The devastating impacts of recent major hurricanes on electrical infrastructure in Puerto Rico have brought fresh attention to the value of hardening substations. In some cases, this involves reinforcing existing structures. In others, it requires designing new facilities to more rigorous standards.
Success depends on establishing the criteria to which those substations should be designed to withstand extreme event loading. It begins with understanding the loading criteria for different extreme events.
Hurricanes are of great concern in Puerto Rico, which is still recovering from damage inflicted by Hurricane Irma and Hurricane Maria in 2017. With a less redundant grid than the continental U.S., damage to electrical infrastructure has led to recurring blackouts. Much of the damaged infrastructure is currently being rebuilt; future work could add redundancy and hardening.
Tsunamis, which are typically of more concern in the Pacific Ocean, should not be discounted as a risk to Puerto Rico, which has experienced some significant seismic activity near its shore in recent years. Tsunamis also produce a flood-like event, but its effects are normally much greater compared to coastal flooding due to higher flow velocities.
Solutions by the Book
Engineers have a few resources at their disposal as they develop loading calculations for hardening substations. No single resource covers the topic completely, but taken together, several resources can help engineers frame an approach.
ASCE 113 focuses specifically on substation structural design, drawing from ASCE 7-05 edition with modifications. It was published in 2007. Although it provides guidelines for some extreme conditions, such as high winds and earthquakes, it does not specifically address the combination of challenges posed by hurricanes and tsunamis.
Because ASCE 113 does not contain provisions for flood loads, the ASCE 7-16 standard and ASCE 24 guide are commonly utilized to inform engineers on how to perform substation structural designs. The former is mainly dedicated to load development and defining the equations used in determining all contributions to the flood load effect. The latter offers more general information, including definitions and general design requirements. Used together, they help assemble a comprehensive understanding of the entire flood load effect.
Another useful tool for flood design is the Coastal Construction Manual, published in two volumes by the U.S. Federal Emergency Management Agency (FEMA P-55). The first volume provides high-level summaries of design requirements and best practices for hazard identification, siting decisions and more. The second contains detailed examples and design requirements for practical applications.
Identifying the Essential Parameters
Extreme winds and flooding are the two main contributors to the overall hurricane load effect. While the wind speeds in Category 1 hurricanes are generally below minimum specified values prescribed in modern building codes, the increasing intensity of higher categories is much more likely to cause severe damage. It’s also important to recognize that sustained wind speed and gust wind speed are separately defined variables. The latter, which is most commonly referred to by engineers as a 3-second gust speed, is used in ASCE 7 to calculate wind pressures during load development. Sustained wind speeds are what weather reports typically reference when classifying hurricanes, but engineers use the gust speeds to develop pressures.
For coastlines, FEMA’s Flood Insurance Rate Maps (FIRM) identify flood zones in most geographic regions in the U.S. and its territories, based on factors like wave height and base flood elevation. The two key parameters used for computing coastal flood loading are flood depth and flood velocity. Similarly for tsunamis, the key parameters are inundation depth and flow velocity. Structures must be hardened to handle a variety of water-based loads, and foundation designs must consider loss of soil strength, erosion, scour and displacement.
Road Map to Resilient Design
Preparing for substation design begins with evaluating the relative merits of a brownfield site versus a greenfield site. For brownfields with reuse potential, a choice must be made between rebuilding on the site or elevating critical items within the existing facility. Greenfields provide an opportunity to consider sites that are more advantageous in terms of extreme weather exposure, but they also can add cost by requiring fresh interconnections.
As a site is being selected, environmental criteria will need to be identified. Relevant FIRM information and Flood Insurance Studies (FIS) will provide information relating to the base flood elevation (BFE), which affects the required elevation of critical equipment and the depth of loads affecting structures.
Next, the designer determines the flood design criteria. The determination of the design flood elevation is defined in ASCE 24. Depending on the flood design class, the minimum elevation of critical equipment should be taken in accordance with the requirements delineated in ASCE 24 chapters 2 and 4, depending on the zone classification of flooding at the site. Selecting the flood velocity depends on consideration of the topography, the distance from the source of flooding, and the proximity to other buildings or obstructions. These provide upper and lower bounds; actual flood velocities are assumed to fall between them, and conservative designs will utilize the upper-bound velocities. Many locations adjacent to existing waterways may have flood velocities defined at certain transects along the waterway. These will be noted in the FIS.
Establishing equipment criticality depends on factors such as the relative cost of hardening versus replacement, equipment lead time, and impact to the grid if the equipment is damaged. For typical primary critical equipment, the control enclosure and power transformers are high-cost items with long lead times, and water entry into the former is likely to be catastrophic. Secondary critical equipment is likely to include capacitor banks, circuit breakers and station power.
A flood mitigation strategy is selected next. This could involve modifying the site topography, elevating critical and vulnerable equipment above the design flood elevation, or developing adequate shielding against the elements via flood walls and diversion devices.
Flood load contributions primarily consist of three types of load effects: hydrostatic (standing or moving water inducing horizontal or buoyancy forces against a structure, particularly when not evenly applied), hydrodynamic (high-velocity flows that can destroy solid walls and dislodging buildings with inadequate foundations) and debris impact. Breaking wave loads also must be considered, as the damage caused by breaking waves against a vertical surface can be 10 times greater than the force created by high winds. ASCE 7 provides procedures for determining these loads.
There are a few other important design elements to evaluate:
- In keeping with guidance from ASCE 24, erosion and scour from flood events make slab-on-grade foundations a poor choice. Deep foundations are preferred in flood hazard areas.
- While most substation and transmission steel structures are galvanized, FEMA suggests specifying thicker protective coatings for structures within 3,000 feet of the ocean.
- Especially when elevating or enclosing equipment, keep in mind accessibility, both for routine maintenance and for after an extreme event.
Combining Strengths
There are many decision points and options for a hardened substation design. Although the substation-specific guidance in ASCE 113 does not adequately cover all extreme weather events, there are additional resources and standards — written or updated more recently — that can shed light on sensible solutions.
Working from a clear understanding of the unique loading cases from hurricanes and similar events, designers can develop new or rebuilt structures that are well prepared to keep power infrastructure operational, even under extreme conditions.
Upgrading electrical equipment and constructing robust infrastructure can help deliver continuous service and save millions of dollars otherwise lost during flooding events.
