Underground Cables.pptx

To understand the construction and types of UG cables for transmission and distribution

1. UNIT – IV UNDER GROUND CABLES 1

2. Under Ground cables-Requirements The conductor used in cables should be tinned stranded copper or aluminium of high conductivity . Stranding is done so that conductor may become flexible and carry more current. The conductor size should be such that the cable carries the desired load current without overheating and causes voltage drop within permissible limits. The cable must have proper thickness of insulation in order to give high degree of safety and reliability at the voltage for which it is designed . 2

3. Under Ground cables-Requirements The cable must be provided with suitable mechanical protection so that it may withstand the rough use in laying it. The materials used in the manufacture of cables should be such that there is complete chemical and physical stability throughout . 3

4. Construction of UG Cables 4

5. Construction of UG Cables Cores or Conductors . A cable may have one or more than one core (conductor) depending upon the type of service for which it is intended. For instance, the 3-conductor cable shown in Figure is used for 3-phase service. The conductors are made of tinned copper or aluminium and are usually stranded in order to provide flexibility to the cable. 5

6. Construction of UG Cables Insulation Each core or conductor is provided with a suitable thickness of insulation, the thickness of layer depending upon the voltage to be withstood by the cable. The commonly used materials for insulation are impregnated paper, varnished cambric or rubber mineral compound . 6

7. Construction of UG Cables Metallic sheath In order to protect the cable from moisture, gases or other damaging liquids (acids or alkalies ) in the soil and atmosphere , a metallic sheath of lead or aluminium is provided over the insulation. Bedding Over the metallic sheath is applied a layer of bedding which consists of a fibrous material like jute or hessian tape . The purpose of bedding is to protect the metallic sheath against corrosion and from mechanical injury due to armouring. 7

8. Construction of UG Cables Armouring Over the bedding, armouring is provided which consists of one or two layers of galvanised steel wire or steel tape . Its purpose is to protect the cable from mechanical injury while laying it and during the course of handling. Armouring may not be done in the case of some cables. 8

9. Construction of UG Cables Serving In order to protect armouring from atmospheric conditions, a layer of fibrous material (like jute) similar to bedding is provided over the armouring. This is known as serving . 9

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11. Advantages of UG Cables It ensures non-interrupted continuity of supply. The possible supply interruptions due to lightning, storms and other weather conditions are eliminated because of UG cables. It requires less maintenance The accidents caused due to breakage of overhead line conductors are eliminated due to use of underground cables. The voltage drop on UG cables is less. The life of UG cables is long compared to OH lines. 11

12. Advantages of UG Cables The beauty of cities and towns get maintained due to UG network of cables. The overhead lines use bare conductors which is unsafe in thickly populated areas. Hence from safety point of view, the UG cables are more advantageous. The only drawbacks of the UG cables are the extremely high initial cost and insulation problems at high voltages. 12

13. Insulating materials for UG Cables The satisfactory operation of a cable depends to a great extent upon the characteristics of insulation used. Therefore , the proper choice of insulating material for cables is of considerable importance. 13

14. Insulating materials for UG Cables In general, the insulating materials used in cables should have the following properties: High insulation resistance to avoid leakage current. High dielectric strength to avoid electrical breakdown of the cable. High mechanical strength to withstand the mechanical handling of cables . Non-inflammable. 14

15. Insulating materials for UG Cables Non-hygroscopic i.e., it should not absorb moisture from air or soil. The moisture tends to decrease the insulation resistance and hastens the breakdown of the cable. In case the insulating material is hygroscopic, it must be enclosed in a waterproof covering like lead sheath . Low cost so as to make the underground system a viable proposition. Unaffected by acids and alkalies to avoid any chemical action. 15

16. Insulating materials for UG Cables The principal insulating materials used in cables are Rubber , Vulcanized India rubber, Impregnated paper, Varnished cambric and Polyvinyl chloride. 16

