Power Transmission

POWER TRANSMISSION & DISTRIBUTION

Electric power transmission is the bulk transfer of electrical energy, from generating power plants to electrical substations located near demand centers. This is distinct from the local wiring between high-voltage substations and customers, which is typically referred to as electric power distribution.

Transmission lines, when interconnected with each other, become transmission networks. The combined transmission and distribution network is known as the "power grid"

Most transmission lines are high-voltage three-phase alternating current (AC), although single phase AC is sometimes used in railway electrification systems. High-voltage direct-current (HVDC) technology is used for greater efficiency at very long distances (typically hundreds of miles (kilometers)), in submarine power cables (typically longer than 30 miles (50 km)), and in the interchange of power between grids that are not mutually synchronized. HVDC links are also used to stabilize and control problems in large power distribution networks where sudden new loads or blackouts in one part of a network can otherwise result in synchronization problems and cascading failures.

Electricity is transmitted at high voltages (115 kV or above) to reduce the energy losses in long-distance transmission. Power is usually transmitted through overhead power lines. Underground power transmission has a significantly higher cost and greater operational limitations but is sometimes used in urban areas or sensitive locations.

A key limitation of electric power is that, with minor exceptions, electrical energy cannot be stored, and therefore must be generated as needed. A sophisticated control system is required to ensure electric generation very closely matches the demand. If the demand for power exceeds the supply, generation plant and transmission equipment can shut down, which in the worst case may lead to a major regional blackout. It is to reduce the risk of such a failure that electric transmission networks are interconnected into regional, national or continent wide networks thereby providing multiple redundant alternative routes for power to flow should such equipment failures occur. Much analysis is done by transmission companies to determine the maximum reliable capacity of each line (ordinarily less than its physical or thermal limit) to ensure spare capacity is available should there be any such failure in another part of the network.

MODE OF TRANSMISSION

(a) Overhead Transmission:

High-voltage overhead conductors are not covered by insulation. The conductor material is nearly always an aluminum alloy, made into several strands and possibly reinforced with steel strands. Copper was sometimes used for overhead transmission, but aluminum is lighter, yields only marginally reduced performance and costs much less. Overhead conductors are a commodity supplied by several companies worldwide. Improved conductor material and shapes are regularly used to allow increased capacity and modernize transmission circuits. Conductor sizes range from 12 mm2 to 750 mm2 with varying resistance and current-carrying capacity. Thicker wires would lead to a relatively small increase in capacity due to the skin effect that causes most of the current to flow close to the

surface of the wire. Because of this current limitation, multiple parallel cables (called bundle conductors) are used when higher capacity is needed. Bundle conductors are also used at high voltages to reduce energy loss caused by corona discharge.

Today, transmission-level voltages are usually considered to be 110 kV and above. Lower voltages, such as 66 kV and 33 kV, are usually considered sub transmission voltages, but are occasionally used on long lines with light loads. Voltages less than 33 kV are usually used for distribution. Voltages above 765 kV are considered extra high voltage and require different designs compared to equipment used at lower voltages.

Since overhead transmission wires depend on air for insulation, the design of these lines requires minimum clearances to be observed to maintain safety. Adverse weather conditions, such as high wind and low temperatures, can lead to power outages. Wind speeds as low as 23 knots (43 km/h) can permit conductors to encroach operating clearances, resulting in a flashover and loss of supply. Oscillatory motion of the physical line can be termed gallop or flutter depending on the frequency and amplitude of oscillation.

(b) Underground Transmission:

Electric power can also be transmitted by underground power cables instead of overhead power lines. Underground cables take up less right-of-way than overhead lines, have lower visibility, and are less affected by bad weather. However, costs of insulated cable and excavation are much higher than overhead construction. Faults in buried transmission lines take longer to locate and repair. Underground lines are strictly limited by their thermal capacity, which permits fewer overloads or re-rating than overhead lines. Long underground AC cables have significant capacitance, which may reduce their ability to provide useful power to loads beyond 50 miles. Long underground DC cables have no such issue and can run for thousands of miles.

OVERHEAD POWER TRANSMISSION:

An overhead power line is a structure used in electric power transmission and distribution to transmit electrical energy along large distances. It consists of one or more conductors (commonly multiples of three) suspended by towers or poles. Since most of the insulation is provided by air, overhead power lines are generally the lowest-cost method of power transmission for large quantities of electric energy.

Towers for support of the lines are made of wood (as-grown or laminated), steel (either lattice structures or tubular poles), and occasionally reinforced wood. The bare wire conductors on the line are generally made of aluminum (either plain or reinforced with steel, or composite materials such as carbon and glass fiber), though some copper wires are used in medium-voltage distribution and low-voltage connections to customer premises. A major goal of overhead power line design is to maintain adequate clearance between energized conductors and the ground so as to prevent dangerous contact with the line, and to provide reliable support for the conductors, resilient to storms, ice load, earthquakes and other potential causes of damage. Today overhead lines are routinely operated at

voltages exceeding 765,000 volts between conductors, with even higher voltages possible in some cases.

