Electricity reliability is a very broad notion. At its simplest it can be defined as “keeping the lights on”. However, this relatively simple definition provides little insight into its multifaceted nature. The concept of reliability needs to be unpacked if it is to be better understood and managed. Reliability in this context encompasses the ability of the value chain to deliver electricity to all connected users within acceptable standards and in the amounts desired. It possesses two fundamental dimensions.1

• system security, which refers to the capability of a power system using its existing resources to maintain reliable power supplies in the face of unexpected shocks and sudden disruptions in real time, such as the unanticipated loss of key generation or network components, loss of fuel, or rapid changes in demand
• adequacy, which refers to the capability of the power system using existing and new resources to meet changes in aggregate power requirements in the present and over time, through timely and flexible investment, operational and end-use responses.

These dimensions are interrelated. For instance, system security policies and practices help to establish the effective adequacy envelope of existing infrastructure in the present, while efficient, timely and well-located investment is needed to maintain power system adequacy and to provide the resources needed to maintain system security into the future. At the same time, access to reliable fuel supplies and efficient use of those supplies is required to ensure generation equipment operates reliably and predictably from a short-term power system security perspective, and to ensure that generation infrastructure is able to meet demand, and hence adequacy requirements, in the present and into the future.

Maintaining secure and stable power systems is fundamentally determined by the physical characteristics of electricity. In particular, unlimited cost-effective storage of electricity is not generally available, and electrical imbalances at any point can have immediate and severe repercussions for the quality and deliverability of electricity throughout a power system. As a result, supply and demand must be balanced in real time at every point across the whole power system to ensure reliable supply that meets defined voltage and frequency requirements. Balancing also needs to be done in near real time, where demand is largely inelastic2 and production is subject to various technical constraints that limit its deployment, such as ramp-up and ramp-down rates.

Maintaining power system security is further complicated by the dynamic nature of power flows which follow the path of least resistance determined by the constantly changing balance of generation and load. Key to success is the simultaneous balancing of electricity flows to maintain frequency and voltage subject to system stability limits and the thermal operating limits of the network infrastructure. Together these technical requirements establish the envelope within which all power systems must be operated to maintain reliable and secure power supplies. 

The unique properties of electricity combined with the technical requirements that have to be met to ensure stable and secure power flows make maintaining power system security a challenging balancing act that can be practically achieved only through centralised, or centrally co‑ordinated, system operation. System operation is generally undertaken by transmission and distribution network owners or independent system operators, with a degree of co‑ordination between them where integrated regional networks incorporate two or more control areas. System operators are also usually primarily responsible for executing emergency procedures to manage extreme events in a manner that minimises the impact on supply while protecting critical electricity infrastructure.

System operators have traditionally used a combination of resources to manage power system security including various forms of technical or operating reserves and services, redispatch, and load shedding. The specific products, or ancillary services, available to help manage system security are usually defined in terms of their function and the time taken to deploy them. Key functions include frequency control, network control and provision for restoration of services following a blackout, usually referred to as black start services.

System operators would typically deploy operating reserves or employ redispatch before load shedding. Load shedding is usually treated as a last resort used to avoid catastrophic system failures. Products used to manage small routine imbalances and to react immediately to emergency events are deployed automatically in response to particular frequency, voltage or stability triggers, while other services tend to be deployed manually by system operators as required. The typical range of operating reserves available to most system operators is summarised in the section below.

System operators also procure black start services to facilitate system restoration following a blackout. These services are usually secured through bilateral contracts with generators, though system operators typically have the authority to requisition necessary services in an emergency.

Some system operators procure additional capacity reserves to help manage supply-demand balances and resource adequacy. This is a more common practice in energy-constrained power systems where the availability of generation is dependent on uncertain fuel supplies– for instance, power systems that are heavily reliant on hydroelectric production and hence are subject to periodic risk of electricity shortages due to drought.

In principle, demand response also has the potential to provide many of these services. System operators across IEA electricity systems are increasingly sourcing operating reserves from demand response providers. Large-scale loads provide the vast majority of these services at present. Considerable scope exists to expand the use of demand response to provide flexibility and system security services.

Some power system security events are of a nature, scale and duration that are beyond the capability of system operators to manage using their normal emergency management practices and resources. Hydro-dependent power systems can be particularly susceptible to these kinds of events due to their exposure to periodic water shortages. Such events are likely to be a recurring challenge in these systems, with implications for the way they prepare for and manage these kinds of emergency events. Some recent examples are identified in the table below.

Understanding the nature and likely duration of a power disruption is critical for deciding how best to respond. The nature of power disruptions can be broadly divided into two dimensions:

• Energy constrained, which occur when a power system lacks the energy required to generate sufficient electricity to meet demand. Hydro-dependent power systems are particularly susceptible to this kind of disruption as a result of exposure to periodic periods of drought.
• Capacity constrained, which occur when power system infrastructure is insufficient to meet peak demand. These kinds of disruptions can occur as a result of rapid and unanticipated growth in demand, infrastructure failures, or a combination of both.

The expected duration of a disruption will also affect the types of measures that can be deployed. For instance, during a relatively short duration sustained emergency event, which may run for a period of days up to a few weeks, supply responses are generally limited to extracting more from the existing infrastructure, while demand restraint typically relies on rationing, voluntary savings and possibly scarcity pricing. More supply and demand options become available to address medium-duration events, which may run for up to six months, including some minor capital replacement (e.g. energy-efficient lighting), a wider range of pricing incentives and some limited fuel switching. While a wide range of supply and demand options become available to help tackle medium- and long-duration disruptions, including limited deployment of new infrastructure and the full range of pricing and regulatory incentives.6

An effective response will incorporate an appropriate mix of supply and demand restraint measures. The appropriate combination of measures will depend on the circumstances, which will reflect the nature and expected duration of the event. International experience suggests that responses will also reflect the feasibility of implementing particular measures and the legal, regulatory and market arrangements governing sector operations.

International experience also suggests some key principles for developing and implementing effective responses to sustained emergency events including:

• Analyse the event. Begin by developing a clear understanding of the causes, nature and likely duration of the event, including insight into how it is likely to affect generation capacity and response, network operation, and consumption patterns including potential for savings by customer class. Initial analysis provides a critical foundation for developing and implementing an effective response.
• Identify and assess potential responses. Effective responses generally incorporate a few key supply, demand and regulatory initiatives, which can be implemented quickly and easily, have the potential to significantly alleviate the “crisis”, are cost-effective and are likely to be socially acceptable. Develop an integrated programme incorporating the most practical, timely and cost-effective regulatory, supply-side and demand-side responses.
• Adopt a timely and effective implementation strategy. International experience shows that implementation has been best managed in jurisdictions that have prepared detailed emergency response plans in advance. Plans need to provide clarity around the roles and responsibilities of all the key stakeholders involved in implementation. They need to ensure that effective co‑ordination and communication is maintained during the event to ensure an adaptable, timely and effective response. They also need to identify key information required to guide implementation and ensure that data collection and analysis capabilities are in place and ready to deploy when required.
• Assess effectiveness and draw lessons. Review the key outcomes and learnings from the event and the event response, and incorporate those lessons into preparations for the next event as part of an ongoing cycle of continual improvement and preparedness. Leading practice jurisdictions also seek to reinforce learning and preparedness through regular emergency response exercises.