Prof. Dr. Magdi El-Saadawi

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1 بسم هللا الرحمن الرحيم رب اشرح لى صدرى ويسر لى أمر واحلل عقدة من لسانى يفقهوا قولى صدق هللا العظيم Prof. Dr. Magdi M. El-Saadawi saadawi1@gmail.com 4/22/2015 Prof. Dr. Magdi El-Saadawi 1

2 Electrical Power (1) Course Code: EE /22/2015 Prof. Dr. Magdi El-Saadawi 2

3 Chapter 4 Components of Overhead Lines Part II 4/22/2015 Prof. Dr. Magdi El-Saadawi 3

4 Chapter 4 Components of Overhead Lines 4.1. Main Components of Overhead Lines 4.2. Conductor Materials 4.3. Line Supports 4.4. Insulators 4.5. Types of Insulators 4.6. Potential Distribution over Suspension Insulator String 4.7. String Efficiency 4.8. Methods of Improving String Efficiency 4.9. Points to keep in mind 4

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7 4.4 Insulators p. 97 They provide necessary insulation between line conductors and supports and thus prevent any leakage current from conductors to earth : They should have the following desirable properties: High mechanical strength in order to withstand conductor load, wind load 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 will be lowered. High ratio of puncture ثقب strength to flashover. 4/22/2015 Prof. Dr. Magdi El-Saadawi 7

8 Insulator Materials 1- Porcelain تضاف للكتاب ص 98 Made of China Clay ) الصينى )الطين, Quartz, Alumina ) األلمنيوم )أكسيد, and Feldspar )سيليكات األلمنيوم( Electric strength is 60 kv/cm Advantages: high Mechanical strength Low leakage current Less affected by temperature and weather Disadvantages Heavier in weight More expensive Affected by air pollutants 4/22/2015 Prof. Dr. Magdi El-Saadawi 8

9 Insulator Materials تضاف للكتاب ص 98 الزجاج المقس ى -2 Toughened Glass Glass is cheaper than porcelain It has very high dielectric strength compared to porcelain. Electric Strength is 140 kv/cm It has lower coefficient of thermal expansion which reduces the strains due to temperature changes Moisture can easily condensed on glass surface and hence air dust will be deposited on the wed glass surface which will provide path to the leakage current of the system. For higher voltage glass can not be cast in irregular shapes since due to irregular cooling internal cooling internal strains are caused, so Glass insulators are used up to 33 kv lines. 4/22/2015 Prof. Dr. Magdi El-Saadawi 9

10 4.5 Types of Insulators 1. Pin type insulators. 2. Suspension type insulators. 3. Strain insulators. 4. Shackle insulators. 4/22/2015 Prof. Dr. Magdi El-Saadawi 10

11 العوازل Pin type insulators المسمارية Pin type insulator consists of a single or multiple units. It is secured (تثبت) to the cross-arm on the pole. There is a groove on the upper end of the insulator for housing the conductor They are used only up to 33 kv. For higher voltages the pin type insulators are very heavy and more costly. 4/22/2015 Prof. Dr. Magdi El-Saadawi 11

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14 Single- unit pin insulator 11 kv Metal pin Double- unit pin insulator 33 kv Triple unit pin insulator > 33 kv 4/22/2015 Prof. Dr. Magdi El-Saadawi 14

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Causes of insulator failure: The electrical breakdown of the insulator can occur either by flash-over or puncture : In flashover, an arc occurs between the line conductor and insulator pin and the

19 Causes of insulator failure: The electrical breakdown of the insulator can occur either by flash-over or puncture : In flashover, an arc occurs between the line conductor and insulator pin and the discharge jumps across the air gaps The insulator will continue to act in its proper capacity unless extreme heat produced by the arc destroys the insulator 4/22/2015 Prof. Dr. Magdi El-Saadawi 19

20 Causes of insulator failure: In case of puncture, the discharge occurs from conductor to pin through the body of the insulator. When such breakdown is involved, the insulator is permanently destroyed due to excessive heat. To avoid puncture by the line voltage: sufficient thickness of porcelain is provided in the insulator. 4/22/2015 Prof. Dr. Magdi El-Saadawi 20