Classification by Operating Voltage:

Overhead power transmission lines are classified in the electrical power industry by the range of voltages:

Low voltage (LV): less than 1000 volts, used for connection between a residential or small commercial customer and the utility.

Medium voltage (MV; distribution): between 1000 volts (1 kV) and to 69 kV, used for distribution in urban and rural areas.

High voltage (HV; sub transmission less than 100 kV; sub transmission or transmission at voltage such as 115 kV and 138 kV), used for sub-transmission and transmission of bulk quantities of electric power and connection to very large consumers.

Extra high voltage (EHV; transmission) – over 230 kV, up to about 800 kV, used for long distance, very high power transmission.

Ultra high voltage (UHV) – higher than 800 kv

STRUCTURES

1. Transmission Tower:

Structures for overhead lines take a variety of shapes depending on the type of line. Structures may be as simple as wood poles directly set in the earth, carrying one or more cross-arm beams to support conductors, or "armless" construction with conductors supported on insulators attached to the side of the pole. Tubular steel poles are typically used in urban areas. High-voltage lines are often carried on lattice-type steel towers or pylons. For remote areas, aluminum towers may be placed by helicopters. Concrete poles have also been used. Poles made of reinforced plastics are also available, but their high cost restricts application.

Each structure must be designed for the loads imposed on it by the conductors. The weight of the conductor must be supported, as well as dynamic loads due to wind and ice accumulation, and effects of vibration. Where conductors are in a straight line, towers need only resist the weight since the tension in the conductors approximately balances with no resultant force on the structure. Flexible conductors supported at their ends approximate the form of a catenary, and much of the analysis for construction of transmission lines relies on the properties of this form.

A large transmission line project may have several types of towers, with "tangent" ("suspension" or "line" towers, UK) towers intended for most positions and more heavily constructed towers used for turning the line through an angle, dead-ending (terminating) a line, or for important river or road crossings. Depending on the design criteria for a particular line, semi-flexible type structures may rely on the weight of the conductors to be balanced on both sides of each tower. More rigid structures may be intended to remain standing even if one or more conductors are broken. Such structures may be installed at intervals in power lines to limit the scale of cascading tower failures.

Foundations for tower structures may be large and costly, particularly if the ground conditions are poor, such as in wetlands. Each structure may be stabilized considerably by the use of guy wires to counteract some of the forces applied by the conductors.

Power lines and supporting structures can be a form of visual pollution. In some cases the lines are buried to avoid this, but this "undergrounding" is more expensive and therefore not common.

For a single wood utility pole structure, a pole is placed in the ground, then three cross arms extend from this, either staggered or all to one side. The insulators are attached to the cross arms. For an "H"-type wood pole structure, two poles are placed in the ground, then a crossbar is placed on top of these, extending to both sides. The insulators are attached at the ends and in the middle. Lattice tower structures have two common forms. One has a pyramidal base, then a vertical section, where three cross arms extend out, typically staggered. The strain insulators are attached to the cross arms. Another has a pyramidal base, which extends to four support points. On top of this a horizontal truss-like structure is placed.

A grounded cable called a static line is sometimes strung along the tops of the towers to provide lightning protection. An optical ground wire is a more advanced version with embedded optical fibers for communication.

2. Circuits

A single-circuit transmission line carries conductors for only one circuit. For a three-phase system, this implies that each tower supports three conductors.

A double-circuit transmission line has two circuits. For three-phase systems, each tower supports and insulates six conductors. Single phase AC-power lines as used for traction current have four conductors for two circuits. Usually both circuits operate at the same voltage.

In HVDC systems typically two conductors are carried per line, but in rare cases only one pole of the system is carried on a set of towers.

A single-circuit line A double-circuit line

Parallel single-circuit lines Four circuits on one tower line

In some countries like Germany most power lines with voltages above 100 kV are implemented as double, quadruple or in rare cases even hextuple power line as rights of way are rare. Sometimes all conductors are installed with the erection of the pylons; often some circuits are installed later. A disadvantage of double circuit transmission lines is that maintenance works can be more difficult, as either work in close proximity of high voltage or switch-off of 2 circuits is required. In case of failure, both systems can be affected.

3. Insulators

Insulators must support the conductors and withstand both the normal operating voltage and surges due to switching and lightning. Insulators are broadly classified as either pin-type, which support the conductor above the structure, or suspension type, where the conductor hangs below the structure. The invention of the strain insulator was a critical factor in allowing higher voltages to be used.

At the end of the 19th century, the limited electrical strength of telegraph-style pin insulators limited the voltage to no more than 69,000 volts. Up to about 33 kV (69 kV in North America) both types are commonly used. At higher voltages only suspension-type insulators are common for overhead conductors.

Insulators are usually made of wet-process porcelain or toughened glass, with increasing use of glass-reinforced polymer insulators. However, with rising voltage levels, polymer insulators (silicone rubber based) are seeing increasing usage. China has already developed polymer insulators having a highest system voltage of 1100 kV and India is currently developing a 1200 kV (highest system voltage) line which will initially be charged with 400 kV to be upgraded to a 1200 kV line.

