We don't know why South Australia's wind farms stopped working, so hold off on the blame game
Friday, Oct 7, 2016, 02:04 AM | Source: The Conversation
By Dylan McConnell
We don't know why South Australia's wind farms stopped working, so hold off on the blame gameDylan McConnell, University of Melbourne
Australia's energy ministers are meeting today in an emergency gathering following South Australia's recent state-wide blackout.
On Wednesday the Australian Energy Market Operator (AEMO) published a preliminary overview of the blackout (known in the jargon as a "black system" event). This document reported first observations based on data available to up to Monday this week.
Importantly, the report concluded that the "root cause is subject to further analysis being conducted", as further information becomes available. As the chief operating operator Mike Cleary made clear, "at this stage we cannot apportion blame".
What does the report say?
The report provides a detailed chronology of the events that occurred in South Australia and led up to the state-wide blackout, and the restart process. This includes details on the three transmission lines lost and changes in generation output.
Here's a shortened summary of the timeline:
Fault on first transmission line: one high-voltage line out of service
Fault on second transmission line: two high-voltage lines out of service
Loss of 123 megawatts of wind power
Fault on third transmission line: three high-voltage lines out of service
Loss of a further 192 megawatts of wind power
Flow on the Heywood interconnector (which links South Australia with Victoria) increases above limit
Heywood interconnector trips off
Torrens Island and other power stations trip off
Supply lost to South Australia.
The time between the second transmission being out of service and the loss of supply to South Australia was approximately eight seconds. A total of 315 megawatts of wind generation disconnected during the this period.
Frequency and Voltage
A unique characteristic of electricity systems is that supply and demand must match instantaneously in real time. If they do not match, the frequency and voltage of the system can deviate from an ideal value. In Australia, AEMO maintains the frequency at 50 hertz (Hz).
Deviations from this value (from sudden changes in supply or demand) can substantially damage equipment. Automatic protection equipment activates to "trip" generators offline.
As the report noted:
generating units are unable to operate (and are not required to do so) where frequency is below 47 Hz. With the frequency below 47 Hz, generating units subsequently tripped off line.
This is likely to be how Torrens Island gas plants and other plants were disconnected from the system, after South Australia was disconnected from Victoria.
Is this why the wind stopped generation?
The report also suggests that:
The magnitude of transmission faults due to weather in a short period of time, resulting in significant voltage dips and loss of load, resulted in system instability. This caused some generators to reduce output, increasing flow on remaining power system equipment, causing power system protection to operate to remove risk of damage.
However, the report makes it very clear that "insufficient analysis has presently been undertaken to determine if everything operated as designed during the event." Crucially, additional analysis is required to determine the reasons for the reduction in generation and observed voltage levels before any conclusions can be drawn.
At this stage, it is unclear if the wind farms tripped off because of voltage dips or other reasons. Other reasons could be related to the extreme storm events, such as a lightning strike. The answer at this stage is we don't know.
What we do know is that the loss of supply from the transmission line faults and the decrease in wind output caused the flow on the Heywood interconnector to increase to approximately 850–900 megawatt – above the interconnector's design limits. The system protection kicked in (in less than a second) to trip the Heywood interconnector, and the rest followed.
Is this scenario unique to wind?
A very similar situation in South Australia in 2005 saw a massive blackout when a similar fault caused a dramatic decrease in brown coal-fired in Port Augusta, with the Northern power station reducing output to 0 megawatt.
This created a similar surge in interconnector flows from Victoria, which ultimately resulted in disconnection from Victoria. Several other generators in South Australia also tripped off as result of this incident, and wide-spread blackout occurred.
The recent storm was was particularly extreme, with reports of 130,000 lightning strikes hitting the state in a matter of hours. The reality is that all parts of the power system are vulnerable to such extreme weather and lightning storms.
Indeed as noted in the AEMO report, one of gas turbines contracted to provide system restart service in South Australia was also affected by the recent storm.
What can the energy ministers do?
The federal energy minister Josh Frydenberg called an urgent meeting of Australia's energy ministers in light of the recent blackout.
The role of interconnectors to provide better links across the National Electricity Market as well as battery storage have been reported to be on the agenda of Friday's meeting.
Increasing transmission was also the focus of a recent energy council meeting, where energy ministers agreed to review of the regulatory test for investment for new transmission assets.
A formal review into the National Electricity Market will also be up for discussion. This may include a review of the National Electricity Objective, which forms the basis of energy policy decisions, and does not include environmental considerations, such as the need to reduce emissions.
Given the increase in moisture and heat expected from climate change, one way that the state and federal governments can help ensure there is not a repeat of blackout is by agreeing to significant emission reductions.
Update: this article was updated October 7 to clarify that it is not known that lightning caused the fault leading to South Australia's 2005 blackout.