Grid Frequency [mains electricity]

It used to be said that they 'made up' the missing cycles just before 9pm so that the BBC news clock and home clocks matched.
It implied off a website quoted above that there was no point in having a conductor more than 18mm in dia. I was under the impression that the high voltage overhead lines are more like 6in dia but hollow segmented tubes as anything inside wouldn't be carrying any current
Just after the blackout I heard a report on the radio from someone who was saying his battery system was called on to supply power for a short time then put in reverse to absorb excess. I've heard Tesla talking about such things but thought they were only in planning stage, are some in operation?

I do believe that unless the clock was three phase this is an Old Wifes Marts tale!

Removed.

Edited By V8Eng on 13/08/2019 14:34:36

Oldmart

reversing the mains input will do nothing to a synchronous motor’s direction but taking it to bits and reversing components will for your old bosses clock

Bazyle

in the guardian on monday was an article about Tesco’s being able to use their fridges to pump back in to the network to the tune of 50MW ..i’m Not sure what they mean by this perhaps they have big back up UPS’S to keep their fridges going in the event of a power cut

https://www.theguardian.com/business/2019/jun/23/cool-running-supermarket-fridges-could-help-power-uk

Edited By Ian Parkin on 13/08/2019 14:37:24

Apparently some systems are up and running, link to relevant story here:-

Back up.

Edited By V8Eng on 13/08/2019 14:38:23

Sorry if this is a bit simple minded, but do wind generators generate 3 phase mains that is synced to the grid or do they generate dc which then feeds a big inverter synced to the grid?

The reason I ask is that if directly synced to the grid then each wind generator has a complex problem matching wind speed to its output. Indeed keeping a few hundred all synced up and matching output looks a bit difficult.

One the other hand if they actually generate dc then I can't see that wind speed would affect the overall output frequency, the inverter(s) sort that out.

So how do they work?

The skin effect is true, but applies to each conductor, so a multi stranded cable has more surface area and therefor has lower resistance at high frequencies.

Audiophile speaker cable has multiple very fine strands for this reason. Even better if it is gold plated.

Incidentally on a different subject, If you have ever wondered why power lines have more than one cable on each arm it is to combat the corona effect. This effect is where electrons gather at sharp points, e.g. sparks start at the point of things. A high voltage cable in cross section looks like a point, so will bleed electrons into the surrounding air. By having more than one cable closely spaced they tend to cancel the electrostatic field. This means to the electrons they look like much fatter (blunter points) cables.

Adrian

Hi Oldmart, I have to agree with Ian. AC clocks that used to be the fashion had a ratchet type of mechanism which would stop the motor from going the wrong way, as it would not know which way it is supposed to go when powered up and it would go in whichever direction the half sinewave drove it when switched on. If it went the wrong way, the ratchet would click in and stop it and it would automatically reverse and carry on going the way it should go. Those clocks that go backwards and the numbers are also backwards have been around for years and my elder sister has had one for many years, you can buy one from **LINK** for example, if you like.

Regards Nick.

Seems like my old boss must have monkeyed with his clock to get it running backwards after all.

Only if the conductors are insulated from each other and then woven/plaited/arranged so that the net field on all conductors is equal. Audiophile multi-strand/gold plated speaker cables are an excellent way to remove money from gullible fools, especially since gold has a lower conductivity than copper. Silver is a better conductor, though.

I believe wind generators match their speed to get the most energy from the wind, with multi pole generators or harmonic gearing to step up between the rotor and alternator. Then rectify and use an inverter locked to mains frequency to generate 3 phase back to the grid. Solar generates dc directly, again will use an inverter.

the Guardian version of the supermarket freezer story pointed out that they can just switch the things off to shed energy when required, they don't I think have any energy storage or generation as such.

Various ways of doing it; a simple scheme might just synchronise naturally with whatever the grid is doing. Not very efficient, mechanically stressful during gusts, and the turbine has to cut in and out whenever the wind speed is too low or too high. Early technology.

