1. High voltage engineering – 19EE702
High voltage engineering by C.L.Wadhwa
High voltage engineering by M. S. Naidu and V Kamaraju
2. Module-1
Introduction: Introduction to HV technology, Advantages of electric power transmission at
high voltages. Need for generating high voltages in a laboratory. Important applications of
high voltages. Types of HV insulators, cables and bushings.
Breakdown phenomena: Classification of HV insulating media. Gaseous dielectrics:
Ionization: Primary and secondary ionization processes. Criterion for gaseous insulation
breakdown based on Townsends theory. Limitations of Townsend theory, Streamer theory,
Corona discharges. Breakdown in electronegative gases. Paschen’s law and its
significance. Time lags of breakdown.
3. Introduction
System voltages around 100kV and above are treated as high voltages. An
integral features of present-day power system are their higher generating
capacity and transmission capability to cope up with the ever-increasing
demand of electrical power. High voltages are preferred for transmission of
bulk power over long distances.
Voltage class Voltage range
Low voltage (LV) ≤ 1kV
Medium voltage (MV) 1 kV ˂ V ˂ 70 kV
High voltage (HV) 110 kV ≤ V ≤ 230 kV
Extra High voltage (EHV) 275 kV ≤ V ≤ 800 kV
Ultra High voltage ( UHV) ˃ 1000kV
4. Advantages of electric power transmission at high voltages
• Increased power transmission ( 𝑃𝑇 ∝ 𝑉2)
• Low specific power loss ( Increased transmission voltage levels will
decrease specific power loss ).
• Decreased investment cost ( cost/unit of power transferred is reduced with
the increase in system transmission voltage).
5. Need for generating high voltages in laboratory
• High voltage testing
• Research in connection with HV technology
• Industrial applications of HV
High voltages in labs are mainly required in the fields of electrical engineering and applied physics.
These high voltages ( ac, dc impulse) are used for many applications.
• Electron microscopes and X-ray units require high DC voltages of the order of 100 kV or more.
• Electrostatic precipitator, particle accelerators in nuclear physics require DC voltages of the order
of kilo-Volts or even mega-Volts.
• For testing power apparatus rated for extra high transmission voltages 400 kV and above, high
voltages of one million volts or even more are required.
6. • High impulse voltages are used to simulate the over-voltages that occur in power system due to
lightening or switching action.
• Most of the research works are oriented in the direction of development of model for a specific study or
purpose.
• A model can be defined as a physical or mathematical construct which can simulate to a certain degree,
with necessary approximations, a given physical phenomena.
A physical conduct could be in the form of experiment and mathematical construct could be in the form of
a computer coding. In either case, the aim is to study:
• Insulation behavior
• Improving the insulation capabilities
• Development of new insulating material.
• More importance is given to the insulation studies, since insulation happens to be the weakest link in the
electrical performance of power system.
Recently, HVDC is also used to get uniform coating of paints even in curved regions
7. • What is Insulation & Why Do We Need It?
• Any item that carries an electrical current is designed to do so safely and with some type of
insulation. The insulation limits the flow of current between the different conductors and
between the conductors to ground. It is therefore very important that the insulation has the
opposite transfer properties of the conductor.
• Conductors are usually metallic. The most common are copper or aluminum, both of which
are known to be very good conductors of electric current due to the metals’ high current
carrying capacity and constant thermal properties.
• Insulation is usually made from a non-metallic material. Most of the electrical insulation is
made of PVC, plastic, or rubber. It should resist current and keep it within the path alongside
the conductor.
• Insulation should not carry electrical current. Obviously, the higher the resistance, the better
the insulation capacity the material has.
8. • A combination of electrical stress and the degradation of insulation is constantly
happening.
• Insulation is subject to many effects which can cause it to fail due to mechanical damage,
vibration, excessive heat or cold, dirt, oil, corrosive vapors, moisture from the air, and
general wear and tear.
• Types of insulation
• Solid insulators : Ebonite, polythene, polyethylene, resins, crystal quartz.
