ABSTRACT

A transmission tower, also known as an electricity pylon or simply a pylon in British English and as a hydro tower in Canadian English, is a tall structure, usually a steel lattice tower, used to support an overhead power line. In electrical grids, they are generally used to carry high-voltage transmission lines that transport bulk electric power from generating stations to electrical substations; utility poles are used to support lower voltage subtransmission and distribution lines that transport power from substations to electric customers. They come in a wide variety of shapes and sizes. The longest span of any hydroelectric crossing ever built belongs to the powerline crossing of Ameralik fjord with a length of 5,376 m (17,638 ft). In addition to steel, other materials may be used, including concrete and wood. There are four major categories of transmission towers: suspension, terminal, tension, and transposition. Some transmission towers combine these basic functions. Transmission towers and their overhead power lines are often considered to be a form of visual pollution. Methods to reduce the visual effect include undergrounding.

INTRODUCTION

Earlier on in the development of electric power, its proponents and developers recognized the importance of economies of scale in power generation. If power could be distributed to a broader customer base, larger, centralized generation facilities could be built providing power at much lower costs. In turn, these lower costs would attract more customers, making even larger scale production possible. However, several factors limit the practical scale of central generation. Most obviously, the practical size of boilers, turbines, and other generating plant equipment is limited by the ability to manufacture and transport this equipment to a plant site. Over the last century, commercial power equipment has evolved such that practical generating station capacities have increased from 5 megawatts (MW) to several thousand megawatts. In the absence of other constraints, central plant size could continue to increase, at least in a modular fashion, by adding more and more units of similar design at a given site. There are other constraints, though, so that the practical size of central generating facilities may actually decline in the future. These constraints include fuel and resource supply at a given site, limits imposed by the natural environment for dissipating waste heat, transport and disposal of waste products, community environmental standards, reliability and security concerns, and the economics of power transmission. As central power station size increased, the plant operators faced myriad challenges in distributing power to customers. Photographs of commercial urban areas in the early years of the twentieth century often reveal a labyrinth of overhead wires from competing suppliers of power (and also of communications). This highly inefficient example of competitive markets was tamed by a system of regulation granting a limited monopoly to selected firms in exchange for providing reliable power service to a community. The development of the regulated industry structure further encouraged centralization of power production and the need for larger distribution networks. By 1910, Samuel Insull had begun rural electrification, so long-distance distribution to rural and other remote customers was needed. In some cases, these developing distribution systems were linked, connecting several generating stations and improving the reliability of power supply (Barthol, 2004; EIA, 2000; Berger, 1995).

HISTORY

The first transmission of electrical impulses over an extended distance was demonstrated on July 14, 1729 by the physicist Stephen Gray. The demonstration used damp hemp cords suspended by silk threads (the low resistance of metallic conductors not being appreciated at the time).

However the first practical use of overhead lines was in the context of telegraphy. By 1837 experimental commercial telegraph systems ran as far as 20 km (13 miles). Electric power transmission was accomplished in 1882 with the first high-voltage transmission between Munich and Miesbach (60 km). 1891 saw the construction of the first three-phase alternating current overhead line on the occasion of the International Electricity Exhibition in Frankfurt, between Lauffen and Frankfurt.

In 1912 the first 110 kV-overhead power line entered service followed by the first 220 kV-overhead power line in 1923. In the 1920s RWE AG built the first overhead line for this voltage and in 1926 built a Rhine crossing with the pylons of Voerde, two masts 138 meters high.

In 1953, the first 345 kV line was put into service by American Electric Power in the United States. In Germany in 1957 the first 380 kV overhead power line was commissioned (between the transformer station and Rommerskirchen). In the same year the overhead line traversing of the Strait of Messina went into service in Italy, whose pylons served the Elbe crossing 1. This was used as the model for the building of the Elbe crossing 2 in the second half of the 1970s which saw the construction of the highest overhead line pylons of the world. Earlier, in 1952, the first 380 kV line was put into service in Sweden, in 1000 km (625 miles) between the more populated areas in the south and the largest hydroelectric power stations in the north. Starting from 1967 in Russia, and also in the US and Canada, overhead lines for voltage of 765 kV were built. In 1982 overhead power lines were built in Soviet Union between Elektrostal and the power station at Ekibastuz, this was a three-phase alternating current line at 1150 kV (Powerline Ekibastuz-Kokshetau). In 1999, in Japan the first powerline designed for 1000 kV with 2 circuits were built, the Kita-Iwaki Powerline. In 2003 the building of the highest overhead line commenced in China, the Yangtze River Crossing (Burgen, 1986; BPA, 2003).

