EE3701 High Voltage Engineering HVE Unit 1 [Autosaved] [Autosaved].pptx

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1. EE3701 - HIGH VOLTAGE ENGINEERING 1

2. HIGH VOLTAGE ENGINEERING In modern times, high voltages are used for a wide variety of applications covering the power system, industry and research laboratories. High voltages are applied in laboratories in nuclear research, in particle accelerators, and Van de Graff generators. For transmission of large bulks of power over long distances, high voltages are essential. 2

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4. CAUSES FOR OVERVOLTAGES External or Lightning Overvoltages Internal Overvoltages They are generated internally by connecting or disconnecting the system, or due to the system faults initiation or excitation. Temporary overvoltages – Power frequency oscillations or harmonics Switching overvoltages 4

5. NATURAL CAUSES FOR OVERVOLTAGES: Lightning: It is peak discharge in which charge accumulated in the clouds discharges into a neighbouring cloud or to the ground. The electrode separation, i.e., cloud-to-cloud or cloud-to-ground is very large, perhaps 10 km or more. The mechanism of charge formation in the clouds and their discharge is quite a complicated and uncertain process. Charge formation in clouds Mechanism of lighting strokes Mathematical modelling for lightning 5

6. 1. Charge formation in clouds During thunderstorms, positive and negative charges become separated by the heavy air currents with ice crystals in the upper part and rain in the lower parts of the cloud. This charge separation depends on the height of the clouds, which range from 0.2 to 10 km , with their charge centres probably at a distance of about 0.3 to 2 km The volume of the clouds that participate in lightning flashover are uncertain, but the charge inside the cloud may be as high as 1 to 100 C. Clouds may have a potential as high as 10 7 to 10 8 V with field gradients ranging from 100 V/cm within the cloud to as high as 10 kV/cm at the initial discharge point. The energies associated with the cloud discharges can be as high as 250 kWh. 6

7. The upper regions of the cloud are usually positively charged, whereas the lower region and the base are predominantly negative except the local region, near the base and the head, which is positive. The maximum gradient reached at the ground level due to a charged cloud may be as high as 300 V/cm , while the fair weather gradients are about 1 V/cm 7

8. Simpson's theory According to the Simpson’s theory there are three essential regions in the cloud to be considered for charge formation 8

9. Below region A: Air currents travel above 800 cm/s and no raindrops fall through. In region A: Air velocity is high enough to break the falling the raindrops causing a positive charge spray in the cloud and negative charge in the air. The spray is blown upwards, but as the velocity of air decreases, the positively charged water drops recombine with the larger drops and fall again. Region B: It becomes negatively charged by air currents. Region C: The temperature is low (below freezing point) and only ice crystals exist. The impact of air on these crystals make them negatively charged , thus the distribution of charge within in the cloud becomes. 9

10. 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 Air currents, moisture, specific temperature range are required for thunder clouds and charge formation. The air currents controlled by the temperature gradient move upwards carrying moisture and water droplets. The temperature is O C at about 4 km from the ground and may reach - 5O C at about 12 km height. Below - 4O C, they freeze as solid particles on which crystalline ice patterns develop and grow. Thus in clouds, the effective freezing temperature range is around - 33 C to - 4O C. The water droplets in the cloud are blown up by air currents and get super cooled over a range of height and temperature. When such freezing occurs, the Crystals grow into large masses and due to their weight and gravitational force start moving downwards. Reynold and Mason theory 10

11. 2. Mechanism of lighting strokes In an active thunder cloud, the large particles possess negative charges and the smaller particles possess positive charges. Thus the base of the thunder clouds carries negative charges and the upper part carries positive charges. Electric field intensity in the charge concentrated cloud exceeds the breakdown value of the moist ionized air (=10 kV/cm), an electric streamer with plasma starts towards the ground with a velocity of about 1/10 times that of the light , but may progress only about 50 m or so before it comes to halt emitting a flash of light. The halt may be due to insufficient build-up of charge of electric charge at its head and not sufficient to maintain necessary field gradient for further process of the streamer. But after a short interval of about 100 µs, the streamer again starts out repeating its performance. This discharge called ‘stepped leader’. From the tip of the discharge a ‘pilot streamer’ starts with low luminosity and a current of few amperes is as shown in figure (a). 11

