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The Future of Reliability - WG C1.27

Electricity supply in most modern economies is considered to be an essential service and accountability for reliable supply has generally rested with network companies and a few large generators.  Consequently, definitions for reliability, security and adequacy have usually been expressed in terms of networks and large generation.

Power systems worldwide are going through a fundamental change. Increasing amounts of renewable generation with different characteristics to thermal generation are being connected. Intermittent generators such as wind farms and rooftop solar have much lower capacity factors than traditional fossil fuel generators. Small scale consumers are developing an increasing ability to generate, store and control their own electricity usage. In Australia, roof top solar systems are becoming the norm and consequently many houses are generating some of their own power.  With storage and load control, these customers may have the potential to become self sufficient or even trade with other similar customers. 

So, what does this mean for our definitions of reliability, adequacy and security? Working Group C1.27 has recently produced Technical Brochure 715, which looks at the future of reliability and the issues that power system designers and operators will need to address as these changes accelerate.  The Australian members on the working group were Graeme Ancell and Phil Southwell. 

To some degree, this Technical Brochure is unusual in that, of necessity, it has looked at likely future scenarios and trends as well as current developments in order to identify the changes needed to existing definitions that will remain current in the likely future power system that designers and operators will need to manage.  In doing this the working group looked at present levels of intermittent renewable generation around the world including small island systems that are targeting one hundred percent penetration.

Research indicated that, while many parts of the world are starting to look at the reliability impacts of these power system changes, modifications to planning and operating criteria were quite limited at this stage.

The Working Group recommended the following new definitions of reliability, adequacy and security: 

Reliability – a measure of the ability of a power system to deliver electricity to all points of consumption and receive electricity from all points of supply within accepted standards and in the amount desired. 

Adequacy – a measure of the ability of a power system to meet the electric power and energy requirements of its customers within acceptable technical limits, taking into account scheduled and unscheduled outages of system components. 


  • Power system includes all elements of the generation, transmission and distribution systems and customer facilities that supply or use power and energy, or provide ancillary services;
  • Customers include all parties that supply power and energy or ancillary services, as well as those who consume them;
  • Requirements of customers include their basic power and energy needs, and agreed use of customers’ ability to vary power supply, adjust demand and provide ancillary services;
  • Acceptable technical limits and scheduled and unscheduled outages are those specified in the applicable planning criteria and standards; and
  • System components include all elements of the supply, delivery and utilization systems regardless of ownership or control. 

Security – the ability of the power system to withstand disturbances. 


  • Power system is as defined for adequacy
  • Ability to withstand will vary depending on specific disturbances and applicable criteria or standards, and includes agreed use of customers’ ability to vary power supply, adjust demand and provide ancillary services; and
  • Disturbances include electric short circuits, unanticipated loss of system facilities, or other rapid changes such as in wind or solar generation. 

The most significant changes to the definitions relate to inclusion of customers and the whole power system from end to end as well as the recognition that individual customers are now able to do more to manage the impacts of power system unreliability. The working group noted that detailed changes to planning and operating criteria will be needed to align with these high level definitions and makes a number of recommendations for system planners and operators.


The Technical Brochure (TB) 715 can be viewed on e-CIGRE and is free to members.  The cost to non members is €150.