UNIT-I OVER VOLTAGES IN ELECTRICAL POWER SYSTEMS
Causes of over voltages and its effect on power system – Lightning – Switching surges and temporary
over voltages – Protection against over voltages.
1. Why protection of transmission line important?
It is essential for electrical power engineers to reduce the number of outages and preserve the
continuity of service and electric supply.
2. What are the causes of over voltages in electric system?
 External Cause – Lightning
 Internal Cause – Switching
3. What are the causes of power frequency over voltages?
 Sudden load rejection
 Disconnection of inductive loads or capacitive loads
 Ferranti effect
 Unsymmetrical fault
 Saturation in the transformers
 Tap changing operations
4. Name the source of switching surges.
 Opening and Closing or switchgears
 High natural frequency of the system
 Damped normal frequency voltage components
 Restriking between the contacts
5. What is lightning phenomenon?
Lightning phenomenon is a peak discharge in which charge accumulated in the clouds discharges into
a neighbouring cloud or to the ground.
6. How does switching over voltage originate?
Switching over voltages originate in the system itself by connection and disconnection of circuit
breaker contact or due to initiation or interruption of faults.
7. What is Isokeraunic level?
Isokeraunic level is defined as the number of days in a year when thunder is heard or recorded in
2
particular location. It is given by, Ng  0.1 to 0.2TD/strokes /km - year
8. State the different methods of protection against over voltages.
Overvoltage due to lightning strokes can be avoided or minimized in practice by,
i.
Shielding the overhead lines by using ground wires above the phase wires,
ii.
Using ground rods and counter-poise wires, and
iii.
Including protective devices like expulsion gaps, protector tubes on the lines, and surge
diverters at the line terminations and substations.
9.
What is a ground wire or shielded wire?
Ground wire is a conductor run parallel to the main conductor of the transmission line supported on
the same tower and earthed at every equally and regularly spaced towers. It is run above the main
conductor of the line.
10. Define tower footing resistance.
It is the effective resistance offered against drainage of the charge and lightning current through the
tower structure and tower foot.
11. Define Shielding Angle.
Shielding angle or Protective angle of the ground wire is defined as the angle between the vertical line
passing through the ground wire and the line passing through the outermost power conductor.
Protective zone is the zone which is a cone with apex at the location of the ground wire and surface
generated by line passing through the outermost conductor.
12. What are the measures to control over voltages due to switching and power frequency?
a. Energization of transmission lines in one or more steps by inserting resistances and
withdrawing them afterwards
b. Phase controlled closing of circuit breakers,
c. Drainage of trapped charges before reclosing,
d. Use of shunt reactors, and
e. Limiting switching surges by suitable surge diverters.
13. What are the factors that influence the lightning induced voltage on transmission lines?
i.
The ground conductivity
ii.
The leader stroke current
iii.
Corona
14. How is transmission lines classified?
i.
Lines with no loss or ideal lines
ii.
Line without distortion or distortion less lines
iii.
Line with small losses
iv.
Lines with infinite and finite length defined by all the four parameters
15. Define attenuation and distortion.
The decrease in the magnitude of the wave as it propagates along the line is called attenuation.
The elongation or change of wave shape that occurs is called distortion.
16. How is attenuation and distortion caused?
Attenuation is caused due to the energy loss in the line and distortion is caused due to the inductance
and capacitance of the line.
17. What are the causes for the change of induction on transmission line?
The changes in the inductance are due to the skin effect, the proximity effect and non-uniform
distribution effect of the currents and the nearness to steel structures.
18. What are the causes for the change of capacitance on transmission line?
The variation in capacitance is due to capacitance change in the insulation nearest to the ground
structures, etc.
19. What is the effect of corona on transmission lines?
The effect of corona is to reduce the crest of the voltage wave under propagation, limiting the peak
value to the critical corona voltage. Hence, the excess voltage above the critical voltage will cause power
loss by ionizing the surrounding air.
20. What are the principles observed in the lattice diagram?
i.