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13. As the leader (negative charges) approaches ground, the electric field between the leader and earth increases. The pilot streamer is about to make contact with the upward positive charges from earth as shown in figure (b) and causes pilot discharges from each objects like trees, tall buildings, etc. At some points, the discharge concentration is high enough to initiate the return stroke (positive streamer) from earth to cloud travelling along the previous path and the negative charge of cloud begins to discharge as shown in figure (c) The first charge centre is completely discharged and streamers begin developing in the second charge centre as shown in figure (d) The return stroke is followed by several strokes. The leader of second and the subsequent strokes is known as the ‘dart leader’. The second charge centre is discharging to ground through the dart leader, distributing negative charges along the path as shown in figure (e). Positive streamers are going up from ground. This is called heavy ‘return stroke’, which begins to discharge negatively charge under the cloud and the second charge centre in the cloud as shown in figure (f). The discharges takes place between clouds is known as ‘sheath lightning’ Total time required for the stepped leader to reach the ground = 20 ms. 13

14. 3. Mathematical modelling for lightning 14

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19. Switching surges and temporary overvoltages For transmission voltages (400 kV and above), the overvoltages generated due to switching is same as that of the magnitude of lightning over voltages. These overvoltages exist for a long time, so it is dangerous to the system. Switching overvoltage increases as the system voltage increases. In extra high voltage line, switching overvoltages determine the insulation level of the lines and their dimensions and costs. 19

20. The making and breaking of electric circuits with switchgear may result in abnormal overvoltage in power systems having large inductance and capacitances. The overvoltage as six times the normal power frequency voltage. In circuit breaking operation, switching surges with a high rate of rise of voltage may cause repeated restriking of the arc between the contacts of a circuit breaker, thereby causing destruction of the circuit breaker contacts. The switching surges may include high natural frequencies of the system, a damped normal frequency voltage component, or the restriking and recovery voltage of the system with successive reflected waves from terminations. Origin of Switching Surges 20

21. Characteristics of Switching Surges The wave shapes of switching surges are quite different and may have origin from any of the following sources. De-energizing of transmission lines, cables, shunt capacitor banks, etc. Disconnection of unloaded transformers, reactors, etc. Energization or reclosing of lines and reactive loads, Sudden switching off of loads. Short circuits and fault clearances. Resonance phenomenon like ferro-resonance, arcing grounds, etc. 21

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23. From the figures of the switching surges it is clear that the overvoltages are irregular (oscillatory or unipolar) and can be of high frequency or power frequency with its harmonics. The relative magnitudes of the overvoltages may be about 2.4 p.u. in the case of transformer energizing 1.4 to 2.0 p.u. in switching transmission lines. 23

24. Switching Overvoltages In EHV and UHV Systems Over voltages are generated in EHV systems when there is a sudden release of internal energy stored either in the electrostatic form (in the capacitance) or in the electromagnetic form (in the inductance). The different situations are summarized as interruption of low inductive currents (current chopping) by high speed circuit breakers interruption of small capacitive currents, such as switching off of unloaded lines etc ferro-resonance condition(   a non-linear resonance phenomenon in electrical power systems, occurring when a circuit containing a saturable (nonlinear) inductor and a capacitor is subjected to a disturbance ) - This may occur when poles of a circuit breaker do not close simultaneously energization of long EHV or UHV lines. 24

25. Transient overvoltages in the above cases can be of the order of 2.0 to 3.3 p.u . and will have magnitudes of the order of 1200 kV to 2000 kV on 750 kV systems . The duration of these overvoltages varies from 1 to 10 ms depending on the circuit parameters. Sometimes the overvoltages may last for several cycles. The other situations of switching that give rise to switching overvoltages of shorter duration (0.5 to 5 ms ) and lower magnitudes (2.0 to 2.5 p.u .) are: single pole closing of circuit breaker interruption of fault current when the L-G or L-L fault is cleared resistance switching used in circuit breakers switching lines terminated by transformers series capacitor compensated lines sparking of the surge diverter located at the receiving end of the line to limit the lightning overvoltages 25

26. The overvoltages are calculated from mathematical modeling using computer scale modeling using transient network analysers conducting field tests The main factors that are investigated in the above studies are (i) the effect of line parameters, series capacitors and shunt reactors on the magnitude and duration of the transients (ii) the damping factors needed to reduce the magnitude of overvoltages (iii) the effect of single pole closing, restriking and switching with series resistors or circuit breakers on the overvoltages, and (iv) the lightning arrester sparkover characteristics. 26

