PROTECTION AGAINST OVER VOLTAGE AND GROUNDING Part 1

LENDI INSTITUTE OF ENGINEERING AND TECHNOLOGY
Jonnada, Andhra Pradesh- 535005
UNIT- VI (PART-1)
PROTECTION AGAINST OVER
VOLTAGE AND GROUNDING
Department of Electrical and Electronics Engineering

SYLLABUS
∟Generation of over voltages in power systems
∟Protection against lightning over voltages
∟Valve type and zinc oxide lighting arresters
∟Insulation coordination
∟BIL
∟Impulse ratio
∟Standard impulse test wave
∟Volt-time characteristics
∟Grounded and ungrounded neutral systems
∟Effects of ungrounded neutral on system performance
∟Methods of neutral grounding
―Solid
―Resistance
―Reactance
―Arcing grounds and grounding Practices

Generation of over voltages in power systems
Overvoltages (or surges) on power systems are due to various causes.
Overvoltages arising on a power system can be generally classified into two main categories as
follows:
1. External Overvoltages
• These overvoltages originate from atmospheric disturbances, mainly due to lightning.
• These overvoltages take the form of a unidirectional impulse (or surge) whose maximum
possible amplitude has no direct relationship with the operating voltage of the system.
They may be due to any of the following causes:
i. Direct lightning strokes
ii. Electromagnetically induced overvoltages due to lightning discharge taking place near the line
(commonly known as ‘side stroke’)

iii. Voltages induced due to changing atmospheric conditions along the line length
iv. Electrostatically induced overvoltages due to the presence of charge clouds nearby
v. Electrostatically induced overvoltages 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
2. Internal Overvoltages
• These overvoltages are caused by changes in the operating conditions of the network.
Internal overvoltages can further be divided into two groups as follows:

A. Switching overvoltages (or transient overvoltages of high frequency)
• These overvoltages are caused by the transient phenomena which appear when the state of the network is
changed by a switching operation or fault condition.
• The frequency of these overvoltages may vary from a few hundred Hz to a few kHz, and it is governed by the
inherent capacitances and inductances of the circuit.
• For example, switching on and off of equipment, such as switching of high voltage reactors and switching of a
transformer at no load causes overvoltages of transient nature.
• Other examples of this type of overvoltage are— if a fault occurs on any phase, the voltage with respect to
ground of the other two healthy phases can exceed the normal value until the fault is cleared; when the contacts
of a circuit breaker open in order to clear the fault, the final extinction of current is followed by the appearance
of a restriking voltage across the contacts whose characteristics are a high amplitude which may reach twice
that of the system voltage, and a relatively high frequency.

B. Temporary overvoltages (or steady-state overvoltages of power frequency)
• These overvoltages are the steady-state voltages of power-system frequency which may result from the
disconnection of load, particularly in the case of long transmission lines.
• Transient overvoltages arising on the power system are assessed by an overvoltage factor which is defined as
the ratio of the peak overvoltage to the rated peak system-frequency phase voltage. This ratio is also referred
to as the amplitude factor.
• The examination of overvoltages on the power system includes a study of their magnitudes, shapes, duration,
and frequency of occurrence.
• This study should be performed not only at the point where an overvoltage originates but also at all other
points along the transmission network through which the surges may travel.
• It is also essential to know the causes and effects of these overvoltages, so that suitable protective measures
can be taken.

Protection against lightning over voltages
Lightning Phenomenon
• The discharge of the charged cloud to the ground is called lightning phenomenon.
• A lightning discharge through air occurs when a cloud is raised to such a high potential with
respect to the ground (or to a nearby cloud) that the air breaks down and the insulating property of
the surrounding air is destroyed.
• This raising of potential is caused by frictional effects due to atmospheric disturbances (e.g.,
thunderstorms) acting on the particles forming the cloud.
• The cloud and the ground form two plates of a gigantic capacitor whose dielectric medium is air.
• During thunderstorms, positive and negative charges are separated by the movement of air
currents forming ice crystals in the upper layer of cloud and rain in the lower part.
• The cloud becomes negatively charged and has a larger layer of positive charge at its top.

• As the separation of charge proceeds in the cloud, the potential difference between concentrations
of charges increases and the vertical electric field along the cloud also increases.
• The total potential difference between the two main charge centres may vary from 100 to 1000 MV.
• Only a part of the total charge is released to the ground by lightning, and the rest is consumed in
inter-cloud discharges.
• As the lower part of the cloud is negatively charged, the ground gets positively charged by
induction.
• Lighting discharge requires breakdown of the dielectric medium, i.e. air between the negatively
charged cloud and the positively charged ground.

