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Renewable Energy: Preventing grid supply in a FITs situation

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    • CommentAuthorGBP-Keith
    • CommentTimeMar 9th 2012
     
    Hi. Does anyone know of any way of preventing grid power coming back along the line and feeding loads in a PV grid connect situation?

    I have been thinking of some sort of blocking diode but I'm betting it won't be that simple.

    I want to run a few small resistive loads during sunshine periods on simple timers but I don't want the grid to feed them if the sun goes behind a cloud.

    I also don't know how the grid connected inverters would cope - I am assuming any blocking diode would prevent them seeing the grid and they would go off

    Clearly an unusual ask but I need to look for a solution.

    Any and all suggestions gladly accepted.

    Thanks, Keith
    •  
      CommentAuthorJSHarris
    • CommentTimeMar 9th 2012
     
    What's needed is a way of detecting export and using that to switch the loads. Some grid tie inverters may have this capability, but to make use of it would need some means of connecting to the inverter, which may not be an easy DIY job.

    Sensing the current flow direction can be done with two clamp on current sensors, one on the grid tie inverter connection to the incoming cable, one on the incoming cable itself. The difference between these two will be an indication of the direction of current flow, and with a bit of circuitry this can be detected and used to switch a load on only when the grid tie inverter would otherwise be exporting.

    AFAIK, there's only one off-the-shelf solution, but it's expensive, the EMMA, (http://www.igloo-eco.com/products.html). There are some cheaper DIY solutions around on the web, but you'd need to be reasonably comfortable with a bit of electronic bodging to have a go at building one, plus you'd need to be happy with building something that would be switching mains voltages.
    • CommentAuthorGavin_A
    • CommentTimeMar 9th 2012
     
    what inverter is it?

    SMA sunny boy HF and TL series have relay functions that will switch on a load circuit at user settable generation levels. TL has it as standard, HF it's an optional extra.
    • CommentAuthorGBP-Keith
    • CommentTimeMar 9th 2012
     
    Thanks Jeremy. I have contacted the Emma people. I don't have the capability to build such a gizmo.

    Cheers Gavin. It is a sunnyboy but I'll check which model i have and look out the user manual. Strange that my installer never mentioned this facility.

    What a great forum this is. I've been scouring the web for a couple of days and got nowhere until now!!

    Cheers, keith
    • CommentAuthorowlman
    • CommentTimeMar 10th 2012
     
    Posted By: moulesnfrites...................We are using SMA 4000TL inverters on most of our PV projects. They have some volt free contacts that can be programmed to open/ close at a certain wattage level. (programming is by any laptop) You can use these contacts to switch a contactor to allow the immersion heaters to use the generated power to provide hot water. We have several of these systems up and working. We have found it better to change the immersions for 1kW versions.
    Total cost to customer is less than £300 ... 1kW immersions are expensive at about £80!

    You could probably get multiple 1 kW immersions into a thermal store tank and get the pv to contibute towards the space heating but we have not had a customer with this requirement yet!




    This may be helpful lead Keith.
  6.  
    sunnyboy multi function relay and optitrac info manual .

    http://files.sma.de/dl/5692/ZusFunktNG-TB-TEN115121.pdf
    • CommentAuthorGBP-Keith
    • CommentTimeMar 10th 2012 edited
     
    My inverters are TL so that's looking good for the inbuilt relays. This is all fantastic feedback guys. It's a shame I had not asked this question here before our new mag went to print as I muse on this point in my latest 'offgrid' story. I'll be sure to give the appropriate credits for your advice in the next installment in the summer issue.

    Just need to talk my installer into configuring it now.
    • CommentAuthorGavin_A
    • CommentTimeMar 10th 2012
     
    it's relatively simple, the complex bit comes if you want to also have a bypass switch on the circuit, so you can manually switch the load on when the sun's not shining. Not hugely complex though for a competent electrician with a bit of nouse.
    • CommentAuthorGBP-Keith
    • CommentTimeMar 11th 2012
     
    Thanks Gavin. I won't want to bypass it manually so that solves that point.

