EL HÉROE

Outdoor electrical substations and switchyards are a collection of various equipment, structures and components where electrical energy (typically high voltage) is modified.

These substation structures support above grade items such as switches, circuit break-ers, insulators, arrestbreak-ers, rigid bus and transformers. In the USA, analysis and design of substation systems is governed by ASCE Substation Structure Design Guide 113 (2008), RUS Bulletin 300 (2001) for rural systems and IEEE Standard 693 (2005).

Substation structures are essentially classified into three types, based on function:

Line Support Structures (LSS) – These are also called take-off or strain structures, deadend or line termination structures and internal strain bus. These can be single or multi-bay, truss or steel pole type. Major forces sustained are conductor and ground wire tensions (full or slack) and wind. LSS are critical components of a substation and are designed to withstand large stresses although on a non-catastrophic failure basis.

Equipment Support Structures (ESS) – These are switch stands, bus support stands, lightning arrestor stands, and line trap stands etc. ESS are designed mainly as ver-tical cantilever beams for short circuit forces and wind. Stresses rarely control but deflections must be checked.

Distribution Structures – Intended for low voltage applications, these are mostly com-prised of steel beams and columns (truss or tube), may have multiple bays but are usually one-bay wide. They support switches and other equipment and are designed for rigidity at equipment location.

Structural profile configurations in substations basically fall under three types: Lat-tice, Solid and Semi-Solid. Lattice-type systems consist of steel angles framing into a box truss, both vertically and horizontally. Solid-type systems are made of wide flange shapes, pipes, round or tapered poles or rectangular tubes. Connections are either bolted or welded. Solid- type configurations are widely used for LSS, in either an A-Frame setup or single pole. Cross arms may be square or rectangular tubes or round/tapered polygonal members. Semi-solid types are made from wide-flange shapes or pipes as main members and use steel angles as braces in-between.

Design loads depend on whether they are intended for LSS or ESS. All LSS and their components, however, must withstand stresses induced by factored loads. For LSS, design loading is similar to that of a transmission line structures. Load Factors (LF) are specified for vertical (V), wind (W) and wire tension (T) forces. For ESS, design loading includes all applicable wind, ice, short circuit and dead loads; wind plus short circuit loads, however, produce maximum stresses. Ice is not expected to control design.

Since all loads contain load factors, Ultimate Strength Design (or USD) is appropriate for LSS. Stiffness is an important requirement for ESS and the goal is to limit deflection under wind. Sections a bit larger than necessary are usually specified. Allowable Stress Design (ASD) is appropriate for ESS, while conforming to NEMA-SG6 (2006) rules, per RUS Bulletin 300.

3.4.6.1 Seismic considerations

Seismic analysis may be necessary at places with high earthquake risk. Seismic loads are generally considered as environmental load situations and are not combined with ice

Structural analysis and design 107 or wind loads, but may be combined with short circuit loading or operational loading.

Substation structures are divided into four types with references to seismic loading:

ST1 Single- or Multi-bay Rack (not supporting equipment)

ST2 Single- or Multi-bay Rack (supporting equipment and conductors) ST3 Rigid Isolated Support (supporting equipment)

ST4 Flexible Isolated Support (supporting equipment)

ST3 and ST4, in voltage classes higher than 121 kV, and within seismic zones 3 and 4, should be designed as per IEEE 693. For other types of structures and situations, design procedures are outlined in ASCE Substation Guide 113.

3.4.6.2 Deflection considerations

In addition to stresses, deflection limitations are imposed on ESS. Excessive movement or rotation of substation structures and components can affect mechanical/electrical operation of the equipment, reduce clearances, induce stresses in insulators/connectors etc. Disconnect switches are highly sensitive to deflections but overhead line dead-ends are not. Lattice-type systems, and A-Frames with solid sections, do not present any deflection problems. ASCE Guide 113 defines the following classes of substation structures for deflection purposes:

Class A – support equipment with mechanical mechanisms where structure deflection could impair or prevent proper operation.

Examples: group switches, vertical switches, ground switches, circuit breaker supports and circuit interrupters.

Class B – support equipment without mechanical components but where excessive structure deflection could result in compromised phase or phase-to-ground clearances, stresses in equipment, fittings or bus.

