In the recent years, millions of Americans across the country lost power at times when they needed it most. As the US power grid deals with an increase of heat waves, winter storms, and stronger hurricanes caused by climate change, these kinds of failures are happening more often, taking longer to fix, and harming more people. Power blackouts, which used to be mostly seasonal occurrences, now occur year-round. One of the most important fixes would be physically hardening the grid to make it a “resilient grid”, which means replacing infrastructure that’s vulnerable to extreme weather with stronger, more resilient upgrades.
Strengthening assets to combat extreme weather events is half the battle. Our approach also included using weather models to predict 1-in-100-year events for radial icing and wind loading on our system. Understanding where the highest winds and icing will allow us to focus our investments in those targeted areas as well as ensuring our design can account for the wind and radial ice cases.
A research program with the Massachusetts Institute of Technology with support of National Grid, was developed to understand changing load patterns (from expansion of renewables and electric cars) and the impacts of climate change (changes in the mean climate characteristics and extreme events) add to the challenges for grid companies to determine capital expenditures and operational changes to prevent outage. The output of this research produced a model highlighting areas of concern for 1-in-100-year events for wind speeds and radial ice loading on the transmission system to help aid in potential problematic areas.
National Grid’s resiliency improvement approach includes inspection and maintenance, updates in design standards, construction practices, material specifications, and restoration practices. These changes improved transmission & distribution performance during extreme weather events in several distinct ways by:
- reducing the number of customers experiencing outages,
- reducing the duration of outages when they are experienced by customers and
- mitigating the impact to customers during outages in the transmission and distribution system
Inspections and Maintenance:
Proactive I&M programs
In order to get ahead of a weakening system, proactive plans are place to inspect the transmission and distribution systems. Comprehensive helicopter inspection with using high-resolution camera to gather detailed information about transmission conductors, hardware, and structures. The results are evaluated and the need for maintenance or capital improvement are then determined. Infrared (IR) testing is completed to sense heat dissipation from sub-transmission and transmission lines. The inspections occur using ground-based cameras or helicopter-mounted cameras. Unmanned Aircraft Systems are used for areas that are sensitive to helicopter noise or only require a limited scope of inspection. This can allow for a quick response and provides high-resolution photos or IR images similar to the helicopter. Cyclical, ground-based foot patrols are also completed in order to verify integrity of the system and capture asset concerns that can impact reliability.
Non-destructive Conductor Testing
National Grid has implemented an inspection program using the LineVue device on energized overhead electrical conductors that contain a steel core such as an ACSR or ACSS conductor or steel shield wire. It is a non-destructive method of testing which can measure the remaining cross-sectional area of steel and detect the presence of any corrosion pits or broken steel wires in the conductor. The traditional destructive method of testing requires a planned outage to cut a physical sample which takes additional time and can impact load on the rest of the grid while introducing a weak point where sample was cut from. Non-destructive testing can allow for entire spans to be assessed and provide more detailed condition information than destructive testing. Preliminary results are available in real-time during the testing which significantly decreases the wait time for critical information. It also does not cause any weak points in the conductor.
Toughened glass disc insulators
Transmission has decided to standardize on toughened glass disc insulators and phase out the use porcelain disc insulators. Toughened glass offers an array of benefits including safety, environmental, reliability and long-term cost savings. Toughened glass disc insulators are functionally equivalent to porcelain – size, dimensions, electrical and mechanical properties are the same. The exception is the insulator shell which is made of toughened glass. The color of toughened glass is typically clear to a greenish hue. It is expected that toughened glass will be used to replace all porcelain-style insulators on the transmission system (gray and brown).
- Lighter weight than porcelain ± 15%
- Unlike porcelain, toughened glass is not susceptible to nearly invisible punctures
- Punctures of porcelain insulators can result in intermittent momentary line flashovers that can be very difficult to locate and fix
- Toughened glass insulators, instead of puncturing, will shatter into small (but not sharp) pieces which makes them easy to detect and replace. This ensures that electrical integrity of a line with toughened glass insulators is maintained
- Fracture shell maintains 80% tensile strength ± broken porcelain strength can be unpredictable
High-Temperature Low Sag conductor efforts
National Grid has installed hundreds of circuit miles of High-Temperature Low Sag (HTLS) conductors on the transmission system and standardized the use of steel poles for all transmission line voltages. HTLS conductors have properties such as high strength lightweight cores, high ampacity annealed aluminum that allow these conductors to run at higher temperatures and limited sag versus standard conductors resulting in increased ampacity. HTLS conductors come at an additional cost compared to standard conductors of equivalent ampacity ratings. However, the high temperature, low sag properties can greatly reduce overall project costs. Lighter-weight HTLS conductors can be used in place of heavier standard conductors greatly reducing the amount of structure reinforcements requires. While there are several types of HTLS technologies available on the market today, carbon core conductors represent the most advanced type in this category. Carbon cores are lightweight, high tension and extremely low in thermal expansion; properties that result in low sag. This, in turn, allows for more current carrying aluminum resulting in high ampacities ratings. A line with higher ampacity can help meet load demand during a weather-related N-1 event.
Transition from wood-to-steel in Transmission
National Grid standardized the use of steel poles for all transmission line voltages. Steel poles are at least 30% lighter than wood poles and require little maintenance, greatly reducing the costs associated with upkeep. Steel poles are more robust versus wood that decays over time, are susceptible to insect and woodpecker damage, and requires increased maintenance. Steel poles can be taller and carry heavier loads, permitting longer spans and requiring fewer poles. The strength capacity of wood is highly variable due to its organic nature. Fiber strength varies across wood species and within the same species due to differences in the growth environment, geometry of the tree and other natural variances. For these reasons, wood pole designs employ strength reduction factors of 0.65 or 0.75 as required by National Electric Safety Code (NESC). The initial strength capacity of steel is accurately established given the controlled manufacturing process. Therefore, the strength reduction factor is 1.0 as compared to wood poles. National Grid utilizes high-strength “Engineered” steel poles for voltage classes 230kV and greater and for transmission lines 115kV and below, “Light Duty” steel poles are utilized where applicable. They are designed to be equivalent strength replacements to wood poles.
Higher-class distribution pole applications
Resilient structures can be designed using stronger poles, cross arms, guys and anchors. National Grid has already taken to use stronger poles for new distribution line construction. As part of design and construction standard changes over the past decade, the typical wood pole used in new construction has increased in strength class from class 4 to class 3. Guys and anchors in the direction of the line could be used, at locations where they would not be used under current design practices, to provide additional strength to allow the structures to survive conductor failures caused by trees and limbs. National Grid Standards currently require the use of an H1 class wood pole for all recloser and switch installations for increased resiliency. H1 Class poles have a 5400 lb breaking strength compared to 3000 lbs for a class 3 pole.
Distribution of dead-end fiberglass cross arms
National Grid has adopted the practice of using fiberglass cross arms for dead-end construction. Compared to wood, Fiberglass Reinforced Polymer (FRP) does not rot or degrade over time. FRP cross arms are lighter, stronger, more reliable, and provide a much longer service life (at least 25% longer than wood). Fiberglass cross arms offer the lowest installed cost over time because two wood cross arms can be replaced with a single fiberglass dead-end cross arm or fiberglass tangent cross arm.
Spacer cable are a standard construction type already used extensively in our distribution system in treed areas because of its tolerance for casual contacts with trees and limbs. This smaller profile construction along with the messenger under tension at top of the pole and covered conductors help avoid tree-related interruptions.
Coastline construction standards
A “Coastline Area” is all areas within 1/2 mile of saltwater coastline and those areas beyond 1/2 mile from a saltwater coastline where experience shows that the area is susceptible to heavy salt spray. In the direct vicinity of the coastline, pollution is deposited onto insulators and equipment mainly by spray, wind and salt fog. A build-up of pollution over a longer term can also occur through a deposit of wind-borne particles, consisting of quick dissolving and inert components that vary depending on local soil characteristics. Natural cleaning of the insulators by rain is typically effective as the active component of the pollution consists mainly of fast-dissolving salts. Reference IEC Standard 60815-1. This standard may also be applied where experience shows that the area experiences heavy road salt contamination overhead construction in “Coastline Areas” is generally the same as in other areas with the exception of some changes in the materials used. For example, the use of 15kV Riser Arresters and 27kV cutouts allow extra creepage, while transformers and switchgear will use a stainless steel casing.
Types of Corrosion & Mitigation
Three overall approaches to Flood Resiliency that we consider are listed below. Based on the different field conditions we determine which of the three are appropriate to implement.
Long-term flood hardening measures:
Largest capital cost; 10- to 50-year service life; limited or no storm response needed; raising equipment and control enclosures is included along with permanent flood walls.
Flood Contingency Plan (FCP):
3- to 10-year service life; some limited storm response is needed to close temporary barriers (at gate entrances, etc.) and to activate pump systems. Minimization of, and preparation for, storm response is an essential component of the FCP.
Interim Measure Plan:
Least capital cost; hardening materials remain in storage until employed; essentially 100% storm response. These plans are in place until such time that the FCPs are complete.
Flood Hardening Guidelines
The critical elevation (coastal) is Base Flood Elevation (BFE) plus two feet, plus one foot or the 500-year BFE plus 1 foot. This is based upon ASCE 24 (Flood Resistant Design and Construction, ASCE Standard 24) and ISO NE guidelines. The critical elevation (inland) is BFE plus two feet or the 500-year BFE. This is based upon ASCE 24 guidelines. The plus two feet is derived from the importance factor of substations and the one foot is derived from a sea level rise over 30 years which has been determined to be the life cycle of the substation equipment.
Coastal flooding and sea water resiliency
National Grid has developed material and design criteria to address areas subject to flooding from rivers, lakes and sea waters. Also, areas where the effect of salt spray from coastal storms and heavy road salt contamination affect the integrity of pad-mounted electrical equipment. The following are recommendations when placing equipment in such areas:
- Increase the elevation above the flood plain of electrical equipment such as transformers and switches in areas subject to flooding. This is achieved by building foundations inside reinforced concrete walls provided with oil containment and handrails for fall protection. Check with the local building authority of the city or town since many locations are starting to adopt building regulations in areas subject to flooding.
- Incorporation of rust-resistant stainless steel cabinets in our designs for all submersible switches and transformers and some pad-mounted switches and transformers.
- Use of sealing foam inside conduits to prevent water intrusion in structures has been made a requirement for all manholes and equipment foundations.
- Use of dead front terminations inside underground switches and transformers, instead of live-front. All switches are designed with 200A and 600A pre-molded elbows and jacket seals that protect concentric neutrals. This helps avoid moisture intrusion in cable terminations, insulation and metallic shield breakdown that could cause outages.
- Use of Molded Vacuum Switches in underground systems to sectionalize and protect loads. These switches allow us to provide fusing off mainline underground circuits.
- Use of insulated copper crabs and flood-seal connectors in manholes and hand-holes resistant to corrosion and that prevent failure of connector. Most cable conductors in our underground systems are copper.
Conduit sealing for distribution UG areas
Where appropriate based on manhole ventilating techniques, water-stop conduit sealing for primary, secondary and spare conduits may be required. Sealing will prevent the flow of water, dirt and gases into the manhole. Sealing is recommended at block intersections to minimize the potential of an event spreading.