Unit - Iv: Protection Against Over Voltages Protection Against Over Voltages

UNIT - IV Protection against Over voltages

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Causes of Over Voltages Over voltages arising on a power system can be generally classified into two main categories as follows:

1)

External Over Voltages

2)

Internal Over Voltages (i) Switching Over Voltages (or transient over voltages of high

frequency)

(ii) Temporary Over Voltages (or steady state over voltages of power frequency)

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1)

External Over Voltages



These over voltages originate from atmosphere disturbances, mainly due to lightning. These over voltages take the form of a unidirectional impulse (or surge) whose maximum possible amplitude has no direct relationship with the operating voltage of the system. Causes for over voltages are

a)

Direct lightning strokes

b)

Electromagnetically induced over voltages due to lightning discharge taking place near the line (commonly known as ‘side stroke’)

c)

Voltages induced due to changing atmospheric conditions along the line length . 3

d)

Electro statically induced over voltages due to the presence of charge clouds nearby

e)

Electro statically induced over voltages due to the frictional effects of small particles such as dust or dry snow in the atmosphere or due to change in the altitude of the line.

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2)

Internal Over Voltages



These over voltages are caused by changes in the operating conditions of the network.

a)

Switching over voltages



These over voltages are caused by the transient phenomena which appear when the state of the network is changed by a switching operation or fault condition.



These over voltages are generally oscillatory and take the form of a damped sinusoid.



The frequency of these over voltages may vary from a few hundred Hz to a few kHz, and it is governed by the inherent capacitances and such as switching of the circuit. 5

b)

Temporary over voltages



These over voltages are the steady-state voltages of power-system frequency which may result from the disconnection of load, particularly in case of long transmission lines.

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Lightning Phenomena 

The large spark accompanied by light produced by an abrupt, discontinuous discharge of electricity through the air, from the clouds generally under turbulent conditions of atmosphere is called lightning.



Representative values of a lightning stroke: Voltage

2 x 108 volts 200 MV (peak)

Current

4 x 44 amp.

Duration 10- 5 sec. kW

8 x 109

kWh

22

Energy = ∫J u . t dt = 22 kWh

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Over Voltages due to Lightning 

Lightning causes two kinds of voltage surges (over voltage), (i) direct stroke to a line conductor, (ii) induced by indirect stroke



A direct stroke to a phase (line) conductor is the most severe lightning stroke as it produces the highest over voltage for a given stroke current.

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Voltage surge magnitude at the striking point (V) is given by V = I Zo /2 where Zo = surge impedance of the line (400 ohm) I = lightning current magnitude (less than 10 kA) V = above 2000kV



Lightning surges on transmission line propagates in the form of a travelling wave towards the end of the line.

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When tower struck by lighting V=RI where R = tower footing resistance I = lightning current in tower V = voltage across insulation at tower



For very high towers v = L (di/dt) where, L = tower inductance di/dt = rate of change of current due to direct stroke 10


Protection of Transmission Lines against Direct Lightning Strokes 

Ground Wire



Ground wires are conductors running parallel to the main conductors of the transmission line, supported on the same towers and adequately grounded at every tower or support.



They are made of galvanized steel wires or ACSR conductors. They are provided to shield the lines against direct strokes by attracting the lightning strokes to themselves rather than allowing them to strike the lines (phase conductors).

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Protection of Transmission… 

When a ground wire is struck by direct lightning stroke, the impedance through which the current flows is very much reduced and a correspondingly higher current is required to cause flashover.



The ground wires serves the following purposes: (i) it shield power conductor from direct lightning strokes (ii) it provide parallel path for the stroke there by surge impedance is reduced. (iii) due to magnetic coupling

between the ground wire and power

conductor , changes of insulation failure are reduced.

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Protection of Transmission…

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Protection of Transmission…


Protective Ratio



It is defined as the ratio of the induced voltage on a conductor with ground wire protection to the induced voltage which would exist on the conductor without ground wire protection.




Protective Angle



It is the angle between a vertical line through the ground wire and a slanting line connecting the ground wire and the phase conductor to be protected. Range is 20o to 45o

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Protective Zone



It is defined as the volume between the base plane and the slanting plane, extending from the ground wire to the plane of the conductors.




