OCPD on PV system - Wiring and the Regulations BS 7671 - IET EngX - IET EngX

OCPD on PV system

This is a basic schematic of a PV connection to a DC bus. There is a 125A MCCB at the bus to which a 35mm2 2-core cable connects from a four channel optimiser. The cables on the other side of the optimiser are 6mm2. They in turn connect to combiner boxes and to the PV strings of approximately 9Kw per string. There are two strings connected to an optimiser channel. You can see that this occurs 13 times so it is a system getting on for 1Mw. The battery is also 1Mw. 

I have no idea what protection exists, if any, in the optimisers. I am just thinking about a short somewhere between the strings and the optimisers where the conductors are 6mm2. Perhaps it is not possible for current from the bus to pass the optimiser and feed such a short?     

  • Two separate thoughts each string may be creating up to 9kW , and that is 6mm cable what voltage are the strings ?

    (Dc power = VI) Probably OK as 6mm is good for about 60A continuous, I assume string voltage is a few hundreds.

    Is there a credible fault that does not trip the 125A breaker that could put the mains onto the DC strings ?

    The solar panels would probably not appreciate it. I'm not sure they would carry enough to blow a 125A breaker. I'd expect the strings to be fused either in line or where they meet the optimaiser.


  • Maybe this is easier to read.

    Voltage is 890v. My error on output; strings are just over 10Kw. I will post system topography when I do a sketch.

  • I don't have any experience of anything on that scale, but solar panels are inherently current limited.  If you short them out, a few amps will flow, the voltage will collapse to near zero, but not much else will happen.  The short circuit current isn't much more than the normal operating current.

  • Simon,

    I appreciate that but I am thinking of the fault current being derived from the DC Bus, not from the pv string. Not the same project but the topography below is almost identical. I have lots of questions about that as well!

  • indeed - the problem is not the solar panels over-generating, (short of an overhead nuclear blast, and then the effect will not be long enough to overheat the cables, and there may  be more pressing considerations than cable life.)

    The concern is the enormous PSSC of the battery, and if that gets back onto the strings of solar  panel wiring. However this quite simliar to the the risk of mains getting back onto the panels in a transformer-less grid tie unit and the fix is similar, only the fusing needs to be DC breaking.


  • Oops, got that last drawing wrong! Will amend after a cup of tea!