I'm just a nosy bastard. But... There are two substations, about a mile apart. One is on a pair of wooden poles; the other is in a big box on the ground. They are both fed from the same 11,000V network. The 235V outputs from these two substations run towards each other along the street, until they meet on the same pole. On that pole there are three fuse holders. It appears that the fuses link the outputs from the two substations. For a while men kept coming and trying to remove these fuses, but they wouldn't unscrew. Finally they replaced the fuseholders, but left the fuses out, dangling on a bit of wire. They are 200A fuses.
Am I right in thinking that the fuses link each of the three phases from the two substations?
Dunno but like you I have a bit of a fascination for for all our unseen utilities and never miss an opportunity to peer down a hole in the road etc.
Perhaps they need to break a ?ring? so as to do repair work and want to minimise loss of power to properties. Jammed fuse holder maybe needed to be replaced first before they could isolate a section?
Maybe they have decided that the two are more likely to fail if connected together so they have left them disconnected. Its one of those questions which only power networks or whatever they are called this week can answer with authority. We are on the edge of an industrial estate and sub stations seem to pop up in little fenced enclosures with amazing regularity, all buzzing away. When one goes down it does seem not to follow any known logic as to which premises lose power. Brian
To be able to quickly backfeed either low voltage distribution circuit in the event of the failure of a substation or part of one of the circuits if the circuit breaks.
No current need to backfeed either low voltage circuit, after a period when there was.
LV distribution in the street is done as a virtual "ring main" with two independent & seperate feeds from the 11 kV HV network via transformers.
The ring main is NEVER fully compelete/closed via all its switches, there are switches all round this virtual ring main. They can sometimes look like fuses but are in fact metal fusible link bars rather than cartridge fuses.
All of the switches are closed are closed but two sets are delibereately left open in the ring main.
All round this ring main are the service cables connecting the households to this ring main.
This splits the ring intwo two distinct parts. One part of this "ring" is fed from one of the HV feeds and the other part is fed from another HV feed.
So things like power cuts can be "managed" or network maintenance work to take place with minimal disruption. Consider the following scenarios:
Say one of the HV feeds fails, the now unpowered transformer is switched off and isolated from the ring main.
Then one or both of the open switches in the ring main is closed so that the part ring is then re-energised using the other HV feed as it has now been combined with the still working part ring.
Say part of the ring main needs isolating for routine maintenance work.
Rather than cut off one HV feed and de-energine the WHOLE of the part ring main, they simply open the switch downstream AND upstream of the works location. The other open switches are then closed to re-energise the remaining parts of that ring and then the works can be undertaken.
If there is a short circuit fault tripping a fuse, rather than fix the fault and let everyone suffer a power cut, the fault can be isolated between two adjacent pairs of switches and the power restored to those upstream and downstream of the two switches before and after the fault. This allows power to be restored to some of teh customers sonner than expected. Then only those fed by the isoloated section of faulty ring main have to have the full power cut experience. :-)
Similarly if a ring main cable goes open circuit, this then creates a new 3rd split ring main. One of the open switches can be closed to power up the 3rd split ring until teh open circuit fault can be repaired.
Similar arrangements exist for the Supergrid ( 400 kV & 275 kV) to the national grid (132kV, 66kV ) and then down to the Local distribution grid ( 33kV and 11kV).
There is also a PE system (Potential Earth) which is a safety system for engineers working on parts of the grid but it does involve quite some complicated switching and involves a permit system.
Lets say the mineral oil in the 11kV to 220V transformer or HV switch needs changing... There is a person that literally drives around in a car with a bunch of keys to all the substations and a check list of the EXACT order and WHICH EXACT switches to open or close or apply potential earth.
All of the switches also have an option to apply potential earth which is a connection to an earth rod on site.
basically I went out for a ride in the car. Basically they:
Drive to Switch A. Close it. This then combines the two parts of the split ring at one end.
Drive to switch B. Close it. This then completes the ring main which is now fed via two HV feeds simultaneously. This will be temporary though....
Drive to switch C. Open it. This will be downstream of the ring main cable coming from the transformer's secondary output
Drive to Switch D. Open it . This will be upstream of the ring main cable coming from the transformer's secondary output
Drive to Switch E. Open it, This then disconnects the transformer's primary from the HV feed.
Then the Transformer's primary is switched to earth.
drive back to switch C. The downstream cable from transformer's secondary is switched to Potential earth.
Drive back to switch D. The upstream cable from transformer's secondary is switched to Potential earth.