Height of Ground Wire



For protection against direct strokes, the ground wire should be located at a height

at least 10% greater than y, calculated from the following

equation. Where

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X  y   y  __  h   h  = 2  −   2  −   H H H H  H 

2

X = horizontal spacing b/w the conductor and ground wire, H = height of cloud; h= height of conductor y = height of ground wire 15




Coupling Factor



The ratio of the induced voltage on the conductor to the ground wire voltage is known as the coupling factor. Coupling factor (C) = √ electro static coupling x electro magnetic coupling

Where

b log a C= 2h log r

C = coupling factor a = distance from conductor to ground wire b= distance from conductor to image of ground wire h = height of ground wire above ground r = radius of ground wire 16



The basic requirement for the design of a line based on direct stroke are (i) the ground wires used for shielding the line should be mechanically strong and should be so located that they provide sufficient shield. (ii) there should be sufficient clearance between power conductors and between power conductor and ground wire or tower structure. (iii) the tower footing resistance should be as low as can be justified economically.

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The tower footing resistance may be reduced by driving rods near the tower and connecting them to the tower base or by burying conductors (counterpoise wires) in the ground and connecting them to the tower base.



Driven rods are usually of galvanized iron, copper weld, or copper-bearing steel. Sometimes driven galvanized iron pipe is also used. Counterpoise wires are usually of copper, aluminum, galvanized steel, stranded cable, or galvanized steel strip.



Deep rods may be driven only in soil free from rocks. Deep-driven rods can be successfully used in the sandy soil.

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Protection of Transmission… 

Counterpoise wires are used extensively because of the difficulty encountered in driving rods. They can be arranged either radially (non continuous) or tower –to-tower (continuous).



Continuous counterpoise wires are more effective than non-continuous counterpoise wires.

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Protection of Transmission…

Figure. Typical arrangement of radial (non-continuous) counterpoise wires 20

Protection of Transmission…

Figure. Typical arrangement of tower-to-tower (continuous) counterpoise wires 21

Protection of Transmission…

Figure. Typical arrangement of tower-to-tower (continuous) counterpoise wires 22


Protection against Travelling Waves 

Severe faults produce high voltages and may cause production of travelling waves which may damage the electrical equipments. The causes for damaging the electrical equipment are

i.

Internal flashover caused by the high peak voltage of the surge may damage the insulation of the winding.

ii.

Internal flashover between inter turns of the transformer may be the steep-fronted voltage wave.

iii. External flashover between the terminals of the electrical equipment caused by the high peak voltage of the surge may result in damage to the insulators. 23

iv.

Resonance and high voltages resulting from the steep-fronted wave may cause internal or external flashover of an unpredictable nature causing building up of oscillations in the electrical equipment



Hence it is absolutely necessary to provide some protective device at the stations or sub-stations for the protection of equipment against the travelling waves (surges) caused by lightning.

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The protective devices used for this purpose are

1)

Rod Gap

2)

Arcing Horn

3)

Lightning arresters or surge diverters (i) Expulsion type lightning arrester (ii) Non-linear surge diverter (iii) Metal oxide surge arrester (MOA)

4)

Surge Absorber

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1)

Rod Gap



A rod gap provides the simplest and cheapest protection to line insulators, equipment insulators and bushings of transformers.



A rod gap consists of two rods of approximately 1.2 cm diameter or square, which are bent at right angles as shown in figure.



One rod is connected to the line while the other rod is connected to ground.

In case of transformers, they are fixed between bushing

insulators. 

In order to avoid cascading across the insulators surface under very steepfronted waves, the rod gap should be adjusted to break down at about 20% below the impulse flashover voltage of the insulation of the equipment to be protected. 26

Rod Gap… 

Further, the distance between the gap and the insulator should be more than one-third of the gap length in order to prevent the arc from being blown to the insulator.



The major disadvantage of the rod gap is that it does not interrupt the power frequency follow current after the surge has disappeared. This means that every operation rod gap creates an L-G fault which can only be cleared by the operation of the circuit breaker. Thus, the operation of the rod gap results in circuit outage and interruption of power supply.

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Figure. Rod gap

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2)

Arcing Horn or Horn Gap



The damage to line insulators from heavy arcs formed due to over voltages is a serious maintenance problem.



Arcing horn is one of such protective devices. It consist of small horns attached to the clamp of the line insulator string.



