BEF Electric Power Transmission and Distribution

BEF 34603 Electric Power Transmission and Distribution
Chapter 2 & 3 MECHANICAL DESIGN OF OVERHEAD LINES Mechanical Design of Overhead Lines Line Insulator BEF Electric Power Transmission and Distribution

BEF 34603 Electrical Power Transmission and Distribution
Outlines Introduction Main Components of Overhead Lines Conductor Materials Line Supports Insulators Corona Sag Some Mechanical Principles BEF Electrical Power Transmission and Distribution

Introduction Electrical power can be transmitted either by means of underground cables or overhead lines. The underground cables are rarely used for power transmission due to: Power is generally transmitted over long distance to load centres. (Costs very high for installation) Electric power has to be transmitted at high voltages for economic reasons. (very difficult to provide proper insulation to the cable to withstand HV) Therefore, as a rule, power transmission over long distance is carried out using overhead lines. BEF Electrical Power Transmission and Distribution

Main Components of Overhead Lines
In general, the main components of overhead line are : Conductor Line Supports Insulators Cross arms Miscellaneous items such as lightning arrestors, phase plates, danger plates, anti- climbing wires and etc BEF Electrical Power Transmission and Distribution

Main Components of Overhead Lines
BEF Electrical Power Transmission and Distribution

Conductors Materials The conductor material used for transmission of electric power should have the following properties: High electrical conductivity High tensile strength in order to withstand mechanical stress. Low cost so that it can be used for long distances Low specific gravity so that weight per unit volume is small. BEF Electrical Power Transmission and Distribution

Conductor Materials The most commonly used conductor materials for overhead lines are: Copper Aluminium Steel –cored aluminium Galvanized steel Cadmium copper The choice of a particular material will depend on cost, the required electrical and mechanical properties and also local conditions. BEF Electrical Power Transmission and Distribution

Conductor Materials Copper An ideal material for overhead lines owing to its high electrical conductivity and greater tensile strength. Has higher current density( current carrying capacity is quite large) Advantages: 1) smaller X-sectional area of conductor is required. 2) the area offered by the conductor to wind loads is reduced. Moreover, this metal is quite homogenous, durable and has high scrap value. BEF Electrical Power Transmission and Distribution

Conductor Materials Aluminium is cheap and light as compared to copper . but it has much smaller conductivity and tensile strength. relative comparison of two materials: the conductivity of aluminium is 60% that of copper- for any particular transmission efficiency, the X- sectional area of conductor must be larger than in copper. for the same resistance, the diameter of aluminium conductor is about 1.26 times the diameter of copper conductor. BEF Electrical Power Transmission and Distribution

Conductor Materials Aluminium the increased X-section of aluminium exposes a greater surface to wind pressure. therefore supporting tower must be designed for greater strength. this often requires the use of higher towers with consequence of greater sag. the specific gravity of aluminium (2.71 gm/cc) is lower than copper (8.9gm/cc)- an aluminium has almost one-half the weight of equivalent copper conductor. (for this reason, the supporting structures for aluminium need not be made so strong as copper conductor) BEF Electrical Power Transmission and Distribution

Conductor Materials Aluminium being light, is liable to greater swings and hence cross arms are required. due to lower tensile strength and higher coefficient of linear expansion of aluminium, the sag is greater in aluminium conductor. considering the combined properties of cost, conductivity, tensile strength, weight, etc, aluminium has an edge over copper. therefore it is widely used as a conductor material. BEF Electrical Power Transmission and Distribution

Conductor Materials Steel Cored Aluminium Due to low tensile strength, aluminium conductors produce greater sag. This prohibits their use for larger span and make them unsuitable for long distance transmission. In order to increase the tensile strength, the aluminium conductor is reinforced with a core of galvanized steel wires. This composite conductor is known as steel cored aluminium and is abbreviated as A.C.S.R (aluminium conductor steel reinforced). BEF Electrical Power Transmission and Distribution

Conductor Materials Steel Cored Aluminium Steel cored aluminium conductor consists of central core of galvanized steel wires surrounded by a number of aluminium strands. Usually diameter of both steel and aluminium wires is the same. BEF Electrical Power Transmission and Distribution

