EE450: High Voltage Engineering
Lecture 5
Farhan Mahmood, PhD
Department of Electrical Engineering
UET, Lahore
January 27, 2025
Outline
Overvoltages in Power Systems
• Introduction
• Lightning overvoltages
• Lightning discharge
• Lightning voltage surge
• Switching overvoltages
• Origin of switching overvoltages
• Energization of an unloaded transmission line
• Temporary overvoltages
• Ferranti effect
• Load rejection
• Ground fault
• Harmonic overvoltages due to magnetic saturation
Page 2
Introduction
• In normal operation, AC (or DC) voltages do not severely stress the insulation of the
power system.
• Overvoltages stressing a power system are classified into two categories:
– External overvoltages: generated by atmospheric disturbances such as
lightning strikes.
– Internal overvoltages: generated by changes in the operating conditions of the
network.
• Internal overvoltages can be further divided into:
– Switching overvoltages
– Temporary overvoltages
Page 3
Introduction
• Lightning strikes and switching operations produce a sudden rise in voltage for a
very short duration, known as a voltage surge or transient voltage.
• The magnitude of these transient overvoltages is sufficiently high to cause insulation
breakdown of the equipment in power systems.
• Therefore, power system engineers always device ways to limit the magnitude of the
overvoltages and to control their effects on the operating equipment.
Page 4
Lightning Overvoltages
• Lightning produces overvoltages in transmission lines.
• Lightning is an electrical discharge between cloud and
earth, between clouds, or between the charge centres
of the same cloud.
• Lightning is a huge spark and takes place when clouds
are charged to such a high potential (+ve or −ve) with
respect to earth or a neighbouring cloud that the
dielectric strength of neighbouring medium (air) is
destroyed.
• When the electric field intensity exceeds the breakdown
strength of air, a lightning discharge is initiated.
Page 5
Lightning Overvoltages
• During thunderstorms, positive and negative charges are separated by the
movements of air currents forming ice crystals in the upper layer of a cloud and rain
in the lower part.
• The cloud becomes negatively charged and has a larger layer of positive charge at
its top. Thus, the surface of the earth will be positively charged by induction.
• In this way, the cloud and the ground form two plates of a gigantic capacitor and the
dielectric medium is air.
• Since, the electric field required to cause breakdown of air is 30 kV/cm (peak).
• However, due to high moisture content in the air is large and high altitude (lower
pressure), the electric field required to cause breakdown of the air may reduce to 10
kV/cm only.
Page 7
Lightning Discharge
• When the electric field reaches a value of approximately 10 kV/cm in the cloud, the
air surrounding the cloud gets ionized.
• At this moment, a first discharge propagates from the cloud towards the air. Thus,
the channel to earth is first established by a stepped discharge called a leader
stroke.
• Depending on the state of ionization of air around the streamer, it is branched to
several paths and thus, called as stepped leader.
• The stepped leader are of the order of 50 m in length. The charge is brought from
the cloud through the already ionized path till it reaches close to the earth.
• As the downward leader approaches the earth, an upward leader (power return
stroke) moves very fast from earth towards the cloud through already ionized path.
Page 8
Lightning Discharge
• The upward leader joins the downward one at a point referred to as the striking point.
At this stage, the negative charge of the cloud is being neutralized by the positive
charge induced on the earth.
• This is the start of the return stroke, which progresses upward like a traveling wave
on a transmission line.
• This is the instant which give rise to lightning flash which we observe with naked
eyes.
• At the earthing point, a heavy impulse current reaching the order of tens of
kiloamperes occurs, which is responsible for the known damage of lightning.
• The lightning current heats its path to temperatures up to 20, 000ºC, causing the
explosive air expansion that is heard as thunder.
Page 9
Lightning Discharge
Pilot Streamer
Stepped Leader Return Streamer
Development stages of a lightning flash and the corresponding surge current
Page 10
Lightning Discharge
• Characteristics of lightning current are:
– Lightning current is unipolar and does not oscillate around current zero
– Peak amplitude of current (10 kA to 100 kA)
– Rate of rise of current (7.5 – 25 kA/μs)
– Velocity of propagation of return stroke may reach half the speed of light.
