Koi Koi Grace
by on February 27, 2019
190 views

1-) Structure spotting

Prior to any design, the engineer must establish the alignment of the transmission line taking into consideration the various criteria. These criteria include property boundary issues, soil conditions, road and other clearances, regulatory issues, environmental concerns, costs, impact on public property, aesthetics and construction access. Potential structure locations are identified along the chosen alignment using structure spotting process. Structure spotting is the design process which determines the height, location and type of consecutive structures on the Plan and Profile (P & P) sheets. The potential points are identified based on terrain, expected access issues and land use. However, structure’s height and strength requirements are not considered as primary criteria. Survey coordinates of the points will be input into computer programs such as PLSCADDTM (2012) to generate the terrain showing graphical view of the transmission line. Structures with known strength ratings (i.e.) maximum allowable span for a given configuration and height are used alternatively to ‘spot’ structures to make best use of their height and strength.

Among the key factors that impact structure spotting are vertical and horizontal clearances, structure capacity, insulator swing, conductor separation, galloping and uplift. Both manual and computerized spotting processes are available. In the case of manual spotting, structure strength is expressed in terms of maximum allowable Weight and Wind Spans.

The weight span (also called vertical span) is the horizontal distance between lowest points on the sag curve of two adjacent spans. The wind span (sometimes called horizontal span) is the horizontal distance between the geometrical midpoints of the adjacent spans. Figure 2.5 shows the definitions of wind and weight spans.

2-) Rulling Spans

The concept of Ruling Span (RS) is used in the design and construction of a transmission line to provide a uniform span length representative of the various lengths of spans within a tension section (i.e.) between deadends. A ruling span is an assumed design span which approximates the mechanical performance of given tension section. This span allows sags and tensions to be defined for structure spotting and conductor stringing.

Mathematically, the Ruling Span is defined as:

Example 2.3 A tension section between two deadends of a 161 kV transmission line contains the following spans: 920 ft (280.4 m), 1060 ft (323.1 m), 1010 ft (307.8 m) and 870 ft (265.2 m). Determine the Ruling Span.

Solution: RS = {[9203 + 10603 + 10103 + 8703]/[920 + 1060 + 1010 + 870]}0.5 = 973.55 ft (296.7 m)

 

3-) Sags and Tensions

Sags and tensions of wires are interdependent. Sags depend on the span length, tension, loading condition, type of wire (conductor, overhead ground wire or optical ground wire) and tensions in turn depend on history of stressing and wire temperature relative to a given weather condition. Long-term stress behavior includes creep which in turn affects tension. Aeolian vibrations of wires induce dynamic stresses. Extreme operating temperatures soften the conductor and induce higher sags. Therefore, the determination of conductor and ground wire sags and corresponding tensions – as a function of temperature and loading – is of fundamental importance in transmission line design. Wire stringing charts showing these sags and tensions are critical to field technicians during installation process.

Wire sags and tensions are generally determined using well known computer program SAG10TM from Southwire (2014). Alternatively, wire sags and tensions can also be calculated within PLS-CADD.

The maximum anticipated sag of a conductor – usually at its highest operating temperature or heavy ice loads also helps determine the required height of a pole or structure at a location to satisfy mandated ground clearance.

 

Example 2.4

The maximum sag of a conductor at a particular level span in a 230 kV transmission line is calculated as 13 ft (4 m). Determine the nominal height above ground of a pole required for that location. If 11% of the total pole is embedded into ground, what is the total pole length needed? Assume a ground clearance buffer of 2.5 ft (0.76 m).

Solution:

This problem serves to illustrate the use of clearance tables discussed in Section 2.2.3. From Table 2.7 – Minimum phase-to-phase separation for a 230 kV line = 9.1 ft (2.77 m)

Minimum separation between OHGW and phase wire = 5.9 ft (1.80 m)

From Table 2.6a-1 – Minimum Ground Clearance required = 22.4 ft (6.8 m) With buffer: Design Ground Clearance = 22.4 + 2.5 = 24.9 ft or 25 ft (7.62 m) Height of pole required = 5.9 + (2)(9.1) + 13 + 25 = 62.1 ft (18.93 m). Total Length of the Pole needed = 62.1/(1.00 – 0.11) = 69.8 or 70 ft (21.34 m) Use 70 ft (21.34 m)

Additionally, the following Sag-Tension relationships will be useful in many situations:

 

The requirement of using iced upper and un-iced lower conductors is related to the issue of differential ice accumulation. Ice on overhead ground wires is unlikely to melt whereas ice on phase wires is prone for melting given the heat generated by passing of current. This unbalanced ice is also a condition to check separation between ground wires and energized phase wires.

Equations 2.4 and 2.6 can be combined to determine maximum allowable horizontal spans for tangent structures based on horizontal separation of conductors. The example below illustrates the process for an H-frame.

 

Example 2.5 The following data apply to a 69 kV line with a ruling span of 800 ft (243.8 m). The suspension insulators each carry 4 bells for a string length of 2.5 ft (0.76 m). Experience Factor is taken as 1.00.

