Special structures

Special situations in transmission lines arise at locations such as river crossings, storm structures and air break switches. These are discussed briefly below.

3.4.7.1 Anti-cascade structures

Overhead Transmission Lines often face extreme events such as severe ice or wind loads, which damage line sections and affect power supply to customers. Even when the best design criteria are employed, there is always a risk to overhead lines when extreme wind or ice storms exceed the design criteria. This damage can occur in poles or supporting structures, insulators or guy wires, depending on the weakest point.

Utilities must therefore consider the possibilities of severe wind storms and icing while planning for High Voltage Lines.

Some approaches to limiting the impact of ice and wind loads on overhead lines include:

a) Better forecasting of maximum wind and ice loads

b) Careful design approaches to minimize the risk of failures, while simultaneously reducing the potential consequences of such events

One preferred way of reducing the chance of several miles of cascading line collapse (domino effect) is to install an in-line or tangent deadend or storm structure at chosen locations. Mitigation approaches also include strengthening existing deadends at crit-ical locations by adding or using stronger guy wires and strain insulators along with better hardware.

110 Design of electrical transmission lines

Figure 3.21 Substation Structures – Classes for Deflections (with permission from ASCE).

Structural analysis and design 111

Figure 3.22 Anti-Cascade Structures.

Storm or damage mitigation structures are typically installed every 4 to 5 miles of a transmission line. Theoretical structure configuration is that of a guyed in-line deadend. This means, even if one section of the line is damaged, the other remains intact. The structure introduced in the line to prevent cascading failures is known as

“Anti-Cascade Structure’’ or ACS. Figure 3.22 shows typical anti-cascade structure locations on a transmission line.

3.4.7.2 Long span systems

Figure 3.23 shows typical river crossing structures in a transmission line. Long span designs are very rare and make complex demands on various design-related items such as loadings, wire strengths and foundations as well as regulatory and environmental impacts. The design criteria fall outside the normal scope used on other routine cases;

long spans mean much larger loads, larger tensions, increased scope for Aeolian vibra-tion and larger foundavibra-tion loads, not to menvibra-tion custom design and installavibra-tion of dampers on all wires. Galloping checks are therefore an important means of accepting a particular conductor or ground wire.

Not all conductors are amenable to a long span situation and special wires may be needed in the span. Special wires in turn demand special attachment hardware and handling. It is also difficult to choose an optimum optical ground wire given the demand for large tensions. The structures themselves at each end of the river (long) span are much taller than the others, requiring transition structures to gradually reduce the height to normal levels. It is common to design these transition structures as a full deadend capable of resisting either large tensions from the special wires or unbalanced tensions due to wire changes at the transition points.

112 Design of electrical transmission lines

Figure 3.23 Long Span River Crossing Structures.

Constructability, including access to large construction vehicles, is a very impor-tant issue after design. Communities living within the vicinity of such large structures often are known to express public opposition, mostly based on aesthetics. Other impediments include special markers, lighting beacons and height constraints if located within the proximity of an airport.

3.4.7.3 Air-break switches

Two-way and three-way phase-over-phase (vertical) and low-profile horizontal phase configuration switches are designed specifically for switching applications on transmis-sion lines. They provide economical sectionalizing, and tap and tie switching points for circuit control. These phase-over-phase switches can be mounted on a single pole, min-imizing ROW requirements. Installation on a single pole significantly reduces costs of land and equipment that a conventional switching substation requires. Switches rated 69 kV and below can be mounted on any suitable structure; for 115 kV and above, they need laminated wood, steel or concrete poles. For side-break style switches, the operating effort to open and close the switch is minimal even at high voltages.

Figure 3.24 shows a steel pole- mounted three-way air break switch (the framing drawing of a 161 kV switch structure is shown later in Section 3.5.3.5). Analysis and design of switch structures is a specialized process. Briefly, the structures are designed as 3-way deadends with the third wire usually a slack span into another line or substation.

Deflection is one of the design criteria and guying is often used for laminated wood poles, especially in the slack span plane.

Structural analysis and design 113

Figure 3.24 Air Break Switch.

3.4.7.4 Line crossings

Figure 3.25 shows a situation where one line crosses another. Crossing clearances are defined on the assumption that the upper circuit is of higher voltage. In most cases, the lower line is a distribution (or another transmission) circuit. Depending on the voltages

114 Design of electrical transmission lines

Figure 3.25 Line Crossings.

involved, this situation demands taller structures due to extra clearance required. A special case arises when the two lines are owned by different utilities. Both the RUS Bulletin 200 and NESC provide guidance for wire and structure clearances for all these cases.