Accuracy of UK power grid time control?

[JANA]

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.

[Mooron]

I read an article a few years ago that discussed how the frequency is regulated in the US. I can't find it now. I think it was by David Mills from the University of Delaware. As I reacall, there are 2 power grids in the US. The Eastern grid is controlled by an automated system at some power plant in Ohio. The Western grid is controlled manually. I wish I could find the article, it had some interesting stuff.

Things are probably done in a similar fashion in the UK.

- Mooron

[Mike Harrison]

You can get standard TXCO modules that are good for a few ppm, often very cheap surplus/ebay. However mains is mostly good enough.

[Dave (from the UK)]

Try asking on time-nuts

[Don Kelly]

---------------- 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.

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------- 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.

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------------ 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.

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--------- Depending on how the clock is driven as indicated above.

[Mathew Newton]

Even a stopped clock doubles that... ;-)

Mathew

[Mike Barnes]

I'd be tempted to use a wall-wart instead of a battery, and probably an MSF mechanism if it was strong enough.

[Andy Dingley]

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.

[Don Kelly]

Correction and apology: I said: ( if one machine is

That is wrong. 3.6 degrees/second implies about

[Andy Dingley]

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.

[Edgar Iredale]

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

[Mark and Gloria Hagwood]

Interesting thought. Would the clock then be driven by the line frequency or the oscillator frequency of the UPS?

Mark

[Michael A. Terrell]

Not if it's digital.

[Andrew Gabriel]

I'm out of touch now, but CEGB used to keep UK power grid at

50Hz +- 0.1Hz. No one ever came up with a good reason it had to be that accurate, but they did it "just because they could", to quote someone I spoke with at the Winnersh control room about this some years back.

I wrote a more detailed article about this a few years ago, which discusses various notable historic events, like how the power grid had to handle the majority of the UK using the toilet at the same instant, which resulted in the largest ever surge in demand on the UK power grid (which with advanced planning, it handled just fine)...

[Michael A. Terrell]

If all those areas are connected to a single power grid they still have to stay in sync, even if the control system is broken into regional centers.

[Michael A. Terrell]

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.

[Jack Denver]

The latter during power cuts, the former the rest of the time, the way most UPS's are designed.

[Michael A. Terrell]

.1 Hz is not an unreasonable standard to keep. The wider the variation that is allowed in the control loop, the easier for the whole system to become unstable and shut down as the controls disconnected equipment from the grid that was too fast, or too slow. You are working with massive mechanical systems that will self destruct if you allow sudden changes while under load. Think of what happens when a long train tries to stop. If the couplings didn't have some play to adsorb the shot, the train would derail when the engineer tried to use the brakes. Its just basic physics.

[Christopher Tidy]

Hi all,

Thanks very much for all the information. Sorry for the original cross-post; I was expecting very few replies.

In answer to some of your questions, the clock is a WWII German "Dehomag" slave clock, originally designed to be driven by a master clock. It has no second hand. The minute hand appears to be balanced, but the hour hand not. The clock is about 14" across, so not huge, but it's exceptionally heavy as nearly everything is made from 1 mm steel plate. Here's a picture of the clock:

can't decide whether to repaint it black, as it was when manufactured, or to leave it grey. It's for my bedroom, so if it were to stop during power cuts I wouldn't mind. It sounds like the 50 Hz grid frequency will be a plenty accurate time keeping source, and it's an interesting solution to the problem. We live in a pretty rural area so I'll try to design a circuit which will be fairly resistant to noise. I had already planned to put a Schmitt trigger on the input from the step-down transformer.

Once again, thanks for all the advice. It's much more than I got from the National Grid - the phone number on their website doesn't even work!

Best wishes,

Chris

[John G]

You did not tell us what the drive mechanism is. If it was similar to the Simplex/IBM master clocks of the time it most likely advanced every minute by a pulse to an electromagnet.

Now there are several ways you could generate that. A Small timer motor with a cam like a washing machine timer. Or an Electronic cct synched by the mains.

If it is for your bedroom you may not want it after the first few hours as the electromagnet noise will drive you bonkers.