Suspension insulators are made of multiple units, with the number of unit insulator disks increasing at higher voltages. The number of disks is chosen based on line voltage, lightning withstand requirement, altitude, and environmental factors such as fog, pollution, or salt spray. In cases where these conditions are suboptimal, longer insulators must be used. Longer insulators with longer creepage distance for leakage current are required in these cases. Strain insulators must be strong enough mechanically to support the full weight of the span of conductor, as well as loads due to ice accumulation, and wind.

Porcelain insulators may have a semi-conductive glaze finish, so that a small current (a few milliamperes) passes through the insulator. This warms the surface slightly and reduces the effect of fog and dirt accumulation. The semiconducting glaze also ensures a more even distribution of voltage along the length of the chain of insulator units.

Polymer insulators by nature have hydrophobic characteristics providing for improved wet performance. Also, studies have shown that the specific creepage distance required in polymer insulators is much lower than that required in porcelain or glass. Additionally, the mass of polymer insulators (especially in higher voltages) is approximately 50% to 30% less than that of a comparative porcelain or glass string. Better pollution and wet performance is leading to the increased use of such insulators.

Insulators for very high voltages, exceeding 200 kV, may have grading rings installed at their terminals. This improves the electric field distribution around the insulator and makes it more resistant to flash-over during voltage surges.

4. Conductor:

The most common conductor in use for transmission today is aluminum conductor steel reinforced (ACSR). Also seeing much use is all-aluminum-alloy conductor (AAAC). Aluminum is used because it has about half the weight of a comparable resistance copper cable (though larger diameter due to lower specific conductivity), as well as being cheaper. Copper was more popular in the past and is still in use, especially at lower voltages and for grounding. Bare copper conductors are light green.

While larger conductors may lose less energy due to lower electrical resistance, they are more costly than smaller conductors. An optimization rule called Kelvin's Law states that the optimum size of conductor for a line is found when the cost of the energy wasted in the conductor is equal to the annual interest paid on that portion of the line construction cost due to the size of the conductors. The optimization problem is made more complex by additional factors such as varying annual load, varying cost of installation, and the discrete sizes of cable that are commonly made.

Since a conductor is a flexible object with uniform weight per unit length, the geometric shape of a conductor strung on towers approximates that of a catenary. The sag of the conductor (vertical distance between the highest and lowest point of the curve) varies depending on the temperature and additional load such as ice cover. A minimum overhead clearance must be maintained for safety. Since the temperature of the conductor increases with increasing heat produced by the current through it, it is sometimes possible to increase the power handling capacity (uprate) by changing the conductors for a type with a lower coefficient of thermal expansion or a higher allowable operating temperature.

Two such conductors that offer reduced thermal sag are known as composite core conductors (ACCR and ACCC conductor). In lieu of steel core strands that are often used to increase overall conductor strength, the ACCC conductor uses a carbon and glass fiber core that offers a coefficient of thermal expansion about 1/10 of that of steel. While the composite core is nonconductive, it is substantially lighter and stronger than steel, which allows the incorporation of 28% more aluminum (using compact

trapezoidal shaped strands) without any diameter or weight penalty. The added aluminum content helps reduce line losses by 25 to 40% compared to other conductors of the same diameter and weight, depending upon electric current. The carbon core conductor's reduced thermal sag allows it to carry up to twice the current ("ampacity") compared to all-aluminum conductor (AAC) or ACSR.

Power lines sometimes have spherical markers to meet International Civil Aviation Organization recommendations.

The power lines and their surroundings must be maintained by linemen, sometimes assisted by helicopters with pressure washers or circular saws which may work 3 times faster. However this work often occurs in the dangerous areas of the Helicopter height–velocity diagram.

Bundled Conductors

A bundle conductor

For higher amounts of current, bundle conductors are used for several reasons. Due to the skin effect, for larger conductors, the current capacity does not increase proportional to the cross-sectional area; instead, it is only with the linear dimension. Also, reactance decreases only slowly with size. But the cost and weight do increase with area. Due to this, several conductors in parallel become more economical.

Bundle conductors consist of several parallel cables connected at intervals by spacers, often in a cylindrical configuration. The optimum number of conductors depends on the current rating, but typically higher-voltage lines also have higher current. There is also some advantage due to lower corona loss. American Electric Power is building 765 kV lines using six conductors per phase in a bundle. Spacers must resist the forces due to wind, and magnetic forces during a short-circuit.

5. Ground wires

Overhead power lines are often equipped with a ground conductor (shield wire or overhead earth wire). The ground conductor is usually grounded (earthed) at the top of the supporting structure, to minimize the likelihood of direct lightning strikes to the phase conductors. In circuits with earthed neutral, it also serves as a parallel path with the earth for fault currents. Very high-voltage transmission lines may have two ground conductors. These are either at the outermost ends of the highest cross beam, at two V-shaped mast points, or at a separate cross arm. Older lines may use surge arresters every few spans in place of a shield wire; this configuration is typically found in the more rural areas of the United States. By protecting the line from lightning, the design of apparatus in substations is simplified due to lower stress on insulation. Shield wires on transmission lines may

include optical fibers (optical ground wires/OPGW), used for communication and control of the power system.