    In another point, I see there is a battery charge option mentioned in the document that James pointed me to. Does this allow for low voltage charging direct from the inverter do you think/know? Or why would they call it 'battery charging' if all it does is switch a remote charger on and off.
    • CommentAuthorGavin_A
    • CommentTimeMar 11th 2012
     
    I'm pretty sure it's the latter. TBH I don't really understand why it needs a different setting to the standard relay function, but I've not tried using the battery charge function so couldn't really say.

    I presume you're aware that the relay needs to be used to trigger a contactor to switch the circuit on. IIRC it only has a 1amp maximum current through the relay circuit itself.
    • CommentAuthorGBP-Keith
    • CommentTimeApr 25th 2012
     
    Does anyone know what kind (make or model number) of 240v switch I would need to use so that the relay from the inverter can turn it on and off?
    •  
      CommentAuthorSteamyTea
    • CommentTimeApr 25th 2012 edited
     
    Be a contactor I should think, or do you mean for isolating?
    Best go for a 2 pole one.
    Something like this:
    http://www.hopkins.biz/shop/spare-parts/hopkins-electrical-range-spares/20a-contactor-2-pole-240v-swit0033/

    Or am I missing something by not re-reading the thread.

    Oh and it is not unusual to have two relays, activated in parallel but taking the load in series for safety.
  13.  
    yep a contactor as in ST s link
    • CommentAuthorGBP-Keith
    • CommentTimeApr 25th 2012
     
    I think I have just found it on the SMA pdf but they look weird. Siemens Insta contactor 5TT5 is one of them. I notice that the relays on the Inverter only have a expected endurance of 100 000 switching cycles.
    •  
      CommentAuthorSteamyTea
    • CommentTimeApr 25th 2012 edited
     
    Posted By: GBP-Keithexpected endurance of 100 000 switching cycles.

    Why two relays should be used, you could go for a solid state relay as the secondary one, though ideally you want the most reliable one in the SMA.
    Relays and Contactors have an uncanny knack of wielding themselves in the closed position, so assume they will not fail safe. One advantage of solid state ones, they are inherently safe.
    • CommentAuthorjamesingram
    • CommentTimeApr 25th 2012 edited
     
    I think the inverter relay is 1A
    I'd have thought any 20A contactor would do , such as
    http://www.hagergroup.ae/energy-distribution/automation-control/modular-contactors/1503.htm
    how many switching cycles do you estimate a day?
  17.  
    Posted By: SteamyTea
    Relays and Contactors have an uncanny knack of wielding themselves in the closed position, so assume they will not fail safe. One advantage of solid state ones, they are inherently safe.


    My experience of solid state ones is that they may not fail safely - might want to check with the manufacturer on specifics for any given device to confirm.
    • CommentAuthorGavin_A
    • CommentTimeApr 26th 2012
     
    Posted By: jamesingramI think the inverter relay is 1A
    I'd have thought any 20A contactor would do , such as
    http://www.hagergroup.ae/energy-distribution/automation-control/modular-contactors/1503.htm" rel="nofollow" >http://www.hagergroup.ae/energy-distribution/automation-control/modular-contactors/1503.htm
    how many switching cycles do you estimate a day?

    them's the ones we use. Other makes are also available etc
    • CommentAuthorGavin_A
    • CommentTimeApr 26th 2012
     
    realistically, how many times are you expecting the contactor to be switching in it's lifetime?

    100,000 should be plenty, and if it's only switching a contactor, then I'd think this would be massively under 1amp anyway, so ought to last longer than that.

    obviously don't set it up to be switching on and off every few seconds.
    • CommentAuthorGBP-Keith
    • CommentTimeApr 26th 2012
     
    Oh sorry it is the inverter that seems good for 100 000 switchings and they say that equates to about 12 switchings per day for 20 years. so seemingly 6 on and 6 off. Does not seem to be many but i guess the trick will be to set the wattage switch point quite high (perhaps at dull but bright level) then I guess it would contain itself to about 4 for my requirements. Thanks for your help again guys.
    • CommentAuthorGBP-Keith
    • CommentTimeApr 26th 2012 edited
     
    If someone can give me a model number for the right contactor in the range that would be a great help.
    My installer is really in the dark on this so I'm going to rig it myself.