Conductor Materials Steel Cored Aluminium The steel cored aluminium conductors have the following advantages: The reinforcement with steel increases the tensile strength but at the same time keeps the composite conductor light. Therefore, steel cored aluminium conductors will produce smaller sag and hence longer span can be used. Due to small sag with steel cored aluminium conductors, towers of smaller height can be used. BEF Electrical Power Transmission and Distribution

Conductor Materials Galvanized Steel
Steel has very high tensile strength. Galvanized steel conductor can be used for extremely long spans or for short line sections exposed to abnormally high stresses due to climatic conditions. Suitable in rural areas where cheapness is the main consideration. Is not suitable for transmitting large power over a long distance due to poor conductivity and high resistance of steel. BEF Electrical Power Transmission and Distribution BEK 4213 Electrical Power Transmission and Distribution

Conductor Materials Cadmium Steel The copper conductor is alloyed with cadmium. An addition 1% and 2% cadmium to copper increases the tensile strength by 50% and conductivity is only reduced by 15%. It is useful for exceptionally long spans However due to high cost of cadmium, this conductor will be economical only for lines of small X-section. i.e. Where the cost of conductor material is comparatively small compared with the cost of supports. BEF Electrical Power Transmission and Distribution

Line Supports Line supports is the supporting structures for overhead line conductors such as poles and towers. In general the line supports should have the following properties: High mechanical strength to withstand the weight of conductors and wind loads. Light in weight without loss of mechanical strength. Cheap in cost and economical to maintain. Longer life. Easy accessibility of conductors for maintenance. BEF Electrical Power Transmission and Distribution

Line Supports The line supports used for transmission of electric power are of various types including: Wooden poles Steel poles R.C.C poles Lattice steel towers The choice of supporting structure for a particular case depends upon the line span, X-sectional area, line voltage, cost and local condition. BEF Electrical Power Transmission and Distribution

Line Supports Wooden poles Made of seasoned wood and suitable for lines of moderate X-sectional area and relatively shorter spans (up to 50m) Cheap, easily available. Providing insulating properties and widely used for distribution purposes in rural areas as an economical proposition. Generally tend to rot below the ground level, causing foundation failure. BEF Electrical Power Transmission and Distribution

Line Supports Wooden poles Double pole structures of H type Double pole structures of A type BEF Electrical Power Transmission and Distribution

Line Supports Wooden poles The main disadvantages are: Tendency to rot below the ground level. (smaller life years) Cannot be used for voltages higher than 20kV Less mechanical strength Require periodical inspection BEF Electrical Power Transmission and Distribution

Line Supports Steel poles Often used as a substitute for wooden poles Have greater mechanical strength, longer life and permit longer spans to be used. Generally used for distribution purposes in the cities. Need to be galvanized or painted in order to prolong its life. Three types of steel poles are rail poles, tubular poles and rolled steel joints. BEF Electrical Power Transmission and Distribution

Line Supports RCC poles (reinforced concrete poles) Very popular as line supports in recent year. Have greater mechanical strength, longer life and permit longer spans than steel poles. Give good outlook, require little ,maintenance and have good insulating properties. The main difficulty is the high cost of transport owing to their heavy weight. Therefore such poles often manufactured at the site in order to avoid heavy cost of transportation. BEF Electrical Power Transmission and Distribution

Line Supports RCC poles (reinforced concrete poles) BEF Electrical Power Transmission and Distribution

Line Supports Steel towers In practice wooden, steel and reinforced concrete poles are used for distribution purposes at low voltages (up 11kV). For long distance transmission at higher voltage , steel towers are used. Have greater mechanical strength, longer life, can withstand most severe climatic conditions and permit the use of longer spans. The risk of interrupted service due to broken insulation is considerably reduced owing to longer spans. BEF Electrical Power Transmission and Distribution

Line Supports Steel towers Tower footings are usually grounded by driving rods into the earth. This minimizes the lightning troubles as each tower acts a lightning conductor. BEF Electrical Power Transmission and Distribution

Cross Arms Cross Arms Hold the insulators and conductors BEF Electrical Power Transmission and Distribution

Insulators The overhead lines conductors should be supported on the poles or towers in such a way that the currents from conductors do not flow to earth through towers/poles. This is achieved by securing line conductors to supports with the help of insulators. The insulators provide necessary insulation between line conductors and tower/poles thus prevent any leakage current from conductors to earth. BEF Electrical Power Transmission and Distribution