• Liquid insulators : Benzen, Hexane, liquid N2, silicon, caster oil.
• Gaseous insulators : Air, N2, CO2, SF6.
9. High Voltage Cables
• High Voltage Cables are used when underground transmission is required. These cables are
laid in ducts or may be buried in the ground. Unlike in overhead lines, air does not form part
of the insulation, and the conductor must be completely insulated.
• Cables are much more costly than overhead lines. Unlike for overhead lines where tappings
can easily given, cables must be connected through cable boxes which provide the necessary
insulation for the joint.
• Cables have a much lower inductance than overhead lines due to the lower spacing between
conductor and earth, but have a correspondingly higher capacitance, and hence a much
higher charging current.
• High voltage cables are generally single cored, and hence have their separate insulation and
mechanical protection by sheaths.
10. High voltage electrical
bushings
High voltage transformer bushings are used to
insulate the incoming or outgoing conductor into
or out of a grounded barrier, in power transformer
case is the transformer main tank. The bushings
connect the windings of the transformer to the
supply line and insulate the feed through
conductor from the transformer main tank.
11. ELECTRICAL BREAKDOWN
What is breakdown in High Voltage?
• Electrical breakdown or dielectric breakdown is when current flows
through an electrical insulator when the voltage applied across it
exceeds the breakdown voltage. This results in the insulator
becoming electrically conductive.
what is the meaning of breakdown voltage?
• The breakdown voltage of an insulator is the minimum voltage that
causes a portion of an insulator to become electrically conductive. For
diodes, the breakdown voltage is the minimum reverse voltage that
makes the diode conduct appreciably in reverse.
12. Gaseous insulation breakdown phenomenon
• N2, CO2, CC2F2 (Freon), SF6 (Sulphur Hexa Fluoride)
• A gas in its normal form is almost a perfect insulator. But when the gas volume is subjected to
high voltage gas starts conducting at higher voltages, which may lead to final electrical
breakdown ( conduction ) with the further increase in applied voltage.
• The process of splitting a neutral atom into a + ve ion and an electron is called ionization. And
their migration to anode and cathode leads to high currents.
What are the physical conditions governing ionization mechanism in gases dielectrics?
1) Pressure
2) Temperature
3) Electrode configuration
4) Nature of electrode surface
5) Availability of initial conducting particles
13.
14. The processes that are primarily responsible for the breakdown of a gas are:
• ionization by collision,
• ionization by photo-ionization, and
• ionization by the secondary ionization processes.
Primary ionization: Electron produced at the cathode by some external means, during its
travel towards the anode due to the field applied, make collisions with neutral
atoms/molecules and liberate electrons & positive ions. The liberated ions make future
collisions and the process continue. The electrons and the ions constitute current.
Ionization by electron collision : The electron moving under the applied field in a gaseous
gap will be possessing energy. If the velocity is high enough, it can collide with neutral atom
resulting in the formation of an ion and an electron.
Photoionization : Ionization of a molecule or atom caused by absorption of radiant energy
15. Ionization by Collision
• The process of liberating an electron from a gas molecule with the simultaneous
production of a positive ion is called ionization.
• In the process of ionization by collision, a free electron collides (hits) with a neutral gas
molecule and gives rise to a new electron and a positive ion.
• If we consider a low-pressure gas column in which an electric field E is applied across
two plane parallel electrodes, then, any electron starting at the cathode will be
accelerated more and more between collisions with other gas molecules during its travel
towards the anode.
16. • If the energy (ɛ) gained during this travel between electrodes, exceeds the ionization
potential Vi , which is the energy required to free an electron from its atomic shell, then
ionization takes place.
• This process can be represented as
• Where, A is the atom, A+ is the positive ion and e - is the electron. A few of the electrons
produced at the cathode by some external means, say by ultra-violet light falling on the
cathode, ionize neutral gas particles producing positive ions and additional electrons.
17. Photo-ionization
The phenomena associated with ionization by radiation, or photoionization,
involves the interaction of radiation with matter.
