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Mechanical security of overhead lines - containing cascading failures and mitigating their effects

Cascade in Poland TB 515

08/08/2012

TB N° 515 summarizes the most up-to-date knowledge gained by Industry on what causes cascading failures of overhead line supports.

This Technical Brochure is available on CIGRE online library "e-Cigre" (TB N° 515, October 2012)

Containing overhead line (OHL) cascades is a high priority for most utilities, especially those that have had to address major incidents in the past. This technical brochure was prepared to help line designers and planners to address this priority through a better understanding of line cascading phenomena. Such an understanding is essential to effective and sustainable cascade mitigation, which must be accounted for in the general context of power grid planning and operations.

Containing overhead line (OHL) cascades is a high priority for most utilities, especially those that have had to address major incidents in the past. This technical brochure was prepared to help line designers and planners to address this priority through a better understanding of line cascading phenomena. Such an understanding is essential to effective and sustainable cascade mitigation, which must be accounted for in the general context of power grid planning and operations.

 

This brochure summarises the most up-to-date industry knowledge on:

  • the causes of OHL support cascading failures,
  • how to increase the mechanical robustness and security of transmission lines.

The brochure also outlines cascade mitigation strategies and techniques that have proven successful. These strategies and techniques are based on scientific research and the experience of several Cigré utility members that have dealt with catastrophic cascading failures during major storms. 

Exceptional vs. accidental loads

To address the mechanical security of overhead lines, utilities must carefully consider the multiple loading conditions that may eventually trigger various types of failures.

 

First, it is important to understand the distinction between exceptional and accidental loads. Exceptional loads are related to extreme natural events to which we can assign a probability of occurrence. Accidental loads result from events that are not specifically accounted for in design; as a result, it is difficult to assign reliable values of probability to their occurrence.

 

From a line security perspective, both exceptional and accidental loads must be considered together. To this end, this brochure’s first chapter provides a list of exceptional loads and identifies the sources of accidental loads that may compromise the mechanical security of overhead lines.

Chapter one also defines specific terminology and classifies line cascades. To understand how cascades occur, it is useful to distinguish between those triggered by vertical, transverse and longitudinal effects. However, utilities must be sure to take all cascade types into account for effective cascade mitigation.

Lessons learned from major tower cascading failures

The brochure’s second chapter reviews the salient features of the most important cascading failures in OHL history. The emphasis is on the catastrophic line cascades that occurred in the last 20 years (summarised in Table 2.1). This review points to triggering events occurring systematically during extreme weather conditions. The potential damage caused by these large-scale weather events presents a strong incentive to intensify maintenance programmes.

 

Current understanding of dynamic line cascading

Chapters three and four present the current understanding of OHL response to accidental loads; they describe the structural dynamics of a cascading line section. This understanding has been greatly enhanced in the last 20 years, thanks to advances in OHL computational dynamic analysis. Careful numerical simulations make it possible to assess the cascading vulnerability of existing lines and to simulate the effects of various techniques or approaches to limit cascades. These simulations are also instrumental in deriving and validating simplified procedures that are more amenable to design practice.

 

Recent developments in OHL cascading mitigation

One critical aspect of line design is the application of effective failure containment measures for cascade prevention. Chapter five reviews several systems and strategies that have been developed and implemented in the last few decades. These have achieved varying degrees of success: unfortunately, OHL systems continue to fail during big storms. Particularly at risk are older lines constructed before strict longitudinal load requirements appeared in design guidelines, as aging makes component resistance more uncertain.

 

Security design criteria to prevent OHL cascades

Chapter six reviews the line security design criteria for new lines as stipulated in the following key documents:

  • IEC 60826
  • CENELEC EN 50341-1
  • ASCE Manual 74

A comprehensive review of the criteria used by all Cigré utility members was beyond the brochure’s scope. However, the document does include survey results that shed light on the particularities of several utilities and countries. Details about these surveys are provided in Appendices B and C of the brochure.

 

Framework for successful design to limit overhead line cascades

Chapter seven presents the general framework of an incremental line security design strategy. This strategy recommends matching the risk levels associated with potential cascade triggering events and the security levels corresponding to various known mitigation measures.

 

Conclusions and recommendations for future action

The global grid reliability approach is now the ruling design method for most electric utilities. In this global context, security measures are specifically encompassed within the analysis framework.

 

One concern for OHL design engineers is that most utilities’ global grid reliability models currently fail to account fully for lines’ mechanical security aspects. In fact, in the global grid context, what matters most is the availability of any OHL line section to transit electricity: if electric power can safely be delivered to clients by an alternative route when a specific section is unavailable, the consequences of its ‘failure’ are not as costly as when strategic ‘backbone’ routes are affected.

The global grid analysis framework does not yet allow for the introduction of different failure containment options for different OHL routes. As a result, OHL engineers experience difficulty in:

1) introducing innovative methods in their structural design

2) developing failure containment strategies that are both technically and cost effective. This can lead to the rejection of interesting options, often because of their costliness.

 

To yield sustainable line security measures, utilities must weigh the costs of preventive/mitigation approaches against the associated gains in reduced risk of service disruptions. For this to happen, experts in mechanical/structural line design must work together with grid reliability analysis experts. The Cigré study committee for the design, construction and operation of OHLs (Cigré SC B2) should lead the way in this collaborative effort.