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What's Ssc?

AJJewsbury

I've just read in the installation instructions for a heat pump...

This unit complies with IEC 61000-3-12 under the prerequis-
ite that the short circuit line Ssc at the connection point for
the customer's installation to the public grid is greater than or
equal to 33. It is the responsibility of the installer or end user
of the unit to ensure, if necessary after consultation with the
network operator, that this unit is only connected to one con-
nection point with an Ssc value that is greater than or equal
to 33.

which got me wondering what on earth Ssc was (I've seen a few other things over they years that have specified a minimum L-N loop impedance for the supply, but at 33 this looked somewhat different).

A bit of research (e.g. Cahier technique no. 158 and this WPD document) seem to suggest that Ssc can be calculated as U²/Zsource - but plugging in some vaguely plausible numbers into that e.g. 400²/0.15Ω gives me values many orders of magnitude away from 33. Working it backwards 33 would imply a Z of around 150kΩ which feels equally implausible.

Obviously there's something I'm not understanding here. Or did they perhaps mean Rsce = 33?

   any ideas?

   - Andy.

Parents

  • Short circuit power ie fault level, units normally MVA

    In this case being used as a measure of grid strength, hence whether harmonic emissions will have a substantial effect on the wider network

    More info in EREC G5/5 from the ENA

Reply

  • Short circuit power ie fault level, units normally MVA

    In this case being used as a measure of grid strength, hence whether harmonic emissions will have a substantial effect on the wider network

    More info in EREC G5/5 from the ENA

Children
  • in reply to Jam

    In this case being used as a measure of grid strength, hence whether harmonic emissions will have a substantial effect on the wider network

    Thanks - that does makes sense.

    Short circuit power ie fault level, units normally MVA

    If it's 33 MVA, then if my maths is right that equates to a loop impedance of less than 0.005Ω, which again doesn't feel that achievable... (>47kA PFC even on single phase), so I'm still struggling to see how the instructions make sense...

       - Andy.

  • in reply to AJJewsbury

    I agree it’s a hefty fault level for a conventional LV supply.

    So what it might effectively be telling you is that it is a relatively noisy piece of kit whose “My harmonic emissions are fine, nothing to see here” declaration only applies if you have a point of common coupling at HV, ie a customer-dedicated transformer, since Ssc of 33MVA is comfortable at 11kV.

    The action therefore arising is that if you’re following the EREC G5/5 assessment process you’d drop into Stage 1C. Which still isn’t massively onerous, but does mean you can’t have all the heat pumps you can eat.

    Personally I doubt it’s a typo as IEC 61000-3-12 requires the minimum Ssc to be declared, if there is one, but I would check with the manufacturer if you’re not sure.

  • in reply to Jam

    if you have a point of common coupling at HV, ie a customer-dedicated transformer, since Ssc of 33MVA is comfortable at 11kV.

    This was for a 5kW output (<2kW input) 230V domestic heat pump!

      - Andy.

  • in reply to AJJewsbury

    Ha - Could do with a filter then?

    Could still be connected at LV of course, just someone (either the installer or DNO depending on where it is) will need to do a more in depth assessment as LV networks may not be strong enough to cope with a street-full.

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