All waves travel downhill, i.e, into the positive time.
ii.
The position of the wave at any instant is given by means of the time scale at the left of the
lattice diagram.
iii.
The total potential at any instant of time is the super position of all the waves which arrive at
that point until that instant of time, displaced in position from each other by time intervals
equal to the time difference of their arrival.
iv.
Attenuation is included so that the amount by which a wave is reduced is taken care.
v.
The previous history of the wave, if desired can be easily traced. If the computation is to be
carried out at a point where the operations cannot be directly placed on the lattice diagram,
the arms can be numbered and the quanti9y can be tabulated and computed.
21. What are the components of switching surges?
Switching surges may include high natural frequencies of the system, damped normal frequency
voltage component or the restriking and recovery voltage of the system with successive reflected waves
from terminations.
22. How does switching surges cause damage to circuit breaker?
In circuit breaking operation, switching surges with a high rate of rise of voltage may cause repeated
restriking of the arc between the contact of a circuit breaker, thereby causing destruction of the circuit
breaker contacts.
23. Give the factor for over voltages generation in EHV system.
Over voltages are generated in EHV system when there is sudden release of internal energy stored
either in the electrostatic form in the electromagnetic form.
24. What are the different method by which switching over voltages of short duration and long
magnitude be calculated?
i. Mathematical modeling of a system using digital computer
ii. Scale modeling using transient network analyzers
iii. By conducting field tests to determine the expected maximum amplitude of the over voltages and
their duration at different points on the line.
25. Where is surge arrester placed in substation?
Surge arresters are devices used at substations and at line terminations to discharge the lightning
over voltages and short duration switching surges. These are usually mounted at the line end at the
nearest point to the substation. They have a flash over voltage lower than that of any other insulation or
apparatus at the substation.
UNIT II-ELECTRICAL BREAKDOWN IN GASES, SOLIDS AND LIQUIDS
Gaseous breakdown in uniform and non-uniform fields – Corona discharge – Vacuum
breakdown – Conduction and breakdown in pure and commercial liquids – Breakdown
mechanisms in solid and composite dielectrics.
1. What are the different gases that are used as insulating medium?
Air, Nitrogen, Carbon dioxide, Freon and Sulphur hexafluoride.
2. What are the various phenomena that occur in gaseous dielectric?
When the applied voltage is low, small currents flow between the electrodes and the insulation retains
its electrical properties. On the other hand, if the applied voltages are large, the current flowing through
the insulation increases very sharply and an electrical breakdown occurs.
3. What is break down voltage?
The maximum voltage applied to the insulation at the moment of breakdown is called the breakdown
voltage.
4. Give the types of electrical discharge in gases.
i.
Non sustaining discharges
ii.
Self sustaining discharges
5. Define spark breakdown and ionization.
The break down in a gas, called spark breakdown is the transition of a non-sustaining discharge into
self sustaining discharge.
The build-up of high current s in a breakdown is due to the process known as ionization in which
electrons and ions are created from neutral atoms or molecules and their migration to the anode and
cathode respectively leads to high currents.
6. Give the theories that explain the mechanism for breakdown.
i.
Townsend theory
ii.
Streamer theory
7. What are the conditions in the gases that govern the ionization process?
(i) Pressure
(ii) Temperature
(iii) Electrode field configuration
(iv) Nature of electrode surfaces
(v) Availability of initial conducting particles
8. Define statistical time lag and formative time lag.
Statistical Time Lag: The time which lapse between the application of the voltage sufficient to cause
breakdown and the appearance of the initiating electron is called a statistical time lag of the gap.
Formative Time Lag: The time required for the ionization processes to develop fully to cause the
breakdown of the gap, and this time is called the formative time lag (tf).
9. What are electronegative gases? Give example.
Electronegative gases are the gases that have affinity towards electrons. When electron comes into
contact with these gas molecules, the gas molecule attracts the electrons and becomes negative ion.