TRANSMISSION LINE COMPONENTS

Towers

Transmission towers are the most visible component of the bulk power transmission system. Their function is to keep the high-voltage conductors separated from their surroundings and from each other. Higher voltage lines require greater separation. The unintended transfer of power between a conductor and its surroundings, known as a fault to ground, will occur if an energized line comes into direct contact with the surroundings or comes close enough that an arc can jump the remaining separation. A fault can also occur between conductors. Such a fault is known as a phase-to-phase fault. The first design consideration for transmission towers is to separate the conductors from each other, from the tower, and from other structures in the environment in order to prevent faults. This requirement and the electrical potential (voltage) define the basic physical dimensions of a tower, including its height, conductor spacing, and length of insulator required to mount the conductor. Given these basic dimensions, the next design requirement is to provide the structural strength necessary to maintain these distances under loading from the weight of the conductors, wind loads, ice loading, seismic loads, and possible impacts. Of course, the structure must meet these requirements in the most economical possible manner. This has lead to the extensive use of variants on a space frame or truss design, which can provide high strength with minimal material requirements. The result is the ubiquitous lattice work towers seen in all regions of the country. The last design requirement is to provide a foundation adequate to support the needed tower under the design loads (U.S Department of Energy, 2005).

Conductors

A variety of conductor compositions and constructions are currently in use to meet a variety of specific requirements. In the early years of the industry, copper was used almost exclusively because of its high electrical conductivity, but cable diameters with copper were determined more by the need for mechanical strength than by the need for improved conductivity. The low strength-to-weight ratio of copper limited the acceptable span length (distance between towers). Aluminum, with its higher strength-to-weight ratio, was introduced as an alternative to copper, allowing for greater span lengths. Though copper has higher conductivity than aluminum, the lower density of aluminum gives it a conductivity-to-weight ratio twice that of copper. The first aluminum transmission lines were installed in the last 5 years of the 19th century (Thrash 2003). An additional incentive favoring aluminum conductors in more recent times is that aluminum is more economical to use than copper, even though aluminum has only 60% of the conductivity of copper (Thrash, 2003).

Substation

Substations as indicated, the voltage required for economical transmission of electric power exceeds the voltage appropriate for distribution to customers. First, customer equipment generally operates at only a few hundred volts, rather than at the hundreds of thousands of volts used for transmission. Second, if high voltages were maintained up to the point of customer connection, fault protection would be extremely expensive. Therefore, distribution from the transmission line to customers is accomplished at much lower voltages, so transformers are required to reduce voltage before the power is introduced to a distribution or subtransmission system. These transformers mark the end of the transmission line and are located at substations. Each transmission line starts from an existing substation and ends at a new substation. If the new transmission line were high-voltage direct current (HVDC), the origin substation would be expanded to accommodate AC-to-DC converters. Intermediate substations may also be required if there is a voltage change along the route, say, from 500 kV to 200 kV (U.S Department of Energy, 2005).

ROWs

A ROW is a largely passive but critical component of a transmission line. It provides a safety margin between the high-voltage lines and surrounding structures and vegetation. The ROW also provides a path for ground-based inspections and access to transmission towers and other line components, if repairs are needed. Failure to maintain an adequate ROW can result in dangerous situations, including ground faults. A ROW generally consists of native vegetation or plants selected for favorable growth patterns (slow growth and low mature heights). However, in some cases, access roads constitute a portion of the ROW and provide more convenient access for repair and inspection vehicles.

