Lessons in resilience from the Australian National Electricity Market

by David Swift
Australian Panel – Study Committee C5

The South Australian region of the NEM has half its supply on average coming from wind and solar and has reached up to 142% of its demand coming these sources. On Wednesday, 28 September 2016, the South Australian region of the NEM suffered a state-wide power outage (referred to as the ‘SA region Black System event’) which affected approximately 800,000 customers and had a total estimated cost to South Australian business of $367 million. The event received worldwide media coverage, sparking debate over the high proportion of renewable energy in South Australia and its purported impact on reliability and security of supply. The Australian Energy Market Operator (AEMO) has undertaken an extensive investigation of the event ^[1], which will not be repeated in this paper. The event and related issues were the subject of several reviews and are currently the subject of litigation. They were also discussed in the Large Disturbance Workshops at the 2016 and 2018 CIGRE Sessions ^[2]. A number of learnings arose from these reviews, which warrant careful consideration. Many of the lessons go beyond the direct issues associated with the Black System.

The Black System event was triggered by:

“... tornadoes with wind speeds in the range of 190–260 km/h occurred in areas of South Australia. Two tornadoes almost simultaneously damaged a single circuit 275 kilovolt (kV) transmission line and a double circuit 275 kV transmission line, some 170 km apart. The damage to these three transmission lines caused them to trip and a sequence of faults in quick succession resulted in six voltage dips on the SA grid over a two-minute period….” [AEMO report1]

The NEM is based on security constrained, optimal dispatch through an energy-only market. Generation in the NEM is dispatched every 5 minutes and is constrained by several thousand equations representing network and other operating limitations to ensure the power system is continuously maintained in a secure operating state. A range of other measures are also in place to support system security including requirements on generators and network service providers to meet technical standards and arrangements to procure ancillary services. All market participants are required to contribute to ensuring system security while AEMO has overall responsibility.

The system security requirements are, however, focussed on ensuring the system is capable of quickly recovering from a ‘credible event’. The NEM rules define the loss of a single circuit line or of one circuit within a double circuit line as credible, but no more severe line losses. The South Australian Black System event was notable in that South Australia has a Mediterranean climate and tornadoes are an extreme weather event. The resulting, almost simultaneous loss of three separate transmission circuits was an extreme power system event as was the subsequent sequence of multiple voltage disturbances.

Wind turbines in the South Australian region successfully rode through the first few grid disturbances but as the number of disturbances grew, nine windfarms reduced their output as turbine protection systems took over. Overall there was a reduction of 456 megawatts (MW) in South Australia in seconds. This caused a significant increase in power flow across the interconnector causing it to trip. Once separated (“islanded”) from the rest of the NEM, frequency collapse and the consequent Black System was inevitable. The observed behaviour of the windfarms implies that they were capable of withstanding a credible contingency. However, the circumstances on the day were more severe than those considered credible and hence beyond that defined as a system security event. Rather, it showed a lack of resilience of the power system to more severe events. The resilience of modern power systems with an emerging mix of renewable, asynchronous, and distributed generation is an issue of international importance.

CIGRE Working Group C4.47 defines power system resilience as follows:

Power system resilience is the ability to limit the extent, severity, and duration of system degradation following an extreme event.
Power system resilience is achieved through a set of key actionable measures to be taken before, during, and after extreme events…

In Australia, resilience is under review and the Australian Energy Market Commission has stated that “In a general sense, the ability of the power system to avoid, survive, and recover from high impact, low probability events (HILPs) can be described as the “resilience” of the power system.”

The Black System event marked a turning point in Australia with reviews of the Black System by the AEMC and the Australian Energy Regulator (AER) in addition to AEMO. Government also commissioned a major review ^[3] by Australia’s Chief Scientist, Dr Alan Finkel. This review made a number of recommendations outlining a blueprint for the future development of the NEM and proposing the formation of a new body, the Energy Security Board (ESB), to drive its implementation. The event and its detailed investigation saw a significant enhancement to AEMO’s power system modelling and provided a range of learnings — both in respect to the specific event and the dynamic behaviour of the power system.

The changing supply mix has challenged security, taking the system into regimes where there is little or no international operating experience. This was seen to require a change in the modelling approach. While AEMO continues to use PSS®E software, it now also uses PSCAD™/EMTDC™ for electro-mechanical transient studies. This has been found to be necessary to adequately emulate dynamic system behaviour with high penetrations of inverter connected plant and has led to a better understanding of the role of system strength in security and resilience.

Since the event, the technical standards for connection of new generating plant have changed and the ability of these generators to ride through multiple faults have significantly improved. That step alone deals with the primary cause of the incident, but much more has been done as an outcome of the experience.

