"Capitance"

[Panrock]

Jeremy at the NBTVA sent me this curiosity this morning. He writes:

"I chanced upon a mystery word today: CAPITANCE. The source is 'the Practical Electrician's Pocket Book of 1961' and I have attached the scanned page. No other reference or index entry appears.

The nearest is Harmsworth's Wireless Encyclopaedia which gives CAPTANCE as a single word for capacitive reactance. But this doesn't seem relevant.

No dictionary I've tried, whether printed or online, or any electrical reference book, or web search has yielded anything at all on the actual word. From the context, all I can think is some mechanism of fault current limitation by the transmission-line behaviour of a close parallel conductors mains cable. But any thought would be most welcome."

Looking at the attached table, it does seem reasonable that the fatter the cable, the longer it would have to be to limit the short circuit current. Under these conditions, the circuit impedance would be very low, and as this is presumably only 50Hz, I don't see how shunt 'capacitance' (say) would be a factor.

Can anyone make sense of this?

Thank you!

Steve

[ppppenguin]

A web search gave this, which looks like a typo: https://www.tandfonline.com/doi/abs/10.3...ode=ijmt20

At the high currents that occur under short circuit conditions I wouldn't be surprised if the transmission line properties of a cable might start to be significant. The L certainly matters at DC: https://www.linkedin.com/pulse/short-cir...n-bigdeli/

[Panrock]

Thanks Jeffrey. Noted about the 'L'.

I would have thought that at 50Hz, (RF-style) transmission line considerations would only come to the fore with very long lines. Mind you, with the National Grid and international connections, we might have these, mightn't we?

Steve

[ppppenguin]

The wavelength at 50Hz is indeed very long and for steady state conditions is probably irrelavent in the UK. In the extreme transient conditions of a short circuit there will be much higher frequencies present. Not sure whether it's true transmission line effects or just lumped L and C but it's going to matter during short circuits.

For very long and international connections it's usual to use DC. This avoids the need to synchronise the 2 grids thus connected. There are no losses due to power factor. You can carry more power on a given cable because an AC cable would need to be insulated for Vpeak, about 140% of Vrms or Vdc.

[Panrock]

Going off-topic but my curiosity won't be denied... it's the sort of question nobody 'normal' ever asks. How 'smooth' is the DC reckoned to be in international DC connections... not even the tiniest bit of 3-phase{?} ripple?

Steve

[ppppenguin]

No idea how smooth. The rectifiers and inverters will be at least 6 phase (think of full wave rectification of 3 phase) and 12 phase is common. So the DC will be relatively smooth to start with. I would expect the capacitance of the cables will give some useful smoothing. I don't know how much extra smoothing is used.

[Panrock]

A great reply. I've attempted to add to your 'reputation'. Hope it's worked.

Steve

[ppppenguin]

Thank you.

I decided to look up polyphase rectifiers and discovered that 18 and even 24 phases can be done.

This wiki pages is a decent intro to HVDC, including stuff on numbers of phases, ripple, smoothing etc: https://en.wikipedia.org/wiki/High-volta...ct_current

[Ed_Dinning]

Hi Jeffrey, indeed polyphase rectification (up to 30 phase) is common on some aircraft systems to improve power factor and help reduce EMC effects of "current chopping").
While working on inverter and converter systems for the A400M we used 18phase rectification from the aircraft 115V 380 to 620Hz bus. This frequency wild system was used so that alternators were directly coupled to the turbo prop engines, saving the weight and expense or the normal variable speed drive.
18 phase gave a good balance between cost and efficiency due to losses in the extra magnetics. We did often wonder if the additional mass of the extra magnetics was greater than the weight saved from the variable speed drives.

Somewhere I have a detailed table of the various phase numbers (all triplen) and the advantages, I'll post it if it can be found

Ed

[Diabolical Artificer]

Capitance? A bad day for a busker or beggar : ) Where's me coat....

Andy.