As has been widely reported, (see here for additional details) an equipment failure and subsequent spot fires in the Torrens Island B switchyard caused the disconnection of three of Torrens Island Power Station’s four ‘B’ units, each of 200 MW capacity, last Friday 3 March. Prior to the incident the three tripped units were producing just over 400 MW, around 17% of the South Australia’s total electricity demand. In addition, the same incident somehow seems to have caused the near simultaneous tripping of the nearby Pelican Point Power Station which was producing about 220 MW.
Referring back to AEMO’s investigation of the SA Black System incident on 28 September last year, the proximate cause identified for the system collapse on that day was the loss of 456 MW of windfarm generation following a series of faults and voltage dips caused by severe storm damage to the transmission system.
This raises the obvious question of why the system was able to withstand the loss of significantly more generation on Friday last week, apparently without any widespread load shedding, than the loss which triggered a complete system shutdown last September.
Even Friday’s near-miss has already led to fairly predictable but unhelpful finger-pointing and political posturing. A quick search on Twitter will find you some.
In this post I’ll look a bit more closely at the sequence of events that is discernible from AEMO’s actual market data, remembering that full investigation into causes and the forensic detail of what happened is ongoing.
Immediately before the event
Using ez2view’s Time Travel functionality to view the state of the South Australian region immediately before the event (this SA Schematic snapshot from Dispatch Interval (DI) ending 15:05 AEST shows generator outputs at the beginning of the 5 minute interval, ie at 15:00 AEST or 3:30pm Adelaide daylight saving time):
we can see that four Torrens Island B units were running – the four filled circles underneath the station name above – each producing just under 140 MW. Pelican Point’s output of 218 MW is shown just below.
In summary, the aggregate supply mix in SA at this time was as follows:
A few points about these numbers for those who are looking closely:
- Wind includes both semi-scheduled and non-scheduled generators;
- Imports are based on actual metered interconnector flows at 15:00, not the end-of-period dispatch targets for 15:05 shown against Heywood (V-SA) and Murraylink (V-S-MNSP1) in the ez2view screenshot above;
- Solar PV (est) is based on AEMO’s after-the-fact half hourly estimates of small scale rooftop PV production – I’ve averaged the values given by AEMO for the two intervals 14:30-15:00 and 15:00-15:30;
- For all these reasons the total supply number shown differs from AEMO’s ‘Scheduled Demand’ value for DI 15:05, but gives a better reflection of total actual supply just before the incident.
5 minutes later
Stepping forward to DI 15:10, the 5 minutes commencing at 15:05 – just two minutes after the incident time of 15:03 reported in AEMO’s Market Notice 57817 as shown above, the SA Schematic view shows this:
We see, in comparison with the previous schematic view, that:
- Two Torrens Island ‘B’ units have tripped – the last two empty circles below the station name, ie units B3 and B4, and station output has reduced by 290 MW;
- Pelican Point output has fallen to 19 MW from its previous 218 MW;
- despite this loss of nearly 500 MW of generation output, we can’t see obvious material changes in the other generator outputs or interconnector dispatch targets that are nearly large enough to compensate – what’s going on??
Another 5 minutes later
Hold that last question while we step forward one more 5 minute Dispatch Interval to show the situation at 15:10, the start time of DI ending 15:15
Now a third Torrens Island unit, B2, has dropped off, reducing station output a further 108 MW (net) leaving only one online unit, and Pelican Point output is zero.
We can also see higher interconnector targets than before, but for those doing the sums in their heads, not nearly high enough to offset the cumulative net loss of 621 MW of generation from Torrens Island and Pelican Point in the few minutes since the incident at 15:03. What’s the cause of this apparent discrepancy?
Part of the reason is that actual interconnector flows can and do differ significantly from AEMO’s dispatch targets. Often it’s the interconnectors – particularly non-controllable AC interconnectors like Heywood – that take up any differences between what AEMO schedules to occur in a NEM region over every 5 minute Dispatch Interval, and what actually happens. AEMO schedules on the basis of its 5 minute ahead demand forecast, calculates end-of-interval dispatch targets for generators according to current output levels, bids, constraints etc then electronically sends those dispatch instructions to generators’ control systems. As second-to-second variations in demand and generation output occur in the actual power system, a variety of mechanisms – principally ancillary services provided by certain generators – are used to maintain real time supply-demand balance. This can and does result in significant variations in actual outcomes from what AEMO’s schedule assumes will happen.
