Accuracy of UK power grid time control?

Don Kelly wrote:


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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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.
<snip>

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
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----------------------------

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.
--

Don Kelly @shawcross.ca
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On Fri, 14 Apr 2006 15:27:23 +0000, Christopher Tidy

It's weird. Frequency is allowed to wobble a bit, but it has to average out very accurately over 24 hours or so, because of the number of clocks in service.
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I probably should have mentioned. If this is outside. You can get GPS, for about that price, and if it's a clockface, you can mount the antenna there, and have it work just fine.
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On Fri, 14 Apr 2006 15:27:23 +0000, Christopher Tidy

Other people have given the rules that are applied (i.e. that the total number of cycles in 24 hours is constant), but the reason is for efficienty of electricty transfer. It is very important that all generators run in sync, so as to minimize transmission losses in the National Grid, AFAIUI.
Paul
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Paul Cooper wrote:

Any generator that is not in sync with the grid will either be slowed down by higher current loading, or it will become a motor and catch up to the other units. The speed AND phase of a generator has to match the grid before it can be connected, or it can literally be ripped loose from its mounts and destroyed. The basic system to do this is a set of lamps connected between the two generators. The new generator has it speed slowly adjusted till the brightness is cycling VERY slowly, then at a time when all the lamps are out it is switched into the grid. After it is connected it synchs itself completely, then the operator increases the fuel or water supply to generate electricity. This has to be monitored to keep the generator below it rated output, to keep the windings from overheating.
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In the 'old' days; mains powered clocks (squirrel cage, synchronous) were common. Many mantel shelves had a two pin round outlet fixed above them for the sole purposes of plugging in an electric clock. Although the _instantaneous_ frequency stability of the mains supply is (relatively) wide; the aggregation of time periods (inverse of frequency) over the course of the day results in a discrepancy that was/is tiny. IIRC, power station control rooms had a red finger which was slaved to a pendulum/GPO time source while the same 'meter' showed the integration of the 50Hz periods - aka 'Electric Time'. It was a responsibility of the Control Room staff to keep 'Electric Time' consonant with 'GPO Time': this could be achieved by altering the instantaneous frequency (speeding up/ slowing down ).
However; as Argos flogs Rugby-synchronised wrist watches for ten-quid-odd, why bother? PS, the 'Klick' wristwatch not only adapts to GMT/BST changes but it functions as a depth-gauge: - if it stops working I must be deeper than fifty feet - but at least I'll have the facility to observe TOD !
--

Brian



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On Fri, 14 Apr 2006 16:23:18 GMT, "Brian Sharrock"

Although a squirrel cage uses a synchronous stator winding, it isn't a synchronous motor, at least not to this level of timekeeping. Their rotors are powered by an induced current in the rotor (the squirrel cage itself), not any sliprings or brushgear. This current is only induced if the rotor experiences a moving or changing magnetic field - i.e. it rotates at a different speed to the field in the stator. They can't generate a torque unless there is some "slip", the speed difference between synchronous speed and actual rotation. It's notable that the more the slip, the more the output torque - so these motors can deliver substantial power under load.
A frictionless, resistanceless, hysteresisless squirrel cage motor doing no work would accelerate up to synchronous speed and then hold that speed spinning freely, doing no work and producing no torque. An idealised but possible motor might always run at a known slip which could be compensated for by gearing. In practice such effects as temperature and lubricant viscosity make this unpredictable, at least for clockmaking accuracy.
Clocks use shaded pole motors, which are synchronous. As these don't have the same increased torque response to slip they're inherently low torque and thus only useful for clocks or other light tasks.
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----------------------------
wrote:

------------------- 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?
--

Don Kelly @shawcross.ca
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On Fri, 14 Apr 2006 16:23:18 GMT, "Brian Sharrock"

This is Usenet isn't it? There'll be complaints....
Of course not all clock motors are shaded pole. The Warren Model A was, back in 1916 (I think the first synchronous clock motor)
However the well-known Westclox / Sangamo design of the '30s and onwards used an induction rotor - effectively a "squirrel cage". This gave good starting torque, but obviously had the slip problem. To keep it locked and synchronous there was also a permanent magnet rotor. This gave adequate torque at synchronous speed, without slip, but wouldn't have been able to start the clock unaided.
Some British designs used a single permanent magnet rotor and required a mechanical pushbutton, or an extra winding for starting.
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Andy Dingley wrote:

I have an old Smith Sectric electric clock that was, I think, new in 1942. It maintains time accurately as compared with a radio clock, so long as the power doesn't fail. When I was very young I remember being the only person in the house who could get it going again after a power cut. One was supposed to set the time and jab in the setting knob to start it, but that seldom worked. At about the age of five I discovered the best way was to remove the motor cover and spin the wheels inside.
Edgar
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It has to be significantly more accurate than that
Power stations have to be in sync with one another - which requires good accuracy and stability
--
geoff

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Actually not - the power grid will work just fine at 49.7Hz average. The way that large generators work, at all times other than when you're starting one up, they are fixed to the grid frequency. If you try to turn one harder, it just generates more electricity, and tends to 'push' the whole system higher in frequency. Of course, one generator can't do this appreciably.
There is no actual need for a national centralised frequency setting, because of the way it works, as long as some power stations switch off/on up/down, when the frequency gets above or below 50Hz. This can be done fine with a 48-52Hz analog meter in the control rooms of each power station.
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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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There won't be any cumulative errors.