27. It is necessary in EHV and UHV systems to control the switching surges to a safe value of less than 2.5 p.u. or preferably to 2.0 p.u. or even less. The measures taken to control or reduce the overvoltages are (i) one step or multi-step energisation of lines by preinsertion of resistors, (ii) phase controlled closing of circuit breakers with proper sensors, (iii) drainage of trapped charges on long lines before the reclosing of the lines, and (iv) limiting the overvoltages by using surge diverters. 27

28. Control of Overvoltages Due to Switching The overvoltages due to switching and power frequency may be controlled by Energisation of transmission lines in one or more steps by inserting resistances and withdrawing them afterwards, Phase controlled closing of circuit breakers Drainage of trapped charges before reclosing Use of shunt reactors Limiting switching surges by suitable surge diverters 28

29. a) Insertion of Resistor It is normal and a common practice to insert resistances R in series with circuit breaker contacts when switching on but short circuiting them after a few cycles. This reduce the transients occurring due to switching. The applied step at the first instance is only 0.5 per unit. When the resistor is short circuited, a voltage step equal to the instantaneous voltage drop enters the line. If the resistor is kept for a duration larger than 5ms it can be shown from successive reflections and transmissions, that the overvoltage may reach as high as 1.2 p.u . for a line length of 500 km. Pre-insertion of suitable value resistors in practice is done to limit the overvoltage to less than 2.0 to 2.5 p.u . Normal time of insertion is 6 to 10 m s. 29

30. b) Phase Controlled Switching Overvoltages can be avoided by controlling the exact instances of the closing of the three phases separately. But this necessitates the use of complicated controlling equipment and therefore is not adopted. 30

31. c) Drainage of Trapped Charge When lines are suddenly switching off, "electric charge" may be left on capacitors and line conductors. This charge will normally leak through the leakage path of the insulators, etc. Conventional potential transformers (magnetic) may also help the drainage of the charge. An effective way to reduce the trapped charges during the lead time before reclosing is by temporary insertion of resistor or shunt reactors and removing & before the closure of the switches. 31

32. d) Shunt Reactors Normally all EHV lines will have shunt reactors to limit the voltage rise due to the Ferranti effect. They also help in reducing surges caused due to sudden energizing . However, shunt reactors cannot drain the trapped charge but will give rise to oscillations with the capacitance of the system. Since the compensation given by less than 100%, the frequency of oscillation will be less than the power frequency and overvoltages produced may be as high as 1.2 p.u . Resistors in series with these reactors will suppress the oscillations and limit the overvoltages . 32

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34. Power Frequency Overvoltages in Power Systems The power frequency overvoltages occur in large power systems and they are of much concern in EHV systems, i.e. systems of 400 kV and above. Overvoltages of power frequency harmonics and voltages with frequencies nearer to the operating frequency are caused during tap changing operations, by magnetic or ferro-resonance phenomenon in large power transformers, and by resonating overvoltages due to series capacitors with shunt reactors or transformers. The duration of these overvoltages may be from one to two cycles to a few seconds depending on the overvoltage protection employed. 34

35. The main causes for power frequency and its harmonic overvoltages are (a) sudden loss of loads (b) disconnection of inductive loads or connection of capacitive loads (c) Ferranti effect (d) unsymmetrical faults (e) saturation in transformers (f) Tap changing operations 35

36. a) Sudden Load Rejection It causes the speeding up of generator prime movers. The speed governors and automatic voltage regulators will restore normal conditions. But initially both the frequency and voltage increase. The approximate voltage rise, neglecting losses, etc. may be taken as where x s is the reactance of the generator x c is the capacitive reactance of the line E’ the voltage generated f is instantaneous increased frequency f o is the normal frequency. 36

37. b) Disconnection of inductive loads or connection of capacitive loads For improving voltage in the transmission lines, inductive loads are disconnected or capacitive loaded are added. Due to these switching operations, power frequency over votages may occur 37

38. c) Ferranti Effect Long uncompensated transmission lines exhibit voltage rise at the receiving end. In long transmission lines and cables, receiving end voltage is greater than sending and voltage during light load or no-load operation. The voltage rise at the receiving end V 2 is approximately given by 38

