Questions about Trace SW4024 and its grid tie interface

joeom
joeom Registered Users Posts: 4
Hi there,

I'm helping with a solar install at a guy's house. When he bought the house it came with some equipment -- a Xantrex Trace SW4024 inverter/charger, a Trace T240 transformer, and a Trace DC disconnect module.

It's all mounted on the wall in what looks to be a pre-packaged unit (PP-SW4024/S), which looks like the first image in this manual except it only has 1 inverter and no charge controller.





The homeowner wants to tie into the grid so he can sell back power to offset his bill, so the first thing I am trying to figure out is whether this system is already capable of grid-tying or if another Grid Tie Interface module is necessary, as seen in this manual. According to the introduction, the GTI is an external module necessary for connecting a SW4024 to the grid in sell mode.
In other places, though, it seems like it's described as an integrated component in the SW4024.

So my question is: would we need to buy a separate GTI module, or is it already present?
Maybe that's what's in the compartment in the right side of the bottom picture? Photos of a SW4024 by itself do not include that box.

I called the local utility and asked if they had restrictions on what types/ages of inverters are allowed, and all they could tell me was that it needs to be UL/IEEE certified. So as far as I know, despite this being 20 years old they might allow it to be used. I'm in SW Colorado. Of course, they might say this one is too old or not allowable for other reasons, and he might have to buy a new one, but his priority is saving money and this inverter seems to have a good reputation so I am trying to figure out if there's any way he can use it.

Thanks for any advice!
Joe

Comments

  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    edited November 2022 #2
    Welcome to the forum Joe,

    As I recall, the GTI board was a quick fix to try and get the (old) SW Trace/Xantrex off grid inverters legal for utility Grid Tie Interface (GTI, GT, utility interactive, etc.)...

    I don't remember for sure--But more than likely the SW GTI would not meet current UL/Utility requirements for GT function--Or at the very least, you may have issues trying to get a utility to "approve" a GT connection.

    Other issues include that this is probably a 15+ year old unit and there is zero support (repairs and parts) from Xantrex/Schneider.

    The suggestion would be to sell the unit to somebody that needs off grid and/or a spare for their existing SW installation.

    Coming back to the present... Does your friend need a Hybrid (GT and Off Grid) capable system, or just looking for pure GT inverter installation (pure GT inverter is much less expensive, no batteries/programming to worry about) and with current production system, they can get warranty, parts, and an easier utility/building department approval.

    If he wants off grid solar (battery backup) and possibly genset backup--There are many better choices today than to try and resurrect this old guy.

    Also, the NEC has been updated a few times over the decade and made solar array installations on Roof Tops a bit more complicated now. Between remote shutdown and spacing for walkway/roof access for fire personnel, etc. and possibly building permit requirements (engineering drawings for structural and electrical wiring, possible utility meter upgrades/reconfiguration)--This is not something to jump into before doing more research.

    All of the above is based on state/county/utility rules and regulations. If this is a first time install--It would not hurt to find/hire somebody knowledgeable about Hybrid/GT/OG solar.

    And for your friend to live there for a few months, review power bills, do some conservation, etc. first before going down the solar power road. Pure GT Solar can be pretty cheap to install and very low maintenance. Off Grid/Hybrid battery systems, a lot more costs and ongoing maintenance costs (hybrid/OG electricity probably costs more than utility power--GT inverters can be much less than utility power--However public utility commission rules and regulations and utility rate plans can dramatically reduce "subsidies" from the power companies--Or is some states, be "illegal" or economically uneconomic.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • littleharbor2
    littleharbor2 Solar Expert Posts: 2,039 ✭✭✭✭✭
    In the event you can use this unit for grid tie and need a GTI grid tie interface I have one or two in my storage. and would be willing to ship you one.
    The box on the right end of the inverter is just a housing/protection for the battery cables.

    2.1 Kw Suntech 175 mono, Classic 200, Trace SW 4024 ( 15 years old  but brand new out of sealed factory box Jan. 2015), Bogart Tri-metric,  460 Ah. 24 volt LiFePo4 battery bank. Plenty of Baja Sea of Cortez sunshine.