True they have to be phased up to the grid exactly before they connect to it, or *BANG* turbine blades through the turbine hall roof. I've been to a power station where it had happened. If they connect successfully then they'll stay in sync.
The exact frequency is not overly critical but is kept as accurate as possible for the benefit of users who depend on the frequency being accurate, steelmills rolling steel for instance.
FWIR the control console at "Grid Control Centres" used to have a standard synchronous electric clock showing grid time, and a clock showing exact GMT (How ??, unless exact 50Hz was distributed about the country) . It was normal for "grid time" to lose a bit during the day, depending on load, and then make up for it overnight.
Quite a manual process in 1969. ;-)
It must be better now !
DG
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For any type of accuracy, depending on the power grid is not a dependable option! There can be noise, interference, and power dips. On the short term, the power grid will be accurate, but over the long term, it can be out by a fair amount.
I have an electric clock that is dependent on the power grid for its timing. This clock can be a few minutes or a few seconds out at times.This is not from the power company being off frequency. It is most likely caused by power dips, or noise in the power delivery.
The accuracy of the power companies timing is very accuracy. It has to be in order to keep synchronized to other systems that they are working with. They have many generating facilities that are working together, therefore all their systems have to be synchronized to within fractions of a degree on the line frequency. From what I am told, here in North America, their margin of error is within about 1 second per month.
The problem when connecting a device at home to the power line, such as a clock, the clock can jump off time very easily. It is susceptible to interference, and power disturbance through their distribution. I was also told this by an engineer from our local power company.
Normally, during peak conditions, they allow the frequency to drop very slightly. It may be as much as 0.5 to 1 Hertz or so. During peak conditions, they will give it back. This means that all the power generation systems working together must also drop, and increase by the same amount.
The clocks that I have that are referenced to the AC line, are not dependable for accurate time. Over a few weeks, I found the quartz clocks to be more accurate. I can check them with the NIST broadcast.
Here in North America many stores are selling quartz clocks with built in time receivers. These are getting the time reference from the NIST. The user only has to put in the approximate time to within about 30 minutes. After 12 hours, the clock will be exactly on time, as long as it can receive the NIST signal. The clock automatically checks in with the NIST about every 12 hours. If the NIST signal is unavailable, the clock will keep time to the spec of any consumer type quartz clock. This is about 5 to 10 seconds per month.
Another thought would be to get a low cost GPS that can accept an external antenna and external power supply. This will give you the most accurate possible time for home use.
You can also go to the NIST site with your computer, and get very accurate time. It should be within about 100 to 200 ms. Going from memory, I believe it will tell you its error to your computer. By hitting the reset key, you can sometimes get it down to within a few milliseconds.
--

JANA
_____


"Christopher Tidy" < snipped-for-privacy@cantabgold.net> wrote in message
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---------------- In fact, long term accuracy is very good- correction is made to ensure that. Short term accuracy may drift but is regularly compensated for. One system that I know had corrections made every minute. Nice homegrown control but wasn't compatible with the overall system when the utility joined the Western Grid. ---------- >

------- And how is this clock driven? If it is a digital clock or electronic then your point may be valid. If it has a hysteresis synchronous motor then it will depend strictly on the frequency and not noise or "dips". However, these may be hard to find nowadays. --------

------------ All machines on the system will be synchronised - at the same frequency- and drifting up and down together if frequency is changing. You will not have frequency differences between machines on a system. There are variations in phase but anything that can be measured as a frequency change -can't be- as by then the system is unstable and it is lights out. ( if one machine is 0.01Hz fast or slow, then instability can occur in less than a second. ) It appears that you are referring to the process of connecting a machine to the system and doing this smoothly does require being within a few degrees in phase and only a small frequency difference in order to minimise "bumps" and heavy power surges when the system pulls the machine into full synchronism. This, of course, has nothing to do with control of time. ------------
> The problem when connecting a device at home to the

--------- Depending on how the clock is driven as indicated above.
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>
> Normally, during peak conditions, they allow the frequency to drop very
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Correction and apology: I said: ( if one machine is

That is wrong. 3.6 degrees/second implies about <25 seconds to steady state instability. This is , of course, too simplistic as such a sustained frequency difference will not occur - either the errant machine is pulled back into synchronism or has been tripped out -hopefully without bringing the whole system down.
This also applies to incoming machines- either they get in line or get kicked out-now!.
--

Don Kelly @shawcross.ca
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JANA wrote:

All wrong.
Clocks essentially filter out all the trash, the long term accuracy is guaranteed by the generating companies, and the one thing that gets you is really power outages only.
But that was teh one thing you don't mention.
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