EE8701-High Voltage Engineering (1).pptx

anna university /EEE/ 7 th semester subject .it deals with unit1 : causes of over voltage in power system.
internal over voltage
external over voltage

1. EE8701-High Voltage Engineering

2. Introduction What is high voltage Engineering ? High voltage(above 1000 V AC and above 1500V DC) Application of high voltage under different medium (gas ,liquid and solid) or Behaviour of high voltage in different medium like gas, liquid and solid Applied design insulator (dielectrics ),protective devices

3. Application of HVE Power System HVAC reduce transmission losses increase transmission effciency Industries Electrostatic Precipitator(ESP) Pollution control Electrostatic painting painting electrical and mechanical machines Electrostatic printing designing PCB Research labs Van de graff generator

4. UNIT -1 OVER VOLTAGES IN ELECTRICAL POWER SYSTEMS

5. Causes of over voltages Examination of over voltages on the power system includes a study of their magnitudes, shapes, durations, and frequency of occurrence. The study should be performed at all points along the transmission network to which the surges may travel. Types of over voltage occur in power system External voltage or lightning Internal overvoltage's Temporary over voltages- power frequency oscillation and harmonics Switching over voltages

6. Internal Causes of over voltage and its effect on power system The over voltage causes may be broadly divided in to two main categories: i ) INTERNAL OVER VOLTAGES CAUSES Switching surges. Insulation Failure. Arcing Ground. Resonance. ii) EXTERNALOVER VOLTAGES CAUSES Lightning.

7. INTERNAL CAUSES OF OVER VOLTAGE: Internal causes of over voltage on the power system are primarily due to oscillations set up by the sudden changes in the circuit conditions. Switching surges: The over voltage, produced on the power system due to switching operations are known as switching surges. the few causes will be discussed here. Case of open line Case of loaded line Current chopping

8. Case of open line: During switching operations of unloaded line, travelling waves are set up to produce over voltage on the line. When the unloaded line is connected to the voltage source a voltage wave is setup which travels along the line.

9. Lightning Over voltages Lightning is produced in an attempt by nature to maintain   dynamic balance between the positively charged ionosphere and the negatively charged earth. Lightning over voltage is natural phenomena, its is peak discharge in which charge accumulated in clouds, discharges in neighbour clouds or to the ground

10. Electrode separation Cloud to cloud Cloud to ground is very large ( more than 10KM)

11. Charge formation in cloud During thunder storm , positive and negative charges becomes separated by heavy air currents with ice crystals in upper parts and rain in lower parts This charge separation depends upon heights of the clouds, which ranges from 0.2km to 10km, with their charge centres probably at distance about 0.3 to 2Km

12. Upper region of the clouds are usually positively charge where as lower region of the clouds are negatively charged except at the local region near the base and head which is positive

13. Theories based on charge separation Wilsons Theory of charge separation Simpons Theory Reynolds and Mason theory

14. Wilsons Theory of charge separation It is based only on assumption A large no.of ions are present in the atmosphere Many of these ions attach themselves to small dust particles and water particles A normal electric field exists in the atmosphere under fair-weather conditions which is generally directed downwards towards the earth. The intensity of the field is approximately 1 volt/cm at the surface of the earth and decreases gradually with height [at 9500m,it is 0.02V/cm] A relatively large rain drop (0.1 cm radius) falling this field becomes polarised, the upper side acquires a - ve charge and lower side + ve charge.

15. Then the lower part of the drop attracts negative charges from the atmosphere. .

16. The upwards motion of air currents tends to carry up the top of the cloud, the + ve air and smaller drops that the wind can blow against gravity. Meanwhile the falling heavier raindrops which are negatively charged settle on the base of the cloud. It is to be noted that the selective action of capturing – ve charges from the atmosphere by the lower surface of the drop is possible. No such selective action occurs at the upper surface. Thus in the original system, both the positive and negative charges which were mixed up, producing essentially a neutral space charge, are now separated.

