Re-Design Thinking

akallen
akallen Registered Users Posts: 10 ✭✭
in Grid Tie and Grid Interactive Systems #1
This question is about doing it right the second time, because I didn't consider all the possibilities the first time.

Currently in place and working well is a grid tied, battery backed system. Xantrex SW5548 panel and associated components. 48 volt system from the 3 arrays up to the 2 charge controllers. The DC run is about 150 feet. Two sets of cable (not 3) running up to the equipment panel and batteries at the house. The Vmp on the arrays is decently matched, so combining two of them into one of the cable runs was not problematic. On a nice day, about 5 kW. The system is about 8 years old now and CRS (can't remember stuff) has me wondering why I put the equipment and batteries at the house. There are no particular security issues.

I started driving electrically in the last few years and am really using up the kW. So, time to put a fourth array in. My oh my have panels gotten better. This fourth array will be around 4kW all by itself. I forgot to mention - they are size limited because they are trackers.

In summary: My stack of Surrettes has a more than ample bulk charging source as they stand.
The underground DC run back to the house was not designed for another 4kW. If I keep doing what I've already done, a whole new wire run needs to go in.
I have an off grid battery setup because SCE (in my area at least) likes to go south on a regular basis.
I'm primarily interested in the additional kW because I want to stay a net producer while driving electrically.
I'm thinking the 4th array will be the last, but then my current situation comes to mind and I remember Sean Connery and "Never say never again"
The arrays are well positioned as far as shading goes, but at either end of the day and winter time, there is some partial shading going on.

Not saying I like this, but the patient would seem to need major surgery in any case. Here's what I've thought of. The people on this forum are considerably more expert than I, so I doubt this list is exhaustive.

1) Relocate all equipment and batteries down to the arrays. MPPT controller on each array (4). Run AC up to the house. Takes care of many distinct evils and can expand well beyond practicality.
2) Don't touch the existing system. Put a pure grid tie inverter on the fourth array and a new wire run up to the house for the AC.
3) Dig up the conduit and put in considerably larger wire to tolerate the new amount of DC. Move charge controllers to arrays (means buy two more) so the voltage mismatch returns to tolerable.

Note is already too long. Opinion time.
Twin Xantrex 5548 (48 V) with two MPPT60 controllers and Rolls batteries, Pre-2008, so the NEC was simple at the time.
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Comments

  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #2
    Re: Re-Design Thinking

    Some fairly universally applicable observations:

    1). Longest wire runs should be on the highest Voltage. Doesn't matter AC or DC. Not knowing what the array Voltage is, it's impossible to say if there would be any gain to be had from switching this around.

    2). If the existing system is maxed out in terms of panels to inverter capacity, there's not much point in adding more panels to it. Once you reach the inverter's rated output, more panels do not increase power being fed to grid/household. If the first system was designed right, the logical solution is adding a supplementary GT only set-up.

    3). How's the service capacity? You can't add more and more back-feeding to a given service. There's this 120% limit that has to be dealt with. If the existing GT system has this at the max you can't add any more no matter how.
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  • akallen
    akallen Registered Users Posts: 10 ✭✭
    #3
    Re: Re-Design Thinking
    Cariboocoot wrote: »
    Some fairly universally applicable observations:

    1). Longest wire runs should be on the highest Voltage. Doesn't matter AC or DC. Not knowing what the array Voltage is, it's impossible to say if there would be any gain to be had from switching this around.

    Agreed and AC better yet as far as the wire gauge is concerned. Panels are series Vmp 32-34 V, so the DC run is in the 64+ V range up to the charge controller.

    2). If the existing system is maxed out in terms of panels to inverter capacity, there's not much point in adding more panels to it. Once you reach the inverter's rated output, more panels do not increase power being fed to grid/household. If the first system was designed right, the logical solution is adding a supplementary GT only set-up.

    Again agreed. The twin SW5548 are probably optimal around 10kW, and I'd be right in there with the fourth array. I should state again for the record that I'm uncomfortable with what now seems to be my poor design, so "If it was designed right..." is the primary question here.

