There have been some recent reports in the media about the possibility that Basslink was operated too close to its physical limitations, so I’ve looked at the 5-minute MW flow data across Basslink on the weeks leading up to the outage on 20th December 2015, to see if they shed any light on the matter.
First of all here is a figure from a previous post, showing the daily minimum, mean and maximum number of MW generated in Tasmania since May 2015:
The typical demand cycle in Tasmania can be seen in the data from late December 2015, a period when Tasmania generated all its own electricity. In the months leading up to the loss of Basslink Tasmania was a major importer of electricity, but there were times when supply came close to meeting (or exceeding) demand, and one period just before the Basslink outage when supply greatly exceeded demand.
The following figure shows Basslink flow data for 1st to 21st December 2015 at 5-minute resolution:
Summing Tasmanian generation over all generators gives the local supply, and subtracting Basslink flow gives demand, both shown in the following figure:
On most days of the period shown above Tasmania maintained a fairly steady supply that was consistently around 450 MW short of demand, with that shortfall coming in via Basslink. In the early days of the month there were brief periods, usually at times of peak demand, when local supply came close to meeting demand. A possible explanation for these supply peaks was Tasmania avoiding spikes in mainland wholesale prices at times of mainland peak demand.
Something different happened at the times of the afternoon/evening peak demand on 15th – 19th December, when Tasmanian supply greatly exceeded its demand, and Basslink flow reversed, with Tasmania becoming a major exporter. A likely explanation for this was the attractive prices on offer during periods of extreme demand experienced on the Southern mainland during a summer heatwave, see the figure below for South Australian demand from a previous post:
Basslink was operated near its electrical limits in December 2015, but those limits were not obviously exceeded, though there were very rapid transitions between high import and high export.
Another comment to make is that at that time Hydro Tasmania was not too worried about the drought that Tasmania had suffered since September 2015, all available hydro generators were run at near maximum capacity to obtain those lucrative exports of electricity, maybe giving an indication of where the balance of power lay between the hydrologists and the economists. Maybe those in charge should have been less guided by wholesale prices and more guided by the rapid depletion of the already low major hydro reservoirs at the end of 2015: