Re: Grand Apagon - Electricity (not) in Spain

On 1/05/2025 1:05 am, Liz Tuddenham wrote:

Bill Sloman <bill.sloman@ieee.org> wrote:
 

On 30/04/2025 8:41 pm, Carlos E.R. wrote:

On 2025-04-30 11:59, Liz Tuddenham wrote:

Bill Sloman <bill.sloman@ieee.org> wrote:
>

... pumped hydro storage has the spinning
turbines, but grid scale batteries have invereters, which can reacta lot
faster than any spinning turbine,

>
I thought the stabilising effect of a spinning turbine was because it
*didn't* react quickly.
>
The grid frequency begins to fall so energy from the moving parts is
converted to electrical power which is fed into the grid to increase.
the frequency.  This results in a loss of stored mechanical energy which
causes the turbine to begin slowing down - which is detected by the
control system and used to feed more water/gas/steam into the turbine so
its speed is returned to normal.

>
I understand that the turbine doesn't actually slow down, because the
generator starts working as a synchronous motor drawing energy from the
network instead; this is detected by the control system and feeds more
water/gas/steam, etc.

>
It doesn't slow down much, but there's no such thing as instantaneous
feedback - you have to an input change before you can start correcting
the output.
>

As long as the network keeps the frequency.

>
The "network" can't keep the frequency - it's the corrections that keep
the low term frequency stable
>

The interface between the stored mechanical energy and the electrical
energy demand has an almost instant response and is inherently stable
without needing elaborate control algorithms.

>
But the stored mechanical energy in the spinning rotor can only get fed
into the grid if the rotor slows down.
>
The generator has to have a control system to control the power being
feed into the rotor to keep it spinning at the same speed while more
energy is being extracted from it.
>
There's nothing magically stable about that kind of control system - it
has to be designed to stable like any other feedback mechanism.

 There are two mechanisms at work here:
 1)  The coupling between the rotating machine and the grid, which is
virtually instantaneous and extracts mechanical energy from the rotating
'store' without any special control system.  It is inherently stable.

Only in the sense that it doesn't do anything unpredictable.

2)  The coupling between the rotating machine and the 'prime mover'
power source, which puts mechanical energy into the rotating 'store'.
This is slower to respond and does need careful control to keep it
stable.

The point about inverter-based controls taking energy from grid-scale batteries (or feeding it into them) is that they can operate much faster. They can force the voltage at their connection to the grid to sinusoidal on a millisecond to millisecond basis.

The difference between these two shows up as a change in the speed of
rotation.

If the current output from the rotating machinery dominates the energy being feed into the grid, that will happen. If most of the energy is coming from solar cells through inverters that isn't a useful way of looking at what's going on.
--
Bil Sloman, Sydney