• When a potential gradient of about 10 kV/cm is set up in the cloud, the
air immediately surrounding the cloud gets ionised and the first
process of the actual lightning discharge starts.
• At this instant, a streamer called a ‘pilot streamer’ starts from the
cloud towards the ground which is not visible.
• The current associated with this streamer is of the order of 100
amperes, and the most frequent velocity of propagation of the
streamer is about 0.15 m/m s (i.e. 0.05 per cent of the velocity of light).
• Depending upon the state of ionisation of the air surrounding the pilot
streamer, it is branched into several paths, a stepped leader is formed,
as shown in Fig. (a).
• It is called a stepped leader because of its zig-zag shape.

• It contains a series of steps about 50 m in length.
• The velocity of propagation of these steps should be more than
16.6 per cent that of light.
• A portion of the charge in the centre from which the strike
originated is lowered and distributed over this entire system of
temporary conductors.
• This process continues until one of the leaders strikes the ground.
• When one of the stepped leaders strikes the ground, an extremely
bright return streamer, as shown in Fig. (b) propagates upward
from the ground to the cloud following the same path as the main
channel of the downward leader.

• The charge distributed along the leaders is thus discharged progressively to the ground, giving rise
to the very large currents associated with lightning discharges.
• The conventional directions of the current in the stepped leader and return streamer are the same.
• The current varies between 1 kA and 200 kA and the velocity of propagation of the streamer is
about 10 per cent that of light.
• It is here that the negative charge of the cloud is being neutralized by the positive induced charge
on the ground.
• It is this instant which gives rise to the lightning flash which is visible with our naked eye.

• After the neutralization of the most of the negative charge on the cloud
any further discharge from the cloud may have to originate from
another charge centre within the cloud near the already neutralized
charge centre.
• Such a discharge from another charge centre will, however, make use of
the already ionised path, and consequently it will have a single branch
and will be associated with a high current.
• This streamer of discharge is called a dart leader as shown in Fig. 16. (c).
• The velocity of propagation of the dart leader is about 3 per cent that of
light.
• The dart leader can cause more severe damage than the return stroke.

• Though the discharge current in the return streamer is relatively large but since it continues only
for a few microseconds, it contains less energy and hence this streamer is called a cold lightning
stroke.
• The dart leader is called a hot lightning stroke because even though the current in this leader is
relatively smaller, it contains relatively more energy since it continues for some milliseconds.
• It has been observed that every thundercloud may consist of as many as 40 separate charged
centres due to which a heavy lightning stroke may occur. This lightning stroke is called multiple
(or repetitive) stroke.
• When a lightning stroke takes place to a point adjacent to the line, the current associated with it
induces a voltage on the line which is in the form of a travelling wave.
• A lightning discharge may have currents in the range of 10 kA to about 100 kA, and the duration of
the lightning flash may be of the order of a few microseconds.

• The current wave form is generally a unidirectional pulse which rises to a peak value in about 3 ms
and decays to a small values in several tens of microseconds.
• Therefore, the overvoltage due to lightning will have travelling waves of very steep wave front
which may be dangerous to the line and equipment of the power system and may cause damage.

WAVE SHAPE OF VOLTAGE DUE TO LIGHTNING
Lightning sets up steep-fronted, unidirectional voltage waves which can be represented as the
difference of two exponentials. Thus
( )
at bt
v V e e
 
 
where a and b are constants which determine the shape
and v is magnitude of the steep voltage and V is equal to
the crest (peak) value of the impulse voltage wave.
The steep is dependent on whether the surge is induced
or is the result of a direct stroke.
The wave shape of this type is shown in Fig. 2.
The wave shape is generally defined in terms of the
times t1 and t2 in microseconds.
Fig. 2 Wave shape of voltage due to lightning

WAVE SHAPE OF VOLTAGE DUE TO LIGHTNING
• Where t1 is the time taken by the voltage wavefront to reach its peak value, and t2 is the time taken for
the tail to fall to 50 per cent of the peak value.
• Both times are measured from the virtual zero time point (i.e., from the start of the wave).
• The wave is then referred to as t1/t2 wave.
• The standard wave chosen for the testing purpose is a 1/50 wave which implies that t1 is 1 ms and t2 is
50 ms.

