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There seems to be a small cable fire in London, has affected key parts of the MV network BBC Local Live: Updates from London on Wednesday 1 April 2015 - BBC News
Discuss Small 11Kv cable fire London... in the Security Alarms, Door Entry and CCTV (Public) area at ElectriciansForums.net
There can be only one possible cause for this......
Plumbers!
True, but there is often a neutral earthing resistor or similar at the star point of the upstream transformer.
The same as LV networks, via earth electrodes....
As far as i'm aware, all of the local MV/LV distribution network TX's are Dyn 11 (delta/wye). N-E Resistors would only be used where it is requirement to limit the fault current on the LV side. Earthing electrodes for MV and LV systems are generally separated at TX locations unless the Ra value of the electrode systems are of a suitably low value (under 1 ohm).
I don't consider MV/MV TX's as being particularly part of a local network distribution system, although in Central and the City of London the networks have been reinforced and evolved many times often in not exactly the best way, so it wouldn't surprise me at all that N-E resistors have been used quite extensively.
Local distribution transformers 11/.433KV Dy11, up to 1000KVA 4.5% imp, 1500KVA 5% imp.
Star point neutral solid earthed. (TN-C)
Primary transformers 33/11KV Dy11+OLTC
Star point impedance earthed. (IT)
A little gem I found last night while reading up on something else.
London’s predominant network assets comprise approximately:
2,000km EHV cables (above 11kV)
8,000km MV cables
19,000km LV cables
100 EHV/MV substations
13,000 distribution substations
2.1m customer connections
Hardly surprising there’s the odd failure.
To increase "reliablity of supply" in the "City" the LV supplies are fed from two MV transformers and have a reverse power relays and ACB at each sub station. This means that the LV fault level is very high, I wonder if this has added to the intenensity of the pavement explosions and so on.
To increase "reliablity of supply" in the "City" the LV supplies are fed from two MV transformers and have a reverse power relays and ACB at each sub station. This means that the LV fault level is very high, I wonder if this has added to the intenensity of the pavement explosions and so on.
From my understanding, the fault levels are very high throughout Central London and the City of London.
The fire as far as i can see is located in the West End (Central London) not in the City...
I was reading about it last night after I found the fault passage indicator info I needed. I got reading about the MV protection and automatic switching.
It’s highly unlikely they’ll run transformers in parallel, purely down to the fault currents local to the substation. You have to take in to account the low system impedance on the MV side feeding in to the transformer.
There was another well publicised incident recently where a LV UG link box failed. It could have been O/L or water ingress which some are prone to. I know one thing, I don’t like them.
The line to line fault currents will be high, at least using NER’s the line to earth current can be limited.
Hi,
I have seen some new 132KV subs being installed on the outskirts of the city (Finsbury) and there have been other changes.
As to Tony's point please read the document I linked to, the two transformers in parrell is exactly what they are doing...!
Hi,
I have seen some new 132KV subs being installed on the outskirts of the city (Finsbury) and there have been other changes.
As to Tony's point please read the document I linked to, the two transformers in parrell is exactly what they are doing...!
Hi,
I don't work for UKPN.
Reply to Small 11Kv cable fire London... in the Security Alarms, Door Entry and CCTV (Public) area at ElectriciansForums.net
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