17. Simpson’s and Scarse Theory

18. Reynolds and Mason proposed modification, According to which the thunder clouds are developed at heights 1 to 2 km above the ground level and may extend up to 12 to 14 km above the ground. For thunder clouds and charge formation air currents, moisture and specific temperature range are required.

19. The temperature is 0ᵒC at 4km & may reach -50ᵒC at 12km. Water droplets do not freeze at 0ᵒC & freeze only when temperature is below -40ᵒC & form solid particles on which crystalline ice patterns develop & grow. Thundercloud consisting supercooled water droplets moving upwards and large hail stones moving downwards. The ice splinters should carry only + ve charge upwards. Water has H+ &OH-ions, the ion density depends on temperature. Lower portion has a net – ve charge density(OH-)& upper portion has a net + ve charge density(H+).

20. Mechanism of Lightning Stroke The cloud and the ground form two plates of capacitor and dielectric medium is air. Since the lower part of the cloud is negatively charged the earth is positively charged by inductions. Lightning discharge will require for break down the Air Electric field required is 30 kv /Cm (peak) Electric field required is 10 Kv /cm( If the moisture content of the air is large)

21. Type of lightning streamer: Pilot streamer Stepped Leader Return Stroke Second Charge center Dart leader Heavy return Streamer

22. Pilot Streamer:

23. After the gradient of approximately 10Kv/cm is set up in the clouds the air surrounding gets ionized. In this condition a streamer starts from clouds towards earth. The current in the streamer is 100 A. and speed is 0.16m/µsec. This streamer is known as pilot streamer. This leads to the lighting phenomenon.

24. Stepped Leader: Depending upon the state of ionization of the air surrounding the streamer it‘s branched to several paths and this is known as stepped leader.

25. Return Stroke: Once stepped layer contact with earth a power return stroke moves very fast up towards the clouds through the already ionized path by the leader.

26. Second charge center :

27. Negative charge of the cloud is being neutralized by the positive induced charge on the earth. This instant gives rise to lighting flash which we observes with our naked eyes. There may be another cell of charges in the cloud near the neutralize charged cell

28. Dart Leader: This charged cell will try to neutralize through this ionized path. This streamer is known as dart leader. The velocity of dart leader is about 3% of the velocity of light. The effect of the dart leader is much more severe that of the return stroke.

29. Heavy return stroke: The second charge centre is discharging to ground through the dart leader. positive streamers are going up from ground. This is called heavy return stroke. This begins to discharge negative charge under the cloud and the second charge centre in the cloud.

30. Rate of charging of thunder clouds

31. Parameters and Characteristics of Lightning Strokes: The parameters and Characteristics of Lightning Strokes include the amplitude of the currents, the rate of rise, the probability distribution of the above, and the wave shapes of the lightning voltages and currents. Typical oscillograms of the lightning current and voltage wave shapes on a transmission line are shown in Figs 8.11 and 8.12. The lightning current oscillograms indicate and initial high current portion which is characterized by short front times up to 10 μs . The high current peak may last for some tens of microseconds followed by a long duration low current portion lasting for several milliseconds. This last portion is normally responsible for damages (thermal damage).

32. Lightning currents are usually measured either directly from high towers or buildings or from the transmission tower legs. The former gives high values and does not represent typical currents that occur on electrical transmission lines, and the latter gives inaccurate values due to non-uniform division of current in legs and the presence of ground wires and adjacent towers. Measurements made by several investigators and committees indicated the large strokes of currents (> 100 kA) are possible (Fig. 8.7). It was shown earlier that tall objects attract a large portion of high current strokes, and this would explain the shift of the frequency distribution curves towards higher currents.