Photo-ionization occurs when the amount of radiation energy absorbed by an
atom or molecule exceeds its ionization potential.
There are several processes by which radiation can be absorbed by atoms or
molecules.
They are (a) excitation of the atom to a higher energy state (b) continuous
absorption by direct excitation of the atom .
18. Just as an excited atom emits radiation when the electron returns to the lower state
or to the ground state, the reverse process takes place when an atom absorbs
radiation.
This reversible process can be expressed as
Ionization occurs when
where, h is the Planck's constant, c is the velocity of light, λ is the wavelength of the
incident radiation and Vi is the ionization energy of the atom.
19. • Substituting for h and c, we get
where Vi is in electron volts (eV).
• The higher the ionization energy, the shorter will be the wavelength of the
radiation capable of causing ionization.
20. Secondary Ionization Processes
(a) Electron Emission due to Positive Ion Impact
Positive ions are formed due to ionization by collision or by photoionization, and being
positively charged, they travel towards the cathode.
If the total energy of the positive ion, i.e., the sum of its kinetic energy and the ionization
energy, is greater than twice the work function of the metal, then one electron will be driven
out (ejected) and a second electron will neutralize the ion.
The possibility of this process is measured as ϒi which is called the Townsend's secondary
ionization coefficient due to positive ions and is defined as the net produce of electrons per
incident positive ion.
ϒi increases with ion velocity and depends on the kind of gas and electrode material used.
21. Electron Emission due to Photons
To cause an electron to escape from a metal, it should be given enough energy to overcome
the surface potential barrier.
The energy can also be supplied in the form of a photon of ultraviolet light of suitable
frequency.
Electron emission from a metal surface occurs at the critical condition
where ⱷ is the work function of the metallic electrode
The frequency (v) is given by the relationship
22. is known as the threshold frequency.
For a clean nickel surface with ⱷ = 4.5 eV, the threshold frequency will be that
corresponding to a wavelength λ.
If the incident radiation has a greater frequency than the threshold frequency, then
the excess energy goes partly as the kinetic energy of the emitted electron and partly
to heat the surface of the electrode.
23. Electron Attachment Process
The types of collisions in which electrons may become attached to atoms or molecules to form negative ions
are called attachment collisions.
Electron attachment process depends on
• The energy of the electron and
• The nature of the gas
All electrically insulating gases, such as O, CO2 , Cl2 , F2 , C2 F6 , C3 F8 ,C4 F10, CCl2 F2 , and SF6
exhibit this property.
An electron attachment process can be represented as
Atom + e - + k → negative atomic ion + (Ea + K)
The energy liberated as a result of this process is the kinetic energy K plus the electron repulsion Ea
24. TOWNSED’S CURRENT GROWTH EQUATION
let us assume that n0 electrons are emitted from the cathode
When one electron collides with a neutral particle, a positive ion and an electron are formed. This
is called an ionizing collision.
Let α be the average number of ionizing collisions made by an electron per centimeter travel in
the direction of the field.
α depends on gas pressure p and E / p, and is called the Townsend's first ionization coefficient.
At any distance x from the cathode, let the number of electrons be nx
25. • When these nx electrons travel a further distance of dx they give rise to (α nx dx)
electrons.
dn = αndx
dn/n = αdx
Or
In n = αx +A
Now at x = 0,n = no. Therefore,
In no = A
or In n = α x + In no
In n/no = αx
At x = d, n= no𝒆𝒂𝒅. Therefore in terms of current
I = Io 𝒆𝒂𝒅
The term 𝒆𝒂𝒅 is called the electron avalanche and it represents the number of electrons
produced by one electron in travelling from cathode and anode.
26. CURRENT GROWTH IN THE PRESENCE OF SECONDARY PROCESSES
• The single avalanche (sudden large amount) process described in the previous section
becomes complete when the initial set of electrons reaches the anode.
• However, since the amplification of electrons [exp (αd)] is occurring in the field, the
probability of additional new electrons being liberated in the gap by other mechanisms
increases, and these new electrons create further avalanches.
