Heat Waves & Soil Dry Out
A heat wave is a situation with daytime and night-time temperatures unusually high compared to average temperatures typical for that period and region and with a duration of several days. Urban heat waves have been increasing in frequency, intensity and duration – especially in the Mediterranean region. Higher ambient temperatures generally degrade performance of a power system and can cause blackouts, sometimes of extended duration. The last type of situation occurs when rainwater does not have time to be absorbed by soil, which remains in a drought condition. Soil dry out reduces its thermal conductivity and resulting heat exchange capability with underground cables. In these situations, even if their ampacity is not exceeded, cables heat up abnormally and degrade to failure of their insulation systems or, more likely, failure of their joints. Italy has about 200,000 km of underground cables of which 60% is insulated with ‘wet insulation’ that has a failure rate (excluding external causes) of 6 to 10 per 100 km per year. In case of failure, a cable (even if its age has exceeded design lifetime) is normally not replaced but rather repaired, adding n.2 joints as part of each repair.
Nearly 70% of internal cable failures (i.e. failures not caused by third parties) occur in joints or terminations. In order to investigate the effects of soil dry-out, RSE installed a series of cable samples over a length of 45 m, under different laydown conditions: the first 10 m (portion A) was laid directly in ground of 1 m width and under a concrete platform; the second 10 m (portion B) was laid in these same conditions but inside a cable pipe; the third 10 m (portion C) was buried in a cable pipe with no concrete platform; the fourth 14 m length (portion D) had no platform and no pipe. All portions were connected in series and rated current was circulated. Conductor temperature was monitored year round. Fig. 20 shows the temperature profile of the cable sample during a soil dry-out event lasting 21 days, i.e. from May 27 when terrain was wet due to persistent rain to June 16 when terrain was dry from 21 days of drought and high ambient temperatures. As can be seen in Fig. 20, heat exchange capability of wet soil is progressively hampered by drought, causing a maximum temperature increase of 26°C for the portion directly buried with no concrete topping, i.e. that section most exposed to changing soil conditions. Increased joint temperature caused by poor heat exchange capabilities is a relatively new threat to power systems and underlines vulnerabilities not previously considered. To avoid or limit contingencies linked with this threat, monitoring and diagnostic techniques are being researched. These are based on pick-up, analysis and interpretation of partial discharge patterns to predict incipient failure of cables and accessories. This will enable distribution utilities to identify weak points in their networks and take corrective action.
Extensive campaigns have been carried out in the field to identify the weakest lines and to subject them to occasional ‘controlled’ failures without load and with artificially increased dielectric stress to avoid stochastic failures under critical service conditions. Increased thermal resistivity induced by soil dry-out is also accompanied by increasing soil resistivity that may adversely influence grounding systems. Vulnerabilities associated with this type of threat are not only linked to safety of personnel from step and contact voltages but could also impact equipment conditions from direct and induced overvoltages, instrumentation errors and harmonic distortions. Worsening grounding conditions linked with soil dry-out can be significant and increases in grounding resistance of as much as 5-fold have been reported when going from ‘normal-wet’ to ‘dry-out’ conditions. This might require reconsidering some of the assumptions made during design of insulation and its coordination, such as protection level of surge arresters, risk of backflashover of line insulators, etc.
Reverse Environmental Constraints: Assessing & Limiting Impact of Power Networks on Local Environments
The electric network is a capillary structure that spreads across substantial territory and its presence can negatively impact local as well as global environments. Indeed, recent public concern about this across Europe has resulted in delays of from 10 to 20 years in the permitting and realization of new transmission infrastructure. Unfortunately, such delays are not acceptable in society’s transition towards a de-carbonized and de-centralized energy system where integration of a high share of renewables requires strong, reliable transmission networks. Visual impact and electromagnetic field generated by lines are among the most cited and debated impacts of overhead lines. Other aspects raised by opponents are hurting wildlife, noise, lower property values and damage to archaeological and other sites of interest.
With the goal of narrowing the divide between power project promoters and opponents, a recent European funded project, INSPIRE-Grid, identified factors that influence public resistance to new infrastructure and proposed methods to overcome these. The primary factors identified were the needs, wants, concerns, and expectations of local stakeholders and also the costs, benefits, risks, and implications of each specific project. Other influential factors identified were linked to the processes by which people learn about, discuss and participate in decisions about a proposed new project. To rationalize and document the needs of the different parties involved, a joint stakeholder map was developed that highlights sources of potential conflict as well as possible entry points for tailor-made solutions. Analysis of past and current project dynamics has confirmed the importance of stakeholder participation starting during the planning phase. A variety of means are used to promote such involvement over the entire process, from planning to construction. These start from information (e.g. at websites, in printed matter, through media, with field visits and open days), to consultation (e.g. round tables, interviews, surveys, public debate, panels) and finally to co-decisions (e.g. advisory committees, mediation, citizen juries and task forces). In fact, current processes often lack when it comes to addressing and demonstrating the need for power lines. They also fail in setting up structured participatory decision-making mechanisms to manage inputs from all the stakeholders. Another area where current processes have been found weak is in rational evaluation of technology alternatives or routes, e.g. overhead versus underground, HVDC versus HVAC. INSPIRE Grid has proposed a formal multi-criteria analysis (MCA) method to select the most suitable alternative in an integrated as well as participatory manner. MCA allows all stakeholders to structure the debate and facilitates participation and negotiation – especially by helping establish a climate of confidence and by providing common understanding of problems. Different methods can be used to describe and assess the possible alternatives considered.
