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National Grid Frequency

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DerekC

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That depends on the output of the station and the type of the station.
A nuclear station requires house loads on order of ten percent of it's net output. But ti should be able to provide many of those from it's emergency generators if the regulator approves the use of them in the black start scenario.

What would normally happen is all the circuit breakers connecting the transmission system to the distribution system would be opened, and then staitons with very low house loads would be started up to provide power to start other plants up.
Once a large number of stations are ready the distribution systems can be reconnected stepwise.

An obvious choice for black start is a hydro scheme, because a station service generator can potentially be started by having someone open a sluice gate with a hand crank.

This process is obviously much less difficult and much faster if some partso f the grid remain intact, which is why some modern nuclear stations are designed to suddenly reject full power to the condenser directly, maintaining only enough flow through the steam generator to keep the plant's house loads operational.
This means the station can stand ready to restore the grid as required.

As the fuel costs of nuclear are very small and sometimes negligible, wasting power output for a few hours is a small price to pay for being able to rapidly restore service. Especially as many freshly shut down nuclear reactors are required by xenon poison out to remain shut down for up to three days if they cannot restart immediately.

This had to be done after the Great Storm of 1987 caused the grid to collapse (electrically speaking) in Southern England. I recall reading an article in an IEE journal about it - the only power station left connected in the area was Isle of Grain, feeding a small amount of power locally, so they started from there.
 
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Bald Rick

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There's a huge difference between lopping load-peaks intra-day and providing providing an alternative source of supply for calm winter days when PV solar and Wind. Pump storage schemes operate for a matter of hours at name-place capacity before the upper reservoir becomes empty. The latter problem is really only solved through geographic diversity of supply if it's renewable- otherwise it's gas turbine plant running only when necessary - and incurring the same maintenance costs to be amortised over less output.

Yep. Pumped storage isn’t good for a windless week. Unless someone linked Thirlmere and Windermere ;)

However the large variations in electricity prices do make it worthwhile, particularly when all the new wind comes on line in 2023-25. On windy days then we will have something like 4-6GW over capacity even with all the CCGT off, and grid prices will often be negative. Great for pumped storage to ‘charge’ as they get paid for it. Then generate when the price goes high - ie most evenings.

This is all subject to the increase in demand overnight from electric cars, which by then could easily hoover up a good chunk of that spare capacity.
 

HSTEd

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Most of the CEGB coal stations eg Ironbridge B, Rugeley B, etc had backup diesel and gas turbine generators to allow for black start. Rugeley for example had two 25MW Rolls Royce Olympus GT generators
The problem with those large gas turbines and diesels is that you then likely need some smaller generator or huge battery bank to ensure you can start them from cold.
 

edwin_m

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I don't think there are any UK 25kV EMUs that have two pantographs that would both be in use at the same time and a bus line between them. A third rail EMU could, though.
I should have made clear in my post that I was referring to third rail. Pantographs can't be interconnected via a bus wire (or if they can, as with Pendolino, then they can't be raised simultaneously). Otherwise when the train runs through a neutral section the different phases would be connected through the bus wire, effectively short circuiting the supply.
 

Belperpete

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I should have made clear in my post that I was referring to third rail. Pantographs can't be interconnected via a bus wire (or if they can, as with Pendolino, then they can't be raised simultaneously). Otherwise when the train runs through a neutral section the different phases would be connected through the bus wire, effectively short circuiting the supply.
Why is it acceptable to interconnect collector shoes but not pantographs? It is not exactly unknown for third-rail trains to run through a section gap and accidentally re-energise a section that has been isolated and earthed. In such cases, the short-circuit current will pass along the train between the collector shoes.
 

HSTEd

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Why is it acceptable to interconnect collector shoes but not pantographs? It is not exactly unknown for third-rail trains to run through a section gap and accidentally re-energise a section that has been isolated and earthed. In such cases, the short-circuit current will pass along the train between the collector shoes.