OVERVOLTAGES DUE TO LIGHTNING
• Lightning causes two kinds of voltage surges (overvoltage), one by direct stroke to a line conductor,
and the other induced by indirect stroke when bound charges are dissipated following a lightning
discharge to an object near the line conductor.
• A direct stroke to a phase (line) conductor is the most severe lightning stroke as it produces the
highest overvoltage for a given stroke current.
• The current in a direct stroke to a phase conductor is hardly affected by the voltage which it sets up
between the conductor and ground, so that a direct lightning stroke approximates to a constant
current source.

If a current I flows in a direct stroke to a phase conductor, forward and backward travelling currents if
and ib, each of magnitude I/2 flow in the conductor away from the point of strike in both directions of
the line as shown in Fig. 3.
Therefore, the voltage surge magnitude at the striking point is given by
0
2
IZ
V 
where Z0 is the surge impedance of the line.

• The lightning current magnitude (I) is rarely less than 10 kA.
• Therefore, for a typical overhead line having a surge impedance Z0 of 400 W, the lightning surge
voltage will probably have a magnitude in excess of 2000 kV.
Fig. 3 Development of lightning overvoltage

• Any overvoltage surge appearing on a transmission line due to internal or external disturbances
propagates in the form of a travelling wave towards the ends of the line.
• The lightning surge is rapidly attenuated, partly due to corona as it travels along the line.
• In this way, corona acts as a natural safety valve to some extent.
• When a tower of the transmission system is directly struck by lightning, the resistance offered to the
lightning current is that of the tower footing and any ground rods or counterpoise wires in parallel.
• In calculation of overvoltage developed due to lightning in this case, certain assumptions must be
made concerning several of the factors such as wave shape and magnitude of lightning current,
surge impedance of lightning stroke channel, shape of potential wave at tower top, effect of surges
on tower and footing impedance, etc.

• All these assumptions can be thrown into the lightning current and tower footing resistance, thus
giving
V IR

where, V = voltage across insulation at tower, R = tower footing resistance, and I = lightning current
in tower.
For very high towers, such as the ones at river crossings, which are subjected to direct strokes of
very high rate of change of current, the tower inductance may be taken into account.
The voltage (v) which may be sufficient to cause insulator flashover is given by
di
v L
dt

where, L = tower inductance, and di/dt = rate of change of current due to direct stroke.

• In the case of an indirect stroke, a negative charge in a cloud base causes bound positive charges on
the conductors of a nearby transmission line, and discharge of the cloud due to a lightning stroke
in the neighbourhood causes the conductor potential to rise to v since high insulation resistance
prevents rapid dissipation of the charge.
• The peak value of the induced voltage surge is given by
v Eh

where, E is the mean electric field near the ground (earth) surface under a thundercloud which
may vary from 0.5 kV/cm to 2.8 kV/cm, and h is the height of the phase conductor above ground.

• If if be the forward travelling wave or the current and ib be the backward travelling wave of the
current, then in a case where the total conductor current i = 0, we have if = ib and Vf = Vb = v/2.
• So, that forward and backward travelling waves of voltage and current set-off in opposite directions
along the transmission line from the place nearest to the cloud discharge.
• The amplitudes of voltages induced on lines indirectly by lightning strokes on a tower, ground wire
or nearby ground or object, are however normally much less than those caused by direct strokes to
lines.
• Such strokes, however, are of real concern on low voltage lines (33 kV and below) supported on
small insulators.
• They are of little importance on high-voltage lines whose insulators can withstand hundreds of
kilovolts without a flashover.

PROTECTION OF TRANSMISSION LINES
AGAINST DIRECT LIGHTNING STROKES
The klydonograph is an instrument for the measurement of surge voltage on transmission lines
caused by lightning.
• Direct lightning strokes are of concern on lines of all voltage classes, as the voltage that may be
set up is in most instances limited by the flashover of the path to ground.
• Increasing the length of insulator strings merely permits a higher voltage before flashover occurs.
• Therefore protective methods must be adopted to avoid flashover or breakdown of line insulators
due to overvoltage caused by direct lightning strokes.
• The most generally accepted and effective method of protecting lines against direct strokes is by
the use of overhead ground wires.
• This method of protection is known as shielding method which does not allow an arc path to
form between the line conductor and ground.