    3). How's the service capacity? You can't add more and more back-feeding to a given service. There's this 120% limit that has to be dealt with. If the existing GT system has this at the max you can't add any more no matter how.

    Yep. Service capacity is not an issue. I'm good for 200A at 240V transformer to main.
    Twin Xantrex 5548 (48 V) with two MPPT60 controllers and Rolls batteries, Pre-2008, so the NEC was simple at the time.
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #4
    Re: Re-Design Thinking

    So your 5kW inverter is pushing only half of your maximum allowable GT connection? (Check this out very carefully with the GT guys here.)

    Now, is there 5kW of array connected to that inverter?
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  • akallen
    akallen Registered Users Posts: 10 ✭✭
    #5
    Re: Re-Design Thinking
    Cariboocoot wrote: »
    So your 5kW inverter is pushing only half of your maximum allowable GT connection? (Check this out very carefully with the GT guys here.)

    I hope I'm understanding your question correctly. The numbers I gave in my original post are design limit numbers. I expect the actuals are maybe 80% of that but I don't have any computer interface to them. That would be *really nice* but these old inverters don't seem to support that. So, 11kW of rated capacity. Arrays are currently a tad over 5kW on the good days. I'm adding another 4kW of array capacity.

    Now, is there 5kW of array connected to that inverter?

    Yes. Inverters (two of them currently).
    Twin Xantrex 5548 (48 V) with two MPPT60 controllers and Rolls batteries, Pre-2008, so the NEC was simple at the time.
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #6
    Re: Re-Design Thinking

    Okay, what I'm saying here:

    40 Amps @ 240 Volts is 9600 Watts. That is probably the maximum you are allowed to back-feed.
    You have two 5kW inverters. Given the first limitation, you can't add any more.
    If both inverters have 5kW of array each, then there's no room for adding panels.

    This is what you have to check out carefully before spending a dime on expansion, as expansion may not be possible.
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  • akallen
    akallen Registered Users Posts: 10 ✭✭
    #7
    Re: Re-Design Thinking

    I'm probably doing a poor job writing about this. My total array capacity is about 5kW. Its a 48 V system. I have two inverters rated at 5.5kW.
    I'm not quite at half my rated capacity. Back to my original question is whether I should continue to use this basic set of equipment with considerable re-engineering in terms of placement, location and quantity of charge controllers, OR just put in a whole new grid tie system for the fourth array? Please don't worry about my service capacity. I designed that out of the equation long ago by paying Edison to upgrade it. The dollars comparison is not obviously different, so I don't have that to fall back on in making a decision.

    Its possible I can infer an answer from the conversation we've already had. If one of the alternatives was glaringly better, someone would say so.
    Twin Xantrex 5548 (48 V) with two MPPT60 controllers and Rolls batteries, Pre-2008, so the NEC was simple at the time.
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #8
    Re: Re-Design Thinking

    Okay, if the two inverters aren't being fed to their maximum capacity then filling out the array so that they can produce peak power would be the thing to do. Got to be cheaper than tearing everything out and starting over.

    The question then becomes what you've got connecting the arrays to the battery bank: wire sizes, run length, type of controller, array Voltage. It may be possible to utilize the existing wiring with some reconfiguration. Too much reconfiguration and it's not worth it, of course.

    Your maximum GT allowance is limited by the service panel rating & NEC. This should all have been documented when the system was installed.
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  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    #9
    Re: Re-Design Thinking

    ok this is my 3rd try posting here. not sure what's going goofy that stopped 2 postings.

    my opinion is to get a separate straight gt inverter for the new 4kw array because you would otherwise need to upgrade the battery bank and add more controllers to the mix. your backed up loads have not really changed so let the car get charged via the grid and your new straight gt system would use the grid like a battery. also, in the case of a failure on your gt backups system the straight gt system will still be operational. only problem may be charging the car batteries during a prolonged outage, but some have successfully back fed their battery backed gt systems and i'll let those more familiar with that scenario address it, but i wouldn't count on this option.

    this assumes the utility panel's bus has the capacity to handle the power involved.
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  • ggunn
    ggunn Solar Expert Posts: 1,973 ✭✭✭
    #10
    Re: Re-Design Thinking
    Cariboocoot wrote: »
    If both inverters have 5kW of array each, then there's no room for adding panels.
    That's not necessarily correct. It is fairly routine to overload an inverter by 20% or more, in part because 5kW of STC DC rated modules is virtually never going to produce 5kW of power because STC virtually never exists in the real world. And some current clipping during peak production times is acceptable if the increased output at off peak times more than makes up for the loss.

