1
UNIT – 5 Travelling Wave and Switching Transients on Transmission Lines Unit-05/Lecture-01 Travelling Waves on Transmission Lines Travelling Waves on Transmission Lines [RGPV/ June 2006, June 2010, Dec 2008/ 10] Introduction: The establishment of a potential difference between the conductors of an overhead transmission line is accompanied by the production of an electrostatic flux, whilst the flow of current along the conductor results in the creation of a magnetic field. The electrostatic fields are due, in effect, to a series of shunt capacitors whilst the inductances are in series with the line. Consider the section of the line adjacent to the generator in Figure 1.
Let the voltage E suddenly applied to the circuit by closing the switch. Under these conditions, the capacitance C1 takes a large initial charging current the whole of the voltage will at first be used in driving a charging current through the circuit consisting of L1 and C1 in series. As the charge on C1 builds up its voltage will increase and this voltage will begin to charge C2 by driving a current through the inductance L2 (Figure 2), and so on, showing that the greater the distance from the generator, the greater will be the time elapsed from the closing the switch to the establishment of the full line voltage E. It is also clear that voltage and current are intimately associated and that any voltage phenomenon is associated with an attendant current phenomenon.
2
The gradual establishment of the line voltage can be regarded as due to a voltage wave travelling from the generator towards the far end and the progressing charging of the line capacitances will account for the associated current wave.
S.NO RGPV QUESTIONS Q.1
What do you understand by term “travelling Dec 2008 waves”?
Q.2
Year
10
June 2010
Explain the cause of travelling waves on June 2006 transmission line?
Marks
10
3
UNIT 05/ LECTURE 02 Velocity of Travelling Waves and Characteristic Impedance Velocity of Propagation of Travelling Waves: If a voltage source u is switched on in a two-wire transmission line at t=0 (see Figure 3.2), the line will be charged by the voltage source. After a small time spant, only a small segmentx of the line will be charged instantaneously with a chargeQ=Cx u. This charge, equally distributed over the line segmentx, causes an electric field E around this line segment and the current, or the flow of charge creates a magnetic field H around the line segmentx.
Figure 1 Electric and magnetic field around a line segmentx of a two-wire transmission line
Ifx is made infinitely small, the expression for the current is
Because the distancexis covered in the timet,x/t is the velocity at which the charge travels along the is∆∅
line.
The
magnetic
flux
present
around
the
line
segment
= 𝐿∆𝑥𝑖. If this is substituted in Equation (3.1) andx is made
infinitely small, the expression for the induced electromotive force emf in the loop enclosed by the two wires over the distance∆x is
Because there cannot be a discontinuity in voltage, this emf equals the voltage source u; this gives an expression for the wave velocity:
The wave velocity depends only on the geometry of the line and on the permittivity and the
4
permeability of the surrounding medium. For a 150-kV overhead transmission line with one conductor per phase, a conductor radius of r=25 mm and an equivalent distance between phases of dm=5.5 m, the inductance and the capacitance values are
This results in a wave velocity of ν line =293.117 km/s. When we calculate the distributed inductance and the capacitance for a single-core cross linked polyethylene (XLPE) –insulated 18/30-kV medium-voltage cable with a 120-mm2copper conductor and a 16-mm 2 earth screen (N2XS2Y-1x120/16-18/30 kV), the values are
This results in a wave velocity of ν cable=114.302 km/s. On an overhead transmission line the electromagnetic waves propagate close to the speed of light, but in an underground cable the velocity is considerably lower. When the wave velocity is substituted in Equation (3.1) we get
We notice that the ratio between the voltage and current wave has a fixed value
This is called the characteristic impedance of a transmission line. The characteristic impedance depends on the geometry of the transmission line and its surrounding medium only. For the 150-kV overhead line, the characteristic impedance is Z line =330, and for the XLPE medium voltage cable, the characteristic impedance isZcable=49 ohm.