39. d) Unsymmetrical Faults Single L-G faults cause rise in voltages in other healthy phases. Usually, with solidly grounded systems, the increases in voltage (phase to ground value) will be less than the line-to-line voltage. With effectively grounded systems (where, Ro and Xo are zero sequence resistance and reactance and X 1 is the positive sequence reactance of the system) The rise in voltage of the healthy phases does not usually exceed 1.4 per unit. 39

40. e) Saturation Effects When voltages above the rated value are applied to transformers , their magnetizing currents increase rapidly and may be about the full rated current for 50% overvoltage. These magnetizing currents are of a peaky waveform. The 3rd, 5th, and 7th harmonic contents may be 65%, 35%, and 25% of the exciting current of the fundamental frequency corresponding to an overvoltage of 1.2 p.u. For higher harmonics, a series resonance occurs between the transformer inductance and line capacitance, which produces overvoltages. 40

41. (f) Tap changing operations Tap changing operations are required when the voltage changes due to load variations. So, during these operations power frequency overvoltage occur. 41

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44. Protection of Transmission Lines against Overvoltages Protection of transmission lines against natural or lightning overvoltages and minimizing the lightning overvoltages are done by suitable line designs, providing guard and ground wires, and using surge diverters. Switching surges and power frequency overvoltages are accounted for by providing greater insulation levels and with proper insulation co-ordination. 44

45. Protection against Lightning Overvoltages and Switching Surges of short Duration Overvoltages due to lightning strokes can be avoided or minimized in practice by shielding the overhead lines by using ground wires above the phase wires using ground rods and counter-poise wires including protective devices like expulsion gaps, protector tubes on the lines, and surge diverters at the line terminations and substations. 45

46. Lightning Protection Using Shielded Wires or 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 It is run above the main conductor of the line. The ground wire shields the transmission line conductor from induced charges, from clouds as well as from a lightning discharge. 46

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48. If a positively charged cloud is assumed to be above the line, it induces a negative charge on the portion below it, of the transmission line. With the ground wire present, both the ground wire and the line conductor get the induced charge But the ground wire is earthed at regular intervals, and as such the induced charge is drained to the earth Only the potential difference between the ground wire and the cloud and that between the ground wire and the transmission line wire will be in the inverse ratio of their respective capacitances 48

49. As the ground wire is nearer to the line wire, the induced charge on it will be much less and hence the potential rise will be quite small. The effective protection given by the ground wire depends on the height of the ground wire above the ground (h) and the protection or and shielding angle (usually 30°) The shielding angle 30° was considered adequate for tower heights of 30 m or less. The shielding wires may be one or more depending on the type of the towers used. But for EHV lines, the tower heights may be up to 50 m, and the lightning strokes sometimes occur directly to the line wires The present trend in fixing the tower heights and shielding angles is by considering the "flashover rates" and failure probabilities. 49

50. Protection Using Ground Rods Ground rods are used to reduce the tower footing resistance. These are buried into the ground surrounding the tower structure. Ground rods are a number of rods about 15 mm diameter and 3 m long driven into the ground. The tower footing resistance can be varied by: Varying the spacings of the rod. Varying the number of rods. Varying the depth to which they are driven. Material used: Galvanized iron or copper bearing steel. 50

51. Protection Using Counter-Poise Wires Counter-poise wires are buried in the ground at a depth of 0.5 to 1 m, running parallel to the transmission line conductors and connected to the tower legs. Wire length may be 50 to 100 m long. 51

52. Using Protective Devices Protective devices are used to protect the power system components against the travelling waves caused by lightning. Basic Requirements of a Lightning Arrester or Surge Diverter The basic requirements of a lightning arresters are: It should not pass any current to the system component which is to be protected at abnormal conditions. It should break down as quickly as possible when abnormal condition occurs. It should discharge the surge current without damaging it. It should interrupt the power frequency follow current after the surge is discharged to ground. 52

53. Shunt Protected Devices 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. When the magnitude of an incoming wave exceeds the gap setting of the rod-gap, a spark-over occurs and the surge is diverted. 53

54. 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. 54

55. Limitations Rod gap is not capable of preventing the power frequency current, so discharge takes place. During transient period, it is not operated for steep fronted surge waves. During climatic changes and the polarity of surge waves will influence the operation of the gap. During the operation of rod gap, more heat is produced and sometimes it will damage the materials of the rod. 55

