Koi Koi Grace
by on February 26, 2019
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3-) Shielding

When lightning strikes a transmission line, it may hit either the overhead ground wire or a phase conductor. If a conductor is hit, there will certainly be a flashover of the insulation. To prevent and minimize such an occurrence, the overhead ground wire is used to intercept the lightning strike, “shielding’’ the conductors. To optimize this shielding, the shielding angle (see Figure 2.4) must be 30◦ or less. If a location is known to have an unusually high exposure to lightning strikes, and structure heights are over 90 ft (27.4 m), even smaller shielding angles should be used. Table 2.10 shows typical shielding angles relative to structure height as well as voltage. Note that for situations where the height and voltage give two different shielding angles, the smaller value shall be adopted.

 

For voltages exceeding 230 kV, the shielding angle often controls the required vertical and horizontal separation between the ground wire and conductor. For areas with a history of lightning strikes, the clearance between the overhead ground wire (OHGW) and the conductor shall be determined with specific reference to the Isokeraunic levels.

3.a) Lightning Performance and Grounding

Design criteria should include the line’s target Isokeraunic Level of the area. This level is usually given in terms of number of thunderstorms in the area per year. An overhead

ground wire (OHGW) should be provided at all places where the Isokeraunic Level is over 20 per RUS Bulletin 200. This wire should be grounded at every structure by way of the structure ground wire. For H-Frame structures with two (2) ground wires, the OHGW must each be connected to the structure ground wire and to one another. This will be beneficial in the event that if one structure ground wire breaks, both overhead ground wires will still remain grounded. RUS Bulletin 200 recommends that lightning outages of 1 to 4 per 100 miles per year is acceptable for lines in the 161 to 230 kV range.

All circuits must be adequately grounded to facilitate protection against lightning strikes. RUS recommends that individual transmission structures should have a footing resistance of less than 25 ohms, measured in dry soil conditions, especially within ½ mile (0.8 km) of a substation in high Isokeraunic areas. Outside of this distance, a resistance of less than 40 ohms is allowed by some utilities at voltages upto 230 kV. Ground rods or counterpoise wires may be used to obtain a footing resistance below the maximum allowable value.

A map of Isokeraunic levels at various locations in the US is given in RUS Bulletin 200.

 

3.b ) Insulation Requirements

Insulators are needed to provide both a mechanical means to hold the line conductors as well as electrical isolation required to withstand impulse voltage events such as lightning impulses and switching impulses. Insulators are basically a physical means of providing an “air gap’’ between an energized conductor and the grounded (earthed) portion of the structure. Toughened glass, porcelain and non-ceramic (polymer) insulators are commonly used on overhead lines. Recommended insulation levels are generally in terms of number of “bells’’ (porcelain) or “sheds’’ (polymer) in an insulator string. The number of bells or sheds also determines the so-called “leakage distance’’ or “dry-arcing distance’’ – the distance measured along the insulating surface from the ground point to energized point.

Tables 2.11, 2.12 and 2.13 provide guidelines for suggested minimum insulation levels in suspension and horizontal post insulators (See also Appendix 11 for more insulator data). Additional insulation may be warranted when required due to higher altitudes, contamination and high soil resistance. Other insulation considerations include critical impulse flashover, switching surge factor flashover and strength requirements. For deadends, where the insulator string is in line with the conductor, two extra bells should be provided relative to the tangent string.

It must be noted that for voltages up to 230 kV, the most severe stress on insulators is generally due to lightning strikes; therefore, the most important characteristic is the impulse flashover. RUS Bulletin 200 recommendations for insulation levels include both positive and negative flashover values.

 3.c ) Conductor Operating Temperature

Clearances from energized conductors to ground are generally evaluated at the wire’s specified Maximum Operating Temperature (MOT). ACSR (Aluminum Conductor

 

Steel Reinforced) wires function well even at an elevated temperature of 100◦C (212◦F) without any significant loss of strength. High performance wires such as ACSS (Aluminum Conductors Steel Supported) can sustain temperatures up to 200◦C (392◦F).

ACCR (Aluminum Conductor Composite Reinforced) conductors often used in long span crossings can withstand temperatures over 200◦C (392◦F). Nominal wire clearances to ground are usually given onP&P (Plan and Profile) drawings in terms of MOT. This in turn will help determine the required structure heights for that location. Conductor operating temperature also influences line ratings (see Ampacity subsection below).

3.d) Corona and Field Effect

Corona is the ionization of the air that occurs at the surface of the conductor and hardware due to high electric field strength at the surface of the metal. Field effects are the secondary voltages and currents that may be induced in nearby objects. Corona is a function of the voltage of the line, conductor diameter and the condition of the conductor and may also result in radio and television interference, light and ozone production.

3.e ) EMF and Noise

Operation of power lines produce electric and magnetic fields commonly referred to as EMF. Noise on transmission lines is also due to the effect of corona. The EMF produced by alternating current in the USA has a frequency of 60 Hz. Electric field strength is directly proportional to the line’s voltage; the higher the voltage, the stronger the electric field. But this field is also inversely proportional to the distance from the conductors. That is, the electric field strength decreases as the distance from the conductor increases.

 3.f ) Galloping

Galloping is a phenomenon where transmission conductors vibrate with large amplitudes and usually occurs when steady, moderate wind blows over a conductor covered with ice. Ice build-up makes the conductor slightly out-of-round irregular in shape leading to aerodynamic lift and conductor movement.

Such movement of conductors results in:

(a) contact between phase conductors or between phase conductors and ground wires resulting in electrical short circuits

(b) conductor failure at support point due to violent dynamic stress caused by galloping

During galloping, conductors oscillate elliptically at frequencies of 1 Hz or less. Shorter spans, usually less than 600 ft (183 m), are anticipated to gallop in a single loop configuration; longer spans are expected to gallop in double loops. Overlapping of these ellipses means possible conductor contact which must be avoided. Therefore, adequate clearance must be maintained between phase wires and between phase and ground wires, to prevent loop contact. Another way to address galloping problems is to adjust the wire tensions to an optimum level so that sag and lateral movement are minimal. Shorter line spans or advanced conductors such as T2 (Twisted Pair) may also help in reducing galloping effects. Anti-galloping devices are also used on existing lines to mitigate galloping issues.

 3.g ) Ampacity

 

Ampacity of a conductor is the maximum current in amperes the wire can carry at its maximum design temperature. This rating is a measure of the conductor’s electrical performance and thermal capability. The maximum conductor design temperature for sags and clearances is also the line’s maximum operating temperature (MOT) for Ampacity. Although the MOT is the primary criteria governing these ratings, wind, ambient temperature and sun (solar heating) conditions are also considered in the calculation of Ampacity. IEEE Standard 738 (2013) provides excellent guidance for determining Ampacity Ratings for conductors.

For example, according to RUS Bulletin 200, the Ampacity ratings for Drake ACSR conductor (795 kcmil 26/7), which is very popular in North America, are: 972 Amps (summer) and 1257 Amps (winter), both calculated at an MOT of 212◦F (100◦C) and with a wind speed of 2 fps (0.61 meters/sec) and at ambient temperatures of 104◦F (summer) and 32◦F (winter).

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