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Renewable Energy: SolarPV and inverters

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    • CommentAuthoralbacore
    • CommentTimeJul 30th 2011
     
    We are planning an installation of PV panels on our bungalow on two roof aspects, just off of S.E and S.W orientation. I seem to remember reading that installations in different orientations should be connected to seperate inverters. (I can't remember where I read it,and can't now find relevant information by googling) Is this correct? and is there any accessible source of information online on such technical aspects of PV installation? I would also like to learn about issues related to cable length, and cables passing through insulation.

    Thank you for your help.
    Albacore
    • CommentAuthoringleside
    • CommentTimeJul 30th 2011
     
    Yes, as low output from one group of panels (or one panel, for that matter) has a deleterious effect on the whole output. Some sort of negative feedback, I think. No doubt someone else can be more technical.
    Tony
    •  
      CommentAuthorSteamyTea
    • CommentTimeJul 30th 2011
     
    It is to do with internal resistance when a panel is shaded, though this is not so much of a problem with new panels.
    Options and opinions are varied on this subject, one inverter that can take two strings, small multi inverters that are supplied by one panel, two separate inverters for the two banks. Look at the price for each option and then consider the reliability of each system. 1 inverter on a single inverter system could totally stop generation, an inverter that can take two strings may or may not stop, separate panel inverters will only stop the supply panel contributing, but may cost more to replace if they are roof mounted.
    Also look at cabling costs and how long the route is and what sort of insulation they have to go though.
    Personally I like panel/inverter combination as you can monitor each panel, but then I am strange like that and like statistics.
    • CommentAuthorJeff B
    • CommentTimeJul 31st 2011
     
    I concur with Steamy Tea in preferring the one inverter per panel scenario. One of the more subtle advantages of this system, which would probably not be noticeable using one inverter per string of panels, is the effect of drifting clouds partially blocking the sun. By being able to monitor each panel individually it can be seen that at any particular instant in time there can be quite a large variation in output from panel to panel. Hence panels that are not obscured by cloud cover continue to deliver a decent output i.e. the output of the whole system is not reduced to the lowest common denominator. Perhaps over a single year that may not account for much but over the 25 year life span of the FIT scheme the difference could be very significant IMO.
    • CommentAuthoralbacore
    • CommentTimeJul 31st 2011
     
    Thank you for this.
    I was only thinking of the difference between one inverter per bank or one for the lot. The idea of one per panel adds another whole layer to the research! My reading on inverters that can take two strings doesn't make it clear if these can be from two different orientations.
    I am a bit concerned about cables; I need a better understanding of why cable length matters and what precautions would be needed going through insulation. Whatever point we choose to bring cables into the house, they will need to pass through six-inch thick polyurethane SIPS panel. Any recommendations on where a person with only moderate technical understanding might read about this would be greatly appreciated.

    Thanks again.
    Albacore
    • CommentAuthorAnke
    • CommentTimeJul 31st 2011
     
    I found this text quite helpful

    http://books.google.com/books?id=fMo3jJZDkpUC&lpg=PP1&dq=photovoltaic%20installation%20handbook&pg=PA166#v=onepage&q=string&f=false
    • CommentAuthorowlman
    • CommentTimeAug 1st 2011
     
    Posted By: ingleside Yes, as low output from one group of panels (or one panel, for that matter) has a deleterious effect on the whole output. Some sort of negative feedback, I think. No doubt someone else can be more technical.
    Tony

    Is that a general thing for all inverters tony, ST and Jeff B ? Even models like the SMA 4000TL which has input facility for two strings with two input areas for each string. I thought panel type, quantity, alignment and tilt, only applied to arrays within each, ( A or B ) input areas.
    •  
      CommentAuthorted
    • CommentTimeAug 1st 2011
     
    Most inverters can handle multiple strings but only have a single MPPT and some have dual (or more) MPPTs.

    You have to check the manufacturers' specs.
    • CommentAuthoralbacore
    • CommentTimeAug 1st 2011
     
    All useful information. Many thanks
    •  
      CommentAuthorSteamyTea
    • CommentTimeAug 1st 2011 edited
     
    Posted By: albacorethey will need to pass through six-inch thick polyurethane SIPS panel