S.NO Q.1
RGPV QUESTIONS
Year
Marks
5
Unit-05/Lecture-03 Attenuation and Distortion of Electromagnetic Waves Attenuation and Distortion of Electromagnetic Waves [RGPV/ June 2007, June 2010/ 10] the following factors should be taken into account for analysis of transmission lines: • The series resistance of the conductors • The skin-effect for higher frequencies • The losses in the dielectric medium between the conductors in a high-voltage cable • The leakage currents across string insulators • The influence of the ground resistance • The corona losses and so on To include these losses in our analysis, we assume a series resistance R and a parallel conductance G, evenly distributed along the wires as the inductance L and the capacitance C. We consider again a line segment∆x. (see Figure 3)
Figure 3 Equivalent circuit of a differential-length line segment∆x of a two-conductor transmission line The losses in line segment∆x at the time to are
When∆x becomes infinitely small, we can write it as dx. An electromagnetic wave with a powerP = i(x0,t0)2 Z entering this infinitely small line segment dx loses (when we differentiate P with respect to i(x,t)) power P = 2i(x0,t0)di Z, while travelling the distance dx. This loss of power has a negative sign because the wave energy decreases and is dissipated in R and G
6
This can be generalised for every instant of time
The solution of this differential equation is
and because of the relation u(x,t) = Zi(x,t) that exists between the voltage and the current wave, we can write for the voltage wave
When the voltage and current waves travel along a transmission line with losses, the amplitude of the waves is exponentially decreased. This is called attenuation and is caused by the properties of the transmission line. For overhead transmission lines, G is a very small number and we can simplify Equation (3.15) for the current and Equation (3.16) for the voltage wave to
The attenuation is small for a line with a low resistance and/or a large characteristic impedance. When the series resistance R and the parallel conductance G can be neglected, both the wave velocity and the characteristic impedance are constant and the transmission line is said to be lossless. When R/L=G/C, we call the transmission line distortion less; the shape of the current and voltage waves is not affected and the wave velocity and the characteristic impedance are constant, similar to a lossless line. When the transmission line is not distortion less, the steepness of the wave front will decrease and the general shape of the waves will be more elongated when they travel along the line.
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S.NO RGPV QUESTIONS Q.1
Year
Explain attenuation of travelling waves on June 2010
Marks 10
transmission lines. Q.2
Explain the terms “attenuation and distortion” June 2007 of travelling waves propagating on overhead lines.
10
8
Unit-05/Lecture-04 Reflection and Refraction of Travelling Waves Reflection and Refraction of Travelling Waves [RGPV/ June 2006, June 2007/ 10] When an electromagnetic wave propagates along a transmission line with a certain characteristic impedance, there is a fixed relation between the voltage and current waves. But what happens if the wave arrives at a discontinuity, such as an open circuit or a shortcircuit, or at a point on the line where the characteristic impedance Z=(L/C)1/2 changes, for instance, when an overhead transmission line is terminated with a cable or a transformer? Because of the mismatch in characteristic impedance, an adjustment of the voltage and current waves must occur. At the discontinuity, a part of the energy is let through and a part of the energy is reflected and travels back. At the discontinuity, the voltage and current waves are continuous. In addition, the total amount of energy in the electromagnetic wave remains constant, if losses are neglected. Figure 3.6 shows the case in which an overhead transmission line is terminated with an underground cable.
When, for the sake of simplicity, both the overhead line and the underground cable are assumed to be without losses, R=0 and G=0, then the expressions for the characteristic impedances are
The forward wave is called the incident wave and it travels on the overhead line toward the cable. The positive x-direction in Figure 6 is from left to right and the line-cable joint is at x0. The incident wave reaches the discontinuityx0 at t=t0. In our equations, the incident waves have subscript 1, the reflected waves have subscript 2, and the waves that are let through have subscript 3.