Insulators In general, the insulators should have the following desirable properties: High mechanical strength in order to withstand conductor load, wind load and etc. High electrical resistance of insulator material in order to avoid leakage currents to earth. High relative permittivity of insulator material in order that dielectric strength is high. The insulator material should be non-porous, free from impurities and cracks otherwise the permittivity is lowered. High ratio of puncture strength to flash over. BEF Electrical Power Transmission and Distribution

Insulators The most commonly used material for insulators of overhead line is porcelain but glass, steatite and special composition materials are also used to a limited extent. Porcelain is stronger mechanically than glass, gives less trouble from leakage current and is less affected by changes of temperature. There are several types of insulators but the most commonly used are pin type insulator, suspension type insulator, strain type insulator BEF Electrical Power Transmission and Distribution

Type of Insulators Pin type insulators is secured to the cross-arm on the pole there is a grove on the upper end of the insulator for housing the conductor. are used for transmission and distribution of electric power at voltages up to 33kV. (beyond 33kV it becomes too bulky and hence uneconomical) BEF Electrical Power Transmission and Distribution

Type of Insulators Causes of Pin type insulators failure is mainly due to electric stresses (line voltage) and may cause the breakdown of the insulator. electrical breakdown of insulator can occur either by flash over or puncture. in flash over, an arc occurs between the line conductor and insulator pin. Discharge jumps across the air gaps, following shortest distance. Figure shows the arcing distance (a+b+c) for the insulation. In case flash over, the insulator will continue to act in its proper capacity unless extreme heat produced by the arc destroys the insulators. BEF Electrical Power Transmission and Distribution

Type of Insulators Causes of Pin type insulators failure In case of puncture, the discharge occurs from conductor to pin through the body of the insulator. When breakdown is involved, the insulator is permanently destroyed due to excessive heat. In practical, sufficient thickness of porcelain is provided in the insulator to avoid puncture by the line voltage. The ratio of puncture strength to flash over voltage is known as safety factor Safety factor of insulator = puncture strength/ flashover voltage * It is desirable that the value of safety factor is high so that flash over takes place before the insulator gets punctured. For pin type, the value of safety factor is about 10. BEF Electrical Power Transmission and Distribution

Type of Insulators Suspension type insulators For high voltage (>33kV), it is usually in practice to use suspension type insulators. They consist of a number of porcelain discs connected in series by metal links in the form of a string. BEF Electrical Power Transmission and Distribution

Type of Insulators Suspension type insulators The conductor is suspended at the bottom end of this string while other end is secured to the cross arm of the tower. The number of discs in series would obviously depend upon the working voltage. Each unit or disc is designed for low voltage, say 11kV. If the working voltage is 66kV, then six discs will be provided on the string. BEF Electrical Power Transmission and Distribution

Type of Insulators Advantages of Suspension type insulators Cheaper than pin type insulators for voltage >33kV. Each unit or disc of suspension type is designed for low voltage (usually 11kV). Depending on the working voltage, the desired number of discs can be connected in series. If any one disc is damaged, the whole string does not become useless because the damaged disc can be replaced. The suspension arrangement provides greater flexibility to the line. BEF Electrical Power Transmission and Distribution

Type of Insulators Advantages of Suspension type insulators In case of increased demand, it is found more practical to increase the line voltage than to provide another set of conductors. The additional insulation required for the increased voltage can be easily obtained in suspension arrangement by adding the desired number of discs. The suspension insulators are generally used with steel towers. As the conductors runs below the earthed cross-arm of the tower, therefore, this arrangement provides partial protection from lightning. BEF Electrical Power Transmission and Distribution

Type of Insulators Strain Insulators Is used when there is dead end of the line or there is corner or sharp curve which the line is subjected to greater tension. BEF Electrical Power Transmission and Distribution

Type of Insulators Shackle Insulators Were used as strain insulators. Today, they are used for LV distribution lines. Can be used either in a horizontal position or in a vertical position. They can be directly fixed to the pole with bolt or to the cross arm. BEF Electrical Power Transmission and Distribution

Potential Distribution over Suspension Insulators String
A string of suspension insulators consists of a number of porcelain discs connected in series through metallic links. BEF Electrical Power Transmission and Distribution

