What is the tech/theory?

stmarstmar Solar Expert Posts: 327 ✭✭✭
What is the theory for an interactive inverter, specifically the Trace SW4024, that keeps the power from back-feeding the grid in an outage? I have set the user inverter and charge values to exclude "Sell", I set them to ON and FLT. But I am curious about how it knows not to have battery/PV power go back into the grid.

Comments

  • BB.BB. Super Moderators, Administrators Posts: 29,368 admin
    Re: What is the tech/theory?

    Basically, the grid is a "dump load".

    The Inverter "looks" at battery voltage. Once it has determined the battery is fully charged, it monitors the float voltage (other charging sources such as solar array, hydro power, etc. are set to charge the battery bank with all available power). If the voltage goes above float, the Hybrid inverter starts drawing current from the bank to keep it at "float" and feeding that excess energy to the grid. Once the sun begins to "go down", the Hybrid inverter feeds less and less energy back into the grid.

    I am sure it is much more complex than that, but that is the basics.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • stmarstmar Solar Expert Posts: 327 ✭✭✭
    Re: What is the tech/theory?

    How does it "know" not to try and power the whole neighborhood during a grid outage? Is there a switch/control/relay that causes it not to backfeed the grid similar to the PV controller that does not allow the array to discharge the battery bank at night? I am sure I have my parameters set correctly but I wanted to know the why/theory behind those settings.
  • jcheiljcheil Solar Expert Posts: 722 ✭✭✭
    Re: What is the tech/theory?
    stmar wrote: »
    How does it "know" not to try and power the whole neighborhood during a grid outage? Is there a switch/control/relay that causes it not to backfeed the grid similar to the PV controller that does not allow the array to discharge the battery bank at night? I am sure I have my parameters set correctly but I wanted to know the why/theory behind those settings.

    It has to SYNC the waveform with the grid power and if it doesn't see any grid power (or if it has issues syncing) it won't send it out. I have heard issues on here with people that had problems getting it to sync due to grid sync issue and had a heck of a time getting it to work.
    Off-Grid in Central Florida since 2005, Full-Time since June 2014 | 12 X Sovello 205w panels, 9 X ToPoint 220w panels, 36x ToPoint 225w panels (12,525 watts total) | Custom built single-axis ground mounts | Complete FP2 Outback System: 3 x FM80, 2 x VFX3648, X240 Transformer, FLEXnet-DC, Mate-3, Hub-10, FW500 AC/DC | 24 x Trojan L16RE-B Batteries 1110ah @ 48v | Honda EU7000is Generator and a pile of "other" Generators | Home-Made PVC solar hot water collector | Custom data logging software http://www.somewhatcrookedcamp.com/monitormate.html
  • BB.BB. Super Moderators, Administrators Posts: 29,368 admin
    Re: What is the tech/theory?

    More or less, there are two major ways of "back feeding" a grid.

    The standard way is to run an Alternator (AC Generator) at 3,600 or 1,800 RPM, and syncronize the output to the AC mains (simple way is to put some light bulbs across the inter-tie switch). The lights will blink slower and slower until the "go out"--Throw the switch connecting the Alternator to the grid (zero voltage/frequency difference between grid and synchronous alternator). Then start applying "torque" to the alternator (try to turn it faster) and it will dump current/energy to the grid.

    A typical AC GT inverter works differently. It measures the AC main frequency/voltage/wave form--And if everything is "in spec." (AC voltage between ~104 and 132 VAC, 60 Hz +/- 0.5 Hz, good quality sine wave) for 5 minutes, the GT inverter "turns on".

    And it does it in a very different way. It looks at the Voltage Sine wave and injects Current based on I=V/R -- Basically when the voltage is at zero volts, it injects zero current and at Vac-max it injects the maximum available current (i.e., the available power from the panels or from the battery bank).

    If the AC voltage falls below spec., there is a voltage/frequency glitch, etc. a GT inverter simply stops outputting current.

    So, if there is no AC main, there is not voltage/frequency for it to "sync" to, so no way it can drive the neighbors AC power.

    For Hybrid Inverters, there is a difference. If the inverter detects the AC mains are out of specifications, it disconnects from the AC mains (typically a relay) and switches over from Current Injection mode to Voltage/Frequency source mode--I.e., it now becomes a "regular" battery powered AC inverter and everything on the "protected" sub circuit continues to receive nominal AC power until the batteries go dead or the inverter is otherwise shut down.

    In engineering, there are many ways to do the same thing. The above are over gross simplifications but represent two "popular" ways of connecting to the grid with the ability to "back feed" current to the grid. And, frankly, I am probably not very qualified to go much deeper into the theory of how these GT/Hybrid/OG inverters work.

    Sufficient to say it does work (in general, and to the satisfaction of UL/NRTLs/building inspectors/utilities/etc.).

    What is usually a bit more useful for the end user is to understand there are many variations of how GT/Hybrid/OG inverters can be configured/designed to solve various problems.

    And for a customer to define their needs first ("know your loads", voltage/watts/Watt*Hours/distance/save money/emergency backup/off grid/etc.)--Then see what hardware is out there that may support those needs in a cost effective manner.

    Solar power is not cheap--And "pure Grid Tied" inverters (no batteries) that feed back power to the utilities are about the only main-stream method today that can compete with the cost of Utility power (and in some cases, even beat the cost of utility power).

    Pretty much all the rest (hybrid, off grid, fuel driven generators, wind turbines, etc.) will not save money as they cost a lot more to run than buying kWH from the utility.

    However, if you have not have "cheap" utility power nearby, and/or can "conserve" significant amounts of energy--It is getting interesting. Some utilities have run up charges and/or modified their tariffs to such an extent (example: high connection charges of $40-$96 per month for the first kWH of power) that even pure off grid solar + batteries + generators are equal to/or cheaper than utility power (and sometimes more reliable too).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • bill von novakbill von novak Solar Expert Posts: 849 ✭✭✭✭
    Re: What is the tech/theory?
    stmar wrote: »
    How does it "know" not to try and power the whole neighborhood during a grid outage? Is there a switch/control/relay that causes it not to backfeed the grid similar to the PV controller that does not allow the array to discharge the battery bank at night? I am sure I have my parameters set correctly but I wanted to know the why/theory behind those settings.

    Yes, there is a switch (relay) that gets thrown. You can hear it click shortly after you start up the inverter. After about a minute it syncs to the input power line and you can hear the "click" when the switch closes. When the grid goes out the relay opens.
  • stmarstmar Solar Expert Posts: 327 ✭✭✭
    Re: What is the tech/theory?

    Thanks for the input, it is probably a complicated system and the inner workings are way beyond my understanding but I like to learn as much as possible. I agree about the cost per kwh, I use my system for back-up to keep the wife happy!! My SW4024 is fairly clear on menu settings and I have not had any problem of it trying to back-feed so it is working properly. When it is set to ON instead of SELL it must know to keep all the power.
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