9
Figure 6 Overhead transmission line terminated with an underground cable Voltage and current waves are continuous at the line–cable joint:
This leads to the conclusion that for t>t0, at the discontinuity at x0, the incident waves and the reflected waves equal the waves that are let through. For the voltage waves, we can write
The reflected current wave has a negative sign:
From Equation (3.48) and Equation (3.49), for the voltage and current waves that are let through
and for the reflected waves
For a transition from overhead line to cable in a substation, an incoming voltage wave is reduced in amplitude. When an incoming overhead line is connected to a substation by means of a cable of short length, the cable protects the substation against overvoltages. When lightning strikes the substation, the opposite occurs – a magnified voltage wave leaves the substation and travels over the overhead line.
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When the incident wave encounters a short-circuit atx0,which means, in our example, that ZC=0, the voltage wave disappears atx0 and the current wave doubles in amplitude; the wave energy is stored in the magnetic field only. When the incident wave encounters an open circuit at x0, the voltage wave is doubled in amplitude and the incident current wave is nullified by the reflected current wave; the wave energy is stored in the electric field only. The voltage doubling that occurs when a voltage source is switched on an unloaded cable or an unloaded overhead line is called the Ferranti-effect, named after the British scientist and engineer Sebastiano Ziani de Ferranti (1864–1930). When a transmission line is terminated with a load impedance ZL, different from the lines characteristic impedanceZ0,theratioofthe complex amplitudes of the reflected and incident voltage waves at the load is called the voltage reflection coefficient
The voltage reflection coefficient is, in general, a complex quantity with a magnitude smaller than one.
S.NO RGPV QUESTIONS Q.1
Year
Explain the following terms related with June 2007
Marks 10
travelling waves on transmission lines. i. Reflection coefficient ii. Refraction coefficient iii. Attenuation factor Q.2
Drive an expression for reflection and refraction June 2006 coefficient for travelling wave at a junction. Hence explain the significance of junction of an overhead transmission line with cable.
10
11
Unit-05/Lecture-05 Effect of Propagation of Travelling Waves on Open Circuit Line [RGPV/ Dec 2009/ 10]
At the Open-Circuited Line: Let a source of constant voltage E be switched suddenly on a line open-circuited at the far end. Then neglecting the effect of line resistance and possible conductance to earth, a rectangular voltage wave of amplitude E and its associated current wave of amplitude I = E/Zc will travel with velocity v towards the open end. Figure 9 shows the conditions at the instant when the waves have reached the open end, the whole line being at the voltage E and carrying a current I.
Figure 9 At the open end, the current must of necessity fall to zero, and consequently the energy stored in the magnetic field must be dissipated in someway. In the case under consideration, since resistance and conductance have been neglected, this energy can only be used in the production of an equal amount of electrostatic field. If this is done, the voltage at the point will be increased by an amount e such that the energy lost by the electromagnetic field (0.5 LI2) is equal to the energy gained by the electrostatic field (0.5Cv2), or:
Hence, the total voltage at the open end becomes 2E. The open end of the line can thus be regarded as the origin of a second voltage wave of amplitude E, this second wave travelling back to the source with the same velocity v. At some time subsequent to arrival of the initial wave at the open end, i.e. the condition shown in Figure 3.a, the state affairs on the line will be as in Figure 10 in which the incoming and reflected voltage waves are superposed, resulting in a step in the voltage wave which will travel back towards the source with a velocity v. The doubling of the voltage at the open end must be associated with the disappearance of the current since none can flow beyond the open circuit. This is equivalent to the establishment of a reflected current wave of negative sign as shown in
12
Figure 10.
Figure 10 At the instant the reflected waves reach the end G, the distribution along the whole line will be a voltage of 2E and a current of zero as in Figure 11.
Figure 11 At G, the voltage is held by the source to the value E, it follows that there must be a reflected voltage of –E and associated with it there will be a current wave of –I. After these have travelled a little way along the line, the conditions will be as shown in Figure 12.
Figure 12 When these reach the open end the conditions along the line will be voltage E and current –I. The reflected waves due to these will be –E and +I and when these have travelled to the end G they will have wiped out both voltage and current distributions, leaving the line for an instant in its original state. The above cycle is then repeated.