Then we drive to the EXACT location of where the transfomer whose oil is to be changed. The safety paperwork is then given to the team doing the work. They then have authorisation to apply all conenctions to and from this transformer to potewntial earth.
All being well there will be no bangs/explosions or sudden power cuts and no one is in trouble for noe following the switch order and location list EXACTLY!
Then when the oil change is completed, the list above is then repeated but in reverse to effectively bring the transformer back into use.
Obviously Potential earth should NOT be connected to a live cable or power applied to a potential earth rod!
How do I know all this? from 1 weeks' work experience spent with the MEB (Midlands Electricity Board) and with the National Grid in 1986 as a spotty 15 year old inquisitive & enquiring lad!....
Nowaways I expect all the switching is now done remotely over the net for much of the network but obviously this will not apply for parts that have not been updated to internet remote management.
But 235W at 200A times three phase ? That's not exactly Hoover Dam.
That's a small amount of power as a backup. There might be a single industrial customer at the end of that.
The fuses stay open, because the line is not to be energized in normal mode.
I have three fuses hanging down like that, a five minute walk from the house. And I've never seen that backup used, ever. Even when our substation was down for construction work, that still wasn't engaged. The wires from it, just run off into the "forest". So you can't see the terminus.
Even when energizing a circuit here is relatively straight forward, the safety checks take forever. Twenty years ago, they were a lot faster at putting things back in working order (they did safety checks, but nobody was standing around and everyone was working to bring it back up). Today, there are a *lot* of guys just standing around. And somehow it's safer.
Now that we have digital power meters, they have visibility at the fused circuit level. So they can tell a fuse is open, just by noting which chunk of houses doesn't have power. Whereas before, part of the truck roll, was driving along the street at night with a search light, and examining the fuses as they went. The substations all have remote monitoring and control. But out at street level, the digital meters added a lot of visibility for the smaller stubs of distribution. They can tell that the fuse at the end of the street which powers five houses, is open. Because five dots disappeared from the "map". If multiple fuses failed, only the outermost one would be identified by the computer.
That doesn't make the repair any easier though. They don't close a fuse here, unless some kind of check says it's ready to go. I've never seen what they use to determine that. But I was told by a lineman one day, that he couldn't do anything just yet, until the line was verified first. And he wasn't doing the checking. He'd loaded a new fuse, had the pole ready to close it, but was... waiting.
235 VOLTS Paul. That's the typical UK low voltage distribution distribution to domestic premises (who have 1 phase and neutral) and small businesses (who probably have all three phases and neutral,nominally 430 Volts). Both might get some sort of energy supplier-provided earth, or not.
The HV stuff, 3.3KV, 6.6Kv, 11KV, 33KV .... never go to any domestic premises. Bigger commercial users might have their own HV/LV transformer on site behind a security fence if ground-mounted
But messing around with "amps" at that level, doesn't make a lot of sense. To distribute the last level of power over large distances requires outsized cables. There's only so much voltage drop in the budget for your 235 volts.
That was my point.
How large a diameter of ring makes sense at 200 amps ?
Using "schemes" at higher levels in the distribution makes a bit more sense, because there's a possibility of using fewer amps. We could probably benefit from a ring scheme at the next level up (that might fix the voltage drop issue seen here).
The lowest level of distribution here, goes directly to a consumer with one fuse in the way at that level.
The next level up is still fused.
The levels above that are automatic switch gear and remote control and monitoring.
I don't know how large 240v distribution circuits are, but certainly a whole street can run on one. Tap changers can compensate for V drop to an extent. I've no idea if they feed distribution runs from more than one point at once.
Turn of the century diagrams showed how to even out v drop on lighting circuits by feeding the L from one end & the N from the other. Result: all lamps see the same voltage. I've no idea whether similar games are played today in distribution.
I just checked my line voltage, and at the moment, it is not compliant with the spec. It's outside the acceptable range, and there is no set of abnormal conditions here today.
That means at least my electric company does not know how to equalize voltages along their "long-string" wiring plan :-( I've put up with this for years, and talking to those people is a waste of time.
To show how little they have in engineering skills, they did attempt to change the wiring plan in my neighborhood. At 10PM at night, they did the cutover. The result was so bad, they had to cut back to the old plan soon after. So much for ohms-law and that hand calculator they own.
Not all of our cities are like that. The operator here, is owned by the city, and the city receives a "dividend" each year from the operation of the power company.
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