• Ground wires are conductors running parallel to the main conductors of the transmission line,
supported on the same towers (or supports) and adequately grounded at every tower or support.
• They are made of galvanised 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).
• 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.
• Overhead ground wires protect the lines by intercepting the direct strokes, keeping them off the
phase conductors (lines), and by providing multiple paths for conducting the strokes current to
ground.
• The multiple paths for the stroke current result in a reduced voltage drop.

• When the ground wire is struck in midspan, the current gets divided and flows towards both
towers.
• At the tower, the current again gets divided between the tower and the outgoing ground wire.
• When the tower is struck by a direct lightning stroke and there is only one overhead ground wire,
the current gets divided into three-path, one through the tower and two through the branches of the
ground wire.
• In addition to the above functions, ground wires also reduce the voltages induced on the conductors
from nearby strokes by increasing the capacitance between conductors and ground.

In order to provide efficient protection to lines against direct strokes, the ground wires must satisfy the
following requirements.
i. There should be an adequate clearance between the line conductors and the ground or the tower
structure.
ii. There should be an adequate clearance between the ground wires and line conductors, especially at the
midspan, in order to prevent flashover to the line conductors up to the protective voltage level used for
the line design.
iii. The tower footing resistance should be as low as economically justifiable.
The ground wires should be placed at such a height above the conductors and so located that they should be
well out on towers and should not be exactly over the conductors in order to avoid any possibility of a short
circuit occurring in the event of conductors swinging under ice loading, etc.

A few terms relating to protection of transmission lines are explained below.
Protective Ratio
The protective ratio 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
The protective angle (a) afforded by a ground wire is
defined as 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. This angle
is shown in Fig.

Protective Zone
The protective zone is defined as the volume between the base plane cbc and the slanting planes ac,
extending from the ground wire to the plane of the conductors.
Figure a shows the cross-section of this volume.
The plane ac cuts the base plane at c, at a distance ky from the point b, vertically under the apex a. The
ground wire is at a height ab = y above the base plane. The ratio ky/y = k is called the protective ratio.
The protective zone is sometimes called the protective wedge or protective shed.
If the line ab in Fig. b represents
a rod or mast with a height y,
then it is said to be protective
cone around the mast. The radius
on the base line is equal to ky
and the protective ratio is ky/y =
k.

Height of Ground Wire
For protection against direct strokes, the ground wire should be located at a height at least 10 per cent
greater than y, calculated from the following equation.
where, X = horizontal spacing between the conductor and ground wire, H = height of cloud, y = height
of ground wire, and h = height of conductor.
Coupling Factor
The ratio of this induced voltage on the conductor to the ground wire voltage is known as the
coupling factor.
Therefore, the voltage across the line insulation is the ground wire voltage times the coupling factor,
neglecting the normal frequency voltage.

The electromagnetic and electrostatic coupling factors are equal unless the effective radius of the
ground wire is increased by a corona, in which case, the electrostatic coupling increases and the
electromagnetic coupling remains unaffected.
The coupling factor with the effect of corona considered is calculated by the following equation.
where, 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 and r = radius of ground wire.

PROTECTION OF STATIONS AND SUB-STATIONS
FROM DIRECT STROKES
• The station structure itself is commonly built of steel members on which lines terminate and which support
the buses, isolating switches, etc.
• The power transformers and circuit breakers usually rest on concrete pads on the ground.
• The task of protection of such stations can be resolved into two parts.
• The first being the protection of stations (including sub-stations) from direct lightning strokes, and the
second is the protection of electrical equipment of stations from travelling waves coming in over the lines.
• Wherever it is likely for direct strokes of lightning to strike the line at or near the station, there is a possibility
of exceedingly high rates of surge-voltage rise and large magnitude of surge-current discharge.
• If the stroke is severe enough, the margin of protection provided by the protective device may be inadequate.
• The installation may, therefore, require shielding of the station and the incoming lines enough to limit the
severity of surges particularly in the higher voltage class of 66 kV and above.
• This can be done by properly placed overhead ground wires, masts, or rods.

