Transfer Switch

czars
czars Registered Users Posts: 16
Does anyone know of a transfer switch that can be used with a grid-tied, battery backed-up PV system to allow the PV system to disconnect from the utility provide back-up power to the load when the utility is down?

Comments

  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    Re: Transfer Switch
    czars wrote: »
    Does anyone know of a transfer switch that can be used with a grid-tied, battery backed-up PV system to allow the PV system to disconnect from the utility provide back-up power to the load when the utility is down?

    Welcome to the forum.

    For the most part, this shouldn't be necessary.
    What inverter is being used? Xantrex?
    Usually there are two "AC" connections: one to feed to/receive from the grid, the other for "critical loads" which will either feed through grid power or supply the battery-backed AC when the grid goes down.
  • czars
    czars Registered Users Posts: 16
    Re: Transfer Switch

    Thanks for the reply. I'm an Electrical Contractor and my client is buying a Xantrex XW4548 and has fed me little info to date. I looked up the manual on the XW4548 and it looks like there is a built-in 60A transfer switch which could be connected to a new sub-panel for loads requiring continuous power. From the brief amount of info in the manual about the transfer switch, I believe that the sub-panel is fed from the utility while the utility is up and via the transfer switch, it is fed from the inverter/battery bank when the utility is down. Is that correct?
  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    Re: Transfer Switch

    That's pretty much how it works all right. Grid available = everything powered by grid. Grid down = sub panel powered by batteries, the rest of the house is dark. :p The grid will also recharge the batteries if there's not enough sun to keep them up. The biggest problem with the XW units is the programming can be a bit tricky. There are a lot of posts on this forum regarding getting the 'SELL' Voltage right and the battery bank size, et cetera. But we certainly have the knowledgeable folk here to help you through it.

    That's not me, btw; I'm an Outback off-grid guy. :D
  • czars
    czars Registered Users Posts: 16
    Re: Transfer Switch

    Thanks for the info. You've been a great help.
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Re: Transfer Switch

    There are actually several AC transfer switches to manage the power flow in the XW Hybrid inverter...

    The XW Hybrid Inverter has both an AC1 and AC2 input... The AC1 it typically connected to the Grid and AC2 is typically connected to the Backup Generator. They both can power AC Out.

    The AC2 input can be qualified very tightly (near grid specs. for voltage and frequency; or very wide open to allow High/Low Freq. and/or Voltage directly through to the protected sub-panel (may require alternative firmware to support this change in generator parameters).

    The XW also has some capabilities to "assist" the Grid Power (not back feeding the grid, but providing some current to support the loads--typically where backfeed/grid tied systems are not legal--Plus it also has the ability to support the generator; if generator has a 10 amp maximum output, the XW can support AC loads by adding current to support AC loads >10 amps of generator output maximum).

    Another couple of things to understand about the design and integration to the home... If you run 120/240 VAC split phase--The generator must also by 120/240 VAC split phase capable too (cannot use the XW charger/generator support with a 120 VAC genset in 120/240 VAC mode).

    Second is the XW can be reconfigured from XkWatts at 120/240 VAC to XkWatts at 120 VAC--the unit can be configured from split phase 120/240 to 120 vac parallel outputs and be a pure 120 VAC unit with 120 VAC genset. I believe one also needs to upload new firmware for this configuration.

    About all I know--You will need to call Schneider directly if you have more questions or need alternate firmware/etc.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • RCinFLA
    RCinFLA Solar Expert Posts: 1,484 ✭✭✭✭
    Re: Transfer Switch
    czars wrote: »
    Does anyone know of a transfer switch that can be used with a grid-tied, battery backed-up PV system to allow the PV system to disconnect from the utility provide back-up power to the load when the utility is down?

    Check out Xantrex AC coupling applications note

    http://www.global-download.schneider-electric.com/852577A4005D7372/all/8FA29D94B918A0FB8525781F0072FCA7/$File/ac-coupling-app-note(976-0240-01-01_rev-a).pdf

    It has several diagrams depending on how local grid tariffing is done.
  • sub3marathonman
    sub3marathonman Solar Expert Posts: 300 ✭✭✭
    Re: Transfer Switch
    czars wrote: »
    Thanks for the reply. I'm an Electrical Contractor and my client is buying a Xantrex XW4548 and has fed me little info to date.