Examples:
 Carbondioxide
 Oxygen
 SF6
10. Name the different secondary ionization process.
a. Electron Emission due to Positive Ion Impact
b. Electron Emission due to Photons
c. Electron Emission due to Metastable and Neutral Atoms
11. Name the three properties of composite dielectrics that are important to their performance.
a. Effect of Multiple Layers
b. Effect of Layer Thickness
c. Effect of Interfaces
12. What are pure liquids dielectrics?
Pure liquids are those which are chemically pure and do not contain any other impurity even in traces
of 1 in 109, and are structurally simple. Examples are n- hexane (C6H14), n-heptane (C7H16) and other
paraffin hydrocarbons.
13. What is breakdown voltage?
The maximum voltage that can be applied on the insulation at the moment of breakdown is called as
breakdown voltage.
14. Define Townsend’s first ionization co-efficient.
Townsends Primary ionization coefficient α is defined as the average number of ionizing collision
made by the free electron per centimeter travel in the direction of applied electric field.
15. Define Townsend’s second ionization co-efficient.
The Townsend’s secondary ionization coefficient  is defined as the net number of secondary
electrons produced per incident positive ion, photon, excited particle or metastable particle.
16. What are the limitations of Townsend’s theory?
Townsend’s mechanism failed to explain the following:
 According to the Townsend theory, current growth occurs as a result of ionization processes only.
But in practice, breakdown voltages were found to depend on the gas pressure and the geometry
of the gap.
 Mechanism predicts time lags of the order of 10-5S, while in actual practice breakdown was
observed to occur at very short times of the order of 10-8S.
 Townsend mechanism predicts a very diffused form of discharge, in actual practice, discharges
were found to be filamentary and irregular.
17. Name the various mechanism of breakdown in solid dielectrics.
Breakdown mechanisms in solid dielectrics can be classified as follows:
(a) Intrinsic or ionic breakdown,
(b) Electromechanical breakdown,
(c) Failure due to treeing and tracking,
(d) Thermal breakdown,
(e) Electrochemical breakdown, and
(f) Breakdown due to internal discharges.
18. What is tracking and treeing?
The spreading of spark channels during tracking, in the form of the branches of a tree is called
treeing.
19. What is tracking index?
The numerical value of voltage that initiates or causes the formation of a track is called the "tracking
index" and this is used to qualify the surface properties of dielectric materials.
20. What is called a composite dielectric?
Composite dielectrics are the different insulating materials in parallel or in series with each other,
such as air or SF6 gas in parallel with solid insulation.
21. Define time lag.
The time difference between the application of a voltage sufficient to cause breakdown and the
occurrence of breakdown itself is called the time lag.
22. On what factors does time lag depend?
(i) Statistical time lag- Pre ionization , size of the gap and quantity of radiation
(ii) Formative time lag – Mechanism of the avalanche growth in the gap, transit time.
23. What is Paschen’s Law?
The breakdown voltage of a uniform field gap is a unique function of the product of p, the gas
pressure and d, the electrode gap, for a particular gas or for a given electrode material.
V=f(pd)
24. What are the effects of corona?
(i) Loss of power
(ii) Deterioration of insulation
(iii) Rise to radio interference.
25. Define corona.
If the field is non-uniform an increase in voltage will first cause a discharge in the gas to appear at
points with highest electric filed intensity namely at sharp points or where the electrodes are curved or on
transmission lines. This form of discharge is called a corona discharge.
(Or)
The phenomenan of faint violet glow, hissing noise and ozone gas produced in the transmission lines
during rainy seasons is called as corona.
26. What are the properties of good gaseous dielectric for the HV application?
(i) High dielectric strength
(ii) Thermal stability and chemical inactivity towards materials of construction.
(iii) Non-flammability and physiological inertness and environmentally non- hazardous.
(iv) Low temperature of condensation
(v) Good heat transfer
(vi) Ready availability at moderate cost.
27. What is vacuum?
A vacuum system which is used to create vacuum is a system in which the pressure is maintained at a
value much below the atmospheric pressure.