    Of course, you are limited in what you can do with an existing system because you can't lengthen strings to the point where Voc at the coldest temperature the array will experience will exceed the maximum voltage allowed by the inverter, and in parallel you can only add modules in complete string increments.
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #11
    Re: Re-Design Thinking
    ggunn wrote: »
    That's not necessarily correct. It is fairly routine to overload an inverter by 20% or more, in part because 5kW of STC DC rated modules is virtually never going to produce 5kW of power because STC virtually never exists in the real world. And some current clipping during peak production times is acceptable if the increased output at off peak times more than makes up for the loss.

    Of course, you are limited in what you can do with an existing system because you can't lengthen strings to the point where Voc at the coldest temperature the array will experience will exceed the maximum voltage allowed by the inverter, and in parallel you can only add modules in complete string increments.

    I'm well aware of the virtues and ramifications of over-sizing arrays. The point I've been trying to get across (apparently without any success) is that you can not just keep adding GT capacity to a service to your heart's content.

    Maybe I've had another stroke and am typing all my posts in Polish. :confused:
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  • inetdog
    inetdog Solar Expert Posts: 3,123 ✭✭✭✭
    #12
    Re: Re-Design Thinking
    Cariboocoot wrote: »
    Maybe I've had another stroke and am typing all my posts in Polish. :confused:

    Looks OK to me. Segueing (not Segwaying) into Reverse Polish Notation: Bumper sticker on car of a Forth (computer language) programmer "YOU FORTH LOVE IF HONK THEN"

    But straying back to the original subject, one way to increase daily yield without any limitation by allowed service amps is to extend the period during which you are producing maximum power. so if the service is maxed out, that argues for adding East and West panels without changing the inverter(s).
    Upsizing the building's service is indeed expensive and has to be figured into any cost/benefit, ROI calculations.
    SMA SB 3000, old BP panels.
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  • ggunn
    ggunn Solar Expert Posts: 1,973 ✭✭✭
    #13
    Re: Re-Design Thinking
    inetdog wrote: »
    Looks OK to me. Segueing (not Segwaying) into Reverse Polish Notation: Bumper sticker on car of a Forth (computer language) programmer "YOU FORTH LOVE IF HONK THEN"
    I loves me my HP-41CX! What's that consarned "=" for? :D
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  • solar_dave
    solar_dave Solar Expert Posts: 2,397 ✭✭✭✭
    #14
    Re: Re-Design Thinking
    ggunn wrote: »
    That's not necessarily correct. It is fairly routine to overload an inverter by 20% or more, in part because 5kW of STC DC rated modules is virtually never going to produce 5kW of power because STC virtually never exists in the real world. And some current clipping during peak production times is acceptable if the increased output at off peak times more than makes up for the loss.

    Of course, you are limited in what you can do with an existing system because you can't lengthen strings to the point where Voc at the coldest temperature the array will experience will exceed the maximum voltage allowed by the inverter, and in parallel you can only add modules in complete string increments.

    My system is pretty much that way, I have a 12.5 KW STC set of panels but a pair of 5200 PVP GT inverters, they will peak right at 10.4 Kw. Each capable of 23 amps max. I have only seen the system clip total watts (and the loss was just a few watts) once in the few years I have had it and that only lasted about 45 minutes, on a cool spring day near the peak angle for my 5/12 roof at solar noon. The original install was about 10.4 Kw and I add the balance after the first year per the design agreed upon with the installer. I have never seen it hit near the max voltages as the string design was set up with the max voltage in mind. It would be a rare occasion to see sub freezing temps and definitely not sub zero.
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  • inetdog
    inetdog Solar Expert Posts: 3,123 ✭✭✭✭
    #15
    Re: Re-Design Thinking
    ggunn wrote: »
    I loves me my HP-41CX! What's that consarned "=" for? :D