17. Classification of UG Cables Cables for underground service may be classified in two ways according to the type of insulating material used in their manufacture the voltage for which they are manufactured. However , the latter method of classification is generally preferred, according to which cables can be divided into the following groups : Low-tension (L.T.) cables — upto 1000 V High-tension (H.T . ) cables — upto 11,000 V Super-tension (S.T.) cables — from 22 kV to 33 kV Extra high-tension (E.H.T.) cables - from 33 kV to 66 kV Extra super voltage cables — beyond 132 kV 17

18. Classification of UG Cables The cable has ordinary construction because the stresses developed in the cable for low voltages ( upto 6600 V) are generally small. 18

19. Cables for Three phase Service For voltages upto 66 kV, 3-core cable ( i . e., multi-core construction) is preferred due to economic reasons. However, for voltages beyond 66 kV, 3-core-cables become too large and unwieldy and, therefore, single-core cables are used . The following types of cables are generally used for 3-phase service : Belted cables — upto 11 kV Screened cables — from 22 kV to 66 kV Pressure cables — beyond 66 kV. 19

20. Belted cables These cables are used for voltages upto 11kV but in extraordinary cases, their use may be extended upto 22kV. Figure shows the constructional details of a 3-core belted cable. The belted type construction is suitable only for low and medium voltages.(beyond 22 kV screened cables are used) 20

21. Screened cables H-type cables: This type of cable was first designed by H . Hochstadter and hence the name. 21

22. Screened cables Advantages of H-type cable : Two principal advantages are claimed for H- type cables. Firstly, the perforations in the metallic screens assist in the complete impregnation of the cable with the compound and thus the possibility of air pockets or voids (vacuous spaces) in the dielectric is eliminated. The voids if present tend to reduce the breakdown strength of the cable and may cause considerable damage to the paper insulation. Secondly, the metallic screens increase the heat dissipating power of the cable. 22

23. Screened cables S.L. (Separate lead) -type cables: 23

24. Screened cables Advantages of S.L. Type Cables: Firstly , the separate sheaths minimize the possibility of core-to-core breakdown. Secondly, bending of cables becomes easy due to the elimination of overall lead sheath . Drawbacks of S.L. Type Cables: However, the disadvantage is that the three lead sheaths of S.L. cable are much thinner than the single sheath of H -cable and, therefore, call for greater care in manufacture 24

25. Limitations of solid type cables The voltage limit for solid type cables is 66 kV due to the following reasons : ( a ) As a solid cable carries the load, its conductor temperature increases and the cable compound ( i . e., insulating compound over paper) expands. This action stretches the lead sheath which may be damaged. ( b ) When the load on the cable decreases, the conductor cools and a partial vacuum is formed within the cable sheath. If the pinholes are present in the lead sheath, moist air may be drawn into the cable . The moisture reduces the dielectric strength of insulation and may eventually cause the breakdown of the cable. 25

26. Limitations of solid type cables ( c ) In practice, voids are always present in the insulation of a cable. Modern techniques of manufacturing have resulted in void free cables. However , under operating conditions, the voids are formed as a result of the differential expansion and contraction of the sheath and impregnated compound. The breakdown strength of voids is considerably less than that of the insulation. If the void is small enough, the electrostatic stress across it may cause its breakdown. The voids nearest to the conductor are the first to break down, the chemical and thermal effects of ionization causing permanent damage to the paper insulation. 26

27. Pressure Cables Oil filled cables: Single core conductor channel Single core sheath channel Three core filler space channel 27

28. Oil filled cables The range being from 66 kV upto 230 kV Advantages : Firstly, formation of voids and ionisation are avoided. Secondly, allowable temperature range and dielectric strength are increased. Thirdly, if there is leakage, the defect in the lead sheath is at once indicated and the possibility of earth faults is decreased . Disadvantages: the high initial cost and complicated system of laying . 28