33. Other important Characteristics of Lightning Strokes are time to peak value and its rate of rise. From the field data, it was indicated that 50% of lightning stroke currents have a rate of rise greater than 7.5 kA/ μs , and for 10% strokes it exceeded 25 kA/ μs . The duration of the stroke currents above half the value is more than 30 μs . Measurements of surge voltages indicated that a maximum voltage, as high as 5,000 kV, is possible on transmission lines, but on the average, most of the Characteristics of Lightning Strokes give rise to voltage surges less than 1000 kV on lines. The time to front of these waves varies from 2 to 10 μs and tail times usually vary from 20 to 100 μs . The rate of rise of voltage, during rising of the wave may be typically about 1 MV/ μs .

34. Characteristics of Lightning Strokes on transmission lines are classified into two groups: Direct strokes and Inducted strokes.

35. Direct strokes When a thunder cloud directly discharges onto a transmission line tower or line wires it is called a direct stroke. This is the most severe form of the stroke. However, for bulk of the transmission systems the direct strokes are rare and only the induced strokes occur.

36. Inducted strokes

37. Consider the three clouds, clouds 1 and 3 are positively charged, and cloud 2 is negatively charged as shown in the figure above. The potential of cloud 3 is reduced due to the presence of the charged cloud 2. On the flash over from Cloud 1 to Cloud 2, both these clouds are discharged rapidly, and class 3 assumes a much potential and flashes to earth very rapidly . It is the most dangerous strokes because it ignores taller building and reaches directly to the ground. This stroke is called the induces strokes.

38. Mathematical Model for lightning discharge During the charge formation process, the cloud may be considered to be a non conductor. Hence, various potentials may be assumed at different parts of the cloud. If charging process continues , it is probable that the gradient at certain parts of the charged region exceeds the breakdown strength of the air or moist air in the cloud. Hence, local breakdown takes place within the cloud. This local discharge may finally lead to a situation where in a large reservoir of charges involving a considerable mass of cloud hangs over the ground, with the air between the cloud and the ground as a dielectric.

39. When a streamer discharge occurs to ground by first a leader stroke, followed by main strokes with considerable currents flowing, the lightning stroke may be thought to be a current source of value I with a source impedance Z discharging to earth.

40. In case a direct stroke occurs over the top of an unshielded transmission line, the current wave tries to divide into two branches and travel on either side of the line.

41. Isokeraunic level or thunderstorm days Thunder storm days (TD) (is known as the Iso Keraunic level) is defined as the number of days in a year when thunder is heard or recorded in a particular location, The incidence of lightning strikes on Tr. Line / substation in related to T.D. T.D is =5 to 10 in Brittan 30 to 50 in USA 30 t0 50 in India

42. Model for Lightning

43. Switching Surges and temporary overvoltage For transmission voltages (400 KV and above) the dis advantages generated due to switching is same as that of the magnitude of lightning over voltages. This over voltages exists for a long time so it‘s dangerous to the system. Switching over voltages increases as the system voltage increases. In extra high voltage line, switching over voltages determine the insulation levels of the lines and their dimensional and cost.

44. Source (or) Origin of switching surges: Open and closing the switch gears High natural frequency of the system Damped normal frequency voltage components Restriking and recovery voltage with successive reflective wave form terminations Repeated restriking of the arc between the contacts off

45. Characteristics of switching surges: Switching surges arise from any one of the following sources. De energizing of lines, cables, and shunt capacitor bank etc. Disconnection of unloaded transformer, reactors etc Opening and closing of protective devices connected to lines and reactive loads Switch off the loads suddenly Short circuit due to insulation failure, line to ground contact, line to line contact, L-L-G contacts, three phase to ground contacts etc Clearing of the faults Arcing ground.

46. Shape of switching surges: Irregular Power frequency with its harmonics Relative magnitude-2.4 p.u for transformer energizing 1.4 to 2.0 p.u for switching transmission line.

47. Temporary over voltage

48. Temporary over voltages represent a threat to equipment as well as to any surge protective devices that may have been provided for the mitigation of surges . The scope of this Guide includes temporary over voltages only as a threat to the survival of SPDs(surge protection devices), and therefore includes considerations on the selection of suitable SPDs.