It serves two purposes (i) the steepness of the wave incident on the equipment to be protected is reduced. (ii) It reflects the voltage surge back to the horn.

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Arcing Horn… 

The protection of line insulators by arcing horn is especially used in hilly areas.



The grading ring when used in conjunction with an arcing horn fixed at the top of the insulator string serves the purpose of an arcing shield.



Disadvantages (i) the time of operation of the gap is quite large as compared to the modern protective gear. (ii) if used on isolated neutral the horn gap may constitute a vicious kind of arcing ground.

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Figure. Suspension string with arcing horns

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3)

Lightning arresters or Surge Diverters



Lightning arresters are also known as surge diverters or surge arresters.



They are connected between the line and ground at the sub station and always act in shunt with the equipment to be protected, and perform their protective function providing

a low-impedance path for the

surge

currents so that the surge arresters protective level is less than the surge voltage with standing capacity of the insulation of equipment being protected. 

The action of the surge diverter can be studied with the help of figure.

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Figure. Voltage characteristics of surge diverter

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An ideal lightning arrester or surge diverter should possess the following characteristics.

i.

It should not draw any current at normal power frequency voltage, i.e. during the normal operation.

ii.

It should break down very quickly when the abnormal transient voltage above its break down value appears, so that low-impedance path to ground can be provided.

iii. The discharge current after breakdown should not be so excessive so as to damage the surge diverter itself. iv.

It must be capable of interrupting the power frequency follow-up current after surge is discharged to ground.

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Types of Lightning Arresters

The Following are the main types of lightning arresters. i.

Expulsion type lightning arresters

ii.

Non-linear surge diverter

iii. Metal-oxide surge arresters

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i.

Expulsion type lightning arresters



This type of arrester is also known as expulsion gap or protector tube. It consists of a fibre tube with an electrode at each end.



The lower electrode is solidly grounded. The upper electrode forms a series gap with the line conductor, as shown in the figure.



When a surge appears on the conductor the series gap breaks down resulting in formation of arc in the fibre tube between the two electrodes.



The heat of the arc vaporizes some of the fibre of the tube walls resulting in the generation of an inert gas. This gas is expelled violently through the arc so that arc is extinguished and the power

frequency current is

prevented from flowing after the surge discharge.

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This type of arrester is mainly used to prevent flashover of line insulators, isolators and bus insulators.

Figure. Expulsion type lightning arresters

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ii.

Non-linear surge diverters



This type of diverter is also called valve type lightning arrester, or conventional non-linear type lightning arrester.



It consists of a divided spark-gap (i.e. several short gaps) in series with non-linear resistor elements.



The divided spark-gap and the non-linear resistor element s are placed in leak tight porcelain housing which ensures reliable protection against atmospheric moisture, condensation and humidity.



The functions of the diverters divided spark-gap are as follows:

a)

It prevents the flow of current through the diverter under normal conditions.

b)

It sparks over at a predetermined voltage.

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c)

It discharges high-energy surges with out any change in spark over characteristics.

d)

It interrupts the flow of power frequency follow current from the power system after the surge has been dissipated.



The functions of the non-linear resistors are as follows:

a)

They provide a low-impedance path for the flow of surge current after gap sparks over.

b)

They dissipates surge energy

c)

They provide a relatively high-resistance path for the flow of power frequency follow current from the power system, there by assisting the divided gap to interrupt the power frequency current. 39

Figure. Valve type lightning arrester

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Figure. A typical valve type lightning arresters 41

iii. Metal-oxide surge arrester (MOA) 

The metal oxide

surge arrester is a recently developed ideal surge

arrester. It is constructed by a series of zinc oxide (ZnO) elements having a highly non-linear resistance. 

The excellent non-linear characteristic of zinc oxide element has enabled to make surge arresters without series connected spark gaps, i.e. fully solid-state arresters suitable for system protection up to the highest voltages.



The metal oxide surge arrester has the following marked advantages over conventional arresters.

i.

Series spark-gap is not required

ii.

It has very simple construction and is fully solid-state 42

iii. Significant reduction in size. iv.

Quick response for steep discharge current

v.

Very small time delay in responding to over voltages

vi.

Superior protective performance

vii. Outstanding durability for multiple operating duty cycle viii. No abrupt transient such as that occurs at the time of spark over in a conventional arrester ix.