At some HVDC converter stations, the ground wire is used also as the electrode line to connect to a distant grounding electrode. This allows the HVDC system to use the earth as one conductor. The ground conductor is mounted on small insulators bridged by lightning arrestors above the phase conductors. The insulation prevents electrochemical corrosion of the pylon.

Medium-voltage distribution lines may also use one or two shield wires, or may have the grounded conductor strung below the phase conductors to provide some measure of protection against tall vehicles or equipment touching the energized line, as well as to provide a neutral line in Wye wired systems.

On some power lines for very high voltages in the former Soviet Union, the ground wire is used for PLC-radio systems and mounted on insulators at the pylons.

Transmission Tower:

The main supporting unit of overhead transmission line is transmission tower. Transmission towers have to carry the heavy transmission conductor at a sufficient safe height from ground. In addition to that all towers have to sustain all kinds of natural calamities. So transmission tower designing is an important engineering job where all three basic engineering concepts, civil, mechanical and electrical engineering concepts are equally applicable.

A power transmission tower consists of the following parts,

1. Peak of transmission tower2. Cross arm of transmission tower3. Boom of transmission tower4. Cage of transmission tower5. Transmission Tower Body6. Leg of transmission tower7. Stub/Anchor Bolt and Base plate assembly of transmission tower.

The main parts among these are shown in the pictures.

Peak: The portion above the top cross arm is called peak of transmission tower. Generally earth shield wire connected to the tip of this peak.

Cross Arm: Cross arms of transmission tower hold the transmission conductor. The dimension of cross arm depends on the level of transmission voltage, configuration and minimum forming angle for stress distribution.

Cage: The portion between tower body and peak is known as cage of transmission tower. This portion of the tower holds the cross arms.

Body: The portion from bottom cross arms up to the ground level is called transmission tower body. This portion of the tower plays a vital role for maintaining required ground clearance of the bottom conductor of the transmission line.

Design of Transmission Tower

During design of transmission tower the following points to be considered in mind,

(a) The minimum ground clearance of the lowest conductor point above the ground level.(b) The length of the insulator string.(c) The minimum clearance to be maintained between conductors and between conductor and

tower.(d) The location of ground wire with respect to outer most conductors.(e) The mid span clearance required from considerations of the dynamic behavior of conductor

and lightening protection of the line.

To determine the actual transmission tower height by considering the above points, we have divided the total height of tower in four parts,

1. Minimum permissible ground clearance (H1)2. Maximum sag of the conductor (H2)3. Vertical spacing between top and bottom conductors (H3)4. Vertical clearance between ground wire and top conductor (H4).

Safety Clearance for Power Transmission Lines

Right of way is one of the important safety clearance in Transmission line and this will mention the line clearance for the ground. Safety clearance is one of the important factor constructions of electrical power transmission line. If transmission line falls to ground what would be the maximum clearance to the ground is address in right of way in transmission line. The following are mentioned the right of way for different voltage level transmission lines.

ROW Width for 132 kV Transmission Line = 27 m ROW Width for 220 kV Transmission Line = 35 m ROW Width for 400 kV Transmission Line = 52 m ROW Width for 765 kV Transmission Line = 85 m

Span of Transmission Line

The Span of transmission line is also do major part to have a better safety clearance for power transmission line and there are several design span lengths of Transmission line calculations. The most commonly used design span lengths are

Basic or Normal Span Rulling or equivalent span Average Span Wind span Weight Span

Basic Span: Basic Span is much economical span and in here the line is designed over leveled ground. The required ground clearance is taking at maximum temperature.

Ruling or Equivalent Span: Ruling span use as a design span between the dead ends of power transmission line. This also uses to calculate the component of tension and tower spot on the profile is done by means of sag template.

Types of Transmission Tower

The vital factor of any power transmission line transmission tower. Main parameters of high voltage transmission lines depend on voltage level. There are various types of towers use for electrical power transmission lines and the major types of Transmission towers can categorized as bellow.

Suspension Tower Tension Tower Transposition Tower Special Tower

Suspension Towers

Suspension towers are on the straight line of transmission line it may vary maximum to degree of 5 angles. The suspension towers design to carry the only weight of the conductor in straight line position. Most of towers in any transmission line is fall into this type of tower category and construction cost of suspension type transmission lines are much cheaper compare to other types of transmission lines.

Tension Towers (Angle)

Tension towers use at locations where the angel of deviation is more than degree of 5. Tension towers are also known as angle towers and these type of tower are designed to take the tension load of the cable. Tension towers are mostly sued for turning points and for the section isolate locations.

The piece of line form one angle tower to other angle tower is known as section and length of the section may vary and depend on the geographical location. All the towers in between the section is suspension towers. Suspension towers are lightweight and much economical compare to angle towers.

According to the angle of deviation there are four types of transmission tower-

1. A – Type tower – angle of deviation 0o to 2o.2. B – Type tower – angle of deviation 2o to 15o. 3. C – Type tower – angle of deviation 15o to 30o.4. D – Type tower – angle of deviation 30o to 60o.