    You have two problems here. The first and simple one is that come PVC cable reacts with polystyrene so you have to make sure that this is addressed.
    The second is that the cable will have to be thicker, may even have to be twice as thick because heat cannot escape where is passes through the insulation. This is known as 'de-rating'. As the PV will be installed by professionals this should not need to concern you, but it is interetsing stuff all the same. Any cable that passes through insulation over a few millimetres thick (can't remember exactly how few and can't find my book at the moment.) is derated, regardless of the overall length of the cable and the load applied. Do not confuse this the 'diversity factor', this is to do with the probability of different loads being applied at the same time.
    As a general guide, wiring has an Installation Number that describes the type of cable and how it is fixed, so there is a bit of cross referencing needed to work out the type of installation and what it is capable of doing.
    This is the latest incarnation of the main book needed:
    http://www.amazon.co.uk/IEE--site-Guide-7671-Regulations/dp/0863418546/ref=sr_1_1?ie=UTF8&qid=1312223313&sr=8-1

    As for panel inverter combinations, there is some scope to run lower amperage cables to a main junction box before the import/export meter, but it will probably make little difference. and is more messy.

    Six inches of SIPs is good though. Does the manufacture 'mind' cables being passed though it?
    • CommentAuthoralbacore
    • CommentTimeAug 2nd 2011
     
    Thank you SteamyTea, that is exactly the sort of information I was hoping someone would pass on. I hadn't thought off the manufacturer having an issue with the cables. Perhaps I had better try and find out.
    • CommentAuthorCWatters
    • CommentTimeAug 2nd 2011
     
    While on the subject of cables..

    Anyone know roughly what voltage drop can be tollerated between a typical inverter output and the CU?
    • CommentAuthorjamesingram
    • CommentTimeAug 2nd 2011 edited
     
    AC side of inverter, cable sized for <1 % volt drop
    •  
      CommentAuthorSteamyTea
    • CommentTimeAug 2nd 2011 edited
     
    Voltage drop is proportional to load and cable resistance. Most of the time (apart from high summer) you can get away with smaller cabling because the energy delivered to the house is small (because you are only generating a small amount). Peak performance is when voltage drop can become an issue, but this happens rarely (see http://www.greenbuildingforum.co.uk/forum114/comments.php?DiscussionID=7643&page=1#Item_10).

    Cables have a resistance based on the material they are made from, their cross sectional area, length and temperature.
    This is known as Resistivity and has the units Ohm meter.
    Using Ohms law and other known parameters you can calculate the likely volt or power loss though any length of cable for any load and any temperature for any light level (it is what Excel was made for).
    Or just make sure that they are a little oversized if you are really worried.

    Be interesting to hear what the SIPs people say about putting cables though their carefully designed and manufactured walls.
    • CommentAuthorCWatters
    • CommentTimeAug 3rd 2011
     
    I know how to do the calculation if I have the starting values. I'm looking at around 4kW so peak current would be circa 19A (mains voltage at lower end of voltage range eg 207V). If the allowed voltage drop is 1% (call it 2V) that means a resistance of around 0.1 Ohms. Unfortunately the length is close to 70m so I'd need better than 1.4mOhms/m eg something over 25mm^2. Obviously I woud do a more accurate calculation but it gives me enought to go on.
    •  
      CommentAuthorSteamyTea
    • CommentTimeAug 3rd 2011
     
    Not sure and guessing a bit here, but does the inverter sense the incoming grid voltage and try and stay above that at all times so that locally produced power is used first (not that I understand how that work)? This would make voltage drop important as you need to consistently be higher than grid voltage.
    Are the inverters by the panels or by the consumer unit? I take it it is by the panels.
    •  
      CommentAuthorDamonHD
    • CommentTimeAug 3rd 2011
     
    ST: frankly even to someone a little familiar with electronics I regard grid-tie inverters as practically black magic. However, basically I think that they regulate the *current* that they inject, monitoring the phase and magnitude of the line voltage while doing it. So, if your A/C cabling to the grid is too thin (impedance too high) then your inverter will disconnect itself from the grid, but that can also happen if you're too close to a substation and thus your RMS voltage is higher than usual anyway.

    Rgds

    Damon
    • CommentAuthorjamesingram
    • CommentTimeAug 3rd 2011 edited
     
    CW , volt drop for multicore armoured thermosetting insulated copper cable, 3+ cores three phase AC from regs table 4D4B
    16mm2 - 2.4 mV/A/m
    25mm2 - 1.5 mV/A/m
    •  
      CommentAuthorSteamyTea
    • CommentTimeAug 3rd 2011
     
    So between 1.68 (25mm^2) and 2.8 (16mm^2) for a 70m run at max current.

