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System Strength Reference Paper

Following the success of a system strength workshop, Study Committee C4 has produced a Reference Paper which was published in the Science and Engineering Journal in February 2021 (Volume 20).  The intent of the reference paper is to promote a common understanding of definitions, problem statements, appropriate solutions, and complementary perspectives relating to system strength.  The authors include a strong Australian contingent led by Babak Badrzadeh and supported by Sachin Goyal, Sorrell Grogan, Andrew Halley, Alan Louis, Tony Morton and Stephen Sproul.

The global power industry is experiencing an unprecedented change, primarily stemming from the increased uptake of renewable inverter-based resources or IBR’s. New IBR’s such as wind and solar farms are frequently situated in remote locations, typically associated with providing limited network capacity and often some distance from traditional synchronous generation. Even if they are electrically near, the traditional generation may not be dispatched due to market forces.  In such situations, maintaining sufficient ‘system strength’ becomes a key challenge and can result in unsatisfactory IBR performance, undesirable plant interactions and power quality issues if not properly managed. System strength is already an issue in many parts of the Australian National Electricity Market (NEM) and is also presenting challenges overseas. 

The paper discusses what is meant by system strength and what the key issues to be considered are. This is critical to ensure that the power system can be planned and operated securely and reliably despite rapid changes in the generation mix and the resulting changes in power system dynamics. Several simulation results and practical examples are presented, demonstrating how a lack of system strength can impact on power system stability, power quality and power system protection.

Some of the weakest and most vulnerable power systems are islanded or can be islanded following credible contingencies.  Examples within Australia include South Australia, Tasmania and areas of Western Australia.  In these cases, low system strength operating conditions can give rise to voltage induced frequency variations due to the reduced system inertia and lower number of active voltage sources as IBR’s displace synchronous generators.  This is exacerbated when a fault results in the loss of a large synchronous generator.  Islanded systems provide leading examples of what may happen in larger systems as the penetration of IBR’s increases.

Specific considerations and challenges associated with power system operation, as well as short and long-term planning are discussed, and similarities and differences between these three time horizons are elaborated on.

Different network and non-network solutions to address the impact of low system strength conditions are overviewed. These include the use of synchronous condensers, advanced grid-following inverters, modifications to controllers on existing network assets, and application of virtual synchronous machines or similar concepts.

The global power industry needs to develop state-of-the-art assessment methods, as well as practical solutions, to address the complex and new power system challenges presented by the rapid technology transition already well underway. The use of appropriate and fit for purpose modelling and real time monitoring tools is fundamental to assessing and addressing the various system strength challenges before they manifest during actual power system operation. Wide-area EMT modelling will be required for assessing most aspects of system strength where phasor-domain modelling may be unable to, for example, reliably predict controller interactions and associated instabilities.

Despite being a highly technical and complex engineering matter, the implications of declining system strength are not just limited to engineering analysis, but are also relevant to regulation (including technical standards and market design), power system economics (at both a system and individual project level) and environmental outcomes.  The latter becomes critical in the context of legislated clean energy targets which are an increasingly common requirement for many power systems to manage.

The reference paper can be downloaded for free from e-cigre