Koi Koi Grace
by on February 25, 2019
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2-) Electrical Design

Overhead transmission lines are designed to satisfy a wide range of local and regulatory requirements. While the demands of these requirements vary from location to location and are often line-specific, the underlying principle is one of legality, safety and reliability.

 

2.a) Regulatory Codes

These relate to the local and national standards pertaining to both electrical operations as well as other land use considerations. For example, in areas close to runways in airports, U.S. Federal Aviation Administration (FAA) has laws limiting the height of a transmission structure to 200 feet (61 m). Protected lands such as wetlands do not permit or severely limit construction activities. In some areas, operating high-voltage transmission lines can be hazardous to raptors (eagles, hawks, falcons and other protected species of birds) and in such areas avian protection guidelines are specified. Other land use restrictions include wild life habitats and environmentally-sensitive locations.

From a design perspective, regulations refer to the NESC to which all designs adhere to. The other code is the RUS Bulletin 200 which is applicable to all rural highvoltage lines funded by the government. Most states in the USA have adopted NESC as governing code without any changes; but some states require supplemental standards via their administrative codes. In California and Hawaii, for example, electrical design is usually governed by GO-95 (2016) and GO-6 (1969), respectively. Supporting these codes are the various material and structural codes, design manuals and design guides

 

(steel, concrete, lattice, foundations etc.). Many individual utilities in the US have their own in-house design standards which meet or exceed the NESC standards. Other international codes, standards and manuals will be briefly discussed in Section 2.7.

Another special situation is when transmission lines of one utility cross those of another or when transmission lines cross distribution circuits. Wire clearances in these cases must satisfy the applicable regulatory code. Lastly, overhead lines near highway or railroad crossings and underground pipelines in the vicinity of transmission lines are regulated by various state or local laws with special permitting and clearance requirements.

 2.b) Right Of Way

The transmission line must be designed with adequate right-of-way (ROW) width to provide legal access to line repair and maintenance crews, vegetation management and to facilitate adequate distance from objects nearby. The actual width of line alignment depends on the voltage, number of circuits, type of structure adopted for the line and clearance from various objects required. Figure 2.1 shows the concept of ROW and how it relates to insulator swing and clearance to objects. Right of Way needs (referring to H-Frame type structures and various voltages) are shown in Table 2.5. RUS Bulletin 200 provides a formula for calculating the required ROW width W for a typical pole structure given the required distance from conductors to a nearby object or such as building and other installation under moderate wind conditions. Referring to Figure 2.2:

 

Sf = Conductor final sag at 60◦F at 6 psf (290 Pa) wind

C = Required horizontal clearance between conductors and nearby objects

∅ = conductor/insulator swing angle under 6 psf (290 Pa) wind

δ = pole deflection under 6 psf (290 Pa) wind

In some cases the structure deflection is very small and negligible, especially for braced H-Frames. In such a case, the equation simplifies to:

W = A + 2(Li + Sf )sin ∅ + 2C

It must be noted that weather conditions other than moderate 6 psf wind are also often considered. The required horizontal clearance C for objects such as buildings and other installations is provided by NESC. Without wind (i.e.) conductor at rest, the required

C values are higher; and therefore, one has to check the ROW for conductor at rest condition also. Note that some utilities allow a buffer of 2 ft (0.61 m) to be added to the required horizontal clearance.

Additionally, ROW width is also required to satisfy the horizontal and radial clearances to

 

Example 2.1 A 69 kV line is planned on an available strip W = 75 ft (22.9 m) wide by an electric utility. The following data is given:

Horizontal distance between outermost insulators = A = 9 ft (2.74 m)

Suspension Insulator with 4 bells, Length = Li = 2 ft-6 in (0.76 m)

Conductor Sag at 60◦F with 6 psf wind = Sf = 5 ft (1.52 m)

The swing angle of the insulator under moderate 6 psf wind at 60◦F =∅= 38◦

The required C per internal standards = 10 ft (3.05 m) away. Neglect pole deflections.

Determine if the strip provides adequate ROW for the line based on Equation 2.3.