28. Define vacuum discharge.
Electrons get multiplied due to the various ionization processes and an electron avalanche is formed. In
high vacuum, even if the electrodes are separated by a few centimeters, an electron crosses the gap
without encountering any collisions.
29. What are the different mechanisms of breakdown in vacuum?
(i) Particle exchange mechanism
(ii) Field emission mechanism
(iii) Clump theory
30. On what factors does liquid dielectric is selected?
(i) Chemical stability
(ii) Space
(iii) Cost
(iv) Previous usage
(v) Susceptibility to the environmental influences.
31. Give different properties of liquid electric.
(a) Electrical properties
(i) Its capacitance per unit volume or its relative permittivity
(ii) Its resistivity
(iii) Its loss tangent or its power factor
(iv) Its ability to with stand high electric stresses.
(b)Heat transfer characteristics
(c) Chemical stability.
32. What are the factors that influence conduction in pure liquid dielectric and in commercial
liquid dielectric?
(i) The nature and condition of the electrodes
(ii) The physical properties of the liquid
(iii) The impurities and gases present in the liquid
33. What are the various theories that explain breakdown in commercial liquid dielectric?
(i) Suspended particle mechanism
(ii) Cavitation and bubble mechanism
(iii) Stressed oil volume mechanism
34. What are the properties of good solid dielectric?
(i) Low dielectric loss
(ii) High mechanical strength
(iii) Should be free from gaseous inclusions and moisture
(iv) Resistant to thermal and chemical deterioration
35. Give different solid dielectric material.
(i) Organic material:
(a) Paper
(b) Wood
(c) rubber
(ii) Inorganic material
(a) Perspex
(b) PVC
(c) Epoxy resins
(d) Mica
(e) Glass
(f) Porcelain
36. Give different breakdown mechanism in solid dielectrics.
(i) Intrinsic or ionic breakdown
(ii) Electromechanical breakdown
(iii) Failure due to treeing and tracking
(iv) Thermal breakdown
(v) Electrochemical breakdown
(vi) Breakdown due to internal charges.
37. What are the different breakdown mechanisms in composite dielectric?
(i) Short term breakdown
(ii) Long term breakdown
UNIT-III GENERATION OF HIGH VOLTAGES AND HIGH CURRENTS
Generation of high DC, AC, impulse voltages and currents – Tripping and control of impulse
generators.
1. Give some uses of HVDC.
 Electron Microscopes
 X-Ray units
 Electrostatic precipitators
 Particle Accelerators in nuclear physics
2. What are the applications of impulse current wave forms of high magnitude?
 Testing of surge diverters
 Testing of non-linear resistors
 Electric arc studies
 Studies of electric plasmas in high current discharges
3. Explain the necessity for generating impulse currents and mention the features of impulse
current generators.
Impulse current generation is required for,
o Testing of surge diverters
o Testing of non-linear resistors
o Electric arc studies
o Studies of electric plasmas in high current discharges
 For producing impulse currents of large value, a bank of capacitors connected in parallel are charged
to a specified value and are discharged through a series R-L circuit.
 Waveshapes used in testing surge diverters are 4/10 and 8/20 μ s. The tolerances allowed on these
times are ± 10% only.
 Rectangular waves of long duration are also used for testing. The rectangular waves generally have
durations of the order of 0.5 to 5 ms, with rise and fall times of the waves being less than ±10% of
their total duration.
4. How are capacitances connected in an impulse current generator?
In high impulse current generation, a bank of capacitors connected in parallel are charged to a
specified value and are discharged through a series R-L circuit.
5. What are the types of wave form will be available in impulse current generator output?
1. Impulse current waves - 4/10 and 8/20 μ s; Tolerances are ± 10%.
2. Rectangular waves - Durations of the order of 0.5 to 5 ms, with rise and fall times of the
waves being less than ±10% of their total duration.
6.
Draw a circuit diagram of a simple voltage doubler.
