What's IMOD_MAX_OCPR ?

I've been trying to make some sense of the newly re-written section 712 (PV) - but I'm struggling with a few things - firstly can anyone tell me what "IMOD_MAX_OCPR" represents? (e.g. in regs 712.431.101 and 102). Unlike most one-off symbols (like Np) it's not explained below, nor can I find it on page 46 (Symbols used in BS 7671). It seems to be something to do with an overcurrent protective device, but seemingly not it's nominal rating, since part of the requirement is that  In ≤  IMOD_MAX_OCPR, nor I guess breaking capacity since 1.35 IMOD_MAX_OCPR ≤ (Ns -1) ISC MAX.

   - Andy.

Parents
  • IMOD_MAX_OCPT is the module's declared maximum current rating for the reverse current protection: Because a PV cell is not a linear device but a diode, under certain conditions (such as partial shading) the array/string/module will be operated at a voltage/current that is not available in the postive-power quadrant of the IV curve of certain strings/modules/cells and they will find themselves in reverse bias... in this case current (and power) goes the wrong way through the PV cell(s), which are also known as photodiodes. Diodes don't like this very much, hence needing overcurrent protection at string level and bypass diodes on a submodule scale.

    It probably helps to consider that historically blocking diodes were an alternative solution to string fuses. They tend not to be used these days.

    For a first, technically inaccurate, approximation this is not unlike using a mix of battery ages/specs together in series/parallel: Sometimes it'll be fine but push it too far and you might be trying to charge a non-rechargable battery which will go pop.

    Reverse current protection is not needed if you have one or two strings because if one module is in reverse bias the maximum current available is from one other string, and obviously the equipment can withstand that because that's normal load current.

    It is worth noting as it is oft forgotten that PV arrays (normally) rely on double/reinforced insulation. The fuses are not for short circuit protection. They provide overcurrent protection for the modules or cables because of the above potential for reverse current resulting in overload. Hence gPV characteristic is needed rather than gG.

Reply
  • IMOD_MAX_OCPT is the module's declared maximum current rating for the reverse current protection: Because a PV cell is not a linear device but a diode, under certain conditions (such as partial shading) the array/string/module will be operated at a voltage/current that is not available in the postive-power quadrant of the IV curve of certain strings/modules/cells and they will find themselves in reverse bias... in this case current (and power) goes the wrong way through the PV cell(s), which are also known as photodiodes. Diodes don't like this very much, hence needing overcurrent protection at string level and bypass diodes on a submodule scale.

    It probably helps to consider that historically blocking diodes were an alternative solution to string fuses. They tend not to be used these days.

    For a first, technically inaccurate, approximation this is not unlike using a mix of battery ages/specs together in series/parallel: Sometimes it'll be fine but push it too far and you might be trying to charge a non-rechargable battery which will go pop.

    Reverse current protection is not needed if you have one or two strings because if one module is in reverse bias the maximum current available is from one other string, and obviously the equipment can withstand that because that's normal load current.

    It is worth noting as it is oft forgotten that PV arrays (normally) rely on double/reinforced insulation. The fuses are not for short circuit protection. They provide overcurrent protection for the modules or cables because of the above potential for reverse current resulting in overload. Hence gPV characteristic is needed rather than gG.

Children
No Data