49. Following considerations are necessary to reach the goal of practical surge immunity: Desired protection Hardware integrity Process immunity Specific equipment sensitivities The power environment Surge characteristics Electrical system Performance of surge protective devices Protection Lifetime The test environment Cost effectiveness

50. Measure to control overvoltage due to switching and power frequency In EHV or UHV lines we should control the switching voltages less than 2.5 p.u the following measures are taken to reduce over voltage. One or multi-step energization of lines by inserting resistors. Phase controlled closing of circuit breaker with proper sensors. Drain the trapped charges before reclosing of the lines Using shunt reactors By using lightning arresters or surge diverters

51. One or multistep energization of lines by inserting resistors During switching of circuit breaker, inserting a series resistance in series with circuit breaker contacts and short circuiting this resistance after a few cycles By using inserting resistance the transients due to switching reduces. If the resistance is inserted for a long time, successive reflections takes place and the over voltage reaches high value. therefore using the pre-inserting resistor limit the over voltage

52. causes for switching and power frequency over voltage In the lines (400 KV and above) power frequency over voltages occurs are caused during tap changing operations in transformer. Causes for power frequency over voltages; Sudden load rejection Disconnection of inductive loads Ferranti effect Unsymmetrical faults Saturation in transformer etc. Tap changing operations.

53. Sudden load rejections: When sudden load rejection in the system cause the speeding up of generator prime movers hence the system frequency will raise. The speed governing system will respond by reducing the mechanical power generated by the turbines. But initially both the frequency and voltage increases. The approximate voltage rise is given by

54. Tap changing transformer: Tap changing operations are required when the voltages changes due to load variations so during these operations power frequency over voltages occurs.

55. Ground wire for protecting power conductor against direct lightning stroke

56. Ground wires: Ground wire is a conductor run parallel to the main conductor of the transmission line, supported on the same tower and earthed at every equally and regularly spaced towers. The different arrangement of ground wires is as shown in below fig.

57. Important considerations of ground wires are: Ground wire selection should be based on mechanical considerations rather than electrical considerations. It should have high strength and non – corrosive. Ground resistance, insulation and clearances between the ground wire and the lines are important in the design.

58. Using Counter – Poise Wires Counter- Poise Wires are buried in the ground at a depth of 0.5 to 1m, running parallel to the transmission line conductors and connected to the tower legs. Wire length may be 50 to 100 m long. The arrangement of counter – poise is as shown

59. Requirements of a Lightning Arrester or Surge Diverter

60. Rod Gap Rod Gap is used to protect the system from lightning or thunderstorm activity is less. A plain air gap usually between 1 inch square rods cut at right angles at the ends, connected between line and earth. The rod gap arrangement is shown.

61. Rod Gap

62. It is a very simple type of diverter and consists of two 1.5 cm rods, which are bent at right angles with a gap in between as shown in Fig. One rod is connected to the line circuit and the other rod is connected to earth. The distance between gap and insulator (i.e. distance  P ) must not be less than one third of the gap length so that the arc may not reach the insulator and damage it. Generally, the gap length is so adjusted that breakdown should occur at 80% of spark-voltage in order to avoid cascading of very steep wave fronts across the insulators.

63. The string of insulators for an  overhead line  on the bushing of transformer has frequently a rod gap across it. Fig 8 shows the rod gap across the bushing of a transformer. Under normal operating conditions, the gap remains non-conducting. On the occurrence of a high voltage surge on the line, the gap sparks over and the surge current is conducted to earth. In this way excess charge on the line due to the surge is harmlessly conducted to earth

64. Advantages Simple in construction. Cheap. Rugged construction. Disadvantages It does not interrupt the power frequency follow current. Every operation of the rod gap results in L –G fault and the breakers must operate to isolate the faulty section. Uses It is used as back – up protection.

65. Expulsion Type Lightning Arrester (Protector Tube)

66. This type of arrester is also called ‘ protector tube ’ and is commonly used on system operating at voltages up to 33kV. The above Fig shows the essential parts of an  expulsion type lightning arrester .