    Damon
    I tried to do an experiment once where I injected a current at a higher voltage into a circuit to see if it worked, never did get it to work, where I find the electrical and water analogy breaks down badly. Is the internal resistance of the inverter the same as the grid, higher or lower, anyone know?
    •  
      CommentAuthorDamonHD
    • CommentTimeAug 4th 2011
     
    The grid has a VERY low impedance (it can supply GW with only a few % loss) and even your branch must have a lower impedance than your grid-tie since you can suck more out of the grid (100A fuse => 23kW) than any domestic inverter you can buy.

    So that's why I think it has to be current regulation involved.

    Rgds

    Damon
    • CommentAuthorCWatters
    • CommentTimeAug 4th 2011
     
    I believe the output power will be regulated so that it matches that available from the panel.

    Both voltage and current will be controlled to some extent but the output voltage will mostly be dictated by the gird.
    • CommentAuthorCWatters
    • CommentTimeAug 4th 2011
     
    <blockquote><cite>Posted By: jamesingram</cite>CW , volt drop for multicore armoured thermosetting insulated copper cable, 3+ cores three phase AC from regs table 4E4A
    16mm2 - 2.5 mV/A/m
    25mm2 - 1.5 mV/A/m</blockquote>

    Thanks. Looks like I might need at least 25mm2. Think that's like £8 a meter.

    Main reason for the interest is that I will soon be digging a trench to run power to an out building and was considering putting a cable for PV in the trench at the same time but given the cost I'll have to make doubly sure I get it sized correctly. Perhaps best I get an installer to advise.
    •  
      CommentAuthorSteamyTea
    • CommentTimeAug 4th 2011
     
    A buried cable will have to be thicker than one in air.
  24.  
    Edited above figure to 16mm2 - 2.4 mV/A/m
    I was looking at 90deg C rather than 70deg C thermoplastic

    16mm2 less than half the price of 25mm2 if you can make that work.
    •  
      CommentAuthorDamonHD
    • CommentTimeAug 4th 2011
     
    CW: output power will certainly be determined by the panel unless you are breaking the laws of thermodynamics, and we don't do that here at GBF (except possibly Tony, but he's got dispensation).

    The subtle point about attempting to drive a very very very low-impedance load such as the grid is that doing it badly will instantly let magic smoke out of something, so one good way to avoid that would be for the inverter to be a current source (with the right phase) and the voltage will take care of itself. Sorta. Not that I could build one.

    Rgds

    Damon
    • CommentAuthorCWatters
    • CommentTimeAug 4th 2011
     
    Yes has to be a current source (they have a high output impedance).
    •  
      CommentAuthorSteamyTea
    • CommentTimeAug 4th 2011
     
    Isn't current set by the load? I=(W.R^-1)^0.5 Tis late and been a long day though.
    •  
      CommentAuthorDamonHD
    • CommentTimeAug 5th 2011
     
    Not with a current source, within reason.

    Try disconnecting an inductor rapidly from a flowing current and see where attempting to 'set' the current to zero abruptly gets you as another example.

    Rgds

    Damon
    • CommentAuthorSteveZ
    • CommentTimeAug 19th 2011
     
    My electrical studies stopped at the 16th Edition Regs, so I will stay out of the cabling part of this discussion. This next bit may add to the confusion but I hope not - I have been looking into a 4kW array for my East-facing roof and I have just decided to go for a different idea to the previous posts here. My preferred system is based on the SolarEdge company's products, although this not an endorsement of the company, it is the only one I have come across in the UK. This uses a small DC-DC PowerBox on each panel performing the Maximum Power Point Tracking (MPPT) and they are then fed into their simpler (no MPPT needed) inverter. The whole system of boxes and inverter cost around the same as a top manufacturer two string inverter, but has advantages over both the string inverter and the distributed micro-inverter (one to each panel) idea. The PowerBox has a smaller parts count than the micro-inverters that I have looked at. Fewer parts to fail on the roof seems sensible and each PowerBox has a 25 years warranty. The inverter only performs the DC to AC function, is simpler in design as a consequence and should be more reliable than usual - this item comes with a 12 year warranty. In addition, each panel can be remotely monitored like a micro-inverter system, which is something I wanted. Add the efficiency figure of 97%, a lot higher than that of a typical micro-inverter, and the on-roof safety feature of each panel only producing one volt (for setting up) when the inverter is switched off and it seems to be a winner. I am now getting a few quotes and will report back if anyone is interested
    • CommentAuthorCoxy
    • CommentTimeAug 23rd 2011
     
    Steve Z

    I would be interested. Looking at whatever I can get on my 30m2 south facing garage roof. It would be good to know if the setup you state is competitive.

    Coxy

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