q regarding generator and grid tie

autoxsteve
autoxsteve Solar Expert Posts: 114 ✭✭✭✭
Referencing this post, I've been wondering.

Could I purchase a 240 vac generator (1500W - small is good) and tie it into my home's electrical system and take advantage of the solar system? I've been considering getting a generator and having it 'trick' the grid tie inverter into thinking the grid is there (so it runs) would be another benefit.

I suspect it would work, but I'm wondering what the generator would react to the extra power from the solar coming back at it.

Thoughts, please...

Comments

  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Re: q regarding generator and grid tie

    Not likely... A grid tie inverter just dumps as much power as it can from the solar panels to the AC lines. If the load on the "local main" is less than the power supplied by the GT inverter--the only place for the power to go is back into the generator--which has no way to handle the back driven energy--and if is mechanically governed, the generator's voltage/frequency will quickly fall out of range and the GT inverter will island.

    However, a standard off grid inverter when connected to a Grid Tied inverter (or similar device) can take the excess power supplied by the GT inverter and actually charge the battery bank (back driving the off-grid inverter).

    The Skystream wind turbine system uses this topology (along with a local "battery" charge detector and feedback to shut down the Wind Turbine to as a "charge controller", and others (via Solar Guppy) have studied using the GT/Off-Grid inverter topology for future systems/products (maybe?).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • crewzer
    crewzer Registered Users, Solar Expert Posts: 1,832 ✭✭✭✭
    Re: q regarding generator and grid tie
    I've been considering getting a generator and having it 'trick' the grid tie inverter into thinking the grid is there
    Steve,

    This won't work.

    Since islanding is dangerous and can even be fatal to unsuspecting utility workers, U-I inverters contain “anti-islanding” features to isolate them from the grid distribution network if the grid operates outside of fairly tight specifications, including failure or disconnection. A further complication is the possibility of connecting asynchronous systems (the grid and the home’s “mini-grid”) when grid power is restored. This “out-of-sync” condition would likely cause equipment damage.

    The National Electrical Code (NEC) para. 690.60 requires that U-I inverters be listed (tested and recognized). The standard listing reference UL-1741. In short, 1741 requires both passive and active anti-islanding features. Passive methods include sampling the grid for line voltage and frequency within certain specifications. For example, the acceptable line frequency window is between 59.3 Hz and 60.56 Hz. This particular specification alone makes it difficult for G-I inverters to sync up to typical residential- or commercial backup generators.

    Active anti-islanding features are contained within the inverter's controller. These features purposely disturb the connected AC grid and then monitor the response. If the grid does not respond as expected, the active circuit causes the inverter to cease power conversion. Again, generators and other unstable power sources may not behave as expected, which could cause the G-I inverters to disconnect.

    However, stand by for some novel "AC coupled" solutions in the near future. ;) See: http://download.sma.de/smaprosa/dateien/5610/SI5048A4-11-BU3307.pdf

    Regards,
    Jim / crewzer
  • mnittler
    mnittler Solar Expert Posts: 63 ✭✭✭
    Re: q regarding generator and grid tie

    Because of the UL1741 grid tie requirement the only generator that might be stable enough to work would be one of the Honda EU (inverter) type. Most of your small gas genertors do not maintain the voltage or frequency anywhere near where it is required to be to satisfy UL1741 so the grid tie inverter will just shut down. I don't know what happens when the actual load is less than the grid tied inverter supply. It will probably let the trapped smoke out of the Honda EU generator or the inverter? Previously in this thread there was much discussion on the possibilities. It seems that each of them ran into a road block at one time or another and required yet another fix. It sounds easy but it is very complicated when considering all the possibilities that can or will happen. Remember that if trying to trick a grid tied inverter to work off grid that you must be totally disconnected from the grid (make your own power island). Dangerous conditions could be created if everything is not working properly between the ears.
    19.76kw Solar/GT Enphase IQ7+ MicroInverters
     5.40kw Solar/GT ABB/Aurora 300 MicroInverters (AC coupled to Schneider/Xantrex XW6048 output)
     6.00kw Solar/Hybrid Xantrex XW6048 Inverter w/2 strings Trojan L-16E-AC Batteries (48VDC)
    18kw Kohler Propane Generator

  • RandomJoe
    RandomJoe Solar Expert Posts: 472 ✭✭✭
    Re: q regarding generator and grid tie
    BB. wrote: »
    However, a standard off grid inverter when connected to a Grid Tied inverter (or similar device) can take the excess power supplied by the GT inverter and actually charge the battery bank (back driving the off-grid inverter).

