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Switched alternative to public supply. Tesla Powerwall and gateway. Have I got this right?

justinneedham
Who here has first-hand of Tesla Powerwall? On cursory inspection, on-line technical info is sparse.


One of my regular domestic customers has had a quote for a Tesla Powerwall, including the Tesla isolating gateway to allow some property consumption in selected backup circuit(s) during network outage. Meanwhile I am quoting for CU upgrade to go in ahead of that work so that it can take place. The latter is basic work, no problems, were it not for the former which is providing me with a bit of a conceptual challenge. Now the first bit below is second-hand from the Tesla installer via my customer. Hence parts of it might be lost in translation, but as far as I have received from Tesla installer:


"Go ahead and to the CU, - it won't affect the Tesla."

The Tesla gateway contains SPD so I "don't need" to fit SPD in my CU. (But Tesla only up to 1KV, so not fully meeting AFAICT, 442.2.2)

Now the biggie: Tesla say they will be fitting a separate earth rod such that (as far as I understand it..), when the gateway entirely disconnects the network including earthing conductor (which after the supply head is no longer PEN 461.2).. then the gateway  "islands" the whole property on the TT via it's switch 537.1.5. 551.6 .


I'm not so familiar with switched alternative to public supplies, and in the absence of anything from Tesla, those who are able to might add to my reasoning below (or otherwise abuse it):


1) The property bonding and MET would still be connected to incoming PEN via gas/water pipe and next-door house(s).

2) What happens to the Tesla earth rod in normal (grid connected) operation? I'm guessing it's wired by the installer such it's effectively just a bonded extraneous conductive part. 

3) Under loss of network power, Tesla gateway disconnects incoming supply and TNCS earthing conductor and now floats on the parallel impedance of it's Ra and the bonding. During this time,the disconnected property would (due to high-ish Ra) likely get bootstrapped close to whatever voltage is on the incoming gas/water/bonding.

4) Although there are two earthing systems, there aren't two simultaneously accessible earthing systems..  Because in normal (network) use the TT electrode would be just a bonded extraneous part, and in "island" mode the TNCS earthing is not connected, - But to make this claim, we'd then need to also state that the TNCS "not-MET" (with installation earth no longer connected) was a bonded extraneous conductive part. 

5) Loss of PEN externally: The Tesla I'm guessing would detect any possible lift in earth potential of the TNCS MET and could then switch into a safe condition (house disconnected entirely, other than bonding to PEN), and un-powered, or even maybe continue to generate for the house backup circuits, while floating on whatever fault condition PEN/bonding voltage was imposed on it. 

Have I reasoned this out right? 












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    AJJewsbury:

    If it helps at all, here are some diagrams (originally done for the old Forum) for the usual layout for switched alternative local generation (the principle is the same whether it's a petrol generator, or battery and inverter, or anything else).


    The first one shows the simplest case of everything TN-S - and how the generator is configured/earthed in just the same way as the DNO supply in a nice symmetric layout. When connected to the DNO's supply (and the generator off), the DNO's electrode provides the means of earthing and the local generator's earth electrode is nothing more than one more extraneous-conductive-part. When the DNO supply is off and the generator running, the local rod provides the means of earthing and the DNO's earthing system is just another extraneous-conductive-part as far as the installation is concerned.

    4ff306a9c315f8f85ae1c8c871974c2f-huge-generatorbasictn-s_zps3cec182e.png


    Then what you're more likely to have in practice, with a PME supply and an RCD very soon after the generator (as most small generators/inverters won't be able to produce the large currents needed to open overcurrent devices for ADS) - so some of the symmetry is lost, but the underlying principle is the same:
    836149818d0b42fd1738c67a374ee52f-huge-generator-detail-tn-c-s-zps660b2d3f.png


       - Andy.




    Andy,


    Whilst these arrangements are absolutely OK for a switched alternative to the normal supply, and illustrate very well the situation about leaving the distributor's means of earthing connected, I feel it's only fair to readers to point out that they won't support either self-use or export of stored energy in connected mode. Therefore, switching for island mode in a prosumers' electrical installation is a little more tricky, involving switching of an N-E bond. This is because you can't have two N-E bonds at the same time ... and BS 7671 doesn't permit you to rely on the supplier's N-E bond even in PME.

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    Whilst these arrangements are absolutely OK for a switched alternative to the normal supply, and illustrate very well the situation about leaving the distributor's means of earthing connected, I feel it's only fair to readers to point out that they won't support either self-use or export of stored energy in connected mode. Therefore, switching for island mode in a prosumers' electrical installation is a little more tricky, involving switching of an N-E bond. This is because you can't have two N-E bonds at the same time ... and BS 7671 doesn't permit you to rely on the supplier's N-E bond even in PME.



    Definitely agreed - Mike's diagram above is far more comprehensive in this case. I just thought that sometimes it's easier to get a handle on things if you can see a relatively simple example in isolation first - it should then be clearer how Mike's more complicated diagram achieves the same thing in 'off grid' mode.


        - Andy.
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    It's only 'my' diagram,  in the sense I have lifted it, but I think it is meant to show the NE link switching on as the mains connection is switching off. That would be ESQR compliant, though from a safety point of view it raises questions about avoiding failure modes that leave the system either un-earthed, or, less dangerously but illegally, with a DNO supply and 2 NE links
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    though from a safety point of view it raises questions about avoiding failure modes that leave the system either un-earthed



    I would have hoped it possible that the control system would be able to monitor the N-PE link contact - and if it failed to close, elect not to close the live contacts (in off-grid mode). Whether such EESS actually do anything that useful of course is another matter.

       - Andy.
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    Something I don't see in that Tesla diagram is how does the inverter know if it is meant to be in grid-tied mode or islanded mode?  A grid-tied inverter should automatically switch off if it sees a mains failure.  I would expect an extra signal, perhaps an additional contact on the big isolator switch.
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    Something I don't see in that Tesla diagram is how does the inverter know if it is meant to be in grid-tied mode or islanded mode?  A grid-tied inverter should automatically switch off if it sees a mains failure.  I would expect an extra signal, perhaps an additional contact on the big isolator switch.



     


     


    I suspect some of the control wiring has been 'omitted for clarity' - there's no mention of the coil that operates the contactor or how it's wired to the powerwall for instance. I would have expected some voltage sensing on the grid side of the contactor - so that the system can tell when grid supplies return - even if the local isolator was left closed during a power cut.

       - Andy.

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