  • joeom
    joeom Registered Users Posts: 4
    BB, thanks for your reply!

    We are aware that the unit is 20 years old (made in 2002) and there will be no support. I think he figures: even if it works for a while and then breaks and we can't fix it, that's still cheaper than buying a new one now? I definitely get your point and if it were my house, I'd rather go with a newer model too, but he seems intent on at least trying to use the existing equipment.

    For the actual install, the owner will go with a legitimate solar install company. He just hired me to help figure out his options. I don't have experience with residential solar, but I used to be an electrical engineer and I have installed my own 640W system on my bus so I figured I could at least help make sense of the process and make some recommendations re: how to size his system. He did consult with a couple of installers and they both wanted to tear out the current equipment and go with all-new stuff. I think he felt like that was a waste, and they were just trying to upsell him and not listening to his preferences. Part of my job might be explaining to him why their reluctance to use 20-year-old equipment is reasonable. That said, I also understand his side... if he has perfectly good, unused equipment that still has a good reputation for being bulletproof, it seems a shame not to use it.

    As far as GT/OG is concerned, his main priority is to save money by generating his own power and feeding it back to the grid when possible. Sometime down the line, he may like to create a backup system in case of blackouts, but for now the goal is to invest as much of his budget as possible into solar panels in order to generate maximum power.

    So, at least for the short term, it seems like he might be open to using a pure GT inverter solution -- especially if the current equipment doesn't work, a pure GT inverter is significantly cheaper, and he wouldn't have to buy batteries or a charge controller. Any recommendations for good models?
    Alternatively, if he has to buy a new one anyway and knows he wants OG backup down the line -- maybe it's reasonable to buy once cry once, and get an inverter that can handle both.

    The first trick is still figuring out whether the current equipment would be accepted. Like I said in the first post, the person I talked to at the utility hadn't heard of this model and said the only thing that mattered was whether it's UL certified. But your point also makes sense that UL standards may have changed in the last 20 years. Maybe I have to get them to come out and look at it?

    Thanks!
  • joeom
    joeom Registered Users Posts: 4
    littleharbor2, that's great, thanks for the offer... about how much would you want for one of your GTIs?
  • littleharbor2
    littleharbor2 Solar Expert Posts: 2,039 ✭✭✭✭✭
    joeom said:
    littleharbor2, that's great, thanks for the offer... about how much would you want for one of your GTIs?
    Message sent

    2.1 Kw Suntech 175 mono, Classic 200, Trace SW 4024 ( 15 years old  but brand new out of sealed factory box Jan. 2015), Bogart Tri-metric,  460 Ah. 24 volt LiFePo4 battery bank. Plenty of Baja Sea of Cortez sunshine.

  • RCinFLA
    RCinFLA Solar Expert Posts: 1,484 ✭✭✭✭
    edited November 2022 #7
    Old SW40xx series use three transformers' series connected outputs to create up to 27 AC output voltage steps to synthesize a sinewave.  Because of its stepped sinewave output the inverter by itself does not meet the harmonic level requirements for grid tied utility injection.

    The GTI interface is a band-aid to clean up the sinewave to a level that will meet harmonic level for grid tie selling.  It is basically just a filter inductor in series with inverter AC output. It also uses the phase shift through the inductor to verify grid presence to meet UL1741 anti-islanding requirements.

    All the extra stuff is just relays to bypass the filter choke when not selling to grid.  The choke adds quite a bit of loss, so it is bypassed when consuming power from grid.

    Most utilities will not allow 120vac L1-N only push to grid.  The T240 autotransformer will take the 120vac inverter output up to 240vac.  It adds a bit more power loss.  I don't know if the GTI interface was ever tested to meet UL1741 with a T240 autotransformer on output.  The T240 might screw up the anti-islanding detection for an open grid situation.

    A 4kW inverter with all the extra band-aid losses is unlikely to make much of a dent in selling power to grid but that depending on local electric rates.