56. Expulsion Type Lightning Arrester (Protector Tube) It is a device consists of a spark gap together with an arc quenching device which extinguishes the current arc when--the gaps break over due to over voltages. When lightning incidents, the series gap and the gap in the tube spark and provide low impedance path for power current to flow. The voltage across the terminals of the arrester drops to a low value after spark over occurs and arrester exerts little opposition to the flow of follow current. The arc struck in the tube volatizes some of the fibre, and emitting gas. This gas rushes out through the vent and are interruption takes place, at zero current. 56

57. Advantages Cheap. To protect small rural transformers where valve type arresters are expensive. Disadvantages It is not suitable for protection of expensive station equipment because of poor volt-time characteristics. Uses To protect transmission line insulators (transmission line type). To protect distribution transformer (distribution type). 57

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60. Valve Type Lightning Arrester (Non-Linear Type) Valve Type Lightning Arresters are used to protect substations and at line terminations to discharge the lightning over voltages and short duration switching surges. A number of non-linear resistor elements made of silicon carbide arc stacked one over the other into two or three sections. They are separated by spark gaps. Spark gaps and resistors are protected by water tight housing. Non-linear resistor possess low resistance to high currents and high resistance to low currents. 60

61. Advantages of Valve type arresters are : To protect station equipments rated 400 kV and above. To protect transmission line rated above 66 kV. To protect motors and generators. To protect distribution transformers. Disadvantages of valve types arresters are: Expensive. Care should be taken. 61

62. Travelling Waves on Transmission Lines Any disturbance on a transmission line or system such as sudden opening or closing of a line, a short circuit or a fault results in the development of over voltages or overcurrents at that point. This disturbance propagates as a travelling wave to the ends of the line or to a termination, such as, a sub-station. Usually these travelling waves are high frequency disturbances and travel as waves. They may be reflected, transmitted, attenuated or distorted during propagation until the energy is absorbed. Long transmission lines are to be considered as electrical networks with distributed electrical elements. 62

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64. Classification of Transmission Lines Transmission lines are usually classified as (a) lines with no loss or ideal lines – R= 0, G = 0 (b) lines without distortion or distortionless lines – R/L = G/C = α (attenuation constant) (c) lines with small losses – R/L = G/C = small value (d) lines with infinite and finite length defined by all the four parameters. 64

65. Bewley Lattice Diagram (Refection and Refraction of Travelling Waves) Bewley lattice diagram from which the motion of reflected and transmitted waves and their positions at every instant can be obtained. It overcomes the difficulty of keeping track of the multiplicity of successive reflections at the various junctions. 65

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67. Refection and Refraction Coefficient 67

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72. Line terminated with natural impedance and surge impedance 72

73. Reflection and Refraction or Transmission at a T- Junction 73

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75. Properties of Bewley Lattice Diagram ✓ All waves travel downhill, because time always increases. ✓ The position of any wave at any time can be deduced directly from the diagram. ✓ The total potential at any instant of time is the superposition of all waves which arrive at that point until the instant of time, displaced in position from each other by time intervals equal to the time difference of their arrival. ✓ Attenuation is included so that the wave arriving at the far end of the line corresponding to the value entering multiplied by the attenuation factor a. ✓ The history of the wave is traced easily. 75

76. Open ended transmission line of surge impedance z 76

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81. Two substations A, B are shown in figure. The attenuation factor is taken as 0.9 and 0.8. Draw the Bewley lattice diagram. 81

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83. Insulation Co-ordination Insulation coordination is the process of carefully selecting and arranging the insulation and protective devices in an electrical power system to ensure reliable operation during overvoltage events. It aims to minimize the risk of equipment damage and service interruptions caused by over voltages, such as those from lightning strikes or switching operations. This involves determining appropriate insulation levels for various components and coordinating them with the characteristics of surge arresters and other protective devices.  83

84. REFERENCES: S.Naidu and V. Kamaraju, ‘High Voltage Engineering’, Tata McGraw Hill, Fifth Edition, 2013. E. Kuffel and W.S. Zaengl, J.Kuffel, ‘High voltage Engineering fundamentals’, Newnes Second Edition Elsevier , New Delhi, 2005. C.L. Wadhwa, ‘High voltage Engineering’, New Age International Publishers, Third Edition, 2010. 84