Small turbines often generate DC converted electronically to AC with an inverter. These have no problem matching to the grid over a wide wind speed range. By using a permanent magnet generator the need for a gearbox can be eliminated, making conversion nearly 10% more efficient. Unfortunately big invertors are pricey.

To keep cost down, big turbines - which can be 150MW plus per unit - drive a three phase generator fitted with an extra set of windings. An electronic controller matches the output of the generator to the grid by rotating and altering the amplitude of the magnetic field inside the extra windings. Again the turbine has to be cut in and out whenever the wind goes out of tolerance, but the more advanced controllers can manage restarts within several cycles.

Other schemes available and I don't think there's a single preferred generator design in view yet. What's suited to a big off-shore generator with reliable wind wouldn't suit a small unit on an hilltop where inland ground turbulence causes lots of variation. But, because the electronics are getting more power capable and considerably cheaper, I'd guess the straightforward inverter approach might become universal one day.

Dave

Not necessarily. Early mains clocks had no ratchet mechanism and had to be started by spinning a small shaft by hand. If you spun it the wrong way, the clock ran backwards. I have an old clock movement of this type which belonged to my late father.

The tower clock of St Mary's Church in Portsmouth made news headlines some years ago when this happened. I wasn't in charge of it at that time, I hasten to add; that came later. The chimes, which were operated by mercury tilt switches, would have continued in the correct sequence but sounded at not-the-quarter and not-the-hour!

George

The school I went to had the then common system of multiple synchronised clocks. The biggest, highest up the wall, most akward to access clock had lost its ratchet. After loss of power, it was pot luck which way ran on restart. Sometimes the caretaker had to get the big ladder out and nudge this clock in the right direction, waiting for the next full sync event.

The clock probably could have been fixed. The headmaster was ex REME. The faulty clock was a good teaching aid. The caretaker was very tolerant.

Perhaps he just bought a Barber's Clock

I was told it was 8am so everybody got to work on time. It could have been both. Whatever the detail, there was no long term drift on a mains driven clock.

Now we tolerate DAB radio 'pips' being several seconds late. Don't get me started...

There was a person being interviewed on R4 PM today, who was indicating that one of the weaknesses in the grid was down to "lack of inertia". Older larger generating stations, take time to wind down. Newer sources trip out very fast, with a big impact on the frequency and the grid is not geared up to absorb this.

Just a couple of points from a retired Station Control and Instrument Engineer:

Governors on conventional, i.e. coal or oil fired, stations had the droop set to 4% That meant that a rise in grid frequency of 4% would shut that unit down and equally a grid frequency 4% low would take it from zero to full power. Obviously that never happened, but that was the "proportional band" of mechanical and electronic governors. Nuclear stations had the droop set as high as it would go, to avoid mucking the reactors about. I believe the term "droop" was derived from the design characteristics of the early mill engines.

As pointed out in previous posts, the grid should have spare capacity available to cover any unexpected shortfall. With a lot of units on-line in the winter, the overall effect of the loss of one is small. In the summer, with fewer sets in service, the loss of one will be more substantial, but can still be covered by another the same size. A new dimension arises when the unit in service and its putative replacement are both wind-powered. Wind farms that are available get paid for their power wether they are on-line or not. This may be a factor in deciding the "Order of Merit" and might be something that needs to be considered.

Contracting the load is an important factor - costs incurred if a generator shuts off too late and they don’t want to be wasting money spinning idly for unnecessary time before they take up the load. Contracts have standard penalty clauses, because any overlap, or ‘underlap’, costs the grid for ‘balancing’ loads during those change-over periods. It is a juggling act where they cannot afford to drop any balls!

Back in the 70s a friend of my dads taught electricity theory for the CEGB.

The 'Nine O'Clock News' story is a myth BUT they did have a policy of making up any lag or surplus of cycles with the aim of balancing out any errors over a 24 hour cycle so that clocks didn't creep out of time.

Neil