Because 25kV is an AC system, they try to connect it to a three phase grid, so different portions of the system are connected to different phases of the supply.

Which means there could be something like ~42kV between the two pantographs if they bridge two sections together. Absolutely enormous short circuit currents would flow.

This si rather more extreme than the third rail situation.
 

Bald Rick

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Or, put another way, the third rail is at a broadly constant voltage in any given area (nominally 750v) therefore anything bridging a gap ‘sees’ broadly the same voltage either side. Even if one side is earthed and at 0v, the maximum potential difference (in voltage) will be the nominal line voltage.

On AC, the current cycles between 25kV and -25kV 50 times a second. This means that all the equipment is rated at 25kV, but there is actually a 50kV difference in voltage. As adjacent supplies can have cycles that are out of phase with each other, a train bridging a gap can experience a potential difference of much more than nominal line voltage, and that voltage would be much higher than the rated capacity of the equipment. In turn that would lead to equipment failures, some potentially spectacular.
 

edwin_m

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I'd like to hope that 25kV equipment (including trains) is specified to withstand the voltages and currents resulting from a phase-to-phase short circuit, at least for the short time it should persist before the protection trips out. That would reduce it from a situation with major safety risks and equipment damage to one of operational inconvenience.
 

edwin_m

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Why is it acceptable to interconnect collector shoes but not pantographs? It is not exactly unknown for third-rail trains to run through a section gap and accidentally re-energise a section that has been isolated and earthed. In such cases, the short-circuit current will pass along the train between the collector shoes.
Others have answered this point but it's worth mentioning transitions between the true LUL 4-rail system and the one used on lines shared by LUL and National Rail where the centre rail is bonded to the running rails. On the latter system the fourth rail is obviously at 0V relative to the running rails and the third rail at +750V (or lower voltage in some obsolete installations). On the LUL system the third rail is at about +450V and the fourth rail at -300V (don't remember the exact figures) relative to the running rail. So there is about 300V difference between the third rail voltages on the two systems, which would cause big problems if train collector shoes bridged between them.

The solution is to have a gap longer than one car. This is possible because LUL stock doesn't have a through DC bus, so the pickups on different cars aren't interconnected. Third rail stock has interconnection between all shoes within a unit.
 

GRALISTAIR

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I'd like to hope that 25kV equipment (including trains) is specified to withstand the voltages and currents resulting from a phase-to-phase short circuit, at least for the short time it should persist before the protection trips out. That would reduce it from a situation with major safety risks and equipment damage to one of operational inconvenience.

I wont derive it for you but phase to phase is nominal 25,000 x sqr root 3 = 43300 volts
 

edwin_m

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I wont derive it for you but phase to phase is nominal 25,000 x sqr root 3 = 43300 volts
Yes I did all those little arrows and complex number thingies, but it was a long time ago, and I didn't say the phase to phase voltage would be 25kV! Designs also need to consider the permitted variation from the nominal voltage due to things such as trains regenerating, which goes up to about 29kV if I remember correctly.
 

apk55

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Neutral sections where a Phase change may occur normally have a short section of earthed contact wire between the two sections so a phase to phase short should not occur.
Some substations i think used "Scott connected" transformer so that there would be a 90 degree phase shift from one section to the other. Between substations it is possible that the sections are wired anti phase and up to 180 degree phase shift may occur. (This may occur in the Channel Tunnel as the UK and French grids are not synchronized)
 

edwin_m

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Neutral sections where a Phase change may occur normally have a short section of earthed contact wire between the two sections so a phase to phase short should not occur.
Some substations i think used "Scott connected" transformer so that there would be a 90 degree phase shift from one section to the other. Between substations it is possible that the sections are wired anti phase and up to 180 degree phase shift may occur. (This may occur in the Channel Tunnel as the UK and French grids are not synchronized)
It would be interesting to know if units with two pantographs interconnected by a 25kV bus (Pendolino, presumably 395s and Azumas too) have the bus wiring specified to withstand the voltage and current that would be seen in the worst case phase to phase short circuit. I think the spacing between these is longer than the typical length of the earthed section. Otherwise the hazard can only be controlled by relying on an interlock (probably software) that prevents both pantographs being up at the same time.