Transposition Tower

Transposition towers are specially use for transpose the conductors of three phase line . Transposition arrangement also called as span transposition. These type of towers are widely used in long transmission line. These types of towers are much less use in recently. Major idea behind transposition is the change the three phase according to determined arrangement to obtain better performance in Transmission line.

As per the force applied by the conductor on the cross arms, the transmission towers can be categorized in another way-

1. Tangent suspension tower and it is generally A - type tower.2. Angle tower or tension tower or sometime it is called section tower. All B, C and D types of

transmission towers come under this category.

Special Towers

Special towers are constructing at location in long spans for example river crossing, Lake crossings, other power line crossings and Vally crossings. The cost for special tower is much higher than suspension tower line costs. The design of special tower is much based on the location. Special Towers are widely use for tapping existing lines, Special termination towers and falling on the line route.

1. River crossing tower2. Railway/ Highway crossing tower

Based on numbers of circuits carried by a transmission tower, it can be classisfied as-

1. Single circuit tower2. Double circuit tower3. Multi circuit tower.

TOWER EXTENSIONS

Power transmission line traverse in various geographical areas and the ground level is not uniform everywhere. So body extension and leg extensions will provide great strength to the transmission tower to hold the load which applies to power transmission tower.

(a) Body Extensions

Body extension is use for increase the tower height and this will also obtain the required minimum ground clearance. These types of body extensions are widely used for road crossings, river crossing and where ground obstacles are happens. Transmission tower heights are normally different in one transmission tower to another tower. The major height different is apply by body extension and leg extensions. Normally body extensions are applied in +3 meters,+6 meters, +9 meters, +12 meters ... and +25 meters in height. There are very few tower which exceed the body or leg extension more than +25 meters. Place where body extensions is higher than 25 m the suitability is checked by reduce the span length and angle of deviation. For the transmission lines traverse in hilly terrain negative body extensions can applied to tensions towers by consider the economic factors.

(b) Leg Extensions

Leg extensions are widely use for any leg or any pair of transmission line tower legs locations were footing the towers are at different levels. These kind of circumstance happens highly in hilly terrain where the ground is not uniform. Unequal leg extension need special care to set the individual stubs using type of stub setting templates. Universal leg extension and Individual leg extensions are the major two types of leg extensions which widely use in power transmission tower construction. Normally spotting power transmission towers in hilly area need more revetment and benching so by

use of leg extension will reduce the benching and revetment of the hilly area and it will provide great stability for Power transmission tower.

TOWER SPOTTING

The efficient location of structures on the profile is an important component of line design. Structures of appropriate height and strength must be located to provide adequate conductor ground clearance and minimum cost. In the past, most tower spotting has been done manually, using templates, but several computer programs have been available for a number of years for the same purpose.

Manual Tower Spotting

A celluloid template, shaped to the form of the suspended conductor, is used to scale the distance from the conductor to the ground and to adjust structure locations and heights to (1) provide proper clearance to the ground; (2) equalize spans; and (3) grade the line.

The template is cut as a parabola on the maximum sag (usually at 49#C) of the ruling span and should be extended by computing the sag as proportional to the square of the span for spans both shorter and longer than the ruling span. By extending the template to a span of several thousand feet, clearances may be scaled on steep hillsides.

The form of the template is based on the fact that, at the time when the conductor is erected, the horizontal tensions must be equal in all spans of every length, both level and inclined, if the insulators hang plumb. This is still very nearly true at the maximum temperature.

The template, therefore, must be cut to a catenary or, approximately, a parabola. The parabola is accurate to within about one-half of 1% for sags up to 5% of the span, which is well within the necessary refinement.

Since vertical ground clearances are being established, the 49#C no-wind curve is used in the template. Special conditions may call for clearance checks. For example, if it is known that a line will have high temperature rise because of load current, conductor clearance should be checked for the estimated maximum conductor temperature.

One crossing over a navigable stream was designed for 88#C at high water. Ice and wet snow many times cause weights several times that of the 1/2-in radial ice loading, and conductors have been known to sag to within reach of the ground.

Such occurrences are not normally considered in line design, and when they occur, the line is taken out of service until the ice or snow drops. Checks made afterward have nearly always shown no permanent deformation.

Lecture 15C.3: Lattice Towers and Masts

1. INTRODUCTION

Towers or masts are built in order to fulfill the need for placing objects or persons at a certain level above the ground. Typical examples are:

Single towers for antennae, floodlight projectors or platforms for inspection, supervision or tourist purposes.

Systems of towers and wires serving transport purposes, such as ski lifts, ropeways, or power transmission lines.

For all kinds of towers the designer should thoroughly study the user's functional requirements in order to reach the best possible design for the particular structure. For example, it is extremely important to keep the flexural and torsional rotations of an antenna tower within narrow limits in order to ensure the proper functioning of the equipment.

The characteristic dimension of a tower is its height. It is usually several times larger than the horizontal dimensions. Frequently the area which may be occupied at ground level is very limited and, thus, rather slender structures are commonly used.