67. It essentially consists of a rod gap AA’ in series with a second gap enclosed within the fiber tube. The gap in the fiber tube is formed by two electrodes. The upper electrode is connected to rod gap and the lower electrode to the earth. One expulsion arrester is placed under each line conductor. On the occurrence of an over voltage on the line, the series gap AA’ spanned and an arc is stuck between the electrodes in the tube. The heat of the arc vaporizes some of the fiber of tube walls resulting in the production of neutral gas. In an extremely short time, the gas builds up high pressure and is expelled through the lower electrode, which is hollow. As the gas leaves the tube violently it carries away ionized air around the arc. This deionizing effect is generally so strong that the arc goes out at a current zero and will not be re-established

68. Value Type Lightning Arrester (Non – Linear Type) Value Type Lightning Arresters are used to protect substations and at line terminations to discharge the lightning over voltages and short duration switching surges. A value type arrester is shown below.

69. Valve type arresters  incorporate non linear resistors and are extensively used on systems, operating at high voltages. Fig shows the various parts of a valve type arrester. It consists of two assemblies ( i ) series spark gaps and (ii) non-linear resistor discs in series. The non-linear elements are connected in series with the spark gaps. Both the assemblies are accommodated in tight porcelain container.

70. The spark gap  is a multiple assembly consisting of a number of identical spark gaps in series. Each gap consists of two electrodes with fixed gap spacing. The voltage distribution across the gap is line raised by means of additional resistance elements called grading resistors across the gap. The spacing of the series gaps is such that it will withstand the normal circuit voltage. However an over voltage will cause the gap to break down causing the surge current to ground via the non-linear resistors.

71. The non-linear resistor discs  are made of inorganic compound such as thyrite or metrosil . These discs are connected in series. The non-linear resistors have the property of offering a high resistance to current flow when normal system voltage is applied, but a low resistance to the flow of high surge currents. In other words, the resistance of these non-linear elements decreases with the increase in current through them and vice-versa.

72. Working of value type surge arrester Under normal conditions, the normal system voltage is insufficient to cause the break down of air gap assembly. On the occurrence of an over voltage, the breakdown of the series spark gap takes place and the surge current is conducted to earth via the non-linear resistors. Since the magnitude of surge current is very large, the non-linear elements will offer a very low resistance to the passage of surge. The result is that the surge will rapidly go to earth instead of being sent back over the line. When the surge is over, the non-linear resistors assume high resistance to stop the flow of current.

73. Merits: To protect station equipment rated 400 KV and above. To protect motors and generators To protect distribution transformer.

74. CORONA AND ITS EFFECTS Definition If the field is uniform , then an increase in voltage(A.C.) directly leads to breakdown without any preliminary discharge. However in non-uniform geometry, the increase in a.c . voltage will cause a luminous discharge with the production of hissing noise at points with highest electric field intensity. Ionization of surrounding air around the conductor , a hissing noise and production of ozone gas in over head transmission line is known as corona effect,

75. Theory of corona formation

76.  Some ionization is always present in the air due to cosmic, ultraviolet rays and radioactivity. So under the normal conditions, the air contains some free electrons, negative ions, and neutral molecules. But when the potential difference between the conductors exceeds than a certain value a potential gradient is set up on the conductor’s surface. When the potential gradient reaches up to 30kv/cm which is sufficient for the free electron to strike with a neutral molecule with enough force to emit an electron. 

77. This process occurs in other molecules which emit more free electrons. This process is cumulative that is why large of electrons jumped into the air surrounding the conductors, the air is now ionized and the spark occurs in between the conductors. Now the corona effect can be divided into two parts, one is the sound and another one is the visual part.