29. Pressure Cables Gas filled cables: designed by Hochstadter , Vogal and Bowden 29

30. Gas filled Cables The triangular section reduces the weight and gives low thermal resistance but the main reason for triangular shape is that the lead sheath acts as a pressure membrane. The cable is laid in a gas-tight steel pipe. The pipe is filled with dry nitrogen gas at 12 to 15 atmospheres . Moreover, maintenance cost is small and the nitrogen gas helps in quenching any flame . However , it has the disadvantage that the overall cost is very high. 30

31. Insulation Resistance of a single core cable The cable conductor is provided with a suitable thickness of insulating material in order to prevent leakage current. The path for leakage current is radial through the insulation. The opposition offered by insulation to leakage current is known as insulation resistance of the cable. For satisfactory operation, the insulation resistance of the cable should be very high . 31

32. Insulation Resistance of a single core cable Consider a single-core cable of conductor radius r 1 and internal sheath radius r 2 as shown in Figure. Let l be the length of the cable and ρ be the resistivity of the insulation . Consider a very small layer of insulation of thickness dx at a radius x . The length through which leakage current tends to flow is dx and the area of X - section offered to this flow is 2π x l . 32

33. Insulation Resistance of a single core cable 33

34. Capacitance of a single core cable A single-core cable can be considered to be equivalent to two long co-axial cylinders . The conductor (or core) of the cable is the inner cylinder while the outer cylinder is represented by lead sheath which is at earth potential. Consider a single core cable with conductor diameter d and inner sheath diameter D . 34

35. Capacitance of a single core cable Let the charge per metre axial length of the cable be Q coulombs and ε be the permittivity of the insulation material between core and lead sheath. Obviously ε = ε ε r where ε r is the relative permittivity of the insulation . Consider a cylinder of radius x metres and axial length 1 metre . The surface area of this cylinder is = 2 π x × 1 = 2 π x m 2 35

36. Capacitance of a single core cable 36

37. Capacitance of a single core cable 37

38. Dielectric stress in a single core cable Under operating conditions, the insulation of a cable is subjected to electrostatic forces . This is known as dielectric stress . The dielectric stress at any point in a cable is infact the potential gradient (or electric intensity) at that point. 38

39. Dielectric stress in a single core cable Consider a single core cable with core diameter d and internal sheath diameter D. T he electric intensity at a point x metres from the centre of the cable is 39

40. Dielectric stress in a single core cable potential difference V between conductor and sheath is 40

41. Dielectric stress in a single core cable 41

42. Dielectric stress in a single core cable It is clear that dielectric stress is maximum at the conductor surface and its value goes on decreasing as we move away from the conductor . It may be noted that maximum stress is an important consideration in the design of a cable . For instance, if a cable is to be operated at such a voltage that maximum stress is 5 kV/mm, then the insulation used must have a dielectric strength of at least 5 kV/mm, otherwise breakdown of the cable will become inevitable. 42

43. Most economical conductor size in a cable It has already been shown that maximum stress in a cable occurs at the surface of the conductor. For safe working of the cable, dielectric strength of the insulation should be more than the maximum stress . Rewriting the expression for maximum stress, we get, 43

44. Most economical conductor size in a cable The values of working voltage V and internal sheath diameter D have to be kept fixed at certain values due to design considerations. This leaves conductor diameter d to be the only variable in the above expression. For given values of V and D, the most economical conductor diameter will be one for which g max has a minimum value. The value of g max will be minimum when d log e D / d is maximum i.e. 44

45. Most economical conductor size in a cable 45

46. Most economical conductor size in a cable For low and medium voltage cables, the value of conductor diameter arrived at by this method ( i.e ., d = 2 V/ g max ) is often too small from the point of view of current density. Therefore , the conductor diameter of such cables is determined from the consideration of safe current density. For high voltage cables, designs based on this theory give a very high value of d, much too large from the point of view of current carrying capacity and it is, therefore, advantageous to increase the conductor diameter to this value. 46