That’s why in the earlier supply mix table for 15:00 I used actual interconnector flows not targets – I’ve updated that table below to show how the mix – and total supply level – changed between 15:00 and 15:10. I’ve also broken out imports across the two separate interconnectors:
(The data for these tables are drawn from AEMO’s published market data, but can require a bit of extra manipulation to extract in comparison with standard views in many tools like ez2view)
‘Missing demand’ and very high interconnector flows may have “saved” the system
What first jumps out from the table above is that in the same interval that the first two Torrens Island B units tripped and Pelican Point lost most of its output – effectively somewhere between 15:03 and 15:05 – demand in South Australia must also have dropped by ~300 MW. Supply and demand must remain in real time balance in any electricity system, so absent gross measurement errors – and there is no obvious sign of these – the supply reduction of 307 MW shown at 15:05 must have corresponded with a matching reduction in power demand.
There have been no reports of general load shedding associated with the incident, and I have contacted both AEMO and SA Power Networks seeking an explanation – the latter has confirmed that there was no distribution system level general load shedding – neither directed (eg rolling blackouts like those initiated on February 8) nor automated (eg Under Frequency Load Shedding protection).
Since the load reduction must have occurred very rapidly between 15:03 and 15:05, and well before there were any general calls for the public to conserve electricity later that afternoon, it cannot have been the result of conscious actions by energy users to reduce usage. It’s also apparent that it was a relatively short-term drop, with ~120 MW of load returning by 15:10 (followed by another 105 MW by 15:15, not shown above). I’m afraid the reasons for this remain a mystery at this early stage – if any readers have a convincing explanation I’d be very glad to hear it!
What’s also obvious at 15:05 and especially 15:10 (as load picked up and another TIPS B unit went offline) is the very large increase in imports on the Heywood interconnector necessary to maintain supply-demand balance in South Australia. The only other controllable thermal generation remaining online at the time was a single TIPS B unit, the Osborne cogeneration facility and the Ladbroke Grove gas turbines in the South East of the state. The latter two stations were essentially at full output already, while the TIPS B1 unit had only about 70 MW spare capacity, which took 20 minutes to ramp up.
Other dispatchable generation started coming online from about 15:10 (principally 60 MW of diesel-fuelled capacity at Port Stanvac followed slightly later by gas turbines at Dry Creek and Quarantine), but in the immediate aftermath of the Torrens Island B and Pelican Point trips it was interconnector flows increasing to (at least) 861 MW on Heywood and by much smaller amounts on Murraylink that balanced the system.
That level of import across the Heywood interconnector is well outside its normal secure operating limits of 600-680 MW, and while those limits provide for short term excursions to higher levels than those to cater for defined contingencies, the level shown above is uncomfortably close to the transfer level of 890 MW reached just before separation from Victoria last September, quoted in AEMO’s report on the SA Black System event (see p45).
I should stress here that the detailed nature and sequence of events on the two days in question is very different, and especially that the Black System report makes it clear that it was not the absolute level of power flow itself across Heywood on 28 September 2016, but a more complex loss of synchronism / transient instability issue that (correctly) triggered separation of SA from Victoria and the consequent system black in SA.
However it remains true that on both days flows on Heywood were pushed well beyond secure operating limits, which are generally set to provide headroom for only “credible” contingencies such as the loss of the largest single generating unit in a region. Losing three Torrens Island B units and Pelican Point in quick succession is well beyond the scope of such credible contingencies.
I have no doubt at all that AEMO, Electranet and many others will be looking very closely at the root causes of this incident, particularly the reasons for the near-simultaneous trip of Pelican Point, its impacts, and its implications for system security in South Australia.
If the apparent loss of 300 MW of system load at around the same time is confirmed (and I can see no reason why it won’t), its nature and underlying cause will also be fascinating to understand – without that near-immediate relief on total supply requirements for South Australia, it is very hard to see how a trip of the Heywood interconnector (which would then have been attempting to transfer well over 1000 MW into SA) would not have occurred.
That in turn would have led with absolute certainty to another system black event, and heavens knows what next-day headlines and political witch-hunts to follow.