S.NO RGPV QUESTIONS Q.1
Year
For travelling waves on a transmission line Dec 2009 starting from fundamentals, obtain expression for reflection and transmission coefficient for
Marks 10
13
voltage and current waves.
14
Unit-05/Lecture-06 Effect of Propagation of Travelling Waves on Short Circuit Line At the Short-Circuited Line [RGPV/ Dec 2010/10] In this case, the voltage at the far end of the line must of necessity be zero, so that as each element of the voltage wave arrives at the end there is a conversion of electrostatic energy into electromagnetic energy. Hence, the voltage is reflected with reversal sign while the current is reflected without any change of sign: thus on the first reflection, the current builds up to 2I. Successive stages of the phenomenon are represented in Figure 13.
Figure 14 A. Original current and voltage waves just prior to the first reflection. B. Distributions just after the first reflection. C. Distributions at the instant the first reflection waves have reached the generator. Note that the whole of the line is at zero voltage. D. Distributions after the first reflection at the generator end. E. Distributions at the instant the first reflectedwaves from the generator reach the far end.
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It will be seen that the line voltage is periodically reduced to zero, but that at each reflection at either end the current is built up by the additional amount I = E/Zc. Thus, theoretically, the current will eventually become infinite as is to be expected in the case of a lossless line. In practice, the resistance of the line produces attenuation so that the amplitude of each wave-front gradually diminishes as it travels along the line and the ultimate effect of an infinite number of reflections is to give the steady Ohm’s law of current E/R.
S.NO RGPV QUESTIONS Q.1
Year
What are the factors affecting the function or Dec 2010 termination of transmission line? Discuss the factors to be considered for them.
Marks 10
16
Unit-05/Lecture-07 Overvoltage in Transmission Line Overvoltage in Transmission Line [RGPV/ June 2005, June 2007, June 2008, June 2010, Dec 2007, Dec 2008, Dec 2010/ 5 & 10]
Each component of the power system, whether it is a simple string insulator or a large power transformer, is continuously stressed by the system operating voltage at power frequency. Occasionally overvoltages occur, having a peak value exceeding the peak value of the system operating voltage. The overvoltages can be divided into three categories:
• Overvoltages caused by lightning discharges (Lightning-Induced Transients), • Switching overvoltages (Switching Transients), and • Sustained AC overvoltages.
The lightning discharge is a current injection either in the vicinity of a line or substation, in the transmission line tower, or directly on the line. The voltages developed across the power system components depend on the characteristic impedance of the components. The waveforms of the lightning-induced overvoltages are all different in amplitude and capricious in shape. For the sake of testing the dielectric impulse strength of power apparatus in the high-voltage laboratory, the impulse voltage wave has been standardised with the 1.2/50-µs waveform.
Switching overvoltages result from switching operations in the network. Switching of a short-circuit current, clearing a short-line fault, disconnecting unloaded transformers, disconnecting or connecting unloaded distribution cables or transmission lines, all result in damped oscillatory voltages, the so-called transient recovery voltage or TRV.
AC overvoltages occur when the 50- or 60-Hz operating voltage temporarily attains a high value at the receiving end of a transmission line after a sudden loss of load. The resistive and reactive voltage drop disappears and the overvoltage stresses the system until the operating voltage is restored. Another situation for AC overvoltages to occur is in the case
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of a single-phase-to-ground fault in an isolated neutral system. The healthy phases rise from phase voltage until the √3 times higher line voltage. Also the capacitance of an unloaded distribution cable in combination with the inductance of a power transformer or generator can increase the system voltage resulting in a sustained AC overvoltage.
S.NO RGPV QUESTIONS Q.1
Year
What are the various causes of overvoltages in Dec 2010 transmission lines?
Marks 10
Dec 2008 Dec 2007
Q.2
What are the various causes of temporarily June 2007 overvoltages in EHC AC transmission line.
Q.3
June 2008
Write a short note on “Overvoltages in June 2005 transmission system”.