    First, there is a very small price differential between the XW4548 and the XW6048, and I think the benefits of the larger XW6048 can far outweigh the added price. I think the XW6048 is also slightly more efficient than the XW4548, although you'll have to check at the specific output levels expected with the PV array.

    As far as battery bank size, a suggested minimum would be 100 ah (48 volts) x array size (kw). So a 3kw system would need a minimum of a 300 ah, 48 volt battery bank. The manual says a 100 ah minimum battery bank, but that is not the real world situation. If you get too small a battery bank there will be major problems with the operation of the inverter, both selling and charging.

    And finally, there is an easy way to figure out everything you need to know about the XW series inverters. Just search the forum for posts by Solar Guppy. He is the expert here.
  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    Re: Transfer Switch

    The price difference between the 4kW XW and the 6 kW XW becomes rather larger if the plan is to use it primarily for grid-tie; you need another 2 kW of panel, and that isn't cheap (or in some cases possible).
  • RCinFLA
    RCinFLA Solar Expert Posts: 1,484 ✭✭✭✭
    Re: Transfer Switch

    The transfer switch is just a closure contact relay between either the AC1in (typically grid connection) or AC2in (typically gen connection) to ACout which is hardwired to inverter. Only one ACin is selected, and which one takes priority if both are present is user settable.

    When the ACin is qualified, meaning it is within freq and voltage specs and the inverter controller establish syncronization, the relay closes putting the inverter in parallel with the grid.

    The inverter can be deactive or active. The inverter is bi-directional. It can take power from the ACin/ACout node to charge the batteries, or push power to the ACin/ACout node. It makes measurement of current flow in and out of ACin, ACout, and inverter so the controller knows which way power is flowing and makes adjustments to the PWM control on inverter to either push or pull power from the battery node.

    When selling to grid is enabled, any attempt to raise the battery voltage (by a PV controller or any other charging source) will cause the inverter to pull current from the battery line to maintain the inverter's charge float voltage and push the excess power to the ACin/ACout node. It will be used by ACout loads or if pushed power is greater then ACout consumes then is will be push excess to grid.

    If selling is not enabled, but grid support is enabled, it will do the same as described above except it will not allow any excess power to be pushed to grid. The battery voltage will be allowed to rise where, presumably, the PV controller takes over battery charge voltage limit control to avoid overvoltaging battery.

    Load shaving is putting a criteria on maximum allowed current draw from ACin (very useful for smaller generators), where if ACout load demand is greater then this set limit, the extra needed ACout load power is supply by inverter, taking power from batteries. This caps the maximum current draw from the ACin.

    Setting of all these functions is sometimes challenging because of the the user interface control naming conventions and how the software manages these functions.
  • sub3marathonman
    sub3marathonman Solar Expert Posts: 300 ✭✭✭
    Re: Transfer Switch
    The price difference between the 4kW XW and the 6 kW XW becomes rather larger if the plan is to use it primarily for grid-tie; you need another 2 kW of panel, and that isn't cheap (or in some cases possible).

    OK, I did some checking. The difference seems to be about $400, which is noticeable. But as far as I know, you don't "need another 2 kW of panels" for it. You can run a 1kW system to the maximum 6kW. If I remember correctly, it is slightly more efficient in the high range of the XW4548 ( which is closer to the mid-high range of the XW6048 ) , but I'll have to look up the charts to make sure. Also, once again if I remember correctly, the standby usage of the XW6048 is less than the XW4548, but of course that is a relatively small benefit.

    Where the big benefit comes, in my opinion, is the future possibilities for expansion. If you set up your system near the maximum of the XW4548, you'll have to buy another inverter, which will be much more than $400. If you have the XW6048, you're already set.
  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    Re: Transfer Switch

    "Primarily for grid tie" i.e. get the most you can fed back to the grid. Usually you have an array larger than the inverter so the inverter maintains close to peak output as much as possible.

    But hey; no one has to listen to me. Spend your money how you want. :roll: