Grid Frequency Trip Setpoint

I am looking at grid-tie inverters for a PV array (probably incorporating battery storage) for a North American-type domestic electricity system. The application is somewhat specialized as I need the inverter to be forgiving of the grid frequency as I am on an island-based power system that is not part of a large mainland power grid and hence are subject to more frequent frequency fluctuations.
As a ballpark figure, I would like that the inverter accommodates a disturbance of 3Hz for duration of 2 seconds, this would happen infrequently and only in exceptional circumstances.
I understand that to obtain UL approval, the inverter has a ‘trip’ level based upon frequency deviation that is quite tight around the nominal 60Hz. Ideally, this setting could be modified by a technician and access controlled.
I am also looking for grid-tie inverters that provide the 240Vac split phase output rather than buying 2 identical 120Vac inverters and paralleling them together.

Comments

  • BB.BB. Super Moderators, Administrators Posts: 31,451 admin
    Re: Grid Frequency Trip Setpoint

    A couple of questions, observations...

    I am sure that the inverters are programmable by the factory--but the expense associated to keep the UL approval with the wider specs. would probably be relatively expensive (if UL would even be willing--might need a letter from your utility to start the process). It would only make sense if you where willing to pay the difference (spread over multiple inverters?) or if you would be willing to wave UL approval (then you have issues with fire, safety, and insurance).

    Also, is your local utility OK with grid tied inverters and will they support Net Metering of some type for you?

    And, in the end if you expect "...this would happen infrequently and only in exceptional circumstances", then the added expenses being stuck with one vendor brand and model for a rare event would hardly seem worth it... For example, a 3kW inverter with expensive $0.30 per kW power:

    3kW * $0.30 * 5min outage/60minperhour = $0.075 per outage...

    If you assume that it costs you $500 extra to get the "right inverter" for the job...

    $500/$0.075 = 6,666 outages or 555 hours of interruptions (5 minute inverter time out per interruption).

    Assume that is 5 hours per day of useful sun, that is over of 3 months with the grid tied turned off in the life of the inverter (say 10 years???) to break even...

    Your other question about 120 vs 240 VAC... Almost all grid tie only inverters are simply 240 VAC--there is no neutral connection at all (and you should not need one). The split 120 VAC units are typically units that are designed to supply both 120 and 240 VAC during power failures (off grid).

    Are you also looking for inverters that support battery backup and off grid applications, or just grid tied operation (which is usually cheaper and more efficient than off grid hybrid operation--and get a small generator for rare multi-hour power failures).

    There are other "interesting" methods of getting 120/240 VAC power and combining inverters using an autotransformer--but we can skip that discussion for now unless you have the need for it.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • crewzercrewzer Registered Users, Solar Expert Posts: 1,832 ✭✭✭✭
    Re: Grid Frequency Trip Setpoint

    Xantrex’ new XW inverter-charger is a 120/240 VAC, utility-interactive, battery-backup unit. See: http://www.xantrex.com/xw/pdf/DS20070618_XW-Hybrid.pdf

    I suspect that getting around the UL 1741 specs for the inverter’s “Sell” mode would be difficult. Note, however, that if the inverter disconnects from the grid while in “Sell” mode, I believe it would try to reconnect after the 1741’s five-minute safety window had expired. Additionally, this tight connection spec only applies when the inverter is operating in “Sell” mode. The voltage and frequency windows are much wider when the inverter is operating in pass-through and/or charging mode.

    HTH,
    Jim / crewzer
  • System2System2 Posts: 6,290 admin
    Re: Grid Frequency Trip Setpoint

    Crewzer & BB,
    both good answers & a great help. The XW series does seem to stand out on its own with support for split-phase supply. I will have to talk to the factory regarding modifying the frequency trip setpoint.
    Powerman
  • crewzercrewzer Registered Users, Solar Expert Posts: 1,832 ✭✭✭✭
    Re: Grid Frequency Trip Setpoint

    Powerman,

    I’ll be surprised if Xantrex is at all interested in helping you modify its utility-interactive (U-I) inverters. The central point of concern is operation known as “islanding”. For the purposes of this discussion, islanding is the potential ability for a power generation system which is normally part of the grid to continue generating power when the grid fails or is otherwise disconnected.

    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 gris 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.

    Best of luck!
    Jim / crewzer
  • System2System2 Posts: 6,290 admin
    Re: Grid Frequency Trip Setpoint

    Crewzer, thanks for the response. I see the purpose of the Anti-islanding feature, however slightly wider frequency bands would probably still detect a dead grid (assuming this depends upon on how far the inverter wishes to disturb the frequency during its islanding test). I do understand that UL have to set the boundaries at some point.

    I am interested in how we protect utility workers if there are a number of G-I systems in the same locale. Can the inverter tests determine whether the section of line is powered from the power station or from a number of G-I inverters in the same district / fused section of line? I know that this is rather hypothetical but it is important if we are to install the right system. Is it commonplace for installations to employ autotransfer switches in a domestic environment? This would be to gain the benefits of isolating the house from a dead grid automatically and then re-attach upon restoration of the grid without having to synchronize first by interrupting the synchronous inverter during restoration.

    Thanks once again for your help
    Powerman
  • mike95490mike95490 Solar Expert Posts: 9,247 ✭✭✭✭✭
    Re: Grid Frequency Trip Setpoint

    As it was explained to me, the grid tie inverters don't really regulate their voltage output, they rely on the presence of an "infinite load" (The Grid) to regulate their voltage, much like connecting a 22V solar panel to a 12V battery, the battery/grid regulates the output to the desired voltage. In an "island" condition, all would be fine, till the [insert large appliance] shuts off, then your unloaded voltage would spike up toward overvoltage shutdown (250V for a 240V system).
    [did I phrase that fairly right S.G.?]
    Powerfab top of pole PV mount | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
    || Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
    || VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

    solar: http://tinyurl.com/LMR-Solar
    gen: http://tinyurl.com/LMR-Lister ,

  • BB.BB. Super Moderators, Administrators Posts: 31,451 admin
    Re: Grid Frequency Trip Setpoint

    In the end, it really is up to you how much backup power is worth to you... If it is just a problem with reprogramming the DVR and clocks--it may be easier and cheaper just to through a $100 UPS on those few appliances you want to have backup power (assuming a few 10's of minutes of power is enough to ride through the power failures--or longer times if the loads are very small--like a small clock or DVR on standby).

    If your failures are longer (i.e., storm wipes out power and it takes two weeks to get it backup), then a real off-grid capable system is better for your needs.

    And these systems may or may not have solar backup/off grid power. You can get an Outback inverter, connect it to the AC mains and a DC battery bank, and you have a very nice, large, UPS for your home, ~20 amps at a time (or whatever the size of the inverter is).

    One minor nit--There are several types of UPS systems... The simple (and RE Inverter types) pass the line energy through directly, and if there is a glitch/drop out, take a 1/2 cycle or so to switch on the backup inverter. Sometimes, this is enough of a delay that it can reboot the average PC/Server. The more expensive types are continuous converters--these systems always have the inverter running, and use the DC battery Charger to power the batteries which power the inverter. There is no switch-over glitches in these type of systems... However, these are more expensive and less energy efficient.

    In general, any loads that you want to run off of battery, inverters, solar, etc., should be as small as possible (conservation). And, remember that small continuous loads (that old stereo system, the PC and laser printer in the corner, the 2nd fridge/freezer in the garage) may draw more power in a day than the microwave in the kitchen.

    The more power you want to supply, the larger and more expensive the systems become. Keeping the off-grid usage low will help keep costs and maintenance requirements down.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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