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22 عوازل التعليق Suspension type insulators For high voltages (> 33 kv), it is a usual practice to use suspension type insulators Suspension insulator consists of a number of disc units mounted one above each other. Each disc is provided with a metal cap at the top and a metal pin under. A string of any number of units can be built according to the line operating voltage. The conductor is suspended below the point of support by means of insulator string 4/22/2015 Prof. Dr. Magdi El-Saadawi 22

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27 Insulator string 4/22/2015 Prof. Dr. Magdi El-Saadawi 27

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30 Advantages of Suspension insulators 1. Cheaper than pin type insulators for voltages beyond 33 kv. 2. Each unit or disc of suspension type insulator is designed for low voltage, usually 11 kv. Depending upon the working voltage, the desired number of discs can be connected in series. 3. If anyone disc is damaged, the whole string does not become useless. 4/22/2015 Prof. Dr. Magdi El-Saadawi 30

31 Advantages of Suspension insulators 4. They provides greater flexibility to the line. The insulator string is free to swing in any direction 5. In case of increased demand on the T.L, it is more satisfactory to supply the greater demand by raising the line voltage than to provide another set of conductors. The additional insulation required for the raised voltage can be easily obtained in the suspension arrangement by adding the desired number of discs. 6. They are generally used with steel towers. The arrangement provides partial protection from lightning. 4/22/2015 Prof. Dr. Magdi El-Saadawi 31

32 disadvantages of Suspension insulators تضاف للكتاب ص Suspension insulator string costlier than pin and post type insulator. 2. Suspension string requires more height of supporting structure than that for pin insulator to maintain same ground clearance of current conductor. 3. The amplitude of free swing of conductors is larger in suspension insulator system, hence, more spacing between conductors should be provided. 4/22/2015 Prof. Dr. Magdi El-Saadawi 32

33 عوازل األجهاد Strain insulators When there is a dead end of the line or there is corner or sharp curve, the line is subjected to greater tension. In order to relieve the line of excessive tension, strain insulators are used. They must have considerable mechanical strength as well as the necessary electrical insulating properties. For high voltage T.L, strain insulator consists of an assembly of suspension insulators. shown in Fig /22/2015 Prof. Dr. Magdi El-Saadawi 33

34 عوازل األجهاد Strain insulators The discs of strain insulators are used in the vertical plane. When the tension in lines is exceedingly high, as at long river spans, two or more strings are used in parallel. They can be considered as special mechanical strong suspension insulators. 4/22/2015 Prof. Dr. Magdi El-Saadawi 34

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39 عوازل األصااد Shackle insulators In early days, the shackle insulators were used as strain insulators. Nowadays, they are frequently used for low voltage distribution lines. They can be used either in a horizontal or in a vertical position. They are directly fixed to the pole or to the cross arm with a bolt The conductor in the groove is fixed with a soft binding wire. 4/22/ Prof. Dr. Magdi El-Saadawi

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46 4.6 Potential Distribution over Suspension Insulator String The porcelain portion of each disc is between two metal links. Therefore, each disc forms a capacitor C (Fig (ii)). This is known as mutual capacitance or self-capacitance. In actual practice, capacitance also exists between metal fitting of each disc and tower or earth (Fig (iii)). This is known as shunt capacitance C 1. 4/22/2015 Prof. Dr. Magdi El-Saadawi 46

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49 Important Points about Potential Distribution p.105 Due to the presence of shunt capacitance, the voltage on a string is not equal on all discs The disc nearest to the line conductor has maximum voltage across it. The unit nearest to the conductor is under maximum electrical stress and is likely to be punctured. Means must be provided to equalize the potential across each unit. If the voltage across the string is d.c., then voltage across each unit would be the same. 4/22/2015 Prof. Dr. Magdi El-Saadawi 49

50 4.7 String Efficiency p.106 The greater the string efficiency, the more uniform is the voltage distribution. Although it is impossible to achieve 100% string efficiency, yet efforts should be made to improve it as close to this value as possible. 4/22/2015 Prof. Dr. Magdi El-Saadawi 50

51 Mathematical expression P. 107 C 1 = K C. Applying Kirchhoff s current law to node A: I 2 =I 1 + i 1 V 2 ω C* = V 1 ω C + V 1 ω C 1 [*current through capacitor = Voltage/Capacitive reactance] V 2 ω C = V 1 ω C + V 1 K ω C V 2 = V 1 (1 + K)...(i) 4/22/2015 Prof. Dr. Magdi El-Saadawi 51