 

Solution: See Figure 2.2. From Eqn. 2.3, we have the required ROW width as

W = 9 + (2)(2.5 + 5)(sin 38◦) + (2)(10) = 38.23 ft (11.7 m)

The 75 ft strip is adequate for ROW.

2.c) Clearances

Design clearances are one of the primary parameters of a transmission line design process. Clearances are primarily provided to safeguard public for activities reasonably anticipated in the vicinity of a transmission line. In general, the typical clearances required for a transmission line design are:

(a) Vertical Clearances of energized conductors above ground and other surfaces

(b) Clearance between Wires – Underbuild

(c) Clearance to Nearby Objects

(d) Clearance to Structure Surface

(e) Clearances between Wires – Phase and Ground Wires

Figure 2.3 shows the typical situations in which clearances are identified.

The NESC and RUS Bulletin 200 provide tables and formulas for minimum recommended clearances in each of the above categories.

One of the parameters influencing clearance calculations is the Maximum Phaseto-Ground Operating Voltage (MOVPG) which is a function of Maximum Phase-toPhase Operating Voltage (MOVPP). In general:

 

 

Clearances for voltages above 230 kV are more complex to evaluate considering the influence of several extra variables such as switching surge factors and electrostatic effects. For more information on electrostatic effects, the reader is referred to RUS Bulletin 62-4 (1976).

Several clearance tables are provided below. In these tables, the voltages are AC and nominal phase-to-phase unless specified otherwise. Altitude correction is not applied to these clearances. Additional buffers are mandatory over the minimum provided in these tables.

 

Vertical clearance of energized conductors to ground and other surfaces The most important of all clearances is the vertical clearance of an energized conductor from the ground which governs structure heights. For voltages exceeding 98 kV AC to ground, Rule 232D of NESC also provides an alternative procedure for calculating vertical clearances of energized conductors above ground and other surfaces. The clearance is defined basically as a sum of a reference height and an electrical component accounting for maximum switching surge factor, non-standard atmospheric conditions and a margin of safety. This procedure also uses the Maximum Crest-to-Ground Operating Voltage (MOVCG) which is defined as:

MOVCG = Sqrt(2) ∗ MOVPG (2.3d)

 

Tables 2.6a-1 and 2.6a-2 show the typical ground and other vertical clearances recommended by NESC and adopted in the US using Rule 232B and Rule 232C. For comparison, Table 2.6a-3 shows typical ground and other vertical clearances for EHV Lines computed per the alternative procedure (Rule 232D) using switching surge factors shown in the table. Utilities themselves also have their own in-house clearance standards, including buffers (adders), based on experience and consideration of various design and construction uncertainties. These standards meet and often exceed NESC values.

Clearance between wires – under-build

 

Dual-use structures (Figure 2.3b) often contain a distribution under-build or communication wires below the transmission circuits. The clearances between wires of the transmission circuit and those of the under-build are very important in determining the relative heights of the structures. Table 2.6b-1 shows these clearances. This table refers to clearances both at the structure as well as in mid-span and is a modified form of clearances from RUS Bulletin 200. The vertical clearances at structure apply regardless of horizontal separation between transmission and under-build conductors. Minimum recommended vertical clearances within span apply to one of the following conditions which yields the least separation between the upper and lower conductors:

 regardless of horizontal separation between transmission and under-build conductors. Minimum recommended vertical clearances within span apply to one of the following conditions which yields the least separation between the upper and lower conductors:

a. Upper conductor final sag at 32◦F (0◦C) with no wind and with radial ice thickness as applicable for the particular loading district;

b. Upper conductor final sag at 167◦F (75◦C)

c. Upper conductor final sag at maximum design temperature with no wind (usually 212◦F or 100◦C for ACSR conductors)

The sag of the under-build conductor to be used is the final sag, measured at the same ambient temperature as the upper conductor without electrical and ice loading.