• The other mechanisms are
• (i) The positive ions liberated may have sufficient energy to cause liberation of electrons
from the cathode when they impact on it.
• (ii) The excited atoms or molecules in avalanches may emit photons, and this will lead to
the emission of electrons due to photo-emission.
• (iii) The metastable particles may diffuse back causing electron emission.
27. • The electrons produced by these processes are called secondary electrons.
• The secondary ionization coefficient ϒ is defined in the same way as α, as the net
number of secondary electrons produced per incident positive ion, photon, excited
particle, or metastable particle, and the total value of ϒ is the sum of the individual
coefficients due to the three different processes.
• i.e., ϒ = ϒ1 + ϒ2 + ϒ3 ϒ is called the Townsend's secondary ionization
• coefficient and is a function of the gas pressure p and E/p
Townsend's procedure for current growth
Let n = Total number of electrons reaching the anode
n0 = Number of initiatory electrons due to uv radiation at the surface of the cathode.
n’0 = number of secondary electrons produced due to secondary (ϒ) processes.
Let n’’0 = total number of electrons leaving the cathode.
Then n”0 = n0 + n’0
28. TIME LAGS FOR BREAKDOWN
• The mechanism of breakdown is considered as a function of ionization processes under
uniform field conditions.
• But in practical engineering designs, the breakdown due to rapidly changing voltages or
impulse voltages is of great importance.
• Actually, there is a time difference between the application of a sufficient voltage to cause
breakdown and the occurrence of breakdown itself.
• This time difference is called the time lag.
• The Townsend criterion for breakdown is satisfied, only if at least one electron is present in
the gap between the electrodes.
• In the case of applied dc or slowly varying (50 Hz a.c) voltages, there is no difficulty in
satisfying this condition
29. • However, with rapidly varying voltages of short duration (≈ 10-6 s), the initiatory electron may not be
present in the gap, and in the absence of such an electron breakdown cannot occur.
• The time t which laps between the application of the voltage sufficient to cause breakdown and the
appearance of the initiating electron is called a statistical time lag (ts ) of the gap.
• After the appearance of the electron, a time tt is required for the ionization processes to cause the
breakdown of the gap, and this time is called the formative time lag (tt ).
• The total time ts + tt = t is called the total time lag. Time lags are of considerable practical importance
• For breakdown to occur the applied voltage V should be greater than the fixed breakdown voltage Vs
as shown in Fig
30. The difference in voltage AV = V- Vs is called the overvoltage,
and the ratio V / Vs is called the impulse ratio.
The variation of tt with overvoltage (AV) is shown in Fig
The volt-time characteristics of different electrical apparatus, which are very important in
insulation co-ordination.
It can be seen from the Fig. that a rod gap will protect a bushing, whereas a sphere gap is
required for the complete protection of a transformer against high voltage surges.
31. Electronegative gas influence on Townsends criterion
• Electronegative gases: in certain gases, the electron get attached to the neutral atoms, there
by forming negative ions.
• Due to this phenomenon , electrons which were supposed to have increased the current
and decreased the breakdown strength are now decreasing the current and increase the
dielectric strength of the gas.
• These gases in which attachment of electron to the neutral atom play an important role.
Are electronegative gases.
• Townsends equation for the current growth involving primary and secondary ionization
coefficients can be used, but with a slight modification so as to include the attachment
phenomenon also.
1. Discuss how breakdown voltage is influenced by
• Nature of gas
• Shape of the electrode
• Pressure
• Humidity
32. Limitations of Townsends Mechanism
• Townsends mechanism fails to explain some of the experimental observation of gaseous insulation
breakdown.
• The most significant weakness is the time required for the formation of a self sustained discharge. The
period of time required for such an irreversible transition from the instant of availability of initiatory
electron is usually referred to as “ Formative time lag of breakdown”.
• Shorter formative time lags, avoids the participation of +ve ions and limit their role in the formation of
electron avalanche process.