Another characteristic feature is that a major part of the tower design load comes from the wind force on the tower itself and its equipment, including wires suspended by the tower. To provide the necessary flexural rigidity and, at the same time, keeping the area exposed to the wind as small as possible, lattice structures are frequently preferred to more compact 'solid' structures.

Bearing in mind these circumstances, it is not surprising to find that the design problems are almost the same irrespective of the purpose to be served by the tower. Typical design problems are:

Establishment of load requirements. Consistency between loads and tower design. Establishment of overall design, including choice of number of tower legs. Consistency between overall design and detailing. Detailing with or without node eccentricities. Sectioning of structure for transport and erection.

In this lecture, towers for one particular purpose, i.e. the high voltage transmission tower, have been selected for discussion.

2. HIGH VOLTAGE TRANSMISSION TOWERS

2.1 Background

The towers support one or more overhead lines serving the energy distribution. Most frequently three-phase AC circuits are used requiring three live conductors each. To provide safety against lightning, earthed conductors are placed at the top of the tower, see Figures 1 and 2.

The live conductors are supported by insulators, the length of which increases with increasing voltage of the circuit. To prevent short circuit between live and earthed parts, including the surrounding environment, minimum mutual clearances are prescribed.

Mechanically speaking, the conductors behave like wires whose sag between their points of support depends on the temperature and the wire tension, the latter coming from the external loads and the pre-tensioning of the conductor. As explained in Section 2.4, the size of the tension forces in the conductor has a great effect upon the tower design.

2.2 Types of Towers

An overhead transmission line connects two nodes of the power supply grid. The route of the line has as few changes in direction as possible. Depending on their position in the line various types of towers occur such as (a) suspension towers, (b) angle suspension towers, (c) angle towers, (d) tension towers and, (e) terminal towers, see Figure 1. Tension towers serve as rigid points able to prevent progressive collapse of the entire line. They may be designed to serve also as angle towers.

To the above-mentioned types should be added special towers required at the branching of two or more lines.

In Figure 2 examples of suspension tower designs from four European countries are presented. Note similarities and mutual differences.

2.3 Functional Requirements

Before starting the design of a particular tower, a number of basic specifications are established. They are:

(a) Voltage.(b) Number of circuits.(c) Type of conductors.(d) Type of insulators.(e) Possible future addition of new circuits.(f) Tracing of transmission line.(g) Selection of tower sites.(h) Selection of rigid points.(i) Selection of conductor configuration.(j) Selection of height for each tower.

The tower designer should notice that the specifications reflect a number of choices. However, the designer is rarely in a position to bring about desirable changes in these specifications. Therefore, functional requirements are understood here as the electrical requirements which guide the tower design after establishment of the basic specifications.

The tower designer should be familiar with the main features of the different types of insulators. In Figure 3 three types of insulators are shown. They are all hinged at the tower crossarm or bridge.

Figure 4 shows the clearances guiding the shape of a typical suspension tower. The clearances and angles, which naturally vary with the voltage, are embodied in national regulations. Safety against lightning is provided by prescribing a maximum value of the angle v. The maximum swing u of the insulators occurs at maximum load on the conductor.

2.4 Loads on Towers, Loading Cases

The loads acting on a transmission tower are:

(a) Dead load of tower.(b) Dead load from conductors and other equipment.(c) Load from ice, rime or wet snow on conductors and equipment.(d) ice load, etc. on the tower itself(e) Erection and maintenance loads.(f) Wind load on tower.(g) Wind load on conductors and equipment.(h) Loads from conductor tensile forces.(i) Damage forces.(j) Earthquake forces.

It is essential to realize that the major part of the load arises from the conductors, and that the conductors behave like chains able to resist only tensile forces. Consequently, the dead load from the conductors is calculated by using the so-called weight span, which may be considerably different from the wind span used in connection with the wind load calculation, see Figure 5.

The average span length is usually chosen between 300 and 450 metres.

The occurrence of ice, etc. adds to the weight of the parts covered and it increases their area exposed to the wind. Underestimation of these circumstances has frequently led to damage and collapse. It is, therefore, very important to choose the design data carefully. The size and distribution of the ice load depends on the climate and the local conditions. The ice load is often taken as a uniformly distributed load on all spans. It is, however, evident that different load intensities are likely to occur in neighboring spans. Such load differences produce longitudinal forces acting on the towers, i.e. acting in the line direction.

The wind force is usually assumed to be acting horizontally. However, depending on local conditions, a sloping direction may have to be considered. Also, different wind directions (in the horizontal plane) must be taken into account for the conductors as well as for the tower itself. The maximum wind velocity does not occur simultaneously along the entire span and reduction coefficients are, therefore, introduced in the calculation of the load transferred to the towers.

The tensile forces in the conductors act on the two faces of the tower in the line direction(s). If they are balanced no longitudinal force acts on a tower suspending a straight line. For angle towers they result in forces in the angle bisector plane, and for terminal towers they cause heavy longitudinal forces. As the tensile forces vary with the external loads, as previously mentioned, even suspension towers on a straight line are affected by longitudinal forces. For all types of towers the risk of mechanical failure of one or more of the conductors has to be considered.