    Here's how I'm going to rig it: (comments welcome)
    I will take a three core line from the grid side of my generation meter to a 30ma breaker box.
    From there on to the contactor close by.
    The contactor will have a low voltage connection from the inverter relay and I think I will set the wattage to about 1000watts as the trigger point.
    From the contactor I will run a 4mm armored three core to my battery room about 40 metres away.
    In the battery room I will terminate with about 3 two gang sockets that have 30ma trips built-in.
    This will then run battery chargers when the inverter switches on the contactor.
    Job done.
    • CommentAuthorjms452
    • CommentTimeApr 26th 2012
     
    I considered the same approach myself until I worked out that I can sell the electricity in the day and they buy it back during economy 7 about breaking even. Given the loads I was interested in were a washing machine and dishwasher this seems simpler and more ethical.
    •  
      CommentAuthorSteamyTea
    • CommentTimeApr 26th 2012
     
    jms452
    I think you are right there, it is about the loads you can run that decides the most effective use of energy. There are a lot less losses (from a metering point of view) selling and buying back than there are taking DC from a panel, though an inverter, through the control/switch gear, through a few metres of cable to a cheap battery charger to charge some batteries that are degrading by the day, then back up a few metres of cable, via an inverter to run what, a TV/few lights or a PC to save 3p/kWH.
    •  
      CommentAuthorDamonHD
    • CommentTimeApr 26th 2012 edited
     
    For me it's the ethics that count: that FiT is there to help displace ff generation, not to encourage use for additional low-grade resistance heating IMHO.

    Rgds

    Damon
    • CommentAuthorSeret
    • CommentTimeApr 27th 2012
     
    Posted By: Gavin_Arealistically, how many times are you expecting the contactor to be switching in it's lifetime?

    100,000 should be plenty


    You've got to be a bit wary with boilerplate reliability figures like this. Often figures like MTBF will just be the average reliability, and averages hide a lot of data. On cloudy days your relay could be switching a lot. It would really depend how your system was set up, and what the implication of it failing either open or closed.

    Damon: I can see where you're coming from, but I'd say people install PV for different reasons. For some ethics would never come into it, as they're purely a money making machine. For some it's all about getting carbon out of the grid. And the whole spectrum of opinion in between; that's an effect of the generosity of the FITs. Who are we to judge?
    •  
      CommentAuthorDamonHD
    • CommentTimeApr 27th 2012
     
    I think intent is important in all sorts of circumstances, and the intent of the FiT is fairly clear, to displace ff generation on the grid not encourage an immediate Jevons-style *extra* consumption via inefficient resistance heating.

    I'm not suggesting that Keith is doing anything Bad(TM), and certainly not illegal, but I'm sure that it's anything but green.

    Rgds

    Damon

    PS. I would avoid relays in anything switching regularly, and their failure modes are often unpleasant/unsafe.
    • CommentAuthorGavin_A
    • CommentTimeApr 27th 2012
     
    Posted By: Seret
    Posted By: Gavin_Arealistically, how many times are you expecting the contactor to be switching in it's lifetime?

    100,000 should be plenty


    You've got to be a bit wary with boilerplate reliability figures like this. Often figures like MTBF will just be the average reliability, and averages hide a lot of data. On cloudy days your relay could be switching a lot. It would really depend how your system was set up, and what the implication of it failing either open or closed.

