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WG C2.25, TB 833 - Operating strategies and preparedness for system operational resilience

The need for resilience in power systems has become important in recent years due to two phenomena; increased frequency and duration of extreme natural events, and a shift in the generation mix towards more inverter based renewable energy resources. The latter has created challenges for power system operators, highlighting the need for resilience and secure operation. There are emerging risks that need to be understood, reviewed frequently, and addressed properly.

CIGRE WG C2.25 has produced Technical Brochure 833 on system operational resilience. Australian members of the WG were Dean Sharafi (who produced this article), Greg Hesse and Mark Miller. 

Operational resilience in the power system refers to timeframes near to real-time operation. It covers the resilience just prior to a High Impact Low Probability (HILP) event, and continues during and after the event until complete recovery is achieved. For example, better forecasting of events such as cyclones or bushfires improves the lead time for measures such as transmission line switching or keeping patrol teams ready with spare parts.

Another key aspect involves the operation and control strategies and risk mitigation preparedness that can support system operators to more effectively manage system disturbances, restore the disrupted power grid to a secure state and eventually to restore the system to its pre-disturbance state.

In engineering terms, resilience is understood as the intrinsic ability of a system to maintain or regain a dynamically stable state, which allows it to continue operations after a major disruption and/or in the presence of a continuous stress. The disruptions are high impact, low probability events that include extreme natural disasters, which have occurred with increased frequency in recent years, as well as manmade threats, such as cyber- or physical attacks.

WG C4.47 recently produced a reference paper which defined power system resilience as, “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 measures to be taken before, during, and after extreme events, such as: anticipation, preparation, absorption, sustainment of critical system operations, rapid recovery, adaptation, and application of lessons learnt.

During a recent bushfire in Western Australia in 2019-2020, the system operator adapted their operations to consider the trip of multiple lines as “credible” during fires which were burning close to or directly underneath these lines; whereas in normal operating conditions such a contingency is considered “non-credible”.

Similarly, after the September 2016 black-out of South Australia, AEMO, considered a new set of conditions in which separation of South Australia from the National Electricity Market (NEM) was considered credible. For those situations, operational practices changed, and the South Australian power system is now operated to continue secure operations when that state separates from the NEM. These operational changes include reducing the output of non-scheduled generation that is exported to Victoria and the dispatch of gas turbines in South Australia to ensure frequency stability.

Further development in power system resilience in Australia has led to the concept of “Indistinct” events, under which the system operator takes a range of different actions to ensure security of the power system in order to increase its resilience. The South Australian blackout highlighted a gap in the frameworks designed to manage the risks from contingency events. These frameworks only considered the failure of a single generating unit or network element as a contingency. Such events are distinct and definable. However, the changing power system risk and resilience profile is seeing an increase in risk from 'indistinct' events. These risks are associated with distributed events, which relate to multiple generation and network assets in an affected area due to a widespread disruptive event. During such events the system security risk does not emerge from failure of a single specific element, but it could arise from different plants which may involve a combination of network element, generation plants and even customer Distributed Energy Resources.

Other jurisdictions around the world have also adapted to the new and emerging risks and have introduced new operating philosophies to manage their power systems during these high-impact events. These risks primarily relate to stability of frequency and voltage, and adequacy of power system inertia and system strength. Frameworks are being developed to ensure power systems continue to be resilient and secure as they evolve and innovate.

Resiliency as a topic is still developing and sometimes is mistaken for reliability, but it will be better understood as a critical concept as power systems evolve to become cleaner and less dependent on conventional generation.

The TB is free for members and 180€ for non-members.