Accuracy of UK power grid time control?

The old brass timeswitches which used to be used on streetlamps (complete with auto seasonal adjustment) used to continue on clockwork for several hours during a power cut. On power restore, the synchronous motor also rewound the clockwork spring. Damn impressive pieces of mechanical engineering those things were.

There's one road near me which obviously still has timeswitches on the lamps, but no evidence of the clockwork standby operation, judging by how all the lights can occasionally go out of sync for a month or so before someone corrects them.

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Andrew Gabriel

Presumably the back up oscillator wasn't adjusted accurately if at all during manufacture, not surprisingly. people were used to having to set the clock.

In the '60's most audio equipment, cheap record players etc around here (Leeds) used to emit a series of peeps at about 6-00 pm, as schoolkids we believed it was to reset the timeswitches governing the streetlights once per day. WCHBW

DG

Of course they run fast. That way the alarm will wake you early enough to reset the clock before you leave for work.

--
Service to my country? Been there, Done that, and I've got my DD214 to
prove it.
Member of DAV #85.

Michael A. Terrell
Central Florida

Thanks for the clarification. Thanks also to everyone else for their help and suggestions.

Best wishes,

Chris

------------------- Shaded pole motors may be induction or may be synchronous. The shaded pole just establishes a rotating field in the right direction to get starting torque. The difference is in the rotor- either conventional induction motor rotor- ie. slip needed for torque: or hysteresis synchronous where the hysteresis-synchronous motor has a core made from two different steels, giving it a nearly constant accelerating torque up to synchronous speed (as an induction motor), at which point it locks into synchronism. Certainly there were some US models (my parents has one and you still see them in antique stores) which needed a mechanical kick for starting-twist a knob to spin the gears) but this may have been to having a single winding and not a shaded pole construction. The rotor doesn't have permanent magnets but generally has permanent magnet material forming poles and the rest filled in with a soft iron. Just as with large synchronous motors, the starting is as an induction motor but running is synchronous. More expensive than the shaded pole induction motor as used for many small fans.

Were the Sangamo designs "reluctance" motors- with salient poles and an induction winding?

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Don Kelly @shawcross.ca
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High voltage DC (HVDC) is used to transmit large amounts of power over long distances or for interconnections between asynchronous grids When electrical energy is required to be transmitted over very long distances, it can be more economical to transmit using direct current (An electric current that flows in one direction steadily) instead of alternating current (An electric current that reverses direction sinusoidally). For a long transmission line, the value of the smaller losses, and reduced construction cost of a DC line, can offset the additional cost of converter stations at each end of the line. Also, at high AC voltages significant amounts of energy are lost due to corona discharge (An electrical discharge accompanied by ionization of surrounding atmosphere) the capacitance (An electrical phenomenon whereby an electric charge is stored) between phases or, in the case of buried cables, between phases and the soil (The part of the earth's surface consisting of humus and disintegrated rock) or water (Binary compound that occurs at room temperature as a clear colorless odorless tasteless liquid; freezes into ice below 0 degrees centigrade and boils above

100 degrees centigrade; widely used as a solvent) in which the cable s buried. Since the power flow through an HVDC link is directly controllable, HVDC links are sometimes used within a grid to stabilize the grid against control problems with the AC energy flow.

Also see

I'm quite familiar with HVDC distribution systems, but more generators are connected via AC than DC and those DO have to be in phase and have the frequency controlled to keep the rest of the grid happy.

BTW: HVDC distribution has been discussed to death on both the news:sci.electronics.design and news:alt.electrical.engineering newsgroups.

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HELP! My sig file has escaped! ;-)

"Michael A. Terrell" wrote in news: snipped-for-privacy@earthlink.net:

Mention of HVDC reminds me that the Channel link is one such (at least, I think that's what I remember). So that prompts me to ask, how closely synchronised are the UK and France, and indeed the other European countries with each other?

(Hope I haven't missed this question somewhere else in this huge thread!)

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Rod

I haven't seen a synchronisation map since the Berlin wall came down. However, before that, most of Western Continental Europe was a single zone controlled from Switzerland, Eastern Europe was controlled from Moscow (can't recall if it was a single zone) and Great Britain was its own synchronisation zone controlled from Reading, Berkshire.

When the Berlin wall came down, West Berlin was very short of power and had been suffering power cuts as a result. East Berlin had a surpless but it was produced by horribly poluting power stations. They couldn't be easily linked as they were in different synchronisation zones. Also, whilst West Berlin was 50Hz +- 0.5Hz, East Berlin's frequency varied by considerably more than that. I don't know how this was eventually resolved -- I read about it only a few months after the wall came down and there was no resolution at that time. Lots of UK power (and gas) engineers got contracted out to the former East Germany just after unification as part of a massive repair of their broken infrastructure (loads of leaking gas mains was a serious problem).

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Andrew Gabriel

So? Note that Michael said "single" power grid.

You are considering a point to point asynchronous connection between two systems. A DC link is often used for this purpose even in some cases where the converter stations are back to back but an asynchronous tie is required because of differing frequencies (Japan)or simply because otherwise there are problems maintaining a synchronous tie between two large systems (Alberta and points west and south-Saskatchewan and points east and south).

It is true that they can be at different frequencies but...

within each system, machines have to be in synchronism. In the case of the NW power pool, a DC backbone is used, as you suggest indirectly, in order to maintain stability of the system which implies that it is used to maintain synchronism in a system which might have problems otherwise.