Determine the maximum horizontal span limited by conductor horizontal separation. H = Horizontal separation of phase wires = 10.5 ft (3.2 m) Ruling Span Sag = 20.4 ft (6.22 m) at 60◦F final φ = Swing angle under 6 psf (290 Pa) wind = 59.4 deg. (sin φ = 0.861)

 

Solution:

Substituting these values in Equation 2.4: 10.5 = (0.025)(1.05)(69) + (1)(Sf) + (2.5)(0.861) or Sf = 42.72 ft (13.02 m) Using this value in Equation 2.6: (Lmax/800)2 = (42.72/20.4) = 2.09 or Lmax = 1157 ft (352.9 m)

 

 

Example 2.6 Given a 138 kV structure with a phase separation at pole of 9.64 ft (2.94 m), a ruling span of 800 ft (243.8 m) with the following additional data:

Sag of lower conductor at without ice = 18.7 ft (5.7 m)

Sag of upper conductor at with ice = 22.4 ft (6.83 m)

Required separation at mid-span = 5.7 ft (1.74 m)

Determine maximum span limited by conductor vertical separation.

Solution:

From Equation 2.8:

Lmcs = (800)(sqrt[(5.7–9.64)/(18.7–22.4)]) = 825.5 ft (251.6 m)

 

 3-a) Galloping

The issue of galloping was discussed in Section 2.2.10 where wire tension control is noted as one of the methods of controlling galloping vibrations. Usually, for long spans in excess of 600 ft (183 m), the optimum wire tension is often governed by galloping movement defined by double loop ellipses. This tension is determined in a trial-anderror procedure by adjusting the tension so that the ellipses do not touch each other. At angles and deadends, a factored value of this wire tension is used to design the structure itself; therefore, it is important to evaluate the impact of galloping while designing transmission lines with large spans.

3-b) Tension Limits

Throughout the life of a transmission line, conductors are subject to a variety of mechanical and climactic loading situations including wind, ice, snow and temperature variations. NESC and RUS Bulletin 200 therefore specify tension limits for conductors and shield wires which refer to state of loading, temperature and climactic parameters. Table 2.14 shows these combined limits.

These limits generally refer to ACSR conductors. For other types of conductors, manufacturer’s guidelines must be followed.

Initial Unloaded Tension refers to the conductor as it is strung initially before any ice or wind is applied.

Final Unloaded Tension refers to the state of the wire after it has experienced ice and wind loads, long term creep and permanent inelastic deformation.

Standard Loaded situation refers to the conductor state when it is loaded with simultaneous ice and wind per NESC loading districts as defined in Section 2.1.2.

Extreme Wind Tension is the tension when wind is acting on the conductor as defined in Section 2.1.1. No ice is allowed on the wire for this condition.

Extreme Ice Tension is the tension when the conductor is loaded with specified amount of radial ice as defined in Section 2.1.3. No wind is allowed on the wire for this condition. This load case is outside of NESC requirements.

Extreme Ice with Concurrent Wind refers to the situation where extreme ice on the wire is accompanied by a moderate amount of wind as defined in Section 2.1.3.

Tension in the wires is also a criterion to determine whether vibration dampers are needed or not (see Section 5.1.1.12 for details).

4-) Insulator

Insulators are an integral component of the mechanical system defining a transmission structure and are employed in several basic configurations: post or suspension, angle and deadend. All insulators specified for a structure must consider both mechanical strength as well as electrical characteristics.

Post insulators are used where insulator swing is not permitted and are usually subject to cantilever forces. Suspension (and angle) insulators are subject to vertical loads due to wire weight and tensile loads due to line angles and are rated by their tensile strength. Deadend insulators carry direct wire tensions and are selected on the basis of tensile strength. NESC recommends the following allowable percentages of strength ratings for line post insulators:

Cantilever 40% (Ceramic and Toughened glass) & 50% (Non Ceremic)

Tension 50%

Compression 50%

For suspension type insulators, NESC recommends allowable strength rating of 50% of combined mechanical and electrical strength in case of ceramic and toughened glass insulators.

For non-ceramic insulators such as polymer insulators, it is 50% of the specified mechanical load. This recommendation applies only for the case of combined ice and wind district loading (Rule 250B) with load factors of unity. These percentages are applied to the insulator manufacturer ratings which are different for ceramic and non-ceramic insulators. For post and braced post insulators, manufacturers also provide interaction strength diagrams showing combined effects of simultaneous application of vertical, transverse and longitudinal loads. For porcelain or glass suspension insulators, the ultimate strength rating is usually denoted as “Combined Mechanical and Electrical’’ whereas for polymer insulators the terms “Specified Mechanical/Cantilever/Tensile Load’’ are used, as appropriate.

 

 5-) Hardware

All components defining a transmission structure also contain various connecting hardware and must meet or match the strength ratings of the main connected part. For example, the anchor shackles and/or yoke plates used to connect deadend insulator strings to the structure must withstand the loads imposed via the insulator string. This also applies to all other associated accessories such as conductor and ground wire splices, suspension and deadend clamps and other hardware.

 6-)  Guy Wires and Anchors

The design of guyed structures involves both guy wires as well as anchors that transfer wire load into the ground. Guy wires are generally galvanized steel (often the same stranding and rating as the overhead ground wire) or aluminum-clad steel. Strength ratings apart, corrosion resistance is also often considered while selecting a guy wire. A wide variety of anchoring systems are available to choose from: log anchors, plate anchors, rock anchors, helical screw anchors etc., each with a specific set of usage and design criteria. The nature of soil/rock at the location plays an important role in anchor design.

As with insulators, limits are placed on allowable loads on guy wires and anchors. Typical allowable percentages of strength ratings for guys and anchors are: Guy Wire 90% (NESC) Anchor 100% (NESC) Guy Wire and Anchor 65% (RUS) In situations where the guy wire may be in close proximity to energized conductors, a guy insulator is often used on the guy wire to provide additional protection. For visual safety, the bottom portion of all guy wires are typically enclosed in colored PVC markers, usually yellow, to render them visible during night time.

 

 

 

 

 

Post in: Business