Fig.3.1. Voltage Doubler Circuit
7. Write the expression to find the optimum number of stages and %ripple in a voltage multiplier
circuit.
noptimum 
Vmax fC
I
8. What is tesla coil?
Tesla coil is the commonly used high frequency resonant transformer, which is a doubly tuned
resonant circuit. The primary and the secondary are wound on an insulated former with no core (aircored) and are immersed in oil.
9. Draw a simple tesla coil equivalent circuit for generation of high frequency AC high voltage.
Fig.3.2. Tesla Coil & Output Waveform
10. What are the advantages of high frequency resonant transformer used in HVAC generation?
Advantages of these high frequency transformers are:
i.
Absence of iron core saves in cost and size,
ii.
Pure sine wave output,
iii.
Slow build-up of voltage over a few cycles and hence no damage due to switching surges, and
iv.
Uniform distribution of voltage across the winding coils due to subdivision of coil stack into a
number of units.
11. Find the percentage ripple in the output voltage produced by an 8 stage Cockcroft-Walton
multiplier circuit with a capacitance all equal to 0.05µF. The supply transformer secondary
voltage is 125kV at a frequency of 150Hz and the load current is 5mA.
Percentage Ripple 
δv  100
2nVmax
Ripple Voltage, δv 
I1 n(n  1)
fC
2
where, No.of Capacitances,n=2 X No.of Stages
n  2  8  16
Load Current , I1  5  10 3 A
5  10 3  16  16  1
 90.67kV
150  0.05  10 6  2
90.67  103  100
% Ripple 
 2.2667%
2  16  125  103
δv 
12. A Cockcroft Walton type voltage multiplier has 8 stages with capacitances all equal to 0.05µF.
The supply transformer secondary voltage is 125kv at a frequency of 150Hz and the load
current is 5mA.Find the optimum number of stages for minimum voltage regulation.
Maximum Voltage, Vmax=125kV=125X103V
noptimum 
Vmax fC
I
125  103  150  0.05  10 5
5  10 3
 13.693  14 Stages

13. A tesla coil has a primary winding rated for 10kV with 2µF capacitance on primary side and
1nF capacitance on secondary side. If the energy efficiency is 5%. Calculate the output voltage.
Output Voltage is given by,
V2  V1 η
C1
C2
V2  10  103 
5 2  10 6

 10  103  10
100 1 10 9
V2  100kV
14. A 12 stage impulse generator has a 0.126µF capacitor. The wave front and wave tail resistances
are 800ohms and 5000ohms respectively. If the load capacitor is 1000pF, find the front and tail
times of the impulse wave produced.
Generator Capacitance, C1=(0.126/12)=0.0105 µF
Load Capacitance, C2=1000X10-12F=0.001 µF
R1=800 and R2=5000
 C1C2 

 C1  C2 
Front Time, t1  3R1Ce  3R1 
 0.0105 106  0.001106 
  2.19μSec
t1  3  800  
6
6 
 0.0105  10  0.00110 
Tail Time, t2  0.7( R1  R2 )(C1  C2 )
t 2  0.7(800  5000)(0.0105  10 6  0.001 10 6 )
t 2  46.69μSec
15. What is voltage multiplier circuit?
 Multiplier circuit is a circuit to generate very high d.c voltages from single supply transformers by
extending the simple voltage doubler circuits.
 This is simple and compact when the load current requirement is less than one milliampere, such
as for cathode ray tubes, etc.
 Multiplier Circuits are designed to overcome the difficulties in higher voltage generation using
cascaded voltage multiplier circuits which require too many supply and isolating transformers.
16. Distinguish between electromagnetic and electrostatic machines.
Electromagnetic machines: Current carrying conductors are moved in a magnetic field, so that the
mechanical energy is converted into electrical energy.
Electrostatic machines: Charged bodies are moved in an electric field against an electrostatic field in
order that mechanical energy is converted into electrical energy.