52 Mathematical expression P. 107 Applying Kirchhoff s current law to node B, I 3 =I 2 + i 2 V 3 ω C = V 2 ω C + (V 1 + V 2 ) ω C 1 V 3 ω C = V 2 ω C + (V 1 +V 2 ) ω K C V 3 = V 2 + (V 1 + V 2 ) K = K V 1 + V 2 (1+ K ) = K V 1 + V 1 (1+ K ) 2 = V 1 [K + V 1 (1+ K ) 2 ] V 3 = V 1 [1 + 3K+ K 2 ]...(ii) 4/22/2015 Prof. Dr. Magdi El-Saadawi 52

53 Voltage between conductor and earth is: V = V 1 + V 2 + V 3 = V 1 + V 1 (1 + K) + V 1 (1 + 3K+ K 2 ) = (3 + 4K+ K 2 ) V = V 1 (1 + K) (3 + K)...(iii) From expressions (i), (ii) and (iii), we get, In this case String efficiency st 4/22/2015 Prof. Dr. Magdi El-Saadawi 53

54 Important points about math. expression p. 109 If K= 0 2 (Say), then we get, V 2 = 1 2 V 1 and V 3 = 1 64 V1. This shows that disc nearest to the conductor has maximum voltage across it. The greater the value of K (= C 1 /C), the more nonuniform is the potential across the discs and lesser is the string efficiency. The inequality in voltage distribution increases with the increase of number of discs in the string. Therefore, shorter string has more efficiency than the larger one. 4/22/2015 Prof. Dr. Magdi El-Saadawi 54

55 3.8 Methods of Improving String Efficiency (i) By using longer cross-arms (ii) By grading the insulators (iii) By using a guard ring 4/22/2015 Prof. Dr. Magdi El-Saadawi 55

56 (i) By using longer cross-arms The value of string efficiency depends upon the value of K i.e., ratio of shunt capacitance to mutual capacitance. The lesser the value of K, the greater is the string efficiency and more uniform is the voltage distribution The value of K can be decreased by reducing the shunt capacitance. This is done by increasing the distance of conductor from tower i.e., longer crossarms should be used. However, limitations of cost and strength of tower do not allow the use of very long cross-arms 4/22/2015 Prof. Dr. Magdi El-Saadawi 56

57 (ii) By grading the insulators Insulators of different dimensions are chosen so that each has a different capacitance. The top unit has the minimum capacitance, increasing progressively as bottom unit is reached. Since voltage is inversely proportional to capacitance, this method tends to equalize the potential distribution across the units in the string. A large number of different-sized insulators are required which is uneconomical and impractical. 4/22/2015 Prof. Dr. Magdi El-Saadawi 57

58 (iii) By using a guard ring A guard ring is a metal ring electrically connected to the conductor and surrounding the bottom insulator Guard ring introduces capacitance between metal fittings and the line conductor The guard ring is contoured in such a way that shunt capacitance currents i 1, i 2 etc. are equal to metal fitting line capacitance currents,... Etc The result is that same charging current I flows through each unit of string. Consequently, there will be uniform potential distribution across the units. 4/22/2015 Prof. Dr. Magdi El-Saadawi 58

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61 Solved Examples P. 112 Example 4.1. In a 33 kv 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 3 insulators and (ii) string efficiency. Solution: Fig shows the equivalent circuit of string insulators. Let V 1, V 2 and V 3 be the voltage across top, middle and bottom unit respectively. If C is the self-capacitance of each unit, then K C will be the shunt capacitance. K = Shunt Capacitance / Self capacitance = 0.11 Voltage across string, V = 33/ 3 = kv 4/22/

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63 Example P. 114 Example 4.2. An insulator string consists of three units, each having a safe working voltage of 15 kv. The ratio of self-capacitance to shunt capacitance of each unit is 8 : 1. Find the maximum safe working voltage of the string. Also find the string efficiency. 4/22/2015 Prof. Dr. Magdi El-Saadawi 63

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رب اشرح لى صدرى ويسر لى أمر واحلل عقدة من لسانى يفقهوا قولى

بسم هللا الرحمن الرحيم رب اشرح لى صدرى ويسر لى أمر واحلل عقدة من لسانى يفقهوا قولى صدق هللا العظيم Chapter 11 Overhead Line Insulators part 2 pp. 370-389 Potential Distribution over a String of Suspension