• Characteristics of lightning overvoltage are:
– Voltage waveform (1000 – 5000 kV, 1 MV/μs)
– Front time (2 – 10 μs)
– Tail time (20 –100 μs)
Page 11
Lightning Discharge
• Lightning may interact with the overhead lines in two ways:
– Direct stroke (lightning directly strikes an overhead line)
– Indirect stroke (lightning stroke reaches the ground near the line)
Page 12
Lightning Voltage Surge
• The most severe lightning stroke is that which strikes a phase conductor on the
transmission line as it produces the highest overvoltage for a given stroke current.
• If lightning hits one of the phase conductors, the return-stroke current splits into two
equal halves, each half travelling in either direction of the line.
• The travelling wave of current produce travelling wave of voltage.
Development of a lightning overvoltage
Page 13
Lightning Voltage Surge
• Therefore, the voltage surge magnitude at the striking point is,
where I is the return-stroke current, Z0 is the surge impedance of the line given
by Z0 = (L/C)1/2, and L and C are the series inductance and capacitance to ground
per meter length of the line.
• If a lightning stroke current (I) of 10,000 A strikes a line of 400 Ω surge impedance
(Z0), it may cause an overvoltage of 4000 kV.
• In case a direct stroke occurs, the current wave would divide into two branches and
travel on either side of the line.
Page 14
Lightning Voltage Surge
• Hence, the effective surge impedance of the line as seen by the wave is Z0/2 and the
overvoltage caused would be 10,000 x (400/2) = 2000 kV.
• If this line were to be a 132 kV line with an eleven disc-type insulator string, the
flashover of the insulator string will take place, as the impulse flashover voltage of
the string is about 950 kV for a 2 µs front impulse wave.
• The insulation of high voltage equipment may experience a break down under the
resulting overvoltage and the subsequent high-energy discharge.
Page 15
Lightning Voltage Surge
Page 16
Switching Overvoltages
• The opening and closing of circuits due to frequent switching operations may give
rise to transient overvoltages in a power system.
• Switching operations will generate an overvoltage up to 2-3 times the normal
voltage.
• With an increase in the transmission voltages to increase the transmitted power,
switching surges have become the governing factor in the design of insulation for
EHV and UHV systems.
• In the meantime, lightning overvoltages come as a secondary factor in these
networks.
Page 19
Switching Overvoltages
• There are two fundamental reasons for this shift in relative importance from lightning
to switching surges as higher transmission voltages are called for:
– Overvoltages produced on transmission lines by lightning strokes are only
slightly dependent on the power system voltages. As a result, their magnitudes
relative to the system peak voltage decrease as the latter is increased.
– External insulation has its lowest breakdown strength under surges whose fronts
fall in the range 50- 500 µs, which is typical for switching surges
• According to the recommendations of International Electrotechnical Commission
(IEC), all equipment designed for operating voltages above 300 kV should be tested
under switching impulses (i.e., laboratory-simulated switching surges).
Page 20
Origin of Switching Overvoltages
• There are several events that would initiate a switching surge in a power system.
• The switching operations of greatest relevance to insulation design can be classified
as follows:
• Energization of transmission lines and cables: The following specific switching
operations are some of the most common in this category:
– Energization of a line that is open circuited at the receiving end
– Energization of a line that is terminated by an unloaded transformer
– Energization of a line through the low-side side of transformer
• Re-energization of a line: This means the energization of a transmission line
carrying charges trapped by previous line interruptions when high-speed reclosures
are used.
Page 21
Origin of Switching Overvoltages
• Load rejection: This is caused by a circuit breaker opening at the far end of the line.
This may also be followed by opening the line at the sending end in what is called a
line dropping operation.
• Switching on and off of equipment: All switching operations involving an element
of the transmission network will produce a switching surge. For example:
– Switching of high-voltage reactors
– Switching of transformers that are loaded by a reactor on their tertiary winding
– Switching of a transformer at no load
• Fault initiation and clearing
Page 22
Energization of an Unloaded Transmission Line
• Suppose that the sinusoidal supply voltage is switched on to an unloaded
transmission line as shown in figure.