• Another weakness of Townsend mechanism lies in the failure to consider the effect of the space charge.
In many instances, the concentration of the positive ions can reach highly appreciable values that distort
the initial field to great extent.
• One more difficulty in justifying in justifying the correctness of townsend mechanism lies in the
interpretation of mechanism of spark formation at high values of pd.
• some difficulty exhists in case of non- uniform field especially at high values of pd.
• All these made it very difficult to recognize much of the experimental observation with the theory of
breakdown based on townsend mechanism although this mechanism supplied excellent interpretation of
observed phenomena.
33. Streamer mechanism( Kanal mechanism) of breakdown in gases
There were few critical drawbacks of Townsends mechanism due to which it failed to explain some of
the experimental observation with respect to the formative time lags measured.
In order to overcome these drawbacks streamer theory is put forth explaining the breakdown process in
gases.
At initial stages, the electron starting from cathode due to UV radiation builds up its own avalanche (
channel of electron flow) by different ionization processes. This initial avalanche crosses the gap. In this
avalanche, since the electrons reach the anode with greater mobility positive ions present in the
avalanche are held more less in the same position.
This results in a + ve ion space charge formation near the anode. The positive ions would remain in a
nearly conical channel with the head at the anode shown. Due to exponential ionization in a avalanche,
the density of ions will be highest near the anode. The field distortion produced by these + ve ions will
be in axial and radial directions and maximum near the head of the avalanche.
34. Also due to photoionization in the space charge regions few electrons are formed which result in few
secondary avalanche. These secondary avalanches form first near the anode where the density of
space charge is maximum. Due to this space charge further increases. Positive ions left behind these
secondary avalanches lengthen and intensify the space charge of the main avalanche towards the
cathode.
This process of lengthening of space charge develops a self propagating streamer. As soon as
streamer touches the cathode, a cathode spot is formed and a stream of electrons rush from the
cathode to neutralise the + ve space charge in the streamer. This leads to an instantaneous spark and
hence breakdown of the gaseous insulation.
35. Corona discharges
If the electric field is uniform and if the field is increased gradually, just when measurable ionization begins, the
ionization leads to complete breakdown of the gap. In non-uniform fields, before the spark or breakdown of the
medium takes place, there are many manifestations in the form of visual and audible discharges. These discharges are
known as corona discharges.
In fact corona is defined as a self-sustained electric discharge in which the field intensified ionization is localized
only over a portion of the distance between the electrodes.
The phenomenon is of particular importance in high voltage engineering where most of the fields encountered are
non-uniform fields unless of course some design features are involved to make the field almost uniform.
Corona is responsible for power loss and interference of power lines with the communication lines as corona
frequency lies between 20 HZ to 20kHZ. This also leads to deterioration of insulation by the combined action of the
discharge ions bombarding the surface and the action of chemical compounds that are formed by the corona
discharge.
When the voltage applied corresponds to the critical disruptive voltage, corona phenomenon starts but it is not visible
because the charged ions in the air must receive some finite energy to cause further ionization by collisions.
36. Module – 2 Generation of High voltage A.C.
Necessity of connecting small units of HV transformers in cascade to generate high
voltage AC by this method.
A single unit of high voltage transformer rated for very high voltages is always discouraged
for three distinct reasons.
Such a large rated transformer will have a high inductive reactance associated with its
windings which in turn leads to poor voltage regulation while in operation.
The cost of the insulation is always proportional to the voltage rating of a transformer and
hence higher the voltage rating more will be the cost of insulation.
Transportation of a big unit of a high voltage transformer is always dangerous and leads to
lot of difficulties even while installing such a big unit.
The schematic diagram describing the cascade connection of 3 independent HV
transformers is as shown.
37. The secondary o/p of the first transformer T1 is rated for V2.
A partial output of T1 is taken between the terminals C1,e1 and fed as the i/p to the primary of transformer T2.
The terminals d2 of the transformer T2 is going to be insulated w.r.t the secondary output of transformer T1. This
makes the total o/p of transformer T2 to be 2V2 w.r.t. ground.