Multiple Lines

The use of a common corridor of ROW for multiple transmission lines is likely to be restricted if it presents a credible risk of a multicircuit outage. Mitigation measures, principally increasing line spacing beyond that required for fault protection, may be used to reduce risk. Multiple lines in a single corridor are subject to the following hazards: 1. A tower from one line falling against conductors of an adjacent line. 2. A shield wire (grounded lightning protector connecting the tops of the towers) being dragged onto adjacent lines by an aircraft. 3. An aircraft damaging more than one circuit. 4. Fire or smoke on the ROW. 5. Lightning strikes. 6. Deliberate malicious damage.

TRANSMISSION LINE TOWER CONSTRUCTION

Tower Construction

Note that this monopole footprint is smaller than that of a lattice tower, but the amount of concrete required is substantially greater to withstand the bending moment at the ground anchor

Substation Construction

Substation construction is expected to take 6 to 9 months and will cover approximately 10 acres for the fenced station plus 3 acres for construction support.

Conductor Stringing

The process of attaching conductor wires to the insulators suspended from the towers is called conductor stringing. It generally involves pulling the conductor off of a truck-mounted spool. This process typically will not result in additional land disturbance beyond that required for tower construction. An exception may occur at diversion towers where severe line direction changes occur.

ROW Restoration

It is general practice to restore the ROW after construction, although the replacement of tall vegetation is not a part of restoration directly within the ROW boundaries. Tall vegetation can create ground-fault hazards, including the risk of fire. Plants consistent with native species are selected, although with consideration of their growth rates and mature plant heights. In some areas, the ROW must remain passable by land vehicles for line inspections (U.S Department of Energy, 2005).

OPERATION AND MAINTENANCE PHASE

 Normal Operation

During normal operation, transmission lines require very little intervention. The only exception is periodic inspections and vegetation management, which are discussed below. Inspections are frequently done from the air using a small plane or a helicopter. However, tracked or other ground vehicles also have a role in line inspections, particularly where air inspections are unsafe or where a closer inspection of a potential hazard is required. It is not clear how “as needed” is determined without inspections (EIA, 2000).

ROW Management

ROW maintenance is used to assure safe clearance between conductors and vegetation and to allow passage for inspections on foot or by vehicles. Vegetation management is a critical function. Failure to perform adequate vegetation management was a major contributing factor to the August 2003 blackout that affected much of the Northeast and Midwest. The combination of heavy electrical loads, high ambient temperature, and low wind speed allowed a critical line to sag close enough to a tree that a ground fault occurred. The subsequent system response resulted in the blackout. The difficulty is that vegetation management involves mechanical cutting and chemical herbicides. In some cases, it involves the replacement of native species with plants that have more favorable growth patterns. In some instances, utilities have reported improvements in local ecosystems due to careful ROW vegetation planning and maintenance (U.S. Department of Energy, 2005).

HIGH-VOLTAGE DIRECT CURRENT TRANSMISSION LINES

Given the same overall transmitted power and practical conductor sizes, low-voltage, high-current transmissions will suffer much greater power losses than high-voltage, low-current transmissions. This holds whether DC or AC is used. Historically, it has been very difficult to efficiently transform DC power to a high-voltage, low-current form, whereas with AC this can be done efficiently with a simple transformer. This was the key to the success of the AC system. Modern transmission grids use AC voltages up to 765 kV. However, technology improvements in the last few decades have allowed reliable generation of high-voltage DC (HVDC), resulting in its reemergence for power transmission systems. HVDC is also more efficient than high-voltage alternating current (HVAC) in that it uses the insulating strength of the line or cable continuously rather than only during the crest voltage, as with AC. Thus for the same level of insulation, the continuous DC voltage can be at least 1.41 ( 2 ) times the RMS AC voltage, with power transfer being increased by the same amount.

The principal advantage of AC is that it allows the use of transformers to change the voltage at which power is used. No equivalent of the transformer exists for direct current, so the manipulation of DC voltages is considerably more complex. With the development of efficient AC machines, such as the induction motor, AC transmission and utilization became the norm. DC transmission remains the exception, rather than the rule, in power transmission. There are environments where HVDC is the conventional solution, such as in submarine cables and in interconnecting unsynchronized AC systems, but for the bulk of situations, AC transmission remains dominant (BPA, 2003).

