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

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.

AJJewsbury said:

Or did they perhaps mean Rsce = 33?

I think that it is a typo. BS EN ISO-3-12:2011+A1:2024 refers to a R_sce limit of 33. (Article 5.2)

For single phase equipment, R_sce = S_sc/3S_equ

S_sc is the short-circuit power, which is equal to (U_nominal)²/Z where U_nominal is the interphase system voltage and Z is the line impedance at the PCC. The PCC is the point of common coupling, i.e. the point in the public system which is closest to the customer concerned and to which other customers are or may be connected.

S_ecu is the rated apparent power of the equipment, i.e. (for single phase) the product of the rated voltage and the rated current.

So let's put in some figures. Take a 32 A heat pump. S_ecu = 32 x 0.23 = 7.36 kW.

I assume that Z would be the line impedance where the service cable joins the street mains, so let's say 0.2 Ω. So S_sc = 400²/0.2 = 800 kW.

Now R_sce = 800k/(3 x 7.36k) = 36.

So, Andy, I think that your original hunch was correct.

I think I asked the same question years ago on an older version of this forum that is no longer accessible and had it explained to me.

As I remember it Ssc is used to determine how much flickering a disturbing load will create.

www.nationalgrid.co.uk/.../229237

Heat pumps are excluded from that document, it looks like you need this two part document 

https://www.nationalgrid.co.uk/downloads/39586

and

https://www.nationalgrid.co.uk/downloads/67627

there is a bit of light reading for you, have fun, as it’s Sunday I’m going into town shopping.

cheers - I'll have a read...

    thanks

          - Andy.

thanks Chris!

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.

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.

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.