unit 1 over voltage in electrical power systems 1. … · 2019-11-22 · or due to initiation or...

EE6701 – HIGH VOLTAGE ENGINEERING (R2013)

UNIT 1 – OVER VOLTAGE IN ELECTRICAL POWER SYSTEMS

1. What is Corona Critical disruptive voltage? (A/M 2017)

Voltage exceeds a certain value is called Critical disruptive voltage. The conductors are

surrounded by a faint violet glow is called Corona. Corona – Hissing sound, Production

of ozone, Power loss and radio interference s.

2. What are the different types of Overhead transmission line protections? (A/M 2017)

Ground wires

Ground Rods

Counter Poise wires

Protective devices

3. Define Isokeraunic level. (N/D 2016)

It is defined as No. of days in a year, when thunder is heard or recorded in a particular

location.

4. What are the abnormalities in high voltage system? (A/M 2017)

Voltage sag, Swell

Power loss

Fault occurrence

Harmonics

Poor power factor

Insulation failures, etc

5. List the factors that influence the induced voltage in transmission line. (N/D 2015)

Ground conductivity

Leader stroke current

Corona

6. What are the characteristics of a lightning voltage? (A/M 2017)

Amplitude of the currents

Rate of rise

Probability distribution of current and rate of rise

Wave shapes of the lightning voltage and current

7. Why a simple spark gap can’t offer full protection against over voltage? (A/M 2015)

There is no current limiting device provided so as to limit the current after spark

over. Hence a series resistance is often used. Without a series resistance the

sparking current may be very high Ant the applied impulse voltage collapses to zero thus

creating steep step voltage which sometime proves to be dangerous to the apparatus to be

protected such as transformer or machine. Their flash over characteristi depends on the at

mospheric condition, polarity of wave and wave shape.

8. What is black flashover? (N/D 2016)

When a direct lightning stroke occurs on a tower, the tower has to carry huge impulses

currents. If the tower footing resistance is considerable, the potential of the tower rises to a


large value, steeply with respect to the line and consequently a flashover may take place

along the insulator strings. This is known as back flashover.

9. Define Basic Impulse level (BIL). It is defined as a reference level expended in impulse crest voltage with a standard wave not

conges then 1.5 x 40 µsec wave. The insulation of apparatus must be equal to or greater than

BIL.

10. Draw the volt – time curve of protective device and the equipment to be protected.

Time per sec →

A = withstand level B = Protection level of protective device

11. What are the various causes of power frequency over voltages?

Sudden loss of loads

Disconnection of inductive loads or connection of capacitive loads.

Ferranti effect

Saturation in transformers.

12. Define coefficient of reflection.

Whenever there is an abrupt change I the parameters of transmission line, such as an

open circuit or a termination, the traveling wave undergoes a transition, part of the wave

is reflected or sent back and may a portion is transmitted forward.

The relation between the reflected waves and the incident waves is called the

coefficient of reflection.

13. What are the characteristics of travelling waves?

These are high frequency disturbances and travel as waves.

They are reflected, transmitted attenuated on distorted during propagation until the

energy is absorbed.

14. Explain the difference between lightning over voltage and switching over voltage.

Lightning over voltage is a natural phenomenon, which switching over voltages

originate in the system itself by the connection and disconnection of CB contacts

or due to initiation or interruption of faults.

15. What are travelling waves? Any disturbance on a transmission line or system such as sudden opening or

closing of a line, a short circuit or a fault results in the development of over voltages

or over currents at that point.

This disturbance proposals and builds up gradually over the transmission line.

This is called the traveling wave.


16. Explain the wave shape of lightning stoke.

Wave Shape of Lightning Stroke: As said earlier the lightning stroke current

rises to crest value very quickly and then starts decaying at a low rate as illustrated in

figure. The generalized wave shape can be characterized as:

(i) Crest or peak value and it has been observed that the maximum value of this

current is kA.

(ii) The wave front line which varies from 1 to 10 µsec

(iii) The time at which the stroke current reduces to 50 per cent value of that crest value

and it has been estimated that the time varies from 10 to 100 µsec.

17. What are the techniques to be adopted for controlling the switching over

voltages?

Installation of shunt reactors.

Use of pre insertion resistor.

Synchronous reclosing and simultaneous operation of CB at both end.

Use of surge arrester and elimination of trapped charges by line

shunting after opening by means of earthling switch.

Use of surge absorber or resistance switching.

18. Define indirect stroke.

In indirect stroke a negative charge in a cloud cause bound positive charges

on the conductor of a nearby transmission line.

19. What are the causes of over voltages in power system?

The causes of over voltages in power system may be internal cause or external cause.

Internal causes of over voltages are

a) Switching Transients

b) Arcing ground

c) Insulation failure

d) Resonance

External cause for over voltages are Lightning.


20. Name the sources of switching surges.

Opening and closing of switchgears.

In circuit breaker operation, switching surges with a high rate of rise voltage

may cause repeated restriking of the arc between the contacts of the CB.

High natural frequencies of the system.

Damped normal frequency voltage components.

Restriking and recovery voltage with successive reflected waves from

terminations.