10
June 2010
05
18
Unit-05/Lecture-08 Lightning Overvoltages Lightning Overvoltages Overvoltages in a power system can be caused by transient currents and by transient voltages after switching actions during normal operation or after clearing fault situations. The overvoltages originate from the state of the system. There are also overvoltages that come from outside the system as a result of atmospheric discharges. Large parts of the power system are formed by overhead transmission lines interconnected by outdoor substations. Only in densely populated areas, the high-voltage transmission and distribution is done with high-voltage cables interconnected by gas-insulated substations (GIS) placed in buildings. When we realise that on an average every commercial aeroplane and every square kilometre of the earth’s surface in a country like the Netherlands is hit by lightning once a year, it is obvious that our power systems should be protected against lightning strokes. For the analysis of the lightning-induced overvoltages, a difference is made between the following:
• Lightning strokes in the vicinity of high-voltage transmission lines, which do not hit the conductors themselves, • Direct lightning strokes on the line conductors injecting a current wave on the line, and • Lightning strokes on the transmission towers or on the ground wires.
THE MECHANISM OF LIGHTNING [RGPV / Dec 2008, June 2006/10] Lightning mostly occurs on summer days when the ambient temperature is high and the air is humid. Because of the temperature difference, the humid air is lifted to higher altitudes with a considerable lower ambient temperature. Cold air contains less water than warm air and raindrops are formed. The raindrops have a size of a few millimetres and are polarised by the electric field that is present between the lower part of the ionosphere and the earth’s surface. The strength of this atmospheric field is on summer days in the order of 60 V/m and can reach values of 500 V/m on a dry winter day. The vertical movement of the raindrops and the wind shear splits the raindrops into negatively charged small drops and positively charged larger raindrops. The larger raindrops fall, under the influence of the gravity, to earth and create a shower. The positive charge of
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raindrops in a shower is confirmed by measurements. The majority of the thunderclouds isnegativelycharged with a potential to earth of several hundreds of megaVolts. The clouds move at great heights and the average field strength is far below the average breakdown strength of air. Inside the thundercloud, the space charge formed by the accumulating negative raindrops creates a locally strong electric field in the order of 10 000 V/m and accelerates the quickly moving negative ions to considerable velocities. Collision between the accelerated negative ions and air molecules, creates new negative ions, which on their part are accelerated, collide with air molecules and free fresh negative ions. An avalanche takes place, and the space charge and the resulting electric field grow in a very short period of time. The strong electric field initiates discharges inside the cloud, and a negative stream of electrons emerges as a dim spark called a stepped leader or dart leader that jumps in steps of approximately 30 meters and reaches the earth in about 10 milliseconds. The stepped leader reaches close to the earth’s surface, reaching an upward positive leader, and forms the main channel.
Because of the stochastic behaviour of the space charge accumulation, the stepped leader also creates branches to the main channel. The main channel carries initially a discharge current of a few hundred Amperes, having a speed of approximately 150 km/s. This discharge current heats up the main channel and the main discharge, the return stroke, is a positive discharge, and it travels at approximately half the speed of light equalising the charge difference between the thundercloud and the earth. The main discharge current can be 100 000 A or more and the temperature of the plasma in the main channel can reach values as high as 30 000 K. The pressure in the main channel is typically 20 bar. The creation of the return stroke takes place between 5 and 10µs and is accompanied by a shock wave that we experience as thunder. A lightning stroke consists of several of these discharges, usually three or four, with an interval time of 10–100µs. The human eye records this as flickering of the lightning and our ear hears the rolling of the thunder. After each discharge, the plasma channel cools down to approximately 3000 K, leaving enough ionisation to create a new conducting plasma channel for the following discharge.
In the majority of the cases, a thundercloud is negatively charged, but positively charged clouds can also be formed. Cloud-to-ground strokes can hit substations, transmission line
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towers, and transmission lines directly, but a considerable number of atmospheric discharges are between clouds. When the charged clouds are floating above, for instance, a high-voltage transmission line, they induce charge accumulation on the line conductors (see Figure 14). When the lightning stroke equalises the charge difference between the clouds, the rather slowly accumulated charge on the conductors has to disappear at once. This results in transient currents and overvoltages.