17. Name the circuits used to generate HVDC.
 High Voltage Half Wave and Full Wave rectifier
 Voltage Doubler and Cascaded Voltage Doubler
 Voltage Multiplier : Cockcroft Walton Multiplier
 Deltatron Circuit
 Electrostatic Machines: a.Van De Graff Generator, b.Electrostatic Generator
18. What are the advantages and disadvantages of Deltatron circuit?
 Can produce high voltages
 Gives high stability
 Small ripple factor
 Fast regulation
19. What are the limitations of Van de graff generator?
 Applicable only in low current applications
 Belt velocity is limited due to vibration
 Difficult to have an accurate grading of fields.
 Maintenance is required due to mechanically stressed parts.
20. What are the advantages and disadvantages of using cascaded transformer?
Advantages:
 Better cooling
 Easy to transport since the units are light and compact
 Construction is identical
 Three phase connection in delta or star is possible for three units
 Available for both indoor and outdoor applications
Disadvantage:
 Costly and requires more space for installation
21. What are impulse wave specifications?
V%
A
100
D
90
G
70
E
50
30
B
F
C
10
O t t’ t
f
1 1
t2
tt
t
O1
Fig.3.3. Impulse waveform and its definitions
tf - Front Time (1.2μSec)
tt – Tail Time (50 μSec)
22. What is the front and tail time of a standard impulse wave? What are the tolerances allowed as
per the specifications?
i.
Front Time: Time required for the wave to rise from 10% to 90% (or 0% to 100%) of the
peak value at first instance.
Tolerance: ± 30%
ii.
Tail Time: Time corresponding to 50% of the peak value during its tail.
Tolerance: ± 20%.
23. How is the wave front and wave tail times controlled in impulse generator circuits?
For a given impulse generator, the generator capacitance C1 and load capacitance C2 will be fixed
depending on the design of the generator and the test object. Hence, the desired waveshape is
obtained by controlling R1 and R2.
 C1C2 

C

C
2 
 1
Front Time, t1  3R1Ce  3R1 
Tail Time, t 2  0.7( R1  R2 )(C1  C2 )
24. What is peak value?
The maximum positive deviation of the output with respect to its desired value is known as peak
value.
25. Give four components of a multistage impulse generator.
1. D.C. Charging Set
2. Charging Resistors
3. Generator Capacitors and Spark Gaps
4. Wave-shaping Resistors and Capacitors
UNIT – IV MEASUREMENT OF HIGH VOLTAGES AND HIGH
CURRENTS
Measurement of high voltages and high currents – Digital techniques in high voltage
measurement.
1. What are the general methods used for measurement of high frequency and impulse
currents?
(i) Potential dividers with a cathode ray oscillograph
(ii) Peak voltmeters
(iii) Sphere gaps
2. What are the high voltage d.c measurement techniques used?
 Series resistance micro ammeter
 Resistance potential divider
 Generating voltmeters
 Sphere and other spark gaps
3. For what measurement are Hall generators normally used?
 Measurement of high direct currents
 Measurement of high frequency and impulse currents.
4. What are the types of measuring devices preferred for measurement of impulse currents of
short duration?
(i) Potential dividers with a cathode ray oscillograph
(ii) Peak voltmeters
(iii) Sphere gaps
5. Draw the simple circuit of peak reading voltmeter and it’s equivalent.
6. List the factors that are influencing the peak voltage measurement using sphere gap.
(i) Nearby earthed objects
(ii) Atmospheric conditions and humidity
(iii) Irradiation
(iv) Polarity and rise time of voltage waveform.
7. What are the advantages of CVT measurement in HVAC?
(i) Simple design and easy installation
(ii) Can be used both as a voltage measuring device for meter and relaying purposes and also as a
coupling condenser for power line carrier communication and relaying.
(iii) Frequency independent voltage distribution along elements as against conventional magnetic
potential transformers which require additional insulation design against surges.
(iv) Provides isolation between the high voltage terminal and low voltage metering.
8. What are the limitations of generating voltmeter?
 Need calibration
 Careful construction is needed
 Any disturbances due to position and monitoring of the electrodes make the calibration
invalid.