For example, CIGRE C3.08 studied an Externalities Approach, consisting of both economic valuation and balance of environmental impact. They demonstrated that the economic value of property, land use and transmission losses can be rationally assessed since these relate to market prices. Landscape visual impact and biodiversity, on the other hand, are more difficult to quantify. A contingent valuation method can be used for visual impact, by involving the public and asking how much they would be willing to pay or to be compensated for the availability or lack thereof of a specific ‘contingent’ environmental service. In regard to biodiversity, in particular bird collision, there is no consensus on economic valuation methods. Application of MCA to real cases has shown its ability to create a reduced set of alternative power line routings by using stakeholder preferences in ranking effects and generating acceptance for the outcome when stakeholders agree on the ranking. However, MCA did not necessarily foster systematic stakeholder acceptance of outcomes. Public involvement in decision-making can take advantage of modern ICT applications. For example, a participative Web-GIS tool was developed by INSPIRE-Grid to enable stakeholders to interact with decision-makers and to express their opinions about possible alternatives or solutions. In particular, a tool was developed for sending comments and documentation related to a specific location and for indicating preferences about any landmarks to be protected. An interference indicator able to measure how much the different options interfere with points preferred by the public was also implemented and Fig. 21 shows an example of a web-GIS. Application to real projects has demonstrated its usefulness for stakeholders to communicate spatially related points of view with owners. However, the method showed limitations when it came to representativeness of the affected population in this type of information exchange. Therefore, there is need for further investigation, ideally with active involvement of those authorities that grant permits, to enhance robustness of the methods proposed in terms of perceived transparency and justice.
Indirect Environmental Impact of Grid Projects: Life Cycle Assessment (LCA)
Life Cycle Assessment (LCA) is a method to assess global environmental impact of products or services over the course of their entire lifetime. Workflow in conducting LCA studies has now become standardized and comprises the following phases: definition of goal and scope, life cycle inventory analysis (LCI), life cycle impact assessment (LCIA) and life cycle interpretation phase, together with related sensitivity analysis and validity checks. Conducting an LCA for power system components can be useful in identifying opportunities to:
1. improve environmental performance throughout their lifecycle;
2. inform and support stakeholders along the process of planning, construction and operation of networks; and
3. provide a set of prominent indicators to assess environmental performance.
The methodology considered in the ISO standard can be applied to all types of products and services and therefore needs specific guidelines to be applied to electrical systems and components. This is the goal of on-going research and RSE has proposed a series of guidelines applicable to LCA for power systems and components. For all systems considered, the various environmental impacts of a power project are due mainly to their indirect effect on electricity production. But impact can also vary depending on different scenarios considered for evolution of a power system. For example, impact due to building, maintenance and dismantling overhead line infrastructure is almost negligible on environmental categories such as climate change, ozone depletion or human toxicity and cancer effects, even though these are a source of resource depletion due to their use of galvanized steel.
Climate change and the various threats linked to it are more and more influencing design and operation of power systems. Traditional mechanical dimensioning criteria as well as insulation coordination approaches should therefore now be questioned in light of the ever-increasing level and frequency of exceptional stresses linked with extreme weather. If one considers the normal system adequacy approach, i.e. the N-1 criteria, lines in most power grids, including in Italy, would require refurbishment to increase robustness in the face of rising threat levels. On the other hand, adopting the resilience approach, where operation of the system in a partly degraded state is contemplated and where measures are in place to assure prompt recovery after failure, would limit need for such interventions. Rather, the focus would shift to operational planning with reliance on increasing observation, forecasting, prevention and reaction. To this aim, a suite of tools has been developed to evaluate risks linked to climate change, forecast events, anticipate failures and identify the best mitigation measures so that preventive and corrective measures can be in place when needed. Such tools, presented above, are linked to wet snow, intense storms, pollution events and droughts and must be looked at as part of a broad, more complete approach to resilience, including dynamic evaluation of the impact of single or cascaded contingencies. This will aid decision-making by network operators. Work is still underway to validate and integrate all these elements. Impact of an electrical system on the environment has also been addressed in light of web-GIS tools and resulting LCA to increase stakeholder participation in all phases of development and realization of new line projects.