Switching live loads between grid and inverter.

RandomJoeRandomJoe Solar Expert Posts: 472 ✭✭✭
I'm starting to collect the pieces I need to switch my few 24x7 loads over to solar power during the day once the batteries are charged. But I have a couple questions that hopefully some here can help with...

Primarily, how fast of a switching time is needed for an operating load? I'm going to do some testing when I get a chance, but would like to hear others' experiences. The main two load types I'll be switching are the fridge and a couple of servers.

The fridge is my biggest worry - if the power bumps while it is running, and the compressor stops, then it won't restart. But how long of a bump is required before that happens? If the 15-30ms it takes a relay to switch isn't fast enough then I'll need to add a time delay during which the fridge is kept off for several minutes or monitor the current draw to know that the fridge isn't running.

I know most computers don't care for power bumps, but have thought about just leaving them on the UPS and switching the input to that. My inverters are true-sine so I don't think the UPS will have a problem with it.

Again, the relays I'm looking at list their action time as 15-30ms (depending on which relay).

Next question, would you just switch the line side (120V circuits) meaning the neutral from the breaker panel would be tied to the neutral from the inverter (these can be properly grounded on the AC side according to their manuals), or would you switch both line and neutral? Not really a big deal either way - the relays I'm looking at are multi-pole and I'm only going to have one load per relay.

And finally, how often could/should I switch? Obviously the equipment already sees bumps and sags from the power company, but how much of a "stress" would switching from one power source to another be? I could make this a one-shot deal, as I get to absorb / float start switching loads over. If the system can't maintain then back off and done. On the other hand, for a partly-cloudy day that could mean losing out on available power later in the day if the sun comes back out. I don't want to be switching back and forth so much that I cause premature failure with anything though.

My idea at this point is to have three roughly 100W (when running) loads to switch on in stages - which lets me make use of available power on overcast days as well - based on available voltage and current as reported by the Outback system during operation. Then, as the system voltage and PV current starts to fall, indicating loss of sunlight, I'll switch the loads back over to grid. Handily, this also means I get an automatic transfer switch for the fridge if the grid fails.

Thanks!
Joe

Comments

  • BB.BB. Super Moderators, Administrators Posts: 29,489 admin
    Re: Switching live loads between grid and inverter.

    The typical switch time advertised for UPS's is 16 msec (1 cycle at 60 Hz).

    In real world power failures--My 2 cents is that PC's and servers will still crash about 1 out of 10 power failures where the UPS transfer did not occur in the time/manner that the computer was expecting.

    Was this because the inverter got confused during a real power failure ("dirty" power during failure) or what--I never found out. I had test gear that would drop a cycle of 60 Hz power--and was never able to reproduce the power / inverter switch over failures.

    So--for computers that need to stay up and never crash on power failure--I would not put them on your transfer switch. If your UPS was a continuous converter (AC to DC to AC) type--then it would be fine on any transfer, as would a laptop with battery installed.

    UPS's usually waste a lot of power, and would not be a great idea to run off of your solar--you will get more wear and tear on your solar battery bank (and possible on the inverter if it pops over to backup operation during the transfer). Just staying on AC mains for UPS power would be the most cost effective. Only transfer over to solar backup if the mains go down (by then the UPS would be on battery anyway and the transfer should not cause any additional problems).

    Unless your fridge has a computer (not a joke anymore)--It should be fine with a relay switchover.

    Switching your Neutral or Not--You have to check with your inverter's manual (or ask the manufacturer). Most MSW do not like grounded neutrals (normal North American wiring standards) and most TSW inverters are OK sharing a grounded neutral.

    Regarding your UPS--I have seen UPS's that check for an ungrounded neutral and will not start if the gorund is not there, or would require a manual start (if the neutral / safety ground bond is open).

    I would keep things as simple as possible. To complex and the chances of failure causing problems for your system may make the whole less reliable (relays, inverters powering inverters, etc.).

    If you have a fair amount of solar power available (2,000 watts or more?) that you want to use while the sun shines--I would look at a Hybrid UPS instead. Feed grid tied power under normal operation and switch over to off-grid UPS power if AC mains fail. If nothing else, your batteries could last ~2x longer (because you are not cycling them every day) and pay for the increased expense of the new Hybrid inverter.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • n3qikn3qik Solar Expert Posts: 741 ✭✭
    Re: Switching live loads between grid and inverter.