 ADVANTAGES OF HVDC OVER HVAC TRANSMISSION

Despite alternating current being the dominant mode for electric power transmission, in a number of applications, the advantages of HVDC makes it the preferred option over AC transmission. Examples include:

• Undersea cables where high capacitance causes additional AC losses (e.g., the 250-km Baltic Cable between Sweden and Germany).
• Endpoint-to-endpoint long-haul bulk power transmission without intermediate taps, for example, in remote areas.
• Increasing the capacity of an existing power grid in situations where additional wires are difficult or expensive to install.
• Allowing power transmission between unsynchronized AC distribution systems.
• Reducing the profile of wiring and pylons for a given power transmission capacity, as HVDC can carry more power per conductor of a given size.
• Connecting a remote generating plant to the distribution grid; for example, the Nelson River Bipole line in Canada (IEEE 2005).
• Stabilizing a predominantly AC power grid without increasing the maximum prospective short-circuit current.
• Reducing corona losses (due to higher voltage peaks) compared to HVAC transmission lines of similar power.
• Reducing line cost, since HVDC transmission requires fewer conductors; for example, two for a typical bipolar HVDC line compared to three for three-phase HVAC (BPA, 1996).

DISADVANTAGES OF HVDC TRANSMISSION

The main disadvantages of HVDC transmission systems, including DC links connecting HVAC systems area, are summarized below:

• Converter stations needed to connect to AC power grids are expensive. Converter substations are more complex than HVAC substations, not only in additional converting equipment, but also in more complicated control and regulating systems. Costs of such stations may be offset by lower construction costs of DC transmission lines, but offsets require DC lines of considerable length.
• In contrast to AC systems, designing and operating multi-terminal HVDC systems is complex. Controlling power flow in such systems requires continuous communication between all terminals, as power flow must be actively regulated by the control system instead of by the inherent properties of the transmission line.
• Converter substations generate current and voltage harmonics, while the conversion process is accompanied by reactive power consumption. As a result, it is necessary to install expensive filter-compensation units and reactive power compensation units.
• During short-circuits in the AC power systems close to connected HVDC substations, power faults also occur in the HVDC transmission system for the duration of the short-circuit. Inverter substations are most affected. During short-circuits on the inverter output side, a full HVDC transmission system power fault can be caused. Power faults due to short-circuits on the rectifier input side are usually proportional to the voltage decrease.
• The number of substations within a modern multi-terminal HVDC transmission system can be no larger than six to eight, and large differences in their capacities are not allowed. The larger the number of substations, the smaller may be the differences in their capacities. Thus, it is practically impossible to construct an HVDC transmission system with more than five substations.
• The high-frequency constituents found in direct current transmission systems can cause radio noise in communications lines that are situated near the HVDC transmission line. To prevent this, it is necessary to install expensive “active” filters on HVDC transmission lines.
• Grounding HVDC transmission involves a complex and difficult installation, as it is necessary to construct a reliable and permanent contact to the Earth for proper operation and to eliminate the possible creation of a dangerous “step voltage.” (BPA, 1996)

HVDC TECHNOLOGIES

Polarity and Earth Return

In a DC system, a constant potential difference exists between two rails. In a common configuration, one of the rails is connected to the Earth (earthed), establishing it at Earth potential. The other rail, at a potential high above or below ground, is connected to a transmission line.

The earthed rail at the source end of a DC circuit may or may not be connected to the corresponding rail at the terminal end of the circuit by means of a second transmission line conductor. A monopole transmission line refers to a transmission line without an accompanying earthed conductor. To complete the circuit, an Earth current (known as a telluric current) flows between the earthed electrodes at the two stations. Such a large Earth current may have undesirable effects in many locations, rendering monopole systems unsuitable (Burgen, 1986).

Polarity and Corona Discharge

Corona discharge involves the creation of ions in the air around transmission line conductors by the presence of a strong electromagnetic field. Corona discharge can cause power loss, create audible and radio-frequency interference, generate ozone, and lead to arcing.

While AC coronas are in the form of oscillating particles, coronas from HVDC lines produce a constant “wind” of ions. With monopolar transmission, the choice of polarity of the energized conductor determines the polarity of the ions making up the corona discharge. Negative coronas generate considerably more ozone than positive coronas, and generate it farther downwind of the power line. Thus, the use of a positive voltage reduces the ozone impacts of monopole HVDC power lines. On the other hand, as negative ions are used in home air ionizers and have purported health benefits, particularly in being responsible for condensing particulate matter, the use of negative potential on monopole lines may be considered (Barthold, 2004).