For completeness I should point out that 700s and other long units that habitually run with two pantographs up are not interconnected at 25kV through the whole train. They are effectively two units in multiple permanently coupled with no cabs at the inner ends, and each pan only feeding the traction equipment in its own half-train.
 

hwl

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It would be interesting to know if units with two pantographs interconnected by a 25kV bus (Pendolino, presumably 395s and Azumas too) have the bus wiring specified to withstand the voltage and current that would be seen in the worst case phase to phase short circuit. I think the spacing between these is longer than the typical length of the earthed section. Otherwise the hazard can only be controlled by relying on an interlock (probably software) that prevents both pantographs being up at the same time.

For completeness I should point out that 700s and other long units that habitually run with two pantographs up are not interconnected at 25kV through the whole train. They are effectively two units in multiple permanently coupled with no cabs at the inner ends, and each pan only feeding the traction equipment in its own half-train.
Vacuum Circuit Breakers are automatically disconnected by the track magnets going through neutral sections (and reconnected afterwards).

If you sit near a pantograph you can hear the loud thud of the VCB being disconnected.
 

Belperpete

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Vacuum Circuit Breakers are automatically disconnected by the track magnets going through neutral sections (and reconnected afterwards).
I understand that they are the same magnets as AWS uses, just mounted outside the track rather than inside. Just as AWS equipment sometimes fails, so I presume these magnets must also sometimes fail to trip the breakers. What happens then?

On a recent scheme, a signal's AWS magnet located adjacent to a set of points had to be moved, as it was tripping the traction breakers on stock taking the diverging route.
 

Bald Rick

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I understand that they are the same magnets as AWS uses, just mounted outside the track rather than inside. Just as AWS equipment sometimes fails, so I presume these magnets must also sometimes fail to trip the breakers. What happens then?

On a recent scheme, a signal's AWS magnet located adjacent to a set of points had to be moved, as it was tripping the traction breakers on stock taking the diverging route.

They are similar magnets. However AWS uses a permenanet magnet (which generates the ‘horn’) and a separate electromagnet (which, when energised, generates the ‘bell’). If the AWS unit fails, then it’s just the electro magnet that stops working, and thus only the ‘horn’ bit works. It is very very rare for the permanent magnet to fail; almost all wrong side AWS events are a problem on the train.

The APC (Automatic Power Cutoff) magnets at neutral sections are only permanent magnets. I’ve never heard of one fail.
 

edwin_m

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Vacuum Circuit Breakers are automatically disconnected by the track magnets going through neutral sections (and reconnected afterwards).

If you sit near a pantograph you can hear the loud thud of the VCB being disconnected.
Absolutely. But that doesn't really affect the possible hazard of both pans being raised (with VCBs closed) while they are connected to separate phases. The VCB for the first pan would open while it passed through the neutral section and re-close on encountering the second magnet, and assuming the second pan hadn't reached the first magnet the bus would be shorting between phases.
 

marko2

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Where there are two pantograph raised on a train they are on separate units which not don't share a bus-line.
 

HSTEd

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Where there are two pantograph raised on a train they are on separate units which not don't share a bus-line.
I think the comment was more questioning what provision is made to either prevent both pantographs being raised at once, or to mitigate the consequences of this occuring.

ie. How much interlocking is provided to ensure only one is up.
 

krus_aragon

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My probably outdated simple layman's understanding of the big benefits of Dinorwic Pumped Storage power station (where the Festiniog railway was diverted) was that it was excellent for filling shortfall in demand at very short notice
Your understanding is correct, except that Dinorwig is the other side of Snowdon, in Llanberis. The Tanygrisiau reservoir was formed for a different pumped-storage power station: the Ffestiniog power station.
 
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