47. Most economical conductor size in a cable There are three ways of doing this without using excessive copper : Using aluminium instead of copper because for the same current, diameter of aluminium will be more than that of copper. Using copper wires stranded round a central core of hemp . Using a central lead tube instead of hemp. 47

48. Grading of cables The process of achieving uniform electrostatic stress in the dielectric of cables is known as grading of cables . The unequal stress distribution in a cable is undesirable for two reasons. Firstly , insulation of greater thickness is required which increases the cable size. Secondly , it may lead to the breakdown of insulation . 48

49. Grading of cables In order to overcome above disadvantages, it is necessary to have a uniform stress distribution in cables. This can be achieved by distributing the stress in such a way that its value is increased in the outer layers of dielectric. This is known as grading of cables. The following are the two main methods of grading of cables : ( i ) Capacitance grading ( ii ) Intersheath grading 49

50. Capacitance Grading The process of achieving uniformity in the dielectric stress by using layers of different dielectrics is known as capacitance grading. 50

51. Capacitance Grading Relative permittivity ε r of any layer is inversely proportional to its distance from the centre . However , ideal grading requires the use of an infinite number of dielectrics which is an impossible task. In practice, two or three dielectrics are used in the decreasing order of permittivity ; the dielectric of highest permittivity being used near the core. 51

52. Capacitance Grading There are three dielectrics of outer diameter d 1 , d 2 and D and of relative permittivity ε 1 , ε 2 and ε 3 respectively. If the permittivities are such that ε 1 > ε 2 > ε 3 and the three dielectrics are worked at the same maximum stress, then, 52

53. Capacitance Grading Potential difference across the inner layer is 53

54. Capacitance Grading If the cable had homogeneous dielectric, then, for the same values of d, D and g max , the permissible potential difference between core and earthed sheath would have been Obviously, V > V ′ i.e., for given dimensions of the cable, a graded cable can be worked at a greater potential than non-graded cable. Alternatively, for the same safe potential, the size of graded cable will be less than that of non-graded cable. 54

55. Capacitance Grading The following points may be noted : ( i ) As the permissible values of g max are peak values, therefore, all the voltages in above expressions should be taken as peak values and not the r.m.s . values. ( ii ) If the maximum stress in the three dielectrics is not the same, then , The principal disadvantage of this method is that there are a few high grade dielectrics of reasonable cost whose permittivities vary over the required range. 55

56. Intersheath Grading In this method of cable grading, a homogeneous dielectric is used, but it is divided into various layers by placing metallic intersheaths between the core and lead sheath. The intersheaths are held at suitable potentials which are in between the core potential and earth potential . This arrangement improves voltage distribution in the dielectric of the cable and consequently more uniform potential gradient is obtained. 56

57. Intersheath Grading 57

58. Intersheath Grading Maximum stress between core and intersheath 1 is Since the dielectric is homogeneous, the maximum stress in each layer is the same i.e., 58

59. Intersheath Grading As the cable behaves like three capacitors in series, therefore, all the potentials are in phase i.e. Voltage between conductor and earthed lead sheath is V = V 1 + V 2 + V 3 Intersheath grading has three principal disadvantages. Firstly, there are complications in fixing the sheath potentials. Secondly, the intersheaths are likely to be damaged during transportation and installation which might result in local concentrations of potential gradient. Thirdly, there are considerable losses in the intersheaths due to charging currents. For these reasons, intersheath grading is rarely used. 59

60. Difficulties in Grading In the intersheath grading, the intersheath has to be thin. Hence there is possibility of damage to it while laying the cable. Similarly intersheath has to carry the charging current which can cause overheating of cable. In capacitance grading, to have uniform distribution of stress it is necessary to select the dielectrics of proper permittivities . But practically it is difficult to get the proper values of permittivities . Similarly the permittivity of dielectric changes with the time which can cause uneven distribution of stress. Such uneven distribution may lead to breakdown at the normal operating voltage. 60