Negligible power follow-up current after a surge operation.



MOA is especially suitable for gas insulated sub stations, since it can be installed directly in SF6.

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Figure. Operation of MOA and conventional arrester responding to an on coming surge 

Non-Linear Resistor: it is that part of the arrester which offers a low resistance to the flow of discharge currents and thus limits the voltage across the arrester terminals and high resistance to power frequency voltage and thus limits the magnitude of follow current. 44

4)

Surge Absorber



A surge absorber is also known as a surge modifier. It is devise which absorbs

the energy contained in a travelling wave and reduces the

amplitude of the surge and the steepness of its wave front. 

The energy absorption in the form of corona loss due to corona formation acts as a kind of a natural safety-valve in the case of travelling waves which rises the line voltage above the corona level.



This method has the following three advantages

i.

The cable gets shielded from all electrostatic phenomena, and from accidental grounds (earths) through various causes.

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ii.

When the travelling waves enter the cable from the overhead line they are reduced in magnitude to about 20% of their incident value because of the ratio of the natural impedance.

iii. When a travelling wave is transmitted into the cable, it gets attenuated rapidly due to the dielectric losses taking place in the cable at high frequencies. 

The most

modern surge absorber is the Ferranti surge absorber. It

consists of an air cored inductor connected in series with each line and surrounded by a grounded metallic sheet called a dissipator. 

The inductor is magnetically coupled but electrically isolated from the dissipator . The inductor is insulated from the dissipator by the air.

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This surge absorber acts as an air cored transformer whose primary is the inductor and the dissipator is the short circuited secondary of a single turn.



Whenever the travelling wave is incident on the surge absorber, the energy contained in the wave is dissipated in the form of heat generated in the dissipator; firstly due to

the current set up in it by ordinary

transformer action, the secondly, by eddy currents. The steepness of the wave front is also reduced because of the series inductance.

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Figure. Ferranti surge absorber

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Comparison of different Protective Devices

Figure. Ferranti surge absorber

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Insulation Coordination 

Insulation coordination is the correlation of the insulation of electrical equipment and lines with the characteristics of protective devices such that the insulation of the power system is protected from excessive over voltages.



The main aim of insulation coordination is the selection of suitable values for the insulation level of the different components in any power system and their arrangement in a reasonable manner so that whole power system is protected from over voltages of excessive magnitude.



The volt-time curves of equipment to be protected and the protective device are shown in figure. curve A is the volt-time curve of the protective device and protected.

curve B is the volt-time curve of the equipment to be 50



From the curve it is clear that any insulation having a voltage withstanding strength in excess of the insulation strength of curve B will be protected by the protective device of curve A.

Figure. volt-time curves of protective device and the equipment to be protected

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Volt-time Curve



The breakdown voltage of any insulation or the flashover voltage of a gap depends upon the magnitude of the voltage and the time of application of the voltage.



The volt-time curve is a graph of the crest flashover voltages plotted against time to flashover for a series of impulse applications of a given wave shape.



The construction of the volt-time curve is based on the application of impulse voltages of the same wave shape but of different peak values to the insulation whose volt-time curve is required.



The value of the voltage corresponding to the point on front of the wave at which flashover occurs is called front flashover. 52



50% of flash over at the tail is called the critical flash over voltage.



If an impulse voltage causes flashover at crest value, it is called crest flash over.



If the flash over does not occurs the wave is called full wave, and if flash over does take place, the wave is called a chopped wave.



The applied impulse voltage reduced to just below the flash over voltage of the test specimen is called the critical withstand voltage.



The rated withstand voltage is the crest value of the impulse wave that the test specimen will withstand without disruptive discharge.

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Figure. construction of volt-time curve and the terminology associated with impulse testing

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Basic Impulse Insulation Level (BIL)



The insulation strength of equipment like transformers, CB, etc. should be higher than that of the lightning arresters and other surge protective devices.



The common insulation level for all the insulation in the station is known as Basic Insulation Level (BIL) which have been established in terms of withstanding voltage of apparatus and lines.



BIL is defined as reference level expressed in impulse crest voltage with a standard wave not longer than a 1.25/50 microsecond wave, according to Indian standards.



Apparatus insulations as demonstrated by suitable tests should be greater than the BIL. 55