The requirement for the system to be able to survive a credible event has been extended to include ‘Protected Events’. Protected Events are HILPs which have been determined to warrant action to mitigate the risks those events present. Those actions will generally be operational measures and emergency frequency control schemes rather than major infrastructure investment. The first declared protected event was the potential loss of the double circuit line into South Australia which is the AC interconnector to the rest of the NEM.

Arrangements have been put in place to ensure adequate synchronous inertia and system strength is available to the system as and where needed. While these are interim measures, they have led to the procurement of synchronous condensers for South Australia to greatly improve system strength. The synchronous condensers are fitted with flywheels and will also provide important inertia to the system when they are implemented later in 2020. The assessment of system strength is now an important consideration in the connection of new generation to the NEM and connecting parties in some areas have been required to include synchronous condensers as part of their generation connection.

The changed generation mix has resulted in poorer frequency control. Some action has, and is being taken, to remedy the situation and a major review is underway to provide longer term solutions to ensuring frequency standards can be maintained across the full range of operating conditions. Non-conventional sources of frequency control services are expected to be important in achieving that in the future. The world’s largest Lithium Ion battery, the Hornsdale Power Reserve Battery Energy Storage System (HPR), was installed in South Australia late in 2017. It has demonstrated very good frequency control capability both for regulating and contingency services ^[4]. The HPR is now being increased in size and upgraded with Tesla’s Virtual Machine Mode, which seeks to emulate the conventional inertia services. Trials have also shown that frequency control services can be provided by renewable energy generators. The Hornsdale windfarm, co-located with the HPR, is a Class 4 windfarm which has demonstrated that it can provide six of the eight frequency control ancillary services currently defined in the NEM ^[5].

Further work is addressing the need for improved primary frequency control and considering requiring generators who are capable of frequency control, to provide that capability if the system frequency exceeds a normal operating bound. The services procured under the current arrangements have been increased and work is continuing on designing enhanced market and regulatory arrangements to procure the range of system services required.

Some areas of the NEM have had a significant capacity of renewable energy generation connect because they are areas with good wind or solar resources. These areas are often different from the areas where conventional coal fired generation is located. This has led to changes in the direction of power flow on parts of the grid which has exposed the need for additional voltage control resources. This is occurring at both the transmission and distribution level. Action is being taken to address these issues.

The challenges of maintaining security and resilience in a power system that includes increased amounts of non-synchronous and inverter-connected plant is an international issue being actively addressed by academics and manufacturers. The new generation mix has different characteristics to conventional plant, and uses active control systems, or complex software, to ride through disturbances. This is leading to reduced resilience to disturbances and lower inertial synchronous strength. There are risks that the detect-respond action in response to specified disturbances may not operate appropriately in the full range of real-world disturbances the plant may experience. Those risks are exacerbated when this plant is operating in areas of the grid with weak system strength.

The NEM has seen strong growth of distributed solar PV generation for some time, and now of battery storage. There are now 1.8 million individual systems in the NEM with an effective total capacity of 8,140 MW. In South Australia, rooftop solar can produce over half of the total demand on some days. There is less information on the location and status of these resources to the system operator and currently no control capability. The distribution system can now be a source of security risk and recent disturbance events have shown their potential to exacerbate disturbances at the transmission level. The large number of small devices adding to an overall important proportion of supply can diversify risks. However, it can also lead to increased risks including exposure to type faults and breaches of cyber security and test the resilience of the system.

The rules for the NEM are currently being changed to provide a ‘sandbox’ or regulatory haven to make it easier for businesses to develop and trial innovative energy technologies and business models. “A regulatory sandbox is a framework within which participants can trial innovative concepts in the market under relaxed regulatory requirements at a smaller scale, on a time-limited basis, and with appropriate safeguards in place ^[6].” This is a welcome enhancement, which will provide the opportunity for the market arrangements to evolve to meet the challenges.

The Council of Australian Governments has charged the Energy Security Board to undertake a major review, the post 2025 market design project, to determine the future market design ^[7] which will be best able to meet the future challenges. A key criterion for the future development of the market arrangements will be that the market supports innovation which is considered critical to achieving security and resilience in the future at the lowest possible cost.

Summary

The Black System event in South Australia was seminal in the development of the Australian NEM. Major reviews and analysis since highlighted not just the cause of the specific event but the increased exposure of the system to severe events — or the resilience of the system — with a changing mix of generation technologies. It has shown that new modelling approaches and a focus on resilience is required to ensure the system continues to meet customer expectations. To minimise the cost to customers, this will require new technologies and potentially new market and regulatory arrangements.

Banner & Thumbnail : Photo by Pat Whelen on Unsplash (Melbourne, Australia)

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