    The inverter I have, has a built in transfer switch. It switches just the line/hot wire. Ground and neutral are connect between the two systems. On cloudy days mine will switch over 3-4 times. No problems with the fridge.

    Note about the inverter powering an UPS. I tried this with an APC 1200VA and it did not work. The UPS could not lock onto the inverter, so it cycled on/off line.
  • SlappySlappy Solar Expert Posts: 251 ✭✭✭✭✭✭
    Re: Switching live loads between grid and inverter.

    have you thought about a DC switching power supply "DC -> DC" for the servers; a little more expensive, but it might be worth it?
  • RCinFLARCinFLA Solar Expert Posts: 1,280 ✭✭✭
    Re: Switching live loads between grid and inverter.

    Syncronous grid tied inverters like XW and Outback switch in less then a cycle. The actual inverter runs in parallel with grid for quick switch over UPS function.

    There is more to quick switchover then relay activation time. You need to have the inverter phased in sync with the grid. A independent non-grid sync'd inverter will have a random phase compared to grid and may happen to be 180 degrees out of phase with grid when the quick relay switches over. This is very bad for motors and compressors, not to mention the possibility of inverter getting subjected to a very large surge current.

    If inverter is non-sync'd to grid it is better to wait several seconds before engaging inverter.
  • RandomJoeRandomJoe Solar Expert Posts: 472 ✭✭✭
    Re: Switching live loads between grid and inverter.
    RCinFLA wrote: »
    Syncronous grid tied inverters like XW and Outback switch in less then a cycle. The actual inverter runs in parallel with grid for quick switch over UPS function.

    Ah... I wondered if they might do that. The 180 degrees out of phase issue is what prompted my original post, I'd never seen anyone mention it but it did seem it could be a real problem for (at least) the compressor.

    Guess that gives a bit more weight to my contemplation of an Outback inverter to go with all my other Outback gear... (Currently just have a couple of standalone true-sine inverters.)

    Slappy wrote: »
    have you thought about a DC switching power supply "DC -> DC" for the servers
    I have done that with one computer, that I wish to keep alive during outages. However, the servers aren't so lucky! :p I actually do NOT want them operating off the battery bank, but they are my only 24x7 continuous loads (the fridge cycles on/off, of course) so wanted to use them as "opportunity" loads on sunny afternoons after the battery bank is topped up.

    n3qik wrote: »
    Note about the inverter powering an UPS. I tried this with an APC 1200VA and it did not work. The UPS could not lock onto the inverter, so it cycled on/off line.
    Ouch. Was that with a true-sine inverter? I had always assumed they would be pretty much indistinguishable from grid power...
  • RandomJoeRandomJoe Solar Expert Posts: 472 ✭✭✭
    Re: Switching live loads between grid and inverter.
    BB. wrote: »
    In real world power failures--My 2 cents is that PC's and servers will still crash about 1 out of 10 power failures where the UPS transfer did not occur in the time/manner that the computer was expecting.
    So far, mine have never done that. And the larger one that powers my servers switches at least daily exercising/testing the batteries so they've seen plenty of switchovers.
    BB wrote:
    UPS's usually waste a lot of power, and would not be a great idea to run off of your solar--you will get more wear and tear on your solar battery bank (and possible on the inverter if it pops over to backup operation during the transfer). Just staying on AC mains for UPS power would be the most cost effective. Only transfer over to solar backup if the mains go down (by then the UPS would be on battery anyway and the transfer should not cause any additional problems).
    This is strictly for an opportunity load during good solar days, the servers will be shut down during outages. But they are typically the only thing that's running during the typical weekday!
    BB wrote:
    If you have a fair amount of solar power available (2,000 watts or more?) that you want to use while the sun shines--I would look at a Hybrid UPS instead. Feed grid tied power under normal operation and switch over to off-grid UPS power if AC mains fail. If nothing else, your batteries could last ~2x longer (because you are not cycling them every day) and pay for the increased expense of the new Hybrid inverter.
    I figured my planned use wouldn't affect the batteries much. The loads wouldn't switch in until the Outback system reported it had hit (have to pick one) Absorb or Float, then would switch out when the controller saw the PV current drop below the load current. (Buffered by a short delay time, since the fridge starting would greatly exceed the PV input capacity, and also to "ride through" clouds passing by.)

