On Wed, 18 May 2016 20:39:03 +0000, Huge wrote:

On 2016-05-18, 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 spinning reserve turbine is spun with electricity, not water.

Indeed, this is so and consumes 4MW from the grid just to keep the
turbine running in air synchronised with the grid so that it is only a
trivial matter of "Opening the Tap" and increasing excitation to reverse
the energy flow from 'motoring' to 'generating'.

The ten seconds time is that required to operate the penstock valve(s)
but I believe it takes another two seconds for the turbine's automatic
governor control to fine tune and stabilise for the generator loading.

The only thing I can't recall is whether this "Hot Standby" mode
(spinning the turbine in dry air) applied only to one of the 6 turbine/
gensets or whether to more than just the one or, indeed, all 6 gensets.

This "Hot Standby" mode is fairly trivial to implement[1] so I'd be
surprised if at least one other genset didn't also have this feature if
only to cover for routine maintenance on the "Primary Hot Standby" genset.
Even if they never intended to run more than any one genset in hot
standby (4MW running cost), including this feature, I would imagine, on
*all* 6 turbines could provide useful diversity of plant usage.

[1] Basically, it's simply a matter of leaving the genset connected to
the grid after being run up and synchronised then shutting off the water
to let the generator motor on as any such generator would do in the
absence of mechanical drive from its prime mover whether steam or gas
turbine or, in this case, high pressure water jets.

The Francis turbine in this case remains dry in order to minimise
unwanted drag (even so, it still needs energy input from the grid at a
work rate of 4MW). The only extra complication that may arise out of such
a motoring mode might be a cooling issue due to the lack of water flow
within the turbine itself.

--
Johnny B Good