    "Parentheses? We don't need no stinking parentheses!"
    SMA SB 3000, old BP panels.
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  • ggunn
    ggunn Solar Expert Posts: 1,973 ✭✭✭
    #16
    Re: Re-Design Thinking
    inetdog wrote: »
    "Parentheses? We don't need no stinking parentheses!"
    My trusty 41CV (with MathPac) got me through engineering school in the 80's, and in a lab I was working in 20 years later someone found a 41CX in a drawer and gave it to me. I love them; I have one on my desk at work and one in my home office and they both still work great. Back in the day there was a bunch of guys who did what they called "synthetic programming" on their 41's. Every keystroke sends a 2 byte word to the calc's CPU, and these guys figured out a way to make it slip a cog and start sending the second byte of one keystroke paired with the first byte of the next to send words outside the documented instruction set to the CPU and make the display do weird stuff.
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  • inetdog
    inetdog Solar Expert Posts: 3,123 ✭✭✭✭
    #17
    Re: Re-Design Thinking
    Cariboocoot wrote: »
    Okay, what I'm saying here:

    40 Amps @ 240 Volts is 9600 Watts. That is probably the maximum you are allowed to back-feed.

    Just to say it again, in different words:
    If you have a 200 A 240 V POCO service, your main panel contains a 200A rated bus component, and you have a 200A main breaker, the largest combined PV backfeed breaker combination the NEC will allow in that panel totals 40A 240V. (120% of 200 = 240; 240 - 200 = 40)

    You can go beyond that even without getting a larger POCO service installed, but it will require some rewiring and new panel components, done by a licensed electrician, potentially costing more than $1000 and requiring a new electrical permit and possibly a new POCO inspection. Just be sure to include that in your cost calculations.
    SMA SB 3000, old BP panels.
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  • solar_dave
    solar_dave Solar Expert Posts: 2,397 ✭✭✭✭
    #18
    Re: Re-Design Thinking
    inetdog wrote: »
    Just to say it again, in different words:
    If you have a 200 A 240 V POCO service, your main panel contains a 200A rated bus component, and you have a 200A main breaker, the largest combined PV backfeed breaker combination the NEC will allow in that panel totals 40A 240V. (120% of 200 = 240; 240 - 200 = 40)

    You can go beyond that even without getting a larger POCO service installed, but it will require some rewiring and new panel components, done by a licensed electrician, potentially costing more than $1000 and requiring a new electrical permit and possibly a new POCO inspection. Just be sure to include that in your cost calculations.

    And of course if you derate the main breaker to 175 amps you can get 65 amp back feed. The big IF here is your loads are low enough to allow a derate of the main breaker, in my case they disallowed this, requiring a service entrance upgrade to 400amps with a pair of 200 amp panels, one derated with a 175amp main breaker.
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  • Rick1
    Rick1 Registered Users Posts: 24 ✭
    #19
    Re: Re-Design Thinking
    Cariboocoot wrote: »
    Okay, what I'm saying here:

    40 Amps @ 240 Volts is 9600 Watts. That is probably the maximum you are allowed to back-feed.
    You have two 5kW inverters. Given the first limitation, you can't add any more.
    If both inverters have 5kW of array each, then there's no room for adding panels.

    This is what you have to check out carefully before spending a dime on expansion, as expansion may not be possible.

    If his 200 amp service has a 225 amp buss could he have 225 x 120% = 70 amps ???
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  • inetdog
    inetdog Solar Expert Posts: 3,123 ✭✭✭✭
    #20
    Re: Re-Design Thinking
    Rick1 wrote: »
    If his 200 amp service has a 225 amp buss could he have 225 x 120% = 70 amps ???

    Yes.
    Although he may have to provide satisfactory documentation on the bus rating to the AHJ if that worthy does not have the information himself.

    Some people, not usually electricians or inspectors, think that the 120% multiplies the main breaker rating or the panel nameplate rating. But the language of the NEC is uncharacteristically clear in this case. :-)
    SMA SB 3000, old BP panels.
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