Aidan, the problem is synchronisation. In an ideal 3 phase network current is 30 degrees out of phase with voltage, which is a necessary function of the way that synchronous machines work. However, when a load is suddenly changed, whether increased or decreased, the current and voltage change their phase relationship. If it moves too far out of whack, the generation plant will stop working and if it isn't protected, it may fail catastrophically.

The protection systems are designed to stop that from happening, as well as to protect all the power conditioning equipment along the way, which includes inductors and capacitors.

The transmission lines are in some ways the least concerning part of the network.

To give you an example of the sort of thing involved, Dreamworld at the Gold Coast has a ride called the Tower of Terror or similar, which uses a linear induction motor to rapidly accelerate a train of cars to about 100km/h in around 2.5s. It has a maximum draw of about 7.5MW and is powered by its own 11kV supply. In order to prevent it from causing catastrophic substation failure Energex installed a huge capacitor bank at the local substation which cost Dreamworld several $million.

Imagine what several hundreds of MW of load taken out in a split second can do...

Craig, the weather bureau was quoted as stating that the wind was
about 85km/hour. That does not mean that the wind that pushed the
towers over was not higher as a local cell I think they call them.
The big blow in 1991 here just missed me by 200 yards and was almost
nothing at my home. So it can be quite common I think that in any storm
one area can get very high winds and other parts quite ordinary.

Australian reports are behind a paywall so not much point in quoting it.
I have just read that AEMO report and it seems clear that increasing
load caused the wind turbines to disconnect although it is possible
that one generator may have changed frequency.
Such an event would I think shut everything very quickly.
Does anyone know if the wind turbines are DC and use choppers to
produce AC ?
It is all very interesting and will I think keep us all arguing for ages!

Bazz, 85km/h is a very high wind speed for turbines, but it may be within the operating range of those in SA, I don't know.

The thing about wind turbines which is different to your roof at home, is that they can be shut down by feathering the blades, thereby minimising the power being delivered to the generators by the blades. If there was a storm approaching, this is standard procedure. I can only assume that there was some imperative overriding the judgement of the engineers in charge and the most likely one is financial: gas turbines cost a lot to run; coal power stations ditto. The operators might have been OK and been regarded as heroes within their firms if the wind hadn't also taken out the distribution network feeding some big loads.

The trick is to minimise the rate of change of both demand and supply.

Aiden said;
That makes a lot more sense, but why would the closure of Hazelwood make any difference?

At present SA relies on feed from Victoria.
They can only get that if Victoria has plenty of spare capacity.
Take Hazelwood out of the picture and the surplice is reduced significantly.
That must increase the risk of problems for SA.
It is all a matter of resilience.

Craig said;
In an ideal 3 phase network current is 30 degrees out of phase with voltage,

That comment staggered me, surely in a resistive load the current
MUST be in phase with the voltage. We go to all sorts of expenditure
with power factor correction to save a bit of money.
Like Dreamworld every fluorescent light has a capacitor installed to correct the power factor.
In a 3 phase system each phase is 120 deg different.
How does 30 deg into the picture ?

You're confusing DC with AC power, Bazz.

In all seriousness, without a comprehensive discussion of "real" vs "complex" power based on a good understanding of phasors, there is simply no point trying to discuss the technicalities.

Suffice to say that power distribution systems are a 3rd year subject in any electrical engineering course, simply because it takes that long to become competent with the maths required.

Craig said;
You're confusing DC with AC power, Bazz.

No I am not. A transmission line is a transmission line no matter what the frequency.
Presume a bank of resistors as the load. Why would the current not be
in phase with the voltage.
I think I have just realised what you are saying, but 30 deg surprises
me, I would have thought 60deg with in a 3 phase system.