The loads and loading cases to be considered in the design are usually laid down in national regulations.

2.5 Overall Design and Truss Configuration

The outline of the tower is influenced by the user's functional requirements. However, basically the same requirements may be met by quite different designs. In general, the tower structure consists of three parts: the cross arms and/or bridges, the peaks, and the tower body.

Statically speaking, the towers usually behave like cantilevers or frames, in some cases with supplementary stays. For transmission lines with 100 kV voltage or more, the use of steel lattice structures is nearly always found advantageous because they are:

Easily adaptable to any shape or height of tower. Easily divisible in sections suitable for transport and erection. Easy to repair, strengthen and extend. Durable when properly protected against corrosion.

By far the most common structure is a four-legged tower body cantilevering from the foundation, see Figure 6. The advantages of this design are:

The tower occupies a relatively small area at ground level. Two legs share the compression from both transverse and longitudinal loads. The square or rectangular cross-section (four legs) is superior to a triangular tower body (three

legs) for resisting torsion. The cross-section is very suitable for the use of angles, as the connections can be made very

simple.

The following remarks in this section relate mainly to a cantilever structure. However, many features also apply to other tower designs.

For a cantilever structure, the tower legs are usually given a taper in both main directions enabling the designer to choose the same structural section on a considerable part of the tower height. The taper is also advantageous with regard to the bracing, as it reduces the design forces (except for torsional loads).

The bracing of the tower faces is chosen either as a single lattice, a cross bracing or a K-bracing, possibly with redundant members reducing the buckling length of the leg members, for example see Figure 6. The choice of bracing depends on the size of the load and the member lengths. The most common type is cross bracing. Its main advantage is that the buckling length of the brace member in compression is influenced positively by the brace member in tension, even with regard to deflection perpendicular to the tower face.

Generally, the same type of bracing is chosen for all four tower body faces, most frequently with a staggered arrangement of the nodes, see Figure 7. This arrangement provides better space for the connections, and it may offer considerable advantage with respect to the buckling load of the leg members. This advantage applies especially to angle sections when used as shown in Figures 10 and 11, since it diminishes the buckling length for buckling about the 'weak' axis v-v.

Irrespective of the type of bracing, the tower is generally equipped with horizontal members at levels where taper changes leg. For staggered bracings these members are necessary to 'turn' the leg forces. Torsional forces, mostly acting at crossarm bottom levels, are distributed to the tower faces by means of horizontal bracings, see Figure 8.

Cross arms and earthwire peaks are, in principle, designed like the tower itself. However, as the load on the cross arms rarely has an upward component, cross arms are sometimes designed with two bottom chords and one upper chord and/or with single lattice bracings in the non-horizontal faces.

2.6 Structural Analysis

Generally, the structural analysis is carried out on the basis of a few very rough assumptions:

The tower structure behaves as a self-contained structure without support from any of the conductors.

The tower is designed for static or quasi-static loads only.

These assumptions do not reflect the real behavior of the total system, i.e. towers and conductors, particularly well. However, they provide a basis from simple calculations which have broadly led to satisfactory results.

Generally speaking, a tower is a space structure. It is frequently modeled as a set of plane lattice structures, which are identical with the tower body planes together with the planes of the cross arms and the horizontal bracings mentioned in Section 2.5.

In a simplified calculation a four-legged cantilevered structure is often assumed to take the loads as follows:

(a) Centrally acting, vertical loads are equally distributed between the four legs.(b) Bending moments in one of the main directions produce an equal compression in the two legs

of one side, and equal tension in the two legs of the other side. The shear forces are resisted by the horizontal component of the leg forces and the brace forces (thus, the leg taper has a significant influence on the design of the bracing).

(c) Torsional moments broadly produce shear forces in the tower body faces, i.e. in the braces.

A classical analysis assuming hinges in all nodes leads to very simple calculations. However, the effect of redundancies should be considered, especially concerning the forces and moments in the brace members.

Although this approach is satisfactory in most cases attention must be drawn to the function of redundant members, which in some cases may change the load distribution considerably. In addition, the effect of fixed connections (as opposed to hinged connections) must be considered, since they produce moments in the bracing members. The effect of eccentricities in the joints should also be taken into account, see Section 2.7.

Finally, the distribution of an eccentric horizontal load is studied. In Figure 9 the force H is acting at the cross arm bottom level. Without horizontal bracing in the tower, three tower body planes are affected by H. The deflections of the plane lattice structures of the tower body deform the rectangle ABCD to a parallelogram A¢ B¢ C¢ D¢ . By adding member AC or BD this deformation is restricted and all four tower body planes participate in resisting the force H.

2.7 Detailing of Joints

The detailed design is governed by a number of factors influencing the structural costs once the overall design has been chosen, such as:

Simple and uniform design of connections. Simple shaping of structural components. Details allowing for easy transportation and erection. Details allowing for proper corrosion protection.

As an introductory example of design and calculation, a segment of a four-legged tower body is discussed, see Figure 10. All members are made of angle sections with equal legs. The connections are all bolted without the use of gussets, except for a spacer plate at the cross bracing interconnection. This very simple design requiring a minimum of manufacturing work is attained by the choice and orientation of the leg and brace member sections.

