Are DNO cables in the UK fused before going into a building? And if so how? What happens if you were to short the cable out before the cutout main fuse?
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Are DNO cables fused?
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You will remember on j3/j4 switches and t3gf3 rmu that silly pivot on the handle, once you operated the switch, you had to remove the handle, and turn the pivot around in order to operate in the other direction.
That was a later addition, prior to that you could operate it straight away.
Unfortunately one operater closed a switch (don't know what make - wasn't one of ours), realising he had closed on a 11kV 350MVA fault, he immediately opened it, being a switch it can make and carry but not break, so it didn't end well.
It was the only retrospective change I know about - every manufacturer had to change their design - we recalled all operating handles and produced new ones with that pivot, that delay gave the protection 3 seconds to clear.
Yes I was very familiar with break back handles, dident the t3gf3s have the bronze trapeziod fitting though.
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Remember also that some of the cables in towns could be up to a hundred years old.
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To the best of my knowledge they dont, at the concept stage they work out maximum lenghts and provide that as guidance to the field guys.
Remember also that some of the cables in towns could be up to a hundred years old.
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Here's one of the link boxes on the corner of my road. 315's in there, not sure why specifically rather than 400's, maybe due to small legacy service cables.
Micktw68
Arms
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Theres plenty of UG subs in London if you know what to look for. Biggest problem is water ingress into the switch gear ,an ocb full of water makes a mess when they trip under fault conditions.
OnlQQker
Esteemed
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Underground cables are great until they go wrong...
Central London electrical fire still burning underground - https://www.theguardian.com/uk-news/2015/apr/02/central-london-electrical-fire-still-burning-underground
Central London electrical fire still burning underground - https://www.theguardian.com/uk-news/2015/apr/02/central-london-electrical-fire-still-burning-underground
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Oh yes, the Holborn fire, the night the west end went dark!
It certainly highlighted the need for emergency lighting in theatre manager's minds!
London uses a solid LV network I think it is the only part of the UK to do so. Bit difficult to explain but basically you have about four transformers with HV and LV protection feeding into a solidly interconnected area. Better utilization of copper but very high fault levels, most faults blow clear but can cause a very big bang.
Micktw68
Arms
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Yep, and all down to a p.poor maintenance regime .
You don't got your MV cables in switch loops like Germany (1:00 as an example)?
I don't really know the range, but I think most built up areas have ones in the 500kVA to 1MVA range feeding something like 100-200 homes.
Remote building, farms, etc, probably have ones in the tens of kVA region depending on their needs, but the majority of UK supplies would be in modest-sized networks around a middle-sized substation.
Hopefully others on here with far more DNO knowledge can comment?
Now think about doing it in a small space, with everything around you made of steel, and rolling about at sea ... and oh yes, DC
I struggled to spot the details as they zoomed in and out of stuff a bit quick. But yes, out here in the sticks it's normal to have loops. I happen to know that we are fed from 2off 132kV feeders into the substation the other side of town. Those feeders go into 132/33kV transformers and 33kV distribution boards. Then a number of 11kV boards are fed from 33/11kV transformers. Having had a private tour of the new 11kV distribution board before it was commissioned, a typical arrangement has the board in two halves - each fed from the separate 132kV feeders via the 33kV system. There's a link between the two halves, so that a loss of one of the feed can be catered for by opening the feeder and closing the link - otherwise it's left open.
Then for the local network, it's typical to have an 11kV ring that goes from a breaker on one side of the board, out in a loop round a few 11k/415V substations, and back to a breaker on the other half of the 11kV board.
The 415V substations will be connected with ring main units - two switches or breakers connecting to the two directions in the ring, and a third for the transformer. Somewhere around the ring, a switch will be left open so that it's running as two radials - but if there's a fault, switches can be opened and closed to feed part of it from the other end. Going back just a decade or two and thes were all manual - so needing an engineer to drive round to operate them. They've been upgrading over the years to add more remote switching so the controll centre (dwon in Manchester for us) can clear (or partially clear) more faults.
Then as already mentioned, depending on the layout, some of the local distribution network has the ability (by fitting/removing links) to be fed from more than one substation.
At my last job I recall watching as a couple of DNO vans pulled up at the substation across the road. One of the guys walked round the corner out of site and then the lights dimmed a little - I recall me and my manager looking at us with that "are the lights going to go out" look. They then pulled the fuses from the output side of the substation and proceeded to do some maintenance on it - sucked the oil of of the breakers, did some work inside (I gather they have had some problems with bad joints), then refilled with oil. When they finished, a guy went round the corner, and the lights went back to normal. The logging from our UPS showed a 10 to 20V drop in voltage while they were working.
As an aside, one of the experiments the DNOs have been doing on load/supply management has involved deliberately linking busses (e.g. the 33kV busses in the above example substation) in main substations and altering transformer taps to create circulating currents. In effect, using the distribution network as a large heater element to deliberately waste energy - just as a very short term measure to compensate for a surge in supply (those darned renewables) or a sudden loass of a large load.
Dunno what came of that.
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Sorry for the delay in answering, yes I think the London system is similar to the drawing you posted. I have never worked on that system only read about it.
Re backfeeding substations, the system is usually designed with 30% of the capacity available via backfeeds this is so that the substation can be switched out for maintenance on a summer day. If the load is greater than the available then the lights get dimmer as per Mr Ohm.
Sorry for the delay in answering, yes I think the London system is similar to the drawing you posted. I have never worked on that system only read about it.
Re backfeeding substations, the system is usually designed with 30% of the capacity available via backfeeds this is so that the substation can be switched out for maintenance on a summer day. If the load is greater than the available then the lights get dimmer as per Mr Ohm.
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