 

Clearance between wires on different supporting structures

 

Transmission lines often cross another, usually a higher voltage (upper) line crossing a lower voltage line. The clearances between wires of the upper level circuit and those of the lower level are very important in determining the relative heights of the structures for the upper line. Table 2.6b-2 shows these clearances. This table refers to the situation where higher voltage wires cross lower voltage wires and where the

 

wires are on different supporting structures. Lower voltage wires crossing over higher voltage wires, though theoretically possible, is not operationally recommended. Note that Table 2.6b-2 is a modified form of RUS clearances from Bulletin 200.

 

Clearance to nearby objects

 

Horizontal clearances to various objects are given in Tables 2.6c-1 (modified RUS) and 2.6c-2. The tables provide clearances required for wires at rest and when wires are displaced under 6 psf (290 Pa) wind at 60 deg. F temperature. The horizontal clearance

 

covers what is commonly known as conductor displacement due to wind. However, the clearances do not cover the blow-out of the conductor (determined with a computer program such as PIS-CADDTM). Vertical clearances to various objects are given in Tables 2.6d-1 (modified RUS) and 2.6d-2.

Buffers of 2.0 ft (0.61 m) to 5.0 ft (1.52 m) can be added to various clearances to provide additional safety. These adders are not absolute but are based on the judgement of the engineer. Some utilities in the US are known to use clearance buffers ranging up to 10.0 ft (3.05 m) on EHV lines.

Codes in various other countries more or less adopt similar clearances, quantitatively and qualitatively. Operating voltages often differ widely between the North American continent, Europe and Asia; so, any attempt to compare and contrast clearances can only be nominal at an informational level.

Clearances between wires – phase and ground wires

 

The separation between energized conductors of the same circuit and between conductors and ground wires should be adequate to prevent swinging contact or flashovers.

 

NESC provides guidelines to determine the minimum required phase separation, both at the structure as well as mid-span. Tables 2.7 (modified RUS) and 2.8 show commonly used phase separation on structures. Clearances for circuits 345 kV and above included in Table 2.8 are based on data from actual lines built at those voltages. These values serve only as a guideline; the actual required clearances are a function of several variables including type of structure adopted, span lengths, voltage, terrain and possibility of Aeolian vibration and galloping. Longer spans are often associated with galloping and require larger separation between phase wires and between phase wires and overhead ground wires.

Insulator swing

Suspension insulator strings are usually free to swing about their points of attachment to the structure. It is, therefore, necessary to ensure that when the insulator strings do swing, minimum clearances are maintained to structure surfaces and guy wires. The amount of swing is a function of conductor tension, wind velocity, insulator weight, line angle etc. The RUS Bulletin 200 provides guidelines for determining minimum swing clearances (i.e.) separation from the structure, both in terms of distance as well

 

as maximum allowed swing angles. These clearances are generally evaluated at the three weather conditions shown below and are derived from the NESC for conditions (a) and (b).

(a) No Wind

(b) Light or Moderate Wind (6 psf minimum)

(c) High Wind

Swing clearance is aimed towards minimizing the possibility of a structure flashover during switching operations. Additionally, it helps protect electrical maintenance workers by providing safety clearances while working on the transmission structures. Table 2.9a shows safe clearances suggested by RUS for voltages up to 230 kV; Table 2.9b gives safe clearances suggested for EHV voltages of 345 and 500 kV. The values also indicate safe distance of energized conductors from guy wires (air gap).

The RUS Bulletin 200 also specifies the required horizontal separation between various wires on a transmission structure. This separation is mandated to prevent swinging contacts or flashovers between phases of the same or different circuit.

 

Elevation effect

 

The various clearances discussed so far are applicable for line locations at altitudes of 3300 ft or less. For locations at a higher altitude, NESC recommends altitude corrections to be added to the given clearances. These corrections include adding a specified amount of clearance for each 1000 ft in excess of 3300 ft.

Example 2.2 For the same data/line of the previous example E2.1, determine horizontal separation required between conductors. Assume Fc = 1.20, Sf = 4.0 ft (1.22 m). Solution: From Eqn. 2.4, we have: H = (0.025)(1.05)(69) + (1.20)(sqrt(4.0)) + (2.50)(sin 38◦) = 5.75 ft (1.75 m) Note: This Sf is defined differently than in Example 2.1.

 

 

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