Substation Structure Design Guide: Recommended Practice for Design and Use

This chapter presents an overview of the key concepts discussed in this book. The book provides structural design guidance and a comprehensive resource for outdoor electrical substation structures and foundations. It also provides an overview of electrical equipment, identifies the various components and structure types, and describes structure configurations. The book presents recommended load factors and load combinations. In addition, recommended deflection limits and structure classes are defined. The book discusses analysis techniques and structural modeling concepts as they relate to substation structures. Both static and dynamic analyses are covered. Guidelines are given for selecting the appropriate analysis method for different structural behaviors such as large versus small displacement theory and nonlinear analysis. The book lists design standards for various materials of construction and considerations for the design of rigid buses, insulators, base plates, structural connections, and other special considerations that include but are not limited to wind and seismic loading.

Substation and switchyard structures are used to support above-grade components and electrical equipment such as cable bus, rigid bus, and strain bus conductors; switches; surge arresters; insulators; and others. This chapter provides an overview of electrical equipment, identifies the various components and structure types, and describes structure configurations. Photographs of selected substation structures and equipment are also included. The chapter provides an overview of typical types of electrical equipment support structures and a brief description of the electrical equipment they support. An understanding of the function, operation, and relationship of electrical equipment to the support structures is a prerequisite for a good structural design. The structure designer is the party responsible for the design of the structures in the substation who may be an agent of the owner or fabricator. The supplier or a fabricator is the producer of the substation structures as directed by the owner or the structure designer.

All substation structures should be designed to withstand applicable loads that consider dead loads, wind, ice, line tensions, earthquakes, short circuits, construction, maintenance, electrical equipment operating loads, and other specified or unusual service conditions. This chapter discusses guidelines for developing substation structure loading criteria. It presents recommended guidelines regarding the application of the structure-loading criteria presented in this manual of practice. The methods for estimating loads—especially those for weather-related events—are mostly based on statistical models. These models, although scientifically correct, have limitations on the precision of their predictions. To ensure structural reliability, load factors are introduced to compensate for this uncertainty. The load factors and load combinations are selected based on the unique characteristics of typical electrical substation structures. Serviceability is the limit state in which the function, maintainability, durability, and appearance of substation structures and electrical equipment are preserved under normal usage.

Deflection and rotation of substation structures and members can affect the mechanical operation of supported electrical equipment, reduce electrical clearances, and cause unpredicted stress in structures, insulators, connectors, and rigid bus conductors. The selection of the deflection limits applicable to a specific application is the responsibility of the owner. Structures supporting stranded conductors or overhead line dead ends typically can withstand structure deflections without detrimental effects on line operation. When evaluating the deflection of a multiple-use structure, the deflection limits applicable to any point on the structure are determined by the classification of the structure from that location upward. The fixity of the substation structure's base plate and foundation can affect the deflection of the structure. Net deflection for horizontal structure members is currently the substation structure industry standard when evaluating deflection.

The design of substation structures requires knowledge of the equipment being supported by the structure, its operation, and electrical and safety codes. The analysis of a structure begins by developing a model of the structural configuration, connection characteristics, support boundary conditions, material properties, and loading cases. An analysis of a structure generates both stresses and deflections, which may be used to determine the adequacy of the structure. The analysis model is a set of mathematical equations to predict the behavior of the structure. In developing a model of a structure, assumptions are made concerning the individual elements, their geometrical and mechanical properties, and the connections, loads, and foundation supports. These are discussed in this chapter. A static analysis may be used for the seismic design of rigid structures in substations. Performing a dynamic analysis requires a thorough understanding of the background material.

Allowable strength design is a method of proportioning structural members such that elastically computed stresses produced in the members by service loads do not exceed specified allowable stresses. Ultimate strength design is the method of proportioning structural members such that the computed forces produced in the members by the factored loads do not exceed the member design strength. In a corrosive environment, water may be absorbed into the open cracks and corrode the reinforcing steel. The use of aluminum for structural supports has a history in applications such as light poles, substation structures, and highway sign structures. Wood structures and poles should be designed and constructed in accordance with MOP 141, Recommended Practice for the Design and Use of Wood Pole Structures for Electrical Transmission Lines and NESC. The seismic qualification report or the equipment manufacturer should be consulted for seismic displacement requirements specific to the particular equipment.

