It looks like a natural disaster but brought on by the fragility of the South Australian power system caused by the size of the variations in wind power.
The failure is most likely to have been triggered by the violent fluctuations from the Snowtown wind farms (Figure 1 and 5). Shortly after 3 pm there was a loss of 200MW with a partial recovery some twenty minutes later of 100MW. The total wind farm supply for South Australia also shows these variations (Figure 2).
This would have put a shock to the system for frequency stability at 50 cycles per second. For most of the day the local gas fired generators were only supplying 100 MW (Figure 3) with the balance to match demand with supply coming from Victoria. But the local generators started to increase and vary their output with first a 150 MW loss at Snowtown just before mid-day and then 50 MW variations that followed. Shortly before 3 pm the Hallett wind farms lost and then recovered 70 MW in a 20 minute interval (Figure 4 and 6). This added to the final Snowtown wind farm 200 MW loss. This detail is shown in Figure 5.
So the system instability could trigger Victoria shutting off the link to South Australia and the blackout followed.
The physical network may not be very robust as can be seen in images of broken pylons. Transmission lines are expensive at $1 to $3 million per km. Each wind farm must be connected to deliver maximum power to the network even though its average performance might be only 33% of maximum so connection costs may have been held to a minimum..
The trouble was north of Adelaide so could it have been isolated with the remaining network continuing or was so much power coming from the north that a blackout would follow no matter what was done
Figure 1 Snowtown `1 2 3 150 km north of Adelaide
Figure 2 Total South Australian wind farm production
Figure 3 Total fossil fuel supply from gas turbines (and diesel?) generators
Figure 4Hallett 1 2 150 km north of Adelaide
Figure 5Snowtown wind farms Data source AEMO
Figure 6 Hallett wind farms Data source AEMO
Post script. AEMO somewhat corroborated this analysis at 9 a.m. on 5 September as follows
"The preliminary report explains how severe weather moved through South Australia on the afternoon of Wednesday 28 September 2016, with high winds, thunderstorms, lightning strikes, hail, and heavy rainfall. The weather resulted in multiple transmission system faults including, in the space of 12 seconds, the loss of three major 275 kV transmission lines north of Adelaide.
"Generation initially rode through the faults, but at 16:18hrs, following multiple faults in a short period, 315 MW of wind generation disconnected, affecting the region north of Adelaide. The uncontrolled reduction in generation increased the flow on the main Victorian interconnector (Heywood) to make up the deficit and resulted in the interconnector overloading.
"To avoid damage to the interconnector, the automatic-protection mechanism activated, tripping the interconnector and resulting in the remaining customer load and electricity generation in SA being lost. This automatic-protection operated in less than half a second at 16:18hrs and the event resulted in the SA regional electricity market being suspended."