    In fact, I may wind up reducing battery load to near-nothing, if I go with the Outback inverter I'll convert to 48V and run everything off AC again...

    I'd still need to do some load switching even with the Outback inverter, because the servers are "opportunity" loads that wouldn't be allowed to run during an actual outage. But the wiring and control becomes much simpler at that point! :cool:
  • peterakopeterako Solar Expert Posts: 144 ✭✭
    Re: Switching live loads between grid and inverter.

    Hi there i caI have two one is whit out problem it is a modern inverter type. the older one does not like the transfer from generator to battery 1 in 30 times, it stalls the motor and for two seconds it draws a high current then the safety from the compressor stops the power supply. after two min. it starts up automaticly whitout a problem.
    The transfer is never a isue for my computers or TV.

    But why are you switching from battery to grid. Why not backfeeding the grid. the losses from charging the battery and inverter losses, you can cain by backfeeding.
    :confused:

    Greetings from Greece8)
  • RandomJoeRandomJoe Solar Expert Posts: 472 ✭✭✭
    Re: Switching live loads between grid and inverter.
    peterako wrote: »
    But why are you switching from battery to grid. Why not backfeeding the grid. the losses from charging the battery and inverter losses, you can cain by backfeeding.

    That would be the most elegant solution, however to do so "legally" would require I go through the paperwork and inspection process, possibly require I have someone "certified" to install it do the work (although for most things, my area allows self-install with a certified inspector giving it a final review) and - just to kick a guy when he's down - I would then have to pay my electric company an extra fee every month for the "privilege" of being grid-tied.

    At this point, I would never be net-positive with GT, however right at the peak of production (when I'm away at work) is also when my house is at its lowest consumption. So odds are I would in fact be pushing power to the grid, although not much. So the electric company would know about it (assuming they are paying any attention yet, although they are starting to roll out smart meters in the area, so within a few years I'm sure I'll have one) so I don't care to just "fly under the radar" with it...

    And, again, I would NOT be using *battery* power to run the loads. The inverter would be running straight off the power coming from the solar panels, if the sun goes down or clouds build to the point I'm dipping into the batteries the loads would be switched back to grid.

    I know this is a strange setup compared to what most people do, but this is as much a hobby for me as it is a functional system. (Yet another reason for not going GT - so I can do whatever I want with the system, without having to fuss with further inspections and whatnot!) I design and install building automation systems at work, and it's just fun for me to design and install this system since it's an area I don't normally deal with at work. I'm also a control freak, and will be adding all kinds of sensors to collect data - nothing I like better than watching my creations at work! :cool:
  • n3qikn3qik Solar Expert Posts: 741 ✭✭
    Re: Switching live loads between grid and inverter.
    RandomJoe wrote: »
    And, again, I would NOT be using *battery* power to run the loads. The inverter would be running straight off the power coming from the solar panels, if the sun goes down or clouds build to the point I'm dipping into the batteries the loads would be switched back to grid.

    I know this is a strange setup compared to what most people do, but this is as much a hobby for me as it is a functional system.

    This is how I currently run my 90% of my system. I have just some LED night lights running off battery at night.

    As to the inverter-> UPS, yes it was a true sine inverter.
  • vcallawayvcallaway Solar Expert Posts: 157 ✭✭✭✭✭
    Re: Switching live loads between grid and inverter.

    I'm using an IOTA switch for my home backup. My inverter requires neutral to be floating, meaning not tied to earth ground.

    I essentially bring hot and neutral into the iota switch and leave earth ground alone in the main box. The iota feeds into a small breaker box where I feed both hot and neutral out to the circuits in my house.

    When I'm on grid power neutral is tied to earth ground, when on the inverter it is not. So far this all works great. There is a switch time delay because the iota is a mechanical relay. It is so quick that my router and dsl modem does not reset.

    I don't run my refrigerator or freezer off of the inverter. They really only need to run a couple of hours a day to keep to temp unless it is really hot out. My outages are usually in the winter. Six hours last night and pretty much any time there is a high wind warning.

    I had an electronic 3-way switch before. It blew out when someone hit a power pole. It also took out my inverter. I decided to go more old school with the iota switches.
Sign In or Register to comment.