Examples: support structures for rigid bus, surge arresters, metering devices, power transformers, hot-stick switches and fuses.

Class C – support equipment relatively insensitive to deflection or stand-alone structures that do not carry any equipment.

Examples: support structures for flexible bus, masts for lightning shielding, dead-end structures for incoming transmission lines. SG-6 does not give any specific limits but deflections here refer to limiting P−  stresses.

Multiple Class – Combination of any above classes

Tables 3.2, 3.3 and 3.4 show the basic load conditions, load cases for ultimate strength design (USD) and deflection limits for substation structures. Figure 3.21 from ASCE Guide 113 defines various structure classes and spans graphically for determining deflections.

Substation structures have a wide range of ground line reactions due to applied forces and therefore a wide variety of foundation types. These include drilled shafts, spread footings and slabs on grade, among others. Drilled shafts are typically used for LSS and steel poles while spread footings are used for circuit breakers and small

108 Design of electrical transmission lines

Table 3.2 Basic Loading Conditions – Substation Structures.

Wire Loaded Switch and Other

Loading Substation Interruption Rigid Bus Equipment

Condition Structures Supports Supports Supports

NESC* YES NO NO NO

Extreme Wind YES YES YES YES

Combined Ice and Wind YES YES YES YES

Earthquake YES YES YES YES

Short Circuit NO YES YES NO**

Construction YES YES YES YES

and Maintenance

Equipment Operation NO YES NO YES

Deflection YES YES YES YES

(with permission from ASCE).

∗Local Codes, such as GO-95, may also be applicable.

∗∗Engineer shall determine if this load effect is significant.

Table 3.3 Ultimate Strength Design Cases and Load Factors – Substation Structure.

Load Case Load Factors and Combinations

1 1.1 D+ 1.2 W IFW+ 0.75 SC + 1.1 TW

2 1.1 D+ 1.2 IWIFI∗+ 1.2 WIIFIW∗∗+ 0.75 SC + 1.1 TW

3 1.1 D+ 1.0 SC + 1.1 TW

4 1.1 D+ 1.25 E (or EFS) IFE+ 0.75 SC + 1.1 TW

(with permission from ASCE).

∗The importance factor for ice is applied to the thickness.

∗∗The importance factor for wind with ice IFIWis 1.0.

D= structure and wire dead load W= extreme wind load

WI= wind load in combination with ice IW= ice load in combination with wind E= earthquake load, FE(without IFW)

EFS= earthquake load reactions from first support imposed on the rest of the structure (without IFW) TW= horizontal wire tension

SC= short circuit load IF= importance factors

IFW,IFI= importance factors for wind and ice loads

Earthquake Load FE= (Sa/R) Wd(IFE) (IMV) applied at center of gravity of structure R= structure response modification factor, a function of structural system

(e.g.: cantilever= 2.0)

IFE= importance factor for earthquake loads Wd= dead load

Sa= spectral response acceleration IMV= 1.0 for single mode behavior

= 1.5 for multiple vibration modes

transformers. Heavy oil-filled transformers need slab-on-grade footings which are gen-erally designed not to exceed the allowable bearing pressure at site as determined by the Geotechnical engineer.

If frost is present at substation location, then spread footings must be seated below frost depth or a minimum of 12 in. (30.5 cm), whichever is maximum. Design loads on most footings include axial compression, uplift, bending and shear. Most utilities

Structural analysis and design 109

Table 3.4 Deflection Limitations.

Maximum Structure Deflection as Ratio of Span Length Structure Class

Member Direction of Deflection Class A Class B Class C

Horizontal* Vertical 1/200 1/200 1/100

Horizontal* Horizontal 1/200 1/100 1/100

Vertical** Horizontal 1/100 1/100 1/50

(with permission from ASCE).

∗Spans for horizontal members should be clear span between vertical supports; for cantilever beams, the distance to the nearest vertical support.

∗∗Spans for vertical members should be vertical distance from foundation to point under consideration.

adopt the Ultimate Strength Design philosophy for structural design of foundations, often nominally increasing the ultimate reactions by 10% or so for additional margin of safety.

RUS recommends that soil borings be taken at critical locations (i.e.) deadend structures and heavy transformers. Bearing capacity, ground water level and other soil parameters must be determined. Possibility of differential settlement in silts and silty sands must be checked.