The porcelain portion of each disc is in between two metal links. Each disc forms a capacitor C as shown in previous figure. This is known as mutual capacitance or self-capacitance. If there were mutual capacitance alone, then charging current would have been the same through all the discs and consequently voltage across each unit would be the same, i.e. V/3. In actual practice, capacitance also exists between metal fitting of each disc and tower or earth. This is known as shunt capacitance C1. BEF Electrical Power Transmission and Distribution

Due to shunt capacitance, charging current is not the same through all the discs of the string. Therefore, voltage across each disc will be different. The disc nearest to the line conductor will have the maximum voltage. (because charging current through the string has the maximum value at the disc nearest to the conductor) Thus referring the figure (iii), V3 will be much more than V2 or V1. BEF Electrical Power Transmission and Distribution

The following points may be noted regarding the potential distribution over a string of suspension insulators: The voltage on a string of suspension insulators does not distribute uniformly across the individual discs due to the presence of shunt capacitance. The disc nearest to the conductor has maximum voltage across it. As we move towards the cross arm, the voltage across each disc goes on decreasing. The unit nearest to the conductor is under maximum electrical stress and is likely to be punctured. If the voltage across the string were d.c., then voltage across each unit would be the same. It is because insulator capacitance are ineffective for d.c. BEF Electrical Power Transmission and Distribution

String Efficiency The voltage applied across the string of suspension insulators is not uniformly distributed across various discs. The disc nearest to the conductor has much higher potential than the other discs. This unequal potential distribution is undesirable and is usually expressed in terms of string efficiency. String efficiency = Voltage across the string/ (n x Voltage across disc nearest to conductor) n is number of the discs in the string. BEF Electrical Power Transmission and Distribution

String Efficiency String efficiency is an important consideration since it decides the potential distribution along the string. The greater string efficiency, the more uniform is the voltage distribution. Thus 100% string efficiency is an ideal case for which the voltage across the each disc will be exactly the same. BEF Electrical Power Transmission and Distribution

Mathematical Expression
Let us suppose that self capacitance of each disc is C. Assume that shunt capacitance C1 is some fraction K of self-capacitance i.e. C1 = KC. Starting from the cross arm or tower, the voltage across each unit is V1, V2 and V3 respective as shown in figure. BEF Electrical Power Transmission and Distribution

Applying Kirchhoff’s current law to node A, we get, Note that current through capacitor = Voltage/ capacitive reactance BEF Electrical Power Transmission and Distribution

Applying Kirchhoff’s current law to node B, we get, BEF Electrical Power Transmission and Distribution

Voltage between conductor and earth (i.e. tower) is BEF Electrical Power Transmission and Distribution

From expression (i), (ii) and (iii) we get Therefore the voltage across top unit, Voltage across second unit from top, Voltage across third unit from top BEF Electrical Power Transmission and Distribution

String Coefficient BEF Electrical Power Transmission and Distribution

Method of Improving string Efficiency
By Using Longer Cross arm The value of string efficiency depends upon the value of K (ratio of shunt capacitance to mutual capacitance). The lesser the value of K, the greater is the string efficiency and more uniform voltage distribution. The value of K can be decreased by reducing the shunt capacitance. In order to reduce shunt capacitance, the distance of conductor from the tower must be increased (longer cross arm) BEF Electrical Power Transmission and Distribution BEK 4213 Electrical Power Transmission and Distribution

By Using Longer Cross arm Limitations of cost and strength of tower do not allow the use of very long cross arm. In practice K=0.1 is the limit that can be achieved by this method. BEF Electrical Power Transmission and Distribution

By Grading the Insulators Insulators of different dimensions are chosen that each has a different capacitance. The insulators are capacitance graded. i.e. they are assembled in the string in such a way that the top unit has the minimum capacitance , increasing progressively as the bottom unit (nearest to conductor) Since the voltage is inversely proportional to capacitance, this method tends to equalize the potential distribution across the discs in the string BEK 4213 Electrical Power Transmission and Distribution BEF Electrical Power Transmission and Distribution

By Grading the Insulators This method has disadvantage that a large number of different sized insulators are required. However a good results can be obtained by using standard insulators for most of the string and larger for that near to the line conductor. BEF Electrical Power Transmission and Distribution