By choosing the design described above, some structural eccentricities have to be accepted. They arise from the fact that the axes of gravity of the truss members do not intersect at the theoretical nodes. According to the bending caused by the eccentricities they may be classified as in-plane or out-of-plane eccentricities. In Figure 11, the brace forces C and T meet at a distance eo from the axis of gravity. The resultant force DS produces two bending moments: Me = DS´ eo and Mf=DS´ e1. These moments are distributed among the members meeting at the joint according to their flexural stiffness, usually leaving the major part to the leg members. As z-z is the 'strong' axis of the leg section, a resultant moment vector along axis v-v will be advantageous. This is achieved, when eo=-e1¢ . In this case C and T intersect approximately at the middle of the leg of the section. Usually this situation is not fully practicable without adding a gusset plate to the joint.

Additional eccentricity problems occur when the bolts are not placed on the axis of gravity, especially when only one bolt is used in the connection (eccentricities ec and et).

The out-of-plane eccentricity causing a torsional moment, V = H´ e2, acting on the leg may be measured between the axes of gravity for the brace members (see Figure 11). However, the torsional stiffness of the leg member may be so moderate - depending on its support conditions - that V cannot be transferred by the leg and, consequently, e2 must diminish. The latter causes bending out-of-plane in the brace members.

The leg joint shown in Figure 10 is a splice joint in which an eccentricity e3 may occur. In this case there is a change of leg section, or the gravity axis for the four (or two) splice plates in common does not coincide with the axis of the leg(s). For legs in compression the joint must be designed with some flexural rigidity to prevent unwanted action as a hinge.

The joint eccentricities have to be carefully considered in the design. As the lower part of the leg usually is somewhat oversized at the joint - this is, in fact, the reason for changing leg section at the joint - a suitable model would be to consider the upper part of the leg centrally loaded and thus, let the lower part resist the eccentricity moment. The splice plates and the bolt connections must then be designed in accordance with this model.

The bolted connections might easily be replaced by welded connections with no major changes of the design. However, except for small structures, bolted connections are generally preferred; as they offer the opportunity to assemble the structural parts without damaging the corrosion protection, see Section 2.8.

This introductory example is very typical of the design with angle sections. Nevertheless some additional comments should be added concerning the use of gussets and multiple angle sections.

The use of gussets is shown in Figure 12. They provide better space for the bolts, which may eliminate the in-plane eccentricities, and they allow for the use of double angle sections. In the latter case out-of-plane eccentricities almost vanish.

For heavily loaded towers it might be suitable to choose double or even quadruple angle sections for the legs. Figure 13 shows some possibilities.

Towers designed with other profiles than angles

In principle any of the commercially available sections could be used. However, they have to compete with the angle sections as regards the variety of sections available and the ease of designing and manufacturing simple connections. So far only flat bars, round bars and tubes have been used, mostly

with welded connections. The use is limited to small size towers for the corrosion reasons mentioned above.

In other contexts, e.g. high rise TV towers, circular sections may be more interesting because their better shape reduces wind action.

Construction joints and erection joints

The tower structure usually has to be subdivided into smaller sections for the sake of corrosion protection, transportation and erection. Thus a number of joints which are easy to assemble on the tower site, have to be arranged. Two main problems have to be solved: the position and the detailing of the joints.

In Figure 14 two examples of the joint positions are shown. The framed structure is divided into lattice structure bodies, each of which may be fully welded, and stays. The cantilevered structure usually is subdivided into single leg and web members.

The two types of joints are lap (or splice) joints and butt plate joints. The former is very suitable for angle sections. The latter is used for all sections, but is mostly used for joints in round tube or bar sections. Figure 15 shows some examples of the two types.

2.8 Corrosion Protection

Today, corrosion protection of steel lattice towers is almost synonymous with hot-galvanizing, possibly with an additional coating. The process involves dipping the structural components into a galvanic bath to apply a zinc layer, usually about 100 m m thick.

No welding should be performed after galvanizing, as it damages the protection. The maximum size of parts to be galvanized is limited by the size of the available galvanic bath.

3. CONCLUDING SUMMARY

The overall design of a lattice tower is very closely connected with the user's functional requirements. The requirements must be studied carefully.

A major part of the design loads on the tower results from the wind force on tower and equipment.

The occurrence of an ice cover on the tower and equipment must be considered in the design. For towers supporting wires, differential loads in the wire direction must be taken into

account. For systems of interconnected towers it must be considered that the collapse of one tower

may influence the stability of a neighboring tower. In most cases a cantilevered tower with four legs is preferred, as it offers structural advantages

and occupies a relatively small ground area. The type of bracing greatly affects the stability of both legs and braces. K-bracings and/or

staggered cross bracings are generally found advantageous. Horizontal braces at certain levels of the tower add considerably to its torsional rigidity. Angle sections are widely used in towers with a square or rectangular base, as they permit very

simple connection design. Both in-plane and out-of-plane eccentricities in the connections must be considered. A proper, long lasting corrosion protection must be provided. The protection method

influences the structural design.

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