This chapter identifies various types of foundations that are commonly used to support substation equipment and structures, discusses geotechnical considerations for the design of substation foundations, and provides design considerations that may be unique to construction with a new or existing substation. It focuses on the design of new, cast-in-place concrete foundations. NAVFAC, while an older document, contains a wide breadth of information and considerations for the design of shallow and deep foundations. Choosing the appropriate type of deep foundation is dependent on the subsurface conditions and local experience and practice. The chapter provides additional design considerations. Such considerations can affect the design and performance of foundations and, where applicable, should be addressed by the geotechnical testing firm with recommendations for design or mitigation in the geotechnical report. Utilities may have unique load combinations and criteria for the design of substation structures and foundations based on regulatory codes, operational requirements, and regional experience.

To resist loads applied to structures and equipment, it is recommended that all substation structures and equipment be positively anchored to their foundations. Different types of anchorage are used to connect substation structures to their foundations. The most common means of transferring structural reaction forces to the foundations is through anchor rods. This type of anchorage provides an effective transfer of load. The approach for the design of anchor rods in this chapter is based on ultimate strength design loads (also referred to as factored loads). All equipment anchorage assemblies, including anchor rods, should be designed for forces resulting from a structural analysis or tests, whichever is applicable. The specification of the anchor material should be in accordance with the appropriate ASTM standard. Anchor rods should be installed with the use of templates to ensure the rods are installed in the proper arrangements in the foundation. Post-installed anchors include a variety of proprietary anchor rods or anchoring systems that are installed in hardened concrete.

To ensure the quality of the product, effective Quality Control and Quality Assurance programs should be instituted by both the fabricator and owner. This will assure the owner that the fabricator has the personnel, organization, experience, procedures, knowledge, equipment, capability, and commitment to produce the specified product. Many of the requirements, procedures, testing, and handling associated with steel structures are also applicable to aluminum structures, except magnetic particle testing. Cast-in-place concrete needs to be in accordance with the latest applicable and most recent industry standards. These requirements should be based on the following specifications: ACI, ASTM, and the Concrete Reinforcing Steel Institute. The material and treatment of wood members should satisfy the requirements of ASCE's MOP 141 on wood using the applicable standard. The fabricator should provide written procedures for handling and storing materials to prevent damage, loss, or deterioration of the structure.

An engineer should anticipate construction loads imposed on substation structures and ensure that proper construction methods and quality materials are used to prevent excessive stresses during construction. The engineer should consider the accessibility of equipment for maintenance and operation. Elevated locations on the structure that require workers to move from one location to another require fall protection devices such as safety cables or attachment points designed and installed for worker fall restraint. Full-scale structural proof tests are rarely performed on substation structures supporting wires. It is not typically cost-effective to perform a full-scale test, because substation structures, unlike transmission structures, are not fabricated in large quantities. Full-scale testing should be considered if a particular substation structure is a standard design and will be used multiple times or if the structure uses a unique structural system not typical of current practice.

This chapter is a guide for the engineer on common drivers and various methods used in the retrofit, analysis, and reuse of an existing substation structure or foundation. Replacing equipment during upgrades or equipment failures may lead to the reuse of a structure or foundation. When retrofitting substations, environmental factors such as asbestos and soil contamination should be considered. Federal, state, and local environmental regulations address the proper handling, management, transport, and disposal of each of these contaminants if they are found in soils or surrounding areas. Any number of security improvements in an existing substation may be necessary for increasing security. A rational approach should be taken when evaluating the impacts of load variations or structural modifications to existing structures. The retrofit of existing structures should include a prior inspection of structurally relevant areas. Any significantly corroded or damaged members need to be repaired or replaced as part of the retrofit.

This chapter is intended to provide utility engineers with information to develop their own substation design criteria and standards for secondary and tertiary oil containments, firewalls, sound walls, ballistic walls, and blast walls. The chapter describes common types of containment systems; however, there also may be many other systems or a hybrid of systems. The chapter introduces engineers to the various barrier walls used in electric substations and their relevant design considerations. Utility companies often conduct site specific studies to look at the vulnerabilities of critical substations. These studies can be used to develop design criteria specific to site security requirements and determine the appropriate type of barrier wall. The barrier walls can be a combination of one or more wall types that are described in this chapter and should be designed with ease of maintenance in mind.