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Don Kelly @shawcross.ca
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>

snipped-for-privacy@cucumber.demon.co.uk (Andrew Gabriel) wrote in news:44441f51$0$663$ snipped-for-privacy@news.aaisp.net.uk:

Haven't been back to Berlin since the wall was put up! (I was there when it was built.)

Thanks for the replies.

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Rod

You seem to be laboring under the idea that all the AC generators tied to the grid have to be carefully regulated to stay in sync with each other through some incredibly precise timing.

That isn't the case. A generator is brought on-line by carefully regulating the speed and getting it in phase. That is a bit tricky. But once tied to the grid, 'keeping in sync' is done by the load current and physics. In fact, base load units don't even have frequency control once on-line. The speed set-point for the governor is run several hz up out of the way and the turbine controls are controlled by a 'load' setting. The operator dials in the amount of MW load they are supposed to carry, and the controls monitor MW and steam flow. They don't respond at all to frequency changes unless the frequency rises to the point the unit is in danger of over-speeding.

During grid disturbances, base load units will naturally speed up/slow-down as grid frequency changes, maintaining their load output based on 'load-set'. Only 'regulating duty' plants monitor generator speed/freq and make any sort of adjustment based on changes in speed/freq. And 'regulating' units make up a fairly small fraction of all AC units.

The vast majority of AC generators will 'stay in sync' just by virtue of the physics of synchronous machines. Only if under-excited, or significant reactance in their output line are they likely to 'pull out' of sync with the grid. (and that's a *bad thing*)

daestrom

I never said that at all, but I did say that the sped and phase have to match to connect a new generator to a grid.

If you would have read the entire thread, I described how the generators are synched, and that the grid keeps them in sync unless something goes wrong. I also stated that the generator was fed more fuel or water to actually produce power for the grid rather than just coasting along, in phase, one it was connected to the grid. I studied the subject with college textbooks on power generation and distribution when I was 13.

Of course the larger the spinning mass in the generator, the more the inertia, and the less likely to be kicked out of phase. The experimental nuclear power plant at Ft. Greely, Alaska was steam driven and unable to adapt to rapid load changes so they blew out quite a few bearings in the turbines before they finally gave up and shut it down. They would barely get it running an synchronized to the Alaskan power grid when someone would fire up another generator and switch it on line without contacting other providers on the grid. The system was unstable, in frequency, voltage and a lot of outages. I watched my 120 VAC feed climb to over 190 volts one day, as circuit breakers all over the complex were tripping out. It took me a couple hours to get all the studio equipment and transmitters back in service that day. It was one of the few times that I was happy that the towers were too short to need lights.

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HELP! My sig file has escaped! ;-)

Yes, this must be the case. From memory, the national grid a long time ago (maybe 30 years or more) was not regulated to ensure long term mean frequency accuracy (ie. the average could drift slowly) and the was noticeable on mains synchronised clocks. Some time after this things changed so that the long term accuracy was controlled.

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The problem is not the steam turbine/governor which responds relatively quickly (much faster than a hydro machine), but likely in the reactor dynamics and control. It appears that the machine was kicked off the system and an uncontrolled shutdown occurred. Wiping of bearings can occur in uncontrolled, coast slowly to a stop, shutdowns. It happened to "Big Ally" in NYC in the '65 blackout.

Most college texts - at least in the past, when you were 13, didn't really discuss synchronisation except for the general concept, nor the problem of load sharing and the effects of governor "droop".

It is true that a heavier machine will respond more slowly for a given accelerating power but this is a mixed blessing as it takes longer to bring it under control and time is of the essence in considering system stability. A smaller machine is more likely to swing rapidly but is normally easier for the system to rein in (system impedances will affect this). I assume that the experimental machine was fairly small as well as being weakly tied to the system but there could well be other, control, problems with the governor system and load sharing coordination.

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Don Kelly @shawcross.ca
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It was a small, poorly designed grid in subzero weather. The plant was built as a feasibility study to see how well a reactor would work in that climate. It had been decommissioned just before I arrived, but I knew the EEs and MEs who maintained it, since most of them had been borrowed from the diesel powered plant that the reactor was supposed to replace. The brick building was still there, right across the street from my barracks.

As far as the textbooks they were published in '60 and '61, so I am sure that there are a lot better text available 45 years later. Like you said, they did cover the basics of how a power grid works, and how they were synchronized. I think they were first and second year texts, but they are long gone. I went into high power RF and microwave communications instead of power generation and distribution, but I still remember the basics.

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Service to my country? Been there, Done that, and I've got my DD214 to
prove it.
Member of DAV #85.

Michael A. Terrell
Central Florida

Then why do you keep repeating....

"generators are connected via AC than DC and those DO have to be in phase and have the frequency controlled to keep the rest of the grid happy."

And similar phrases about AC generators needing extremely accurate frequency control. As I pointed out, most generators on the grid are *not* frequency regulating. Once tied in, they just follow the grid frequency. Base load units are a prime example of this. The governors are run up out of the way so they are not controlling the turbine at all once connected.

Not familiary with Ft Greely, but I would guess that such a system was not really part of a large grid. Sounds more like a small number of units with a relatively small amount of load (< 500 MW???). Yes, controlling such a setup does require more coordination. Especially since one unit can make a rapid change to the system's frequency.

daestrom