By Using Guard Ring The potential across each disc in a string can be equalized by using a guard ring which is a metal ring electrically connected to the conductor and surrounding the bottom insulator. BEF Electrical Power Transmission and Distribution

By Using Guard Ring The guard ring introduces capacitance between metal fittings and the line conductor. The guard ring is contoured in such way that shunt capacitance currents i1 and i2 are equal to metal fitting capacitance currents i’1 and i’2. The result is that same charging current I flows through each unit of string. Consequently there will be uniform potential distribution across the discs. BEK 4213 Electrical Power Transmission and Distribution BEF Electrical Power Transmission and Distribution

Important Points!!!!!! While solving problems relating to string efficiency, the following points must be kept in mind: The maximum voltage appears across the disc nearest to the conductor The voltage across the string is equal to phase voltage. i.e. Voltage across string = voltage between line and earth = phase voltage. Line voltage = √3 x voltage across the string BEK 4213 Electrical Power Transmission and Distribution BEF Electrical Power Transmission and Distribution

BEK 4213 Electrical Power Transmission and Distribution
Example Example 1 In a 33kV overhead line, there are three units in the string of insulators. If the capacitance between each insulator pin and earth is 11% of self-capacitance of each insulator, find (i) the distribution of voltage over 3insulators (ii) string efficiency BEK 4213 Electrical Power Transmission and Distribution

Example Example 2 A 3 phase transmission line is being supported by three disc insulators. The potentials across top unit (near to the tower) and middle unit are 8kV and 11kV respectively. Calculate (i) the ratio of capacitance between pin and earth to the self-capacitance of each unit (ii) the line voltage (iii) string efficiency BEF Electrical Power Transmission and Distribution BEK 4213 Electrical Power Transmission and Distribution

Example Example 3 Each line of 3 phase system is suspended by a string of 3 similar insulators. If the voltage across the line is 17.5kV, calculate the line to neutral voltage. Assume that the shunt capacitance between each insulator and earth is 1/8th of the capacitance of the insulator itself. Also find the string efficiency. BEF Electrical Power Transmission and Distribution

Corona Corona is the phenomena of violet glow, hissing noise and production of ozone gas in an overhead transmission line. Corona are caused when air around an energized conductors get ionized causing a discharge. Factors affecting corona Atmosphere Conductor size Spacing between conductors Line voltage BEK 4213 Electrical Power Transmission and Distribution BEF Electrical Power Transmission and Distribution

Corona Advantages and Disadvantages of Corona Advantages
Due to corona formation, the air surrounding becomes conducting and virtual diameter of the conductor is increased. The increased diameter reduces the electrostatic stresses between the conductors. Corona reduces the effects of transients produced by surges. BEF Electrical Power Transmission and Distribution BEK 4213 Electrical Power Transmission and Distribution

Corona Advantages and Disadvantages of Corona Disadvantages
Corona is accompanied by a loss of energy. This effects the transmission efficiency of the line. Ozone is produced by corona and may cause corrosion of the conductor due to chemical action. The current drawn by the line due to corona is non-sinusoidal and hence non-sinusoidal voltage drop occurs in the line. This may cause interference with neighboring communication lines. BEF Electrical Power Transmission and Distribution BEK 4213 Electrical Power Transmission and Distribution

Corona Method of reducing Corona effect By increasing conductor size
-the voltage at which corona occurs is raised and hence reduce the corona effect considerable. By increasing conductor spacing ↑ spacing ↑the voltage at which corona to occur. Hence reduce the corona effect. (increase too much will increase the cost of supporting structure (i.e. bigger cross arm and tower) BEK 4213 Electrical Power Transmission and Distribution BEF Electrical Power Transmission and Distribution

Sag in Overhead Lines Figure shows a conductor suspend between to supports A and B. The conductor is not fully stretched but is allowed to have a dip. The lowest point on the conductor is O and sag is S. BEK 4213 Electrical Power Transmission and Distribution BEF Electrical Power Transmission and Distribution

Sag in Overhead Lines While erecting an overhead line, it is very important that conductors are under safe tension If the conductors are too much stretched between supports, the stress in the conductor may reach unsafe value and might cause conductor to break. In order to permit safe tension, the conductors are not fully stretched but are allowed to have a sag. Sag can be defined as the difference in level between points of supports and the lowest point on the conductor. BEK 4213 Electrical Power Transmission and Distribution BEF Electrical Power Transmission and Distribution