78. Critical disruptive voltage:  It is the minimum phase-neutral voltage at which corona occurs. consider two conductors of radii r and space b/w them is d. if V has applied voltage then the potential gradient at the surface of conductors are,

79. Visual critical voltage:   It is the minimum phase-neutral voltage at which a glow appears around all along the conductors. It has been seen that in the case of parallel conductors, the corona glow does not begin at the disruptive voltage Vc but at a higher voltage, Vv called visual critical voltage. The phase neutral effective value of visual critical voltage is given by the following formula,

80. Factors affecting corona The phenomenon of corona can be affected by the physical state of the atmosphere as well as by the condition of the line. The following are the factors upon which corona depends. Atmosphere : As corona formed due to the ionization of the air surrounding the conductors, therefore corona is affected by the physical state of the atmosphere. In stormy weather, the number of ions is more than the normal, so the corona can occur at much less voltage as compared to the fair weather. Conductor size & shape : The corona can be affected by the physical shape and size of the conductor as well. If the surface of the conductor is irregular then it will give rise to the corona compare to the solid conductor. Because the unevenness of the conductor produces more chances of corona than a smoothly surfaced conductor.

81. Spacing b/w the conductors : The spacing between the transmission line conductors must be greater than the diameter of the conductors because if the spacing b/w the conductors are less the air surrounding the conductors can be ionized at low voltage. Line voltage : The line voltage greatly affects the corona. If the line voltage is low then there will be no change in the air surrounding and hence no corona is formed. However, if the line voltage exceeds than a certain value the electrostatic stresses developed at the conductors surface which ionized the air and corona is formed.

82. Methods of Reducing Corona Effect It has been observed that intense corona effects are seen at working voltage of 33kv or above; therefore the care should be taken while designing the transmission lines and substations to avoid this kind of enormous and destructive effects of a corona. The following are the methods of reducing the effects of corona discharge.

83. By increasing the conductor size : By increasing the size of the conductor, the voltage at which corona occurs is raised and hence corona effects are considerably reduced. By increasing conductors spacing : By increasing the space between the conductors of transmission lines can considerably reduce the corona effect. We should increase the space b/w conductors from the space at which corona occurs. Increasing space accommodates more particles b/w the conductors.

84. Merits and demerits of corona effect Corona has many advantages and disadvantages. In the correct design of a high voltage overhead line, the following merits and demerits are considered the most important. Merits Due to corona the space b/w conductors is ionized and become conducting path, so the virtual diameter of the conductor is increased. Corona reduces the effects of transients  produced by surges . Demerits Corona is accompanied by a loss of energy. This affects the transmission lines efficiency. Ozone is also produced in the corona and may cause corrosion. The current drawn by the line due to corona is non-sinusoidal and hence non-sinusoidal voltage drop occurs in the line.

85. This form of discharge is termed as Corona discharge and is accompanied by the formation of ozone , as is indicated by the characteristic order of this gas. If the voltage is d.c. , then the appearance will be different . The positive wire will be having a uniform glow and negative wire has a more patchy glow often accompanied by streamers. An important point in connection with corona that it is accompanied by a loss of power and this means that there is a flow of current to the wire.

86. The current waveform is non-sinusoidal and the non-sinusoidal drop of volts caused by it may be more important than loss of power. It gives rise to radio interference. Attenuation due to corona: The effect of corona is to reduce the crest of the voltage wave under propagation, limiting the peak value to the critical corona voltage. Hence, the excess voltage above the critical voltage will cause power loss by ionizing the surrounding air.

87. Practical Importance of Corona: 1.)Under normal conditions the loss of power due to corona is of no good importance , and consequently corona calculations do not enter directly into transmission line design. The basis of such design is entirely financially the most economical line being the most acceptable. 2.)The non-sinusoidal coronal current causes a non-sinusoidal drop of volts and these may cause some interference with neighbouring communication circuits due to electromagnetic and electrostatic induction .The current contains large third harmonic. 3.)Average corona loss on several lines from 345 KV to 750 KV gave 1 to 20 KW/Km in fair weather the higher values referring to higher voltages . In foul-weather the losses can go upto 300 KW/Km. 4)When a line is energized and no corona is present , the current is a pure sine wave and capacitive