    I have seen this comment made on several occasions, and finally just had to ask. Curiosity overwhelms me!

    When you say "a standard off grid inverter", do you really mean *any* "off-grid" style inverter would work this way? Even the little Samlex inverter I have sitting here on my desk, or the 1500W version in the garage?

    Or do you mean an inverter more intended for full-time household use (Outback FX series, whatever)?

    I'll readily admit I don't know much about the circuits inside power inverters. Based on electronic circuits I *have* worked with, it just seems backfeeding an electronic circuit producing 120VAC from 12VDC is going to cause much smoke and damage to the circuits inside! :p It just doesn't make sense to me that a fully electronic inverter would be able to work in both directions...! I do know if I were to try feeding DC power into my Astron power supply, it's going to pop the voltage regulator chip on the control board (at worst - a design flaw on earlier models) or just sit there doing nothing (at best).

    Not that I mind being wrong! :cool: Just trying to understand what's going on there...
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Re: q regarding generator and grid tie

    Solar Guppy has done the testing (and design)--not me... But, as I understand, a TSW inverter has a stable enough frequency and "true enough" sine wave that a GT inverter will sync up and run.

    The interesting thing with the the inverter is that the switches (FETs/Transistors, etc.) don't care which direction the current flows--just that it flows. So, I would guess, that driving a TSW inverter backwards is, more or less, acting like a syncronous rectifier (you can used diodes as rectifigers--but they are "one way check valves", and they are have significant voltage drops of 0.2-1.0+ volts. A syncronous rectifier uses electronics to "switch on" the FET/Transistor at the correct part of the sine wave cycle; can be more efficient because you don't have the diode drop--and in this case, if the GT inverter has higher voltage output==really current output regardless of voltage==it can "push" the current backwards through the inverter and back to the battery--through the transformers and various taps used to make a sine wave, etc.).

    There is no feed back/switching circuit in the TSW inverter to care about the direction of energy flow.

    MSW inverters have (as I understand) a wave form that is "too square" for a code approved GT inverter to lock onto the wave form--so it will never sync.

    The "problem" with driving energy backwards through a TSW inverter (standard, meaning unmodified) is two fold (as I see it).
    1. The inverter was not designed, tested, and approved (if UL/NRTL approved) for being back driven. So--while it may work, it may "not be legal" (not code approved, could cause issues with insurance if there was ever a fire).
    2. The inverter has no battery charge controller/feedback circuit. And the GT inverter is designed to pump out every solar watt it can... This leaves the battery being charged in an uncontrolled manner. You can over charge the battery, boil it dry, and cause a meltdown/fire in the worst case.
    Setting up a charge controller to "cut" the GT inverter's output (hit the inhibit line or just open a relay in the AC output) would be one way around the charge controller issue. Another would be to put a dump controller and load on the battery bank... NEC would probably require two independent charge control systems to prevent fire (primary and backup).

    Skystream (wind turbine) was offering variations of this setup for their off-grid system setup (Skystream is only offered as a Grid-Tied Inverter setup--no DC output option). In fact the first version (if I recall correctly) used their radio link to turn off the GT inverter in the wind turbine. Later versions cut the AC power connection to the GT inverter in the turbine (Skystream is notorious for RF link problems--you don't want that as a charge controller feed back circuit).

    The above contains much guess-work by your truly... Perhaps somebody else with more direct knowledge can add/correct my information.

    Any more questions (that I can bluff my way through :roll:)?

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • RCinFLA
    RCinFLA Solar Expert Posts: 1,484 ✭✭✭✭
    Re: q regarding generator and grid tie

    Most TSW stand alone inverters are not bi-directional. The H-bridge output devices are but the boost supply that creates 190 vdc for the H-bridge is not.

    An inverter-generator has a permanent magnet alternator with six full wave, three phase rectifiers to produce >200 vdc for the TSW inverter. Backfeeding from a GT inverter into an inverter-generator will just cause gen-inverter's internal D.C. supply to rise, likely to a point that blows out something in the inverter circuitry, probably the electrolytic caps. No backfeed power torque will be fed to alternator.

    The only thing that may save the inverter-gen is the grid-tie inverter output voltage going above max voltage limit, shutting down the grid-tie inverter. The engage-disengage process on the GT inverter will repeat in an endless loop.