Transmission Lines and Cables

For bulk power transmission over land, overhead transmission lines are most frequently used. These lines most often employ a bipolar configuration using two conductors with opposite polarity. HVDC cables are also normally used for submarine power transmission. The most common types of cables are the solid and the oil-filled types. Solid cables have insulation that consists of paper tapes impregnated with high-viscosity oil. No length limitation exists for this type, and designs are available today for depths of about 1,100 yards. Oil-filled cable is completely filled with a low-viscosity oil that is maintained under pressure. The maximum practical length for this type of cable is limited to around 37 miles, due to the limitations of oil systems. Recent developments have produced a new type of HVDC cable, which is available for HVDC underground or submarine power transmissions. This cable is made using extruded polyethylene insulation, and is used in voltage sourced converter (VSC)-based HVDC systems (Berger, 1995).

CONCLUSION

HVDC transmission lines have reduced impacts compared to HVAC transmission lines for many environmental impact measures. These advantages may appear as lower costs for mitigating such impacts when installing HVDC lines compared to HVAC lines. If land use is taken as an overall measure of the comparative environmental impacts of HVAC and HVDC transmission lines of the same relative capacity, HVDC line impacts are roughly two-thirds of those of HVAC lines. Thus, a transmission system that incorporates HVDC power transmission will, as a whole, have reduced impacts compared to one that exclusively employs HVAC transmission lines (Koshcheev 2003)

RECOMMENDATION

During HVDC transmission line project planning, most of the same environmental impact characteristics that are considered in planning a HVAC transmission line project should be taken into consideration. These characteristics include impacts from electrical and magnetic fields, radio interference, audio noise, potential accelerated corrosion of buried metal installations due to ground currents, visual impacts, and land use impacts from siting transmission line towers and substations and limitations imposed on land use in transmission line corridors.

 

REFERENCES

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Berger, R.P. (1995), Fur, Feathers, & Transmission Lines, Winnipeg, MB, Manitoba Hydro,

System Planning and Environment Division. BLM and USFS (Bureau of Land Management and U.S. Forest Service) (2006). Surface Operating Standards and Guidelines for Oil and Gas Exploration and Development, U.S. Department of the Interior and U.S. Department of Agriculture, BLM National Science and

Technology Center Branch of Publishing Services. BPA (Bonneville Power Administration), (1996). Electrical and Biological Effects of Transmission Lines: A Review, Portland, Oregon, December.

BPA (2003). Schultz-Hanford Area Transmission Line Project Final Environmental Impact Statement, Bonneville Power Administration, U.S. Department of Energy, Bureau of Land

Management, Bureau of Reclamation, Fish and Wildlife Service, U.S. Department of Interior, Department of Army, U.S. Department of Defense.

Burgen, A.R. (1986). Power Systems Analysis, Prentice Hall, Inc., Englewood Cliffs, New Jersey.

DOE (U.S. Department of Energy) (2005). Tucson Electric Power Company Sahuarita-Nogales Transmission Line Final Environmental Impact Statement, DOE/EIS-0336, Office of Fossil Energy, U.S. Department of Energy, January.

EIA (Energy Information Administration) (2000). The Changing Structure of the Electric Power Industry 2000: An Update, Energy Information Administration, U.S. Department of Energy, Washington, D.C.

Koshcheev, L.A. (2003). Environmental Characteristics of HVDC Overhead Transmission Lines, St-Petersburg, High Voltage Direct Current Power Transmission Research Institute, prepared for the Third Workshop on Power Grid Interconnection in Northeast Asia, Vladivostok, Russia, September 30–October 3. Available at http://www.nautilus.org/archives/energy/grid/ 2003Workshop/Koshcheev_paper_final1.pdf. Accessed July 12, 2006.

Thrash, F.R. (2003). “Transmission Conductors — A Review of the Design and Selection Criteria.” Available at http://www.southwire.com. Accessed September 12, 2006.