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Old 3rd Nov 2010, 8:43 pm   #21
Heatercathodeshort
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Default Re: National grid frequency

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Originally Posted by merlinmaxwell View Post
In the last hour I have got a large moving coil meter. Guess what for?
Similar to this one I pulled off the tip around 15 years ago? It's a Sangamo Weston. The movement is a fantastic bit of engineering. It is fitted with the old silk covered flex and a 5amp 2pin plug. It reads around 1 cycle low but this is probably due to leaky internal wax capacitors. There are five in all.
It still has its original box. No idea of it's age. J
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Old 3rd Nov 2010, 9:11 pm   #22
Sean Williams
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Default Re: National grid frequency

Er yes, synching a generator out of phase is a very bad thing, and does give a loud bang!

Think of trying to stop your car engine by throwing a steel bar through the flywheel.....

The peak current is quite impressive - certainly enough to phisically lift several tons of alternator.....
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Old 3rd Nov 2010, 9:50 pm   #23
AlanBeckett
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Default Re: National grid frequency

When I was at college we carried out just such an experiment. We had to synchrinise a local motor/generator set to the incoming three phase and then put a plug into a set of sockets to connect them together. We were then meant to vary the phase difference and observe the current flows. My 'colleague' got the wrong end of the stick and banged the plug in when they weren't properly phased. It took out every breaker in the Lab block.
Still, we got off early
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Old 3rd Nov 2010, 11:42 pm   #24
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Default Re: National grid frequency

Quote:
Originally Posted by Heatercathodeshort View Post
Quote:
Originally Posted by merlinmaxwell View Post
In the last hour I have got a large moving coil meter. Guess what for?
Similar to this one I pulled off the tip around 15 years ago?
We used to use a thing with vibrating reeds in it, the ends visible through a series of little holes in the dial. As I recall it operated from about 45 - 55 Hz.

I have synchronised alternators (and a humungous synchronous motor) in the past with the aid of a synchroscope and 'lamps, dim' (as opposed to 'lamps, bright' - another method), although the generators I deal with nowadays are auto-synching and autonomous: not parallelled with the grid!
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Old 3rd Nov 2010, 11:58 pm   #25
AlanBeckett
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Default Re: National grid frequency

Russell,
Some of it comes back to me. I think that's where he got it wrong
Alan
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Old 4th Nov 2010, 8:11 am   #26
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Default Re: National grid frequency

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When I was at college we carried out just such an experiment. We had to synchrinise a local motor/generator set to the incoming three phase.....
Sounds familiar. In Eric Laithwaite's lab at Imperial College we did something very similar, using an antiquated 3 lamp synchroniser. One group (not mine!) got it 180 out and blew the lab fuses. Even a few degrees out will make the alternator complain as a large rotating lump of metal tries to move instantly to a different angle.
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Old 4th Nov 2010, 9:40 am   #27
John M0GLN
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Default Re: National grid frequency

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Originally Posted by russell_w_b View Post
We used to use a thing with vibrating reeds in it, the ends visible through a series of little holes in the dial. As I recall it operated from about 45 - 55 Hz.
I used one when I worked at Mullard, when you switched it on, for the first few seconds the white visible ends used to produce a slowly moving sine wave, like a 'scope slightly out of sync'.

John
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Old 4th Nov 2010, 10:04 am   #28
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Default Re: National grid frequency

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Originally Posted by russell_w_b View Post
We used to use a thing with vibrating reeds in it, the ends visible through a series of little holes in the dial. As I recall it operated from about 45 - 55 Hz.
One of these things? I want one...
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Old 4th Nov 2010, 10:07 am   #29
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Default Re: National grid frequency

I was working on some generators a while ago that had RCD's fitted as an after thought. When the RCD was reset, it threw the contactor straight in without Syncing up first, quite scary to see several tonne of machinery jump six inches across the yard.
I still think it's wonderful that by varying the excitation you can make an alternator act like a capacitor or inductor.
Rob.
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Old 4th Nov 2010, 1:49 pm   #30
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Default Re: National grid frequency

When syncing an incoming generator to an established supply, using a synchroniser or the lamp method, is it not possible to connect via low-value resistors which are then switched out of circuit? The incoming generator will then be pulled gently into full sync. Afterwards, the excitation can be fine-adjusted to set the load.

The resistors would only take a hammering if sync was significantly out. And in this case, a bank of sacrificial resistors would be much cheaper and quicker to replace than a wrecked alternator.
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Old 4th Nov 2010, 1:53 pm   #31
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Default Re: National grid frequency

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One of these things? I want one...
Nearly... It was much older than that, and circular.
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Old 4th Nov 2010, 2:19 pm   #32
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Default Re: National grid frequency

I think a series resistor for syncing would need to be rather large (a few MW?), even though the generator excitation would presumably be set for low power until synced. Someone who was likely to wreck a generator would not be set loose on a unit desk!
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Old 4th Nov 2010, 4:41 pm   #33
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Default Re: National grid frequency

Not really required if syncing is carried out properly. If anything you want the generator coming on stream to be running slightly faster than synchronous speed to prevent reverse power. The excitation is adjusted to give the same voltage as that on the bus before syncing, once synced it is adjusted to give equal power factor on all sets. The "throttle" is then opened to load up the set (with real power).
Rob.
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Old 4th Nov 2010, 11:53 pm   #34
Zelandeth
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Default Re: National grid frequency

Ah, "twanger" meters! I've got one of them rattling around somewhere - unfortunately it's 115V and 60Hz though!

Was going to dig it out and snap a photo of it - until I realised how deeply buried in the wardrobe it is...can wait till the weekend!

It's really interesting though actually being able to see the grid's behaviour in real time though.
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Old 5th Nov 2010, 2:04 pm   #35
Lucien Nunes
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Default Re: National grid frequency

Anybody who like fidding with big alternators etc should be well at home at Electrokinetica once we're up and running. In fact one of the interactive demo's that I've been pondering about setting up is a fool-proof real-time demonstration of the process, that people can experiment with by following the instructions and prompts from the (hidden) controller. Not a mock-up though - a running generator delivering real power to the museum mains.

It's something for the tech heads to get to grips with while the rest of the family are comparing vacuum cleaners and playing 70's video games!

Lucien
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Old 5th Nov 2010, 3:09 pm   #36
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Default Re: National grid frequency

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while the rest of the family are comparing vacuum cleaners and playing 70's video games!
Nowt wrong with vacuum comparisons...
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Old 5th Nov 2010, 3:31 pm   #37
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Default Re: National grid frequency

Hi,
In reply to G8HQP Dave's reply, synchronising an alternator when the excitation is weak runs the risk of "pole slipping" when the grid can pull an alternator out of synch a phase at a time instead of in. I've never witnessed it at the stations I worked at, but the engineers reckon it can be almost as spectacular as not synching properly in the first place.
One day we had a 500MW unit trip which took us off the bars & shut the steam off to the turbine - but - the breaker feeding the station supplies didn't trip and for a few seconds the whole station slowed down as the alternator ran down while still generating. I was in the auxiliary boilerhouse at the time and it was a strange experience watching (and hearing) all the motors slowly grind to a halt followed by an eerie silence before the panic set in.
Sorry if I've de-synched the thread
Cheers, Pete
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Old 5th Nov 2010, 4:45 pm   #38
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Default Re: National grid frequency

Some years back, during a bad winter with power cables down, Lynton was running from its local standby generator (gas turbine based - off grid) which ran out of fuel. All lights everywhere dimmed out to blackness over about 10 seconds.

It would have been great to have been watching the frequency, or the mains waveform, at the time - obviously with independently powered equipment
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