61. Capacitance of Three Core cables The capacitance of a cable system is much more important than that of overhead line because in cables ( i ) conductors are nearer to each other and to the earthed sheath ( ii ) they are separated by a dielectric of permittivity much greater than that of air. 61

62. Capacitance of Three Core cables 62

63. Capacitance of Three Core cables 63

64. Measurement of C e and C c The following two measurements are required : In the first measurement, the three cores are bunched together ( i.e. commoned ) and the capacitance is measured between the bunched cores and the sheath. The bunching eliminates all the three capacitors C c , leaving the three capacitors Ce in parallel. Therefore, if C 1 is the measured capacitance, this test yields : Knowing the value of C 1 , the value of C e can be determined. 64

65. Measurement of C e and C c In the second measurement, two cores are bunched with the sheath and capacitance is measured between them and the third core. This test yields 2 C c + C e . If C 2 is the measured capacitance , then , As the value of C e is known from first test and C 2 is found experimentally, therefore, value of C c can be determined. 65

66. Measurement of C e and C c It may be noted here that if value of C N (= C e + 3 C c ) is desired, it can be found directly by another test . In this test, the capacitance between two cores or lines is measured with the third core free or connected to the sheath. This eliminates one of the capacitors C e so that if C 3 is the measured capacitance, then , 66

67. Power factor and Heating of cables For a single core cable the insulation resistance is given by Here ρ is the specific resistance and l is the length of the cable. Assuming ρ to remain constant, the impressed voltage V will send a current (in phase with V) through the insulation. Capacitive current drawn from the supply, Where, C is the cabale capacitance. The charging current (I C )leads V by 67

68. Power factor and Heating of cables The equivalent circuit with its phasor diagram is shown in figure. The resultant current I leads V by ϕ (= ) where cos ϕ gives the power factor of the cable. 68

69. Power factor and Heating of cables But The cable power factor thus can be expressed as, Dielectric losses are causes by dielectric absorption or polarization. It is small for voltages upto 33 kV, but for higher voltages has an increasingly important effect on current rating. The loss in dielectric is due to loss in the equivalent leakage resistance. 69

70. Power factor and Heating of cables 70

71. Heating of Cables Under working condition, the temperature of the cables increases due to the following factors The heat produced within the cables The dissipation of heat up to the periphery of the cables The heat dissipation to the surrounding medium The current carried by the cables. The various load conditions like continuous, distributed, intermittent etc. 71

72. Heating of Cables The heat is produced with in the UG cables due to the following losses Copper loss which is also called I 2 R loss (temperature, length, area of cross section) Dielectric loss (W=V 2 ῳ Ctan δ ) Sheath loss (sheath eddy current, sheath circuit current) 72

73. Heat flow in Cables Heat flowing radially out from core to ground through dielectric, sheath, bedding, armouring and serving. 73

74. Thermal characteristics of Cable The current in an electric circuit is given by I= potential difference/resistance = V/R Similarly heat flow H is given by H= temperature difference/Thermal resistance = θ /S Thermal resistance S is defined as the resistance which allows the heat flow of 1 watt when a temperature difference of 1 o C is maintained. It is given by S= kl/A thermal ohms Where k= thermal resistivity of the material l= length of the path of heat flow A= area of section through which heat flow 74

75. D.C. Cables When a cable is used for d.c. transmission then its voltage and current carrying capacity increases. This is because, In d.c. system, there is no charging current hence the corresponding copper losses are absent. In d.c. system the dielectric hysteresis loss is absent. The only loss present is copper loss due to leakage current. Hence for the same temperature rise of the insulation, d.c. cables can carry much more current. 75

76. D.C. Cables The skin effect is absent in d.c. hence corresponding power loss is absent. In d.c. system, no voltage gets induced in the sheath hence sheath losses due to the induced current is zero. Due to all these reasons, the current carrying capacity of d.c. cable is higher than a.c . cables. Also the d.c. breakdown stress of the dielectric is more than the a.c . breakdown stress hence the voltage rating of d.c. cables is much higher compared to a.c . cables. 76