9. State the demerits of CVT measurement for HVAC measurements.
 Voltage ratio is susceptible to temperature variations.
 In the presence of capacitance, the problem of ferroresonance occurs in power system.
10. What are the methods available for measuring ac voltages of power frequency?
(i) Series impedance ammeters
(ii) Potential dividers
(iii) Potential transformers
(iv) Electrostatic voltmeters
(v) Sphere gaps
11. What are the methods available for measuring dc current?
(i) Resistive shunts with milli ammeter
(ii) Hall effect generators
(iii) Magnetic links
12. What is generating voltmeter?
A generating voltmeter is a variable capacitor electrostatic voltage generator which generates current
proportional to the applied external voltage. The device is driven by an external synchronous or constant
speed motor and does not absorb power or energy from the voltage measuring source.
13. What are the advantages and limitations of generating voltmeter?
Advantages:
(i) No source loading by the meter
(ii) No direct connection to high voltage electrode
(iii) Scale is linear and extension of range is easy
(iv) A very convenient instrument for electrostatic devices
Limitations:
(i) They require calibration
(ii) Careful construction is needed and is cumbersome instrument requiring an auxiliary drive
(iii) Disturbance in position and mounting of the electrodes make the calibration invalid.
14. What are the sources that contribute to the error?
(i) The effective value of the capacitance being different from the measured value of C.
(ii) Imperfect rectifiers which allows small reverse currents
(iii) Non-sinusoidal voltage waveforms with more than one peak or maxima per half cycle.
(iv) Deviation of the frequency from that of the value used for calibration.
15. How resistance shunt is usually designed?
(i) Bifilar flat strip design
(ii) Coaxial tube or Park’s shunt design
(iii) Coaxial squirrel cage design.
16. What are the different techniques for impulse current measurement?
(i) Rogowski coil
(ii) Magnetic links
(iii) Hall generators
(iv) Faraday generator
(v) Current transformer
17. What is hall voltage and hall coefficient?
If electric current flows through a metal plate located in a magnetic field perpendi9cular to it, Lorenz
forces will deflect the electrons in the metal structure in a direction normal to the direction of both the
current and the magnetic field. The charge displacement generates an emf in the normal direction, called
the “Hall voltage”. The Hall voltage is proportional to the current I a, the magnetic flux density B and the
reciprocal; of the plate thickness, the proportionality constant R is called the “Hall coefficient”.
VH = R (Bi / d)
UNIT-V HIGH VOLTAGE TESTING AND INSULATION COORDINATION
High voltage testing of electrical power apparatus – Power frequency, impulse voltage and DC
testing – International and Indian standards – Insulation coordination.
1. Name the different types of standard tests conducted on high voltage apparatus.
 Type Test - To check the design features
 Routine Test - To check the quality of the individual test piece.
 High Voltage Tests Include
o Power frequency tests
o Impulse tests
2. What is the test conducted on bushings?
 Power Factor-Voltage Test
 Internal or Partial discharge Test
 Momentary Withstand Test at Power frequency
 One Minute withstand Test at Power Frequency
 Visible Discharge Test at Power Frequency
 Impulse voltage tests- a. Full wave Withstand Test , b. Chopper Wave withstand Test
3. Define withstand voltage.
The voltage which has to be applied to a test object under specified conditions in a withstand test is
called the withstand voltage [as per IS: 731 and IS: 2099-1963].
4. Define impulse voltage.
 Impulse voltages are characterized by,
o Polarity,
o Peak value,
o Time to front (tf), and
o Time to half the peak value after the peak (tt).
 According to IS: 2071 (1973), standard impulse is defined as one with tf = 1.2μSec, tt =50 μSec
(called 1/50 μSec wave).
 The tolerances allowed are ±3% on the peak value, ±30% in the front time, and ±20% in the tail
time.
5. Differentiate type test and routine test. (or) What do you mean by type tests and routine test?
Type Tests:
 These tests are intended to prove or check the design features and quality
 Type tests are done on samples when new designs or design changes are introduced.