Figure 14 Effect of a charged cloud on a high-voltage transmission line
WAVESHAPE OF THE LIGHTNING CURRENT Lightning currents differ in amplitude and shape. The majority of the cloud-to-ground lightning strokes vary from kilo Amperes to several tenths of kiloamperes. Strokes above 100 000 amperes are rare, and the highest reported peak value of the return stroke current is 200 000 A. The shape of the current wave and the related voltage wave is rather capricious and different for every stroke. To facilitate testing in the laboratory and computations either by hand or by computer, the shape of the current wave of the return stroke is standardised.
The IEC has standardised the so-called 1.2/50-µs waveform (Figure 14).
The rise time tf =1.2µsis defined as being 1.67 times the time interval between 30 percent and 90 percent of the peak value of the current wave.
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The tail value tt =50µs is defined as the time it takes until the wave drops till 50 percent of the peak value. When the current wave travels in the power system, there is, of course, a related voltage wave also present.
The ratio between the voltage wave and the current wave at a certain place in the system is the characteristic impedance at that particular part of the network. System components can be exposed to very high lightning-induced overvoltages.
Basic Insulation Level [RGPV/ Dec 2009/ 05] The name plate of high-voltage equipment shows the Basic Insulation Level or BIL, which is a standardised figure for each voltage rating related to the voltage level at which the equipment should operate.
Figure 14 Standardised waveform of a lightning-induced voltage wave he lightning-induced voltage wave can be described mathematically as the difference of two exponential functions:
In this expression, the parameter βis associated with the rise time tf and α with the tail time tt. When choosing for α the value 1.4∗E4s−1 and for β the value 4.5∗E6s−1, the double exponential expression of Equation (7.1) results in a 1.2/50-microsecond waveform. The double exponential wave is easy to manipulate in mathematical analysis
22
and results in an acceptable degree of accuracy.
S.NO RGPV QUESTIONS
Year
Marks
Q.1
Explain the mechanism of lightning stroke.
Dec 2008
10
Q.2
Describe with neat sketch the mechanism of June 2006
10
lightning stroke contacting a tower.. Q.3
Write a short note on “Overvoltages in June 2005 transmission system”.
Q.4
June 2010
Define BIL. Explain its signicance in power Dec 2009 system.
10
05
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Unit-05/Lecture-09 Switching Transients Switching Transients When load break switches, circuit breakers, disconnectors, or fuses operate, a switching action takes place in the network and parts of the power system are separated from or connected to each other. The switching action can be either a closing or an opening operation in the case of a switching device. Fuses can perform opening operations only. After a closing operation, transient currents will flow through the system, and after an opening operation, when a power-frequency current is interrupted, a transient recovery voltage or TRV will appear across the terminals of the interrupting device. The configuration of the network as seen from the terminals of the switching device determines amplitude, frequency, and shape of the current and voltage oscillations. When capacitor banks for voltage regulation are placed in a substation, the switching devices interrupt a mainly capacitive load when operating under normal load conditions. The current and voltage are approximately 90° out of phase and the current is leading the voltage. When a large transformer is disconnected in a normal load situation, current and voltage are also approximately 90° out of phase but now the current is lagging. Closing a switch or circuit breaker in a dominantly capacitive or inductive network results in inrush currents, which can cause problems for the protection system.
The short-line fault is of special importance for SF6 circuit breakers. A fault is called a short-line fault when the short-circuit, usually a single line-to-ground fault, occurs on a high-voltage transmission line, a few hundred meters to a few kilometres from the breaker terminals. A very steep triangular-shaped oscillation immediately after current zero puts stress on the still-hot arc channel and can easily cause a thermal breakdown.