It may be noted that transformers T1,T2,T3 are identical in nature. Similarly, the input requirement of transformer
T3 is met by taking a partial output between C2,e2 of transformer T2.
The terminal d3 of transformer T3 will have to be insulated against the o/p of transformer T2. Thus, the net o/p
voltage w.r.t ground of such three units of transformer connected in cascade is V0 = 3V2.
Similarly , if there are n such units connected in cascade , we can anticipate the total o/p voltage of nV2, where
V2 secondary voltage rating of each transformer.
38. Principle of generating high voltage ac using a series resonant circuit
A series resonant circuit comprises of a high voltage transformer having a variable reactor
and a pure capacitive load in series with its secondary winding. The output voltage of such a
circuit under resonance condition yield higher voltages compared to the input applied to the
primary winding of the transformer.
By varying the inductance, it is possible to achieve the resonance condition in the circuit.
At resonance, the load terminal voltage is given by
V0 = IXc
= (V1/Re)Xc
= V1(Xc/Re)
V0 = QV1
Thus, at resonance the voltage a/c, the load terminals is increased to Q times that of the input
voltage. So, the voltage required at primary is 1/Q times the load terminal voltage. In other
words, the input required is just 1/Q times the rated kVA of the transformer.
39. But in practice, it is impossible to make a high voltage by continuously varying the reactor
which provide low voltage inductance variation is used along with a step-up transformer
incorporated in parallel to meet the voltage requirement.
To supply the regulator gives the input from the mains through a feed transformer. Two or
more transformer/reactor units may be connected in series shown in fig.
40. Advantages of a series resonant circuit used in the generation of high voltage ac.
Since the voltage on the HV side of a series resonant transformer is Q times magnified the low voltage side
requirement is Q times reduced. So the input kVA required is just 1/Q times the total secondary rating of the
transformer. The total kVA required is only about 5% of the main kVA.
The voltage waveshape is improved not only by the elimination of unwanted resonance but also by attenuation of
harmonics present in the supply. Good wave shape always gives a high degree of accuracy in the high voltage
measurements.
By continuously varying the inductance, the resonance is achieved. When the test object fails power arc does not
occur in most of the cases since the resonance is lost immediately. Even if the power arc does occur it is going to be
self extinguishing in nature, thereby it is possible to observe the path of arc flashover.
For heavy current testing it is possible to parallel the transformers of different impedances. For higher voltage
requirements different high impedance transformers can be connected in cascade.
High voltage connections used in the circuit need no heavy busbars; only a small gauge wire will suffice the need.
41. High frequency, high voltages are required for testing of electrical operators against switching surges.
A resonant transformer used for such a purpose is known as tesla coil.
A tesla coil is doubly tuned resonant circuit in which the primary can be fed either from dc or ac
supply through capacitor C1. The tesla coil consists of 2 air cored concentrically arranged coils. The
high voltage winding consists of relatively larger number of turns compared to low voltage winding
both of which are wound on an insulating frame.
The voltage to which C1 is charged depends on the supply voltage and the setting of trigger gap, S.
when the spark gap breakdown C1 discharges and high frequency damped oscillations are produced
in the primary circuit of tesla coil. If L1 is the inductance of the primary circuit the frequency of
oscillations is
The usual value of frequency needed for the test is around 100 kHz. The oscillation circuit in the
primary induces oscillations in the secondary circuit of tesla coil. The frequency of these induced
oscillations can be made equal by tuning of the 2 circuits by setting L1C1 = L2C2 where L2 is the
inductance of the secondary circuit.
Both primary and secondary windings are immersed in coil. By adjusting the values of C1 and C2, it
is possible to tune the circuit from 10 to 100kHz.
If V1 is the maximum voltage to which C1 is charged and V0 is the maximum voltage to which C2 is
charged. A relation developed for the efficiency of energy transferred is shown.
The chief use of tesla coil is in the field of testing high voltage insulators and bushings.
