Accuracy of UK power grid time control?

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

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.

Try asking on time-nuts

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

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

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

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

Mathew

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

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.

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

That is wrong. 3.6 degrees/second implies about

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.

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

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

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

Mark

Not if it's digital.

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)...

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.

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.

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

.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.

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:

I 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