Designing for Reliability Versus Resilience

Projections on climate change confirm that power systems will be exposed to increasing threats of the type that jeopardize continuity and quality of supply. An example was the complete collapse of the transmission grid in Puerto Rico after a powerful hurricane last year. How best to respond to this continually evolving situation? Do entire electric systems need to be reinforced to deal with increased levels of stresses that were not considered during the design phase? Moreover, is the conventional process of system planning and operation, founded on the concept of reliability, still sufficient or must design philosophy be changed toward the broader concept of resilience?

There seems little doubt that the impact of climate change can dramatically influence design and operation of electrical networks. Mechanical dimensioning criteria and insulation coordination approaches adopted in the past are now brought more and more into question based on increasing level and frequency of environmental stresses linked with extreme weather. As example, if one considers the normal system adequacy approach based only on reliability, several lines in the Italian grid may require extensive refurbishment to increase their robustness towards new potential threat levels. On the other hand, adopting the resilience approach accepts operation of the system even in a partly degraded state. Then, measures to ensure prompt recovery in case of failure have to be contemplated. While physical interventions to the current system are only limited in this scenario, what is implied instead are changes in operations planning to require higher levels of observability, forecasting, prevention and response.

To this end, RSE, an Italian Organization dealing with Research on Electrical Systems, has developed tools to evaluate risk of climate change and to forecast events. Mitigation measures are identified that anticipate failures with the goal of setting up all preventive and corrective measures in a timely manner, as needed. These tools are linked with events such as wet-snow, storms, pollution, fires and drought – all conditions made more likely following climate change. These offer a complete approach to resilience, including dynamic evaluation of the impact of single and cascade contingencies as well as aids to decision-making by grid operators. Examples of such tools are shown below, indicating their impact within such a ‘resilience-oriented’ approach.

power system Designing for Reliability Versus Resilience transmission line
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Among the different threats that can affect overhead power lines in general and insulators in particular, one of the most critical is ‘wet snow’. A good example of this was a widespread regional blackout in northern Japan a decade ago due to heavy deposition of wet snow blowing in from the sea. Another was massive power disruptions in southeastern China due to heavy snowfall. Although these types of event cannot be prevented, efforts have been made to predict its occurrence, to limit resulting contingencies and to mitigate the consequences. It is also important in this case to prioritize all interventions to be made on the transmission system (e.g. installation of pendulum de-tuners, network meshing, dedicated reinforcement, load management, etc.). RSE has developed an integrated monitoring and alert system named WOLF (Wet snow Overload aLert and Forecasting). WOLF, developed over a GIS platform, provides wet snow forecasting over a 72 hour period, estimates associated mechanical loads and estimates the anti-icing current (i.e. level of current in conductors that would avoid formation of the sleeve through the heating effect caused by Joule losses). This supports grid operators in adopting pro-active mitigation strategies.

Violent and deep thermal atmospheric disturbances are now occurring more often than in the past. These generate intense localized storms, characterized by extreme lightning activity, whirlwinds and even tornados that directly or indirectly affect power systems. Once again, a resilience approach can be envisaged with the goal of reducing the consequences of any resulting contingencies. Here, a well-designed alert system can prove valuable in terms of preventing associated injuries while precise localization of such events can help reduce the time needed to dispatch rescue crews. In this regard, RSE has set up STAF (Storm Track Alert and Forecast), a ‘now-casting’ system based on Radar and MSG (Meteosat Second Generation) data that selects only severe thunderstorms, tracks them and then sends alert messages to users. STAF calculates the probability of damage from any thunderstorm in light of several parameters derived from radar and satellite observations (with special reference to the reflectivity of the cumulonimbus along a vertical axis).

Environmental pollution and its impact on the reliability of the electricity system insulation is a known threat for power engineers. Surface contamination from salt deposits or other pollutants can, under certain circumstances, lead to flashover of line and substations insulators and thus result in additional contingencies. Given the impact of climatic change and continued industrial development, a country’s pollution map is not static but needs to be adjusted over time. Here, RSE has been following evolution of the pollution map of Italy by systematic use of special diagnostic apparatus. The latest version of the AMICO (Artificially Moistened Insulator for Cleaning Organization) device is able to measure site pollution severity. Through systematic use of advanced air quality modelling to assess dust and pollution accumulation on insulator surfaces, AMICO also alerts system operators whenever meteorological conditions threaten pollution flashover.

Heat waves see unusually high daytime and nighttime temperatures compared to average for the period and region and can last several days. Urban heat waves are increasing in frequency, intensity and duration – especially in the Mediterranean region. That means forecasting them is now a necessity given that higher ambient temperatures generally degrade performance of power systems and can lead to blackouts. For example, worsening of grounding conditions linked with soil dry-out can be widespread and increase in grounding resistance of as much as 5 times between normal-wet and dry-out conditions have been reported. This could mean reconsidering assumptions made during past design of insulation and its coordination, such as protection level of surge arresters, risk of back-flashover of line insulators, etc.

power system Designing for Reliability Versus Resilience New pollution map of Italy
New pollution map of Italy.
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power system power system Designing for Reliability Versus Resilience Example of snow sleeve build up on medium voltage line in central Italy
Example of snow sleeve build up on medium voltage line in central Italy (January 2017).
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