SPINNING RESERVE CONCERNS
- This discussion from another list may be of interest.
From: Andrew Tuckey [mailto:tuckey@...]
Sent: Friday, July 06, 2001 5:23 AM
Subject: SPINNING RESERVE CONCERNS
A few comments below
> I think the real function of spinning reserve is to be a pro-activemonitor/stabiliser of grid frequency, which is inherently unstable and
influenced by dynamic load changes. Any spinning reserve ("wind turbine"?) must
therefore be programmed to increase its power output when the grid frequency
drifts slightly below 50 Hz, and conversely decrease its output when the
frequency drifts above 50 Hz.
>will actually perform the exact opposite function: for example a squall front
> Large wind farms forming say 20 per cent of a system's generating capacity
entering a compact wind farm would cause a big surge in generation with no
reference to the grid frequency, and unless matched by a similar increase in
system load, would inevitably tend to increase the system frequency. I regret to
say that this is really the opposite of spinning reserve, and a potentially
destabilising influence on the grid.
>inversely with reference to grid frequency? I have never heard of such a place,
> Does anyone know of a wind farm which modulates its output second-by-second
but perhaps we should start developing them. Just need a mighty big load dump
and some fancy electronics.
You are exactly right that spinning reserve is there to stiffen the grid by
being "a pro-active monitor/stabiliser of grid frequency." And yes, a wind
turbine can do the opposite. We have found that the problem is not when a
sudden squall hits the wind turbine, since modern wtg have good dynamic
maximum output power control (they can limit their output power very quickly
and accurately). The problem occurs when there is a lull in the wind and the
output power drops. Then the frequency of the system decreases since supplied
power < load power.
Therefore utilities often request (demand) that there is enough spinning
reserve in the system to cover all or 2/3 of the total wind power. This may
not be a problem in a large stiff grid, but in smaller grids, like minigrids
in Australia, it may mean that generators must run to supply spinning reserve
when there is no need for them; i.e. you can never turn them off, even if
there's enough wind, just in case the wind drops for 30 seconds. Further,
some rotating plant requires a minimum load to avoid excessive maintenance.
In this case, you can't use the wind power even if it's there.
What we are developing is spinning reserve to cover wind fluctuations. This
stiffens the (mini)grid, and allows enough storage for extra plant to come
online. So you can effectively turn off rotating plant when there is enough
wind, and you don't have to fear a blackout if there is a wind drop.
>It would make more sense to contract with distribution network owners tomodulate their bus voltage according to grid frequency, to provide a matched
number of MW of load control to offset the fluctuating generation of a local
wind farm. Do they make tap changers that respond quickly enough? Customers
would get a bit suspicious if their light globes always failed on windy nights!
I don't know about varying the voltage, but in some minigrids you do have the
freedom to do some load shedding so less spinning reserve is needed. This is
the ideal solution. We use this where we can.
Dr Andrew Tuckey
Renewable Energy R&D
Powercorp Pty Ltd
Ph: +61 8 8947 0933
Fax: +61 8 8947 0925
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