• R, L and C represents the total series resistance, inductance and capacitance to
ground from source up to the far end of line including the transformer.
• The switching operation is performed at an instant T seconds beyond that of zero
voltage.
• The voltage across the capacitor C is the one under study here, as it represents the
voltage at the open-circuit end of the line.
Page 23
Energization of an Unloaded Transmission Line
• The sinusoidal supply voltage vs (t) is given by,
• Applying KVL around the closed loop,
• The expression for the voltage across the line capacitance takes the form:
Page 24
Energization of an Unloaded Transmission Line
Energization switching transient
Page 25
Standardization of Testing Voltage Waveforms
• In practice, the shape and magnitudes of lightning and switching overvoltage
waveform differs with every event.
• However, field measurements have shown that lightning and switching overvoltages
are characterized by short front duration, and then slowly decreasing to zero.
• The standard lightning impulse voltage has been accepted as an aperiodic impulse
that reaches its peak value in 1.2 µsec and then decreases slowly (in about 50 µsec)
to half its peak value.
• The standard switching impulse voltage has been accepted as an aperiodic impulse
that reaches its peak value in 250 µsec and then decreases slowly (in about 2500
µsec) to half its peak value.
Page 27
Standardization of Testing Voltage Waveforms
General waveshape of lightning General waveshape of switching impulse
impulse voltage (1.2/50 µs) voltage (250/2500 µs)
Page 28
Temporary Overvoltages
• Temporary overvoltages (i.e., sustained overvoltages) differ from transient switching
overvoltages in that they last for longer durations, typically from a few cycles to a few
seconds.
• They take the form of undamped or slightly damped oscillations at a frequency equal
or close to the power frequency.
• Some of the most important events leading to the generation of temporary
overvoltages are:
– Ferranti effect
– Load rejection
– Ground fault
– Harmonic overvoltages due to magnetic saturation
Page 29
Ferranti Effect
• The sending-end voltage of a long transmission line is given by,
where VR and VS are the receiving-end and sending-end voltages, respectively.
• If the line is operating under no load conditions or lightly loaded, IR = 0,
where l is the line length (km)
β is the phase shift constant of the line per unit length.
L and C are the inductance and capacitance of the line per unit length.
Page 30
Load Rejection
• When a transmission line or a large inductive load that is fed from a power station is
suddenly switched off, the generator will speed up and the busbar voltage will rise.
• The equivalent network is shown as:
Equivalent circuit during load rejection
Page 31
Load Rejection
• The amplitude of the overvoltage can be evaluated approximately by,
where E is the voltage behind the transient reactance, which is assumed to be
constant over the subtransient period and equal to its value before the
incident
Xs the transient reactance of the generator in series with the transformer
reactance
Xc the equivalent capacitive input reactance of the system
Page 32
Ground Fault
• A single line-to-ground fault will cause the voltages to ground of the healthy phases
to rise.
• In the case of a line-to-ground fault, systems with neutrals isolated or grounded
through a high impedance may develop overvoltages on healthy phases higher than
normal line-to-line voltages.
• Solidly grounded systems, on the other hand, will only permit phase-to-ground
overvoltages well below the line-to-line value.
Page 33
Harmonic Overvoltages due to Magnetic Saturation
• Ferroresonance is a resonance phenomenon caused by the nonlinear inductance
and system capacitance.
• Harmonic oscillations in power systems are initiated by system nonlinearities whose
primary source is that of the saturated magnetizing characteristics of transformers
and shunt reactors.
Page 34
Harmonic Overvoltages due to Magnetic Saturation
• The magnetizing current of these components increases rapidly and contains a high
percentage of harmonics for voltages above the rated voltage.
• Therefore, saturated transformers inject large harmonic currents into the system.
Page 35
Harmonic Overvoltages due to Magnetic Saturation
• Suggested Reading: Chapter 14 (Mazen Abdel-Salam’s book)
Page 36
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