FROM DIRECT STROKES
• If supports are available for overhead ground wires, these may be run over the station in such a way that
the station and all apparatus will lie in the protected zone.
• Sufficient ground wires should be strung so that the entire station is covered, including any apparatus
outside the main structure and any apparatus on top of the structure as shown in Figure.
Fig. Protection of a station by overhead ground wires (elevation view)

FROM DIRECT STROKES
• For a small station, it is sometimes sufficient to run one or two ground wires across the station from adjacent
line towers as shown in Fig. (a).
• For a more extensive station, extra ground wire may be used, fanning out from the towers to the station
structure, as also over the station if mechanically feasible as shown in Fig. (b).
• If there are no isolating switches or other apparatus above the station structure, then the steel of this structure
will act as a grounded object to protect the apparatus and buses below it.

PROTECTION AGAINST TRAVELLING WAVES
Shielding the lines and stations (including sub-stations) by overhead ground wires, masts or rods
undoubtedly provides adequate protection against direct lightning strokes and also reduces electrostatically
or electromagnetically induced over voltages, but such shielding docs not provide protection against
travelling waves which may come in over the lines and reach the terminal equipment.
Such travelling waves can cause the following damage to electrical equipment.
i. Internal flashover caused by the high peak voltage of the surge may damage the insulation of the
winding.
ii. Internal flashover between interturns of the transformer may be caused by 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.
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.

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

Valve type and zinc oxide lighting arresters
• Lightning arresters are also known as surge diverters or surge arresters.
• They are connected between the line and ground at the substation and always act in shunt
(parallel) with the equipment to be protected, and perform their protective function by
providing a low-impedance path for the surge currents so that the surge arrester’s protective
level is less than the surge voltage withstanding capacity of the insulation of equipment being
protected.
• A lightning arrester’s protective level is the voltage appearing across the terminals of the
arrester at sparkover or during the flow of current through the arrester after sparkover.
• The main purpose of lightning arresters is to divert or discharge the surge to the ground.

• The action of the surge diverter can be studied with the help
of Fig. .
• When the travelling surge reaches the diverter, it sparks over
at a certain prefixed voltage as shown by point P and
provides a relatively low-impedance path to ground for the
surge current.
• The current flowing to ground through the surge impedance
of the line limits the amplitude of the overvoltage across the
line and ground known as ‘residual voltage’ (as shown by
point Q in Fig.) to such a value which will protect the
insulation of the equipment being protected.

It is however, essential that the low-impedance path to ground must not exist before the overvoltage
appears and it must cease to exist immediately after the voltage returns back to its normal value.
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 breakdown very quickly when the abnormal transient voltage above its breakdown value
appears, so that a 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 the surge is
discharged to ground.

Impulse Ratio of Lightning Arresters
It is defined as the ratio of the breakdown impulse voltage of a wave of specified duration to the
breakdown voltage of a power-frequency wave.
The impulse ratio of any lightning arrester is therefore a function of time duration of the transient
wave.
Types of Lightning Arresters (or Surge Diverters)
The following are the main types of lightning arresters.
(i) Expulsion Type Lightning Arrester
(ii) Non-linear Surge Diverter
(iii) Metaloxide Surge Arrester (MOA)

(i) Non-linear surge diverter
• 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) in series with non-linear resistor
elements.
• The divided spark-gap and the nonlinear resistor
elements are placed in leak tight porcelain housing
which ensures reliable protection against atmospheric
moisture, condensation and humidity (see Fig.).
• Figure shows a typical valve type lightning arrester.

The functions of the diverter’s 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.
(c) It discharges high-energy surges without any change in sparkover 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 dissipate surge energy.
(c) They provide a relatively high-resistance path for the flow of power frequency follow current from the
power system, thereby assisting the divided gap to interrupt the power frequency current (i.e. reseal
against system voltage).

(ii) Metal-oxide surge arrester (MOA)
• The metal oxide surge arrester abbreviated as MOA is a recently developed ideal surge arrester.
• It is a revolutionary advanced surge protective device for power systems.
• It is constructed by a series connection 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 (MOA) which consists of a series connected stack of discs of zinc
oxide elements, operates in a very simple fashion.
• It is dimensioned so that the peak value of the phase to ground voltage in normal operation never
exceeds the sum of the rated voltages of the series-connected discs.

• The resistive losses in the arrester in normal operation are therefore very small.
• When an overvoltage occurs, the current will rise with the wavefront according to the
characteristics without delay.
• No breakdown occurs but a rather continuous transition to the conducting state is observed.
• 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 a fully solid-state protective device.
(iii) Significant reduction in size.
(iv) Quick response for steep discharge current.