    Getting approval for grid selling with this old unit depends on local utility and county inspectors.  It does meet UL1741.

    This arrangement would only allow DC coupling of PV directly to batteries.  DC coupling of PV to battery then using inverter to create AC output to push to grid creates a lot of continuous 120Hz ripple current on batteries.  Battery is providing the filter capacitor to take smooth illumination-based PV DC charge controller power profile to the 120 Hz DC input power profile for 60Hz sinewave single phase AC inverter.  This puts stress on battery due to the ripple current even with no net average power in or out of battery. 

    You also need large enough battery AH size to take the ripple current without excessive battery stress.  Minimum would be 400-600 AH battery for a 4kW inverter.

    Using lead-acid battery for DC coupled grid selling will likely more than eat up any grid savings for the ongoing replacement costs on batteries.

    In short, the picture looks like a nice clean setup for backup power for house, but I would not recommend using it for selling to grid.  If you leave it for that purpose, remove the GTI unit as it just adds more losses.

  • littleharbor2
    littleharbor2 Solar Expert Posts: 2,039 ✭✭✭✭✭
    In the event he goes the newer GTI route keep in mind that there is a strong market for Trace/Xantrex equipment. I recently bought a T-240 transformer and must say it's the first one I have come across in over 15 years of looking. The inverter should sell for about $1,200.00. DC disconnect, probably $200

    2.1 Kw Suntech 175 mono, Classic 200, Trace SW 4024 ( 15 years old  but brand new out of sealed factory box Jan. 2015), Bogart Tri-metric,  460 Ah. 24 volt LiFePo4 battery bank. Plenty of Baja Sea of Cortez sunshine.

  • Dave Angelini
    Dave Angelini Solar Expert Posts: 6,730 ✭✭✭✭✭✭
    Hey Joe, lot's of good advice on this very old unit. My 2 cents are make sure you get paid for your time. Depending on what state you are in, you should have been able to get the basic No from the building department. Good Luck!
    "we go where power lines don't" Sierra Nevada mountain area
       htps://offgridsolar1.com/
    E-mail offgridsolar@sti.net

  • joeom
    joeom Registered Users Posts: 4
    Hey everyone, thanks for your advice!

    An update:

    I contacted someone at the utility who was able to tell me the unit WON'T work for grid-tie, as expected.

    I have communicated this to the homeowner and although he's disappointed not to be able to use the equipment that came with the house, I think he understands why it's not feasible.

    The idea now is to sell the old equipment and install a simple grid-tie inverter.
    I understand that there are inverters that allow for a simple configuration: power from the solar panels tied straight into the inverter, and the inverter feeding the grid. I think this will be the best solution as it will allow my client to offset his power usage while spending as little as possible.
    However, he may be interested in adding a battery/generator backup system in the future, so if the inverter also has the ability to integrate that kind of functionality later on that would be great too.

    The guy I talked to at the utility was, off the top of his head, able to recommend Enphase micro inverters and Solar Edge inverters as options that have proven easy to integrate into the grid here.

    Would anyone mind giving me some recommendations for a good value inverter?


    He sent me a graph of his household's power usage over time, and it looks like it peaked out in August at exactly 1,000kWh.
    So, just over 30kWh per day.

    The utility will buy back power at the same rate they sell it -- up to your total monthly power usage. They pay much less for power generated beyond that. In other words, in August if he had fed the grid 1,000kWh, it would have completely offset his bill (other than the fixed fees), but if it was 2,000kWh then he would have offset his bill but not earned significant additional money. So, for maximum cost effectiveness we'll need a system that exceeds his power consumption by only a small margin.

    We're in Durango, CO, so we get a pretty good amount of sunlight. The panels will likely go on a west-facing roof.
    I'm thinking if he wants the ability to generate about 35kWh/day in August then maybe you count on west-facing panels being illuminated for 6 hours? So a 6kW system? Maybe an 8kW inverter?
    Or maybe you plan for winter instead... he used about 700kWh last month, and there's a lot less light now.

    Thanks for any feedback :)