    Damon: I can see where you're coming from, but I'd say people install PV for different reasons. For some ethics would never come into it, as they're purely a money making machine. For some it's all about getting carbon out of the grid. And the whole spectrum of opinion in between; that's an effect of the generosity of the FITs. Who are we to judge?

    pretty much answered by this bit though isn't it?

    obviously don't set it up to be switching on and off every few seconds.


    all such settings are ajustable within the inverter software.

    we'd usually advice setting it to only switch on above eg 3kW for a 1-1.5kW load, and only after a few minutes of it being at that load, and then to remain on for a minimum 5-10 minutes, but probably longer.

    tbh it's a while since I've done it myself, so would have to check the settings, but you get the idea.

    the relay is also replaceable btw.
    • CommentAuthorSeret
    • CommentTimeApr 28th 2012
     
    Posted By: DamonHDI think intent is important in all sorts of circumstances, and the intent of the FiT is fairly clear, to displace ff generation on the grid


    Actually I disagree. If the current FIT regime is intended to do this (at least directly) then it's not fit for purpose (no pun intended). A FIT system designed to do this would be more like a straight swap between the current feed-in and export rates, and would mandate an export meter.

    The FIT system we actually have is designed to do one thing only: sell PV systems. That's why the feed-in rate is high and the export one is low: because it's much easier for people selling systems to forecast production than export, so makes them easier to sell, and allows buyers to estimate the risk better. The actual reduction of grid carbon intensity is a secondary effect of sales, but the primary intention of the FIT is to drive down the capital cost of PV. You may call that splitting hairs, but it's the only explanation for the oddball pricing system we have, that seems to directly encourage behaviour which, as you say, runs counter to green "best practice".

    My opinion: anyone who stumps up several thousand pounds of their hard-earned to install the panels has the right to do whatever they want with them. From the collectivist viewpoint, even if they export nothing it's still a (small) win as they're still pulling in less from the grid, and their purchase still pushes the price of panels down.

    And swerving back to the topic:

    As for relays: most machines in the world are still controlled by some amount of relay logic. Go for it, especially if as Gavin says you can get software to do some of the work. I've worked on some hoary old machines with waaaaay too much relay logic in them, but this is basically a closed loop.
    • CommentAuthorbillt
    • CommentTimeApr 28th 2012
     
    I agree that the only practical effect of the FITs has been to sell PV installations, however, the ostensible (if completely unrealistic) reason was to contribute to carbon emmission reductions. The politicians claim that enlarging the UKs PV industry would have an effect on PV capital cost is risible. PV is a global industry of which the UK is a tiny and uninfluential part. Capital costs have fallen while the UK FITs scheme has been in operation, but that's likely to be due to the collapse of other countries PV industries and the resulting equipment oversupply.

    If you stump up capital without any other direct reward, yes you should be free to do as you like. That is not the case with FITs. Over 25 years you are going to be paid a lot more than your initial capital outlay out of the pockets of other electrity consumers. There is an ethical duty to use the systems as was originally intended, to contribute to the grid rather than burning any excess off in heating water.

    Of course, I appreciate that most systems were installed by people whose only interest was the financial return. Some of them seem to be so small minded that they don't want anyone else to get any benefit from "their" power. It can't be the few extra pennies they might save by heating a bit of water can it?
    •  
      CommentAuthorSteamyTea
    • CommentTimeApr 28th 2012 edited
     
    I have recently started working directly in the PV industry (after years of talking about it from an academic background) and I am amazed how our customers think. They start of from purely the 'investment' side (but this is really to do with the low returns from banks) but soon start to understand what their bills are telling them. They often ask what their PV system can run and seem a bit shocked when I tell them that it is hard pushed to run a kettle for two minutes. I then have to explain that a kettle is one of the heaviest loads that they draw (put less water in it), but a TV on all day may use more kWh overall.
    The best customers are the ones that hate buying in electricity at 15p/kWh and have a daughter that spends an hour in a 10kW shower. That brings it home to them.
    Reducing waste is the way to go.

    As for the ethics of denying others what they are contributing to, think NHS and other public services, would you consider that fair?

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