case resolution match to the instantaneous grid sine wave is about 7.5 volts for a 120 vrms grid tie. 

    The SW series have three transformers with their secondary winding series connected.  Relying on low leakage inductance transformers produces a very low impedance output on the secondary winding when the primary side winding is shorted.  By this mechanism the SW series mixes the three possible voltage states possible on three transformers in total combinations generating the 27 possible net output voltage possiblities.  Primary side voltage on the transformers are 24 v for 4024's and 48 v for 4048's.  Each transformer is driven by a full wave H-bridge FET array that applies + battery, 0 volts, or - battery to the respective transformer's primary winding.

    The net result is the largest transformer's secondary produces +135, 0, or -135 volts, the mid-sized transformer produces +45, 0, or -45 volts, and the small transformer produces +15, 0, or -15 volts.   These voltage are slightly variable based on input battery voltage and load on the inverter.  Mixing the 3 possible output voltage levels from each of the three transformer yields the 27 possible series combination total voltage range.

    As reference to the 34 to 52 possible timeline change increments, when the battery voltage is low more time steps can be used, when battery voltage is high, less time increment steps must be used.  This AC input voltage variation is also involved in the time increment selection.  The primary objective is to match the transformer voltage stack as close to the instantaneous input AC waveform as possible.

    The magic is the difference between drawing from the batteries or charging the batteries is just the slight program sequence difference of whether the transformer combination stack voltage is slightly below or above the instantaneous AC input waveform voltage.
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