24
Unit-05/Lecture-09 Protection Against Lightning Protection Against Lightning Transients or surges on the power system may originate from switching and from other causes but the most important and dangerous surges are those caused by lightning. The lightning surges may cause serious damage to the expensive equipment in the power system (e.g. generators, transformers etc.) either by direct strokes on the equipment or by strokes on the transmission lines that reach the equipment as travelling waves. It is necessary to provide protection against both kinds of surges. The most commonly used devices for protection against lightning surges are :
(i) Earthing screen (ii) Overhead ground wires (iii) Lightning arresters or surge diverters
Earthing screen provides protection to power stations and sub-stations against direct strokes whereas overhead ground wires protect the transmission lines against direct lightning strokes. However, lightning arresters or surge diverters protect the station apparatus against both direct strokes and the strokes that come into the apparatus as travelling waves. We shall briefly discuss these methods of protection.
The Earthing Screen The power stations and sub-stations generally house expensive equipment. These stations can be protected against direct lightning strokes by providing earthing screen. It consists of a network of copper conductors (generally called shield or screen) mounted all over the electrical equipment in the sub-station or power station. The shield is properly connected to earth on atleast two points through a low impedance. On the occurrence of direct stroke on the station, screen provides a low resistance path by which lightning surges are conducted to ground. In this way, station equipment is protected against damage. The limitation of this method is that it does not provide protection against the travelling waves which may reach the equipment in the station.
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Overhead Ground Wires The most effective method of providing protection to transmission lines against direct lightning strokes is by the use of overhead ground wires as shown in Fig. 7. For simplicity, one ground wire and one line conductor are shown. The ground wires are placed above the line conductors at such positions that practically all lightning strokes are intercepted by them (i.e. ground wires). The ground wires are grounded at each tower or pole through as low resistance as possible. Due to their proper location, the ground wires will take up all the lightning strokes instead of allowing them to line conductors. When the direct lightning stroke occurs on the transmission line, it will be taken up by the ground wires. The heavy lightning current (10 kA to 50 kA) from the ground wire flows to the ground, thus protecting the line from the harmful effects of lightning. It may be mentioned here that the degree of protection provided by the ground wires depends upon the footing resistance of the tower. Suppose, for example, tower-footing resistance is R1 ohms and that the lightning current from tower to ground is I1 amperes. Then the tower *rises to a potential Vt given by ;
Figure 7
Since Vt (= I1R1) is the approximate voltage between tower and line conductor, this is also thevoltage that will appear across the string of insulators. If the value of Vt is less than that required to cause insulator flashover, no trouble results. On the other hand, if Vt is excessive, the insulator flashover may occur. Since the value of Vt depends upon tower-
26
footing resistance R1, the value of this resistance must be kept as low as possible to avoid insulator flashover.
Lightning Arresters The earthing screen and ground wires can well protect the electrical system against direct lightning strokes but they fail to provide protection against travelling waves which may reach the terminal apparatus. The lightning arresters or surge diverters provide protection against such surges. A lightning arrester or a surge diverter is a protective device which conducts the high voltage surges on the power system to the ground.
Fig. (i) shows the basic form of a surge diverter. It consists of a spark gap in series with a non-linear resistor. One end of the diverter is connected to the terminal of the equipment to be protected and the other end is effectively grounded. The length of the gap is so set that normal line voltage is not enough to cause an arc across the gap but a dangerously high voltage will break down the air insulation and form an arc. The property of the nonlinear resistance is that its resistance decreases as the voltage (or current) increases and vice-versa. This is clear from the volt/amp characteristic of the resistor shown in Fig. (ii).
27
Types of Lightning Arresters 1. Rod gap arrester 2. Horn gap arrester 3. Multigap arrester 4. Expulsion type lightning arrester 5. Valve type lightning arrester
S.NO RGPV QUESTIONS Q.1
Year
Discuss in brief the various method of Dec 2007
Marks 10
overvoltage protection. Q.2
Write a short note on “Control of lightning and
10
switching transients”. Q.3
Write a short note on “Lightning Arrestor.”
Q.4
How
lightning
and
switching
controlled in a power system.
surges
June 2005 are Dec 2010
10 10