UNIT 2 - DIELECTRIC BREAKDOWN

1. What are the properties required for gaseous di-electric? (A/M 2017)

High dielectric strength

High thermal stability

Non – flammability

Low toxicity

Low boiling point

Good heat transfer properties and

Low cost

2. What are commercial liquid dielectric. (A/M 2017)

The commercial liquids, oil are not chemically pure, normally consists of mixture of

organic molecules.

3. Name the secondary ionization process involves in gaseous dielectric breakdown?

(N/D 2015)

Electron emission due to Positive ion impact

Electron emission due to Photon

Electron emission due to Meta stable and neutral atoms

4. What is electro negative gas? Give its significance. (N/D 2015)

An electro negative gas is an electron attached gas in which long lived negative ions

(Parent / fragment) are produced.

Significant : Direct attached and dissociative attachments.

5. Define Paschen’s Law. (A/M 2017)

It is defined as the break down voltage which is directly proportional to the product of

gas pressure and the gap distance between them.

V = f (Pd)

6. Define Townsends first & second ionization coefficient. (A/M 2017)

Townsend‟s first ionization coefficient is the average number of ionizing collisions

made by an electron per centimeter travel in the direction of the field.

The Townsend‟s secondary ionization coefficient is defined as the net number of

secondary electrons produced per incident positive ion, photon, excited particle or

meta stable particle.


7. What is ionization by collision? (N/D 2016)

In the process of ionization by collision, a free electron collides with a neutral gas

molecule and gives rise to a new electron and a positive ion.

8. Define Gas Law. (N/D 2016)

It consists of many laws. The following laws are very important.

a)Charle‟s law P a T Volume is constant

b)Boyle‟s law P a 1/V T is constant

General equation: PV = nRT

Where

P-pressure ; V-Volume; n-number of moles; R-molar gas constant;

T-temperature.

9. Which insulation is used in high voltage circuit breakers of large power

rating?

Now a day‟s most of all circuit breakers that are in operation use SF6 gas or vacuum

as insulating medium. SF6 circuit breakers are manufactured up to the higher

transmission voltage of 800 kV and current range of 63 kA and 80 kA. However it is

an expensive gas and at the normal operating pressure of 6 bar it condenses at

temperatures lower than 20°C.

10. Define uniform and non uniform field and give examples of each.

In uniform field the applied field remains constant across the gap.

Example: The field between the two plane electrodes.

In non uniform field the applied field varies across the gap.

The examples are coaxial cylinders, point- plane and the sphereplane gaps.

11. What is Time lag in the breakdown of dielectrics? The time that elapses between the application of the voltage to a gap sufficient to

cause breakdown and actual breakdown is called time lag.

12. What is meant by Townsend discharge and explain its main feature?

When the voltage between anode and cathode is increased the current in the anode

equals the current in the external circuit. Therefore the current becomes infinitely

large under the above mentioned condition but practically it is limited by the

resistance of external circuit and practically by the voltage drop in the arc. The

condition Veαc

= 1 defines the condition for the beginning of spark and is known

Townsend criterion for spark formation or breakdown.

13. Distinguish between insulators and dielectrics and give examples for each.

A dielectric is a non conducting substance ie, an insulator. Although the dielectric

and insulator are generally considered synonymous the term dielectric is more

often used to describe the material where the dielectric polarization is important

like the insulating material between the metallic plates of a capacitor while insulator is m

ore often used when the material is being used to prevent a current flow across

it.

Examples of insulators: Glass and porcelain

Examples of dielectric: Paper and Mica

14. What is composite dielectric?

It is difficult to imagine complete insulation system in electrical equipment which

does not consist of more than one type of insulation. If insulation as a whole is

considered, it will be found that more than one insulating material is used. These


different materials can be in parallel with each other such as air or SF6 in parallel

with solid insulation or in series with one another. Such insulation systems are

called composite dielectrics.

15. Name some of the gaseous insulating media.

Nitrogen(N2)

Carbondioxide(CO2)

Freon(CCl2F2)

Sulphur Hexa fluride(SF6)

16. List out the factors that affect ionization.

Humidity, Pressure, Temperature, nature of applied voltage, imperfections into dielectric

electrode field configuration, nature of electrode surface initial condition of particles.

17. What are the drawbacks of Townsend mechanism?

The current growth occurs due to result of ionization process only. But

in practice breakdown voltage were found to depend on the gas pressure and

the geometry of the gap.

The mechanism predicts time lags up the order for 10-5 s, while in actual

practice breakdown was observed to occur at very short times of the order of

10-8s.

This predict a very differed from of discharge in a actual practice discharges were

found to the filamentary and irregular.

18. State the preferred properties of a gaseous dielectric for high voltage

applications.

High dielectric Strength

Thermal stability and chemical inactivity towards material of construction.

Non-flammability and physiological inertness.

Low temperature of condensation.

Good heat transfer

Ready availability at moderate cost 19. What are the different mechanisms for breakdown in vacuum?

Particle exchange mechanism

Field emission mechanism

Clump Theory. (Mechanism)

20. Give some examples of liquid dielectrics.

Petroleum oils (transformer) are the most commonly used liquid dielectrics.

Synthetic hydrocarbons and halogenated hydrocarbons are also used for certain

applications. For very high temperature application, silicone oils and fluorinated

hydrocarbons are employed.

UNIT – 3 GENERATION OF HIGH VOLTAGE & HIGH