Routine Tests:
 Routine tests are intended to check the quality of the individual test piece.
 Routine tests are done to ensure the reliability of the individual test objects and quality and
consistency of the materials used in their manufacture.
6. Define the term ‘ac test voltage’ referred to HV testing.
7. Give the values of reference atmospheric conditions as per Indian standard specification.
8. Define disruptive discharge voltage.
9. What are the demerits of synthetic testing of circuit breakers?
10. Define creeping distance.
It is the shortest distance on the contour of the external surface of the insulator unit or between two
metals fitting on the insulator
11. What is insulation co-ordination?
The process of bringing the insulation strengths of electrical equipment and buses into the proper
relationship with expected overvoltages and with the characteristics of the insulating media and surge
protective devices to obtain an acceptable risk of failure
12. Define 50% and 100% flashes over voltage.
50% Flashover Voltage:
This is the voltage which has a probability of 50% flashover, when applied to a test object. This is
normally applied in impulse tests in which the loss of insulation strength is temporary.
100% Flashover Voltage:
The voltage that causes a flashover at each of its applications under specified conditions when
applied to test objects is specified as hundred per cent flashover voltage.
13. Differentiate flashover and puncture.
Flashover: It is a discharge over the surface of the insulation systems.
Puncture or Spark over: It is a discharge through the insulation systems. If the insulation is solid, it
could not able to regain its insulation strength after puncture.
14. What are the different tests done on insulators?
 Type Test - To check the design features
 Routine Test - To check the quality of the individual test piece.
 High Voltage Tests Include
o Power frequency tests
o Impulse tests
15. What are impulse tests?
16. What is the significance of impulse tests?
17. What is an isolator?
 It is an off-load or minimum current breaking mechanical switch.
 Explained according to “IS:9921 Part-1, 1981”.
 Interrupting small currents(0.5A): Capacitive currents of bushings, busbars etc.,
18. What are the test conducted on isolators and circuit breakers?
The main tests conducted on the circuit breakers and isolator switches are
i.
Dielectric tests or overvoltage tests,
ii.
Temperature rise tests,
iii.
Mechanical tests, and
iv.
Short circuit tests
19. What is the test conducted on transformer?
 Induced Overvoltage Test
 Partial Discharge Tests
 Impulse Test
20. What are partial discharges?
These are the discharges due to presence of void or any other inclusions inside of the dielectrics. The
partial discharges may not be suddenly bridge the electrode; but is increasing with the duration of the
operation.
21. What is the test conducted on surge arresters?
 Power frequency spark over test
 Impulse sparkover test
 Residual voltage test
 Impulse current withstand test
22. What is the test conducted on cables?
Different tests on cables are
i.
Mechanical tests like bending test, dripping and drainage test, and fire resistance and corrosion
tests
ii.
Thermal duty tests
iii.
Dielectric power factor tests
iv.
Power frequency withstand voltage tests
v.
Impulse withstand voltage tests
vi.
Partial discharge test
vii.
Life expectancy tests
23. Why is insulation coordination needed?
 To ensure the reliability & continuity of service
 To minimize the number of failures due to over voltages
 To minimize the cost of design, installation and operation
24. State the principle that is followed in the insulation design of EHV and UHV substations.
In EHV and UHV substations, the insulation design is mainly based on the consideration of switching
surges whereas in high voltage substations consider lightning surges.
25. Explain the reasons for conducting wet tests on high voltage apparatus and give the
specifications for the water used for wet tests.
The wet test is carried to satisfy the service condition of the HV apparatus. The test object is
subjected to spray of water with the following specifications:
 Precipitation Rate :310% (mm/min)
 Direction of Spray : 450 to the vertical
 Conductivity
: 100 micro-siemens 10%
 Water Temperature : Ambient 15%
26. What are the equipment and devices needed for conducting impulse test on HV equipments?
i.
Impulse Generator
ii.
Sphere Gap
iii.
Potential Dividers
iv.
Protection Gap
v.
Probes
vi.
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