On 18/05/16 20:50, Johnny B Good wrote:
On Wed, 18 May 2016 09:29:04 +0100, Chris Hogg wrote:

On Wed, 18 May 2016 09:13:12 +0100 (BST), "Dave Liquorice"
wrote:

On 16 May 2016 21:31:51 GMT, Huge wrote:

What's even more amazing is that with the press of a button
marked
"MAXGEN"[1], it can go from zero to full output in 15 seconds.

I think that's from in sync spinning in air, rather than
stationary.

They keep one turbine spinning all the time in order to provide
"instant" output. I think it's rather less that 15 seconds, too.

The information about the various start times seems to have disappeared
from the web. A turbine on line and using water I should imagine can go
from "tickover" to full chat as fast as the inlet valve can open. As you
say I'd expect rather less than 15 seconds.

The 10 seconds figure I mentioned related to the time it took to open or
close the giant penstock gate valve(s) feeding the turbines.


I wonder how many tonnes of water per second each turbine uses at
maximum output?

At maximum flow to the turbine hall is 60 cu.m/sec. There are 6
turbines, so 10 cu.m/sec. i.e. 10 tonnes/sec. each
http://tinyurl.com/4nj8a9

I get the impression that they can keep them spinning with compressed
air, which reduces the time to full output compared with a standing
start. But whether all of them spin that way, all of the time, I don't
know.

They don't use compressed air to keep them spinning whenever they need
them in 'Hot Standby' running in air. The turbines use the generator as a
motor to maintain synchronous speed so all that is required to change
from motoring mode to generator mode is basically just a matter of
"Turning on the tap" and adjusting the excitation current to raise the
stator output voltage.

This rather neatly avoids the need to synchronise from a standing start
but at a cost of 4MW per turbine. I'm afraid I can't recall whether all
six turbine sets had this hot standby running in air capability or not.
One thing is for certain, the 90MW turbine sets at Ffestiniog had no such
hot standby feature, hence their much longer 60 seconds run up time.

Pretty much any directly coupled AC generator generator is a motor and
will normally sit there happily taking mains power to spin, until
something causes it to try and spin faster when its internal phase
advances and it starts to push the current along as a true generator.

However in grid terms 60 seconds is 'instant' anyway. There is enough
energy storage in all those spinning rotors on the grid* to cope with
fluctuations in demand on that sort of timescale.

*'intermittent renewable' energy excepted. The electronic converters
have no sort of storage in them such as that afforded by huge spinning
turbines.

Years ago I was on board an aircraft used as a research tug by Decca
radar. There was this whining noise in the cabin... 'what's that?'
'that's the rotary converter to generate mains voltage from the
batteries' 'Ugh! why not have a (then newfangled) transistorised
inverter!?' 'because that rotary convertor can survive the 50% voltage
drop we get when pulling up the undercarriage, for 30 seconds'



--
The biggest threat to humanity comes from socialism, which has utterly
diverted our attention away from what really matters to our existential
survival, to indulging in navel gazing and faux moral investigations
into what the world ought to be, whilst we fail utterly to deal with
what it actually is.