Calculation of Sag The sag should be adjusted so that the tension in the conductors is within safe limits. The tension is governed by conductor weight, effect of wind, ice loading and temperature variations. In standard practice , it is always to keep conductor tension less than 50% of the ultimate tensile strength. The sag can be calculated based on two cases: when supports are equal levels. when supports are at unequal levels. BEK 4213 Electrical Power Transmission and Distribution BEF Electrical Power Transmission and Distribution

Calculation of Sag BEF Electrical Power Transmission and Distribution

Calculation of Sag When supports are at equal levels. S = Sag
l = Length of span w = Weight per unit length of conductor T = Tension in the conductor BEK 4213 Electrical Power Transmission and Distribution BEF Electrical Power Transmission and Distribution

Calculation of Sag When supports are at unequal levels. We can see this condition at hilly areas. l = Length of span h = Difference in levels between two supports x1 = Distance of support at lower level (A) from O. x2 = Distance of support at higher level (B) from O. T = Tension in the conductor W = Weight per unit length of conductor. BEF Electrical Power Transmission and Distribution

BEK 4213 Electrical Power Transmission and Distribution
Calculation of Sag Effect of Wind and Ice loading In actual condition, a conductor may have ice coating and simultaneously subjected to wind. The weight of ice acts vertically downwards in the same direction as the weight of conductor. The force due to wind is assumed to act horizontally ( at right angle to the projected surface of the conductor) Hence the total force on the conductor is the sum of horizontal and vertical forces. BEK 4213 Electrical Power Transmission and Distribution

Calculation of Sag Effect of Wind and Ice loading Total weight of conductor per unit length is where BEF Electrical Power Transmission and Distribution

Calculation of Sag Example 4 A 132kV transmission line has the following data: Wt. of conductor = 680kg/km Length of span = 260m Ultimate strength = 3100kg Safety factor = 2 Calculate the height above ground at which the conductor should be supported. Ground clearance required is 10 meters. BEF Electrical Power Transmission and Distribution

Calculation of Sag Example 5 A transmission line has a span of 150m between level supports. The conductor has a cross-sectional area of 2 cm2. the tension in the conductor s 2000kg. If the specific gravity of the conductor material is 9.9 gm/cm3 and wind pressure is 1.5 kg/m length, calculate the sag. What is the vertical sag. BEF Electrical Power Transmission and Distribution

Calculation of Sag Example 6 The towers of height 30m and 90m respectively support a transmission line conductor at water crossing. The horizontal distance between the towers in 500m. If the tension in the conductor is 1600kg, find the minimum clearance of the conductor and water and clearance mid-way between the supports. Weight of conductor is 1.5kg/m. bases of the towers can be considered to be at water level. BEF Electrical Power Transmission and Distribution

Some Mechanical Principles
Some important points in the mechanical design of overhead transmission lines. Tower height Tower height depends on the length of the span. With long spans, relatively few towers are required but they must be tall and costly. It is not usually possible to determine the tower height and span length on the basis of direct construction costs because the lightning hazards increase greatly as the height of the conductor is increased. BEK 4213 Electrical Power Transmission and Distribution

Conductor clearance to ground The conductor clearance to ground at the time of greatest sag should not be less than some specified distance (usually between 6 and 12 m) depending on the voltage, nature and local laws. Sag and tension The tension is governed by the effects of wind, ice loading and temperature variations. The relationship between tension and sag is dependent on the loading conditions and temperature variations. For example, the tension increases when the temperature decreases and there is a corresponding decrease in the sag. BEF Electrical Power Transmission and Distribution

Stringing Charts For use in the field work of stringing the conductors, temperature-sag and temperature tension charts are plotted for the given conductor and loading conditions. These charts is very useful while stringing overhead lines. BEF Electrical Power Transmission and Distribution

Conductor Spacing Spacing of the conductors should be able to provide safety against flash-over when the wires are swinging in the wind. The proper spacing is a function of span length, voltage and weather condition. Conductor Vibration wind exerts pressure on the exposed surface of the conductor. If the wind velocity is small, the swinging of conductors is harmless provided the clearance is sufficiently large so that conductors do not approach within the sparking distance of each other. BEF Electrical Power Transmission and Distribution

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