(v) Very small time delay in responding to overvoltages.
(vi) Superior protective performance.
(vii) Outstanding durability for multiple operating duty cycle.
(viii) No abrupt transient such as that occurs at the time of sparkover in a conventional arrester.
(ix) Negligible power follow-up current after a surge operation.
MOA is especially suitable for gas insulated sub-stations (GIS), since it can be installed directly in
SF6.

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 whole power system is protected
from excessive overvoltages.
• 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 the
whole power system is protected from overvoltages of excessive magnitude. Thus, the insulation
strength of various equipment, like transformers, circuit breakers, etc. should be higher than that of the
lightning arresters and other surge protective devices.
• The insulation coordination is thus the matching of the volt-time flashover and breakdown characteristics
of equipment and protective devices, in order to obtain maximum protective margin at a reasonable cost.

INSULATION COORDINATION
• The volt-time curves of equipment to be protected and
the protective device are shown in Fig.
• Curve A is the volt-time curve of the protective device
and curve B is the volt-time curve of the equipment to
be protected.
• From volt-time curves A and B, 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.
Fig. Volt-time curves of protective
device and the equipment to be
protected

Volt-Time Curve
• The breakdown voltage of any insulation or the
flashover voltage of a gap depends upon both 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 and the
terminology associated with impulse testing are shown
in Fig. Fig. illustrates for an assumed impulse, its wave front flashover,
crest flashover, critical flashover voltage, critical withstand
voltage, rated flashover voltage, wavetail flashover and a volt-
time curve.

Volt-Time Curve
• 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.
• If an impulse voltage of a given wave shape and polarity is adjusted so that the test specimen (i.e. a particular
insulation) flashesover on the front of the wave, the value of voltage corresponding to the point on front of
the wave at which flashover occurs is called front flashover.
• If an impulse voltage of the same wave shape is adjusted so that the test specimen flashes over on the tail of
the wave at 50 per cent of the applications and fails to flashover on the other 50 per cent of the applications,
the crest value of this voltage is called the critical flashover voltage.
• If an impulse voltage causes flashover of the test specimen exactly at the crest value, then it is called crest
flashover.
• If flashover does not take place, the wave is called a full wave and if flashover does take place, the wave is
called a chopped wave.

Volt-Time Curve
• The applied impulse voltage reduced to just below the flashover 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.

BASIC IMPULSE INSULATION LEVEL (BIL)
• The insulation strength of equipment like transformers, circuit breakers, etc. should be higher than that of
the lightning arresters and other surge protective devices.
• In order to protect the equipment of the power system from overvoltages of excessive magnitude, it is
necessary to fix an insulation level for the system to see that any insulation in the system does not
breakdown or flashover below this level and to apply protective devices that will give the apparatus
effective protection.
• Several methods of providing coordination between insulation levels in the station and on the line leading
to the station have been offered.
• The best method is to establish a definite common level for all the insulation in the station and bring all
insulation to or above this level.
• This limits the problem to three fundamental requirements, namely, the selection of a suitable insulation
level, the assurance that the breakdown or flashover strength of all insulation in the station will equal or
exceed the selected level, and the application of protective devices that will give the apparatus as good
protection as can be justified economically.

• This limits the problem to three fundamental requirements, namely, the selection of a suitable insulation
level, the assurance that the breakdown or flashover strength of all insulation in the station will equal or
exceed the selected level, and the application of protective devices that will give the apparatus as good
protection as can be justified economically.
• The common insulation level for all the insulation in the station is known as Basic Impulse Insulation Level
(BIL) which have been established in terms of withstanding voltages of apparatus and lines.
• Basic impulse insulation level can be defined as reference level expressed in impulse crest voltage with a
standard wave not longer than a 1.2/50 microsecond wave, according to Indian standards (1.5/40 ms in
USA and 1/50 ms in UK).
• Apparatus insulation as demonstrated by suitable tests should be equal to or greater than the BIL.

• The basic impulse insulation level for a system is selected such that the system could be protected
with a suitable lightning protective device, e.g. a lightning arrester.
• The margin between the BIL and the lightning arrester should be fixed such that it is economical
and it also ensures protection to the system.
• The BIL chosen must be higher than the maximum expected surge voltage across the selected
arrester.

PROTECTION AGAINST OVER VOLTAGE AND GROUNDING Part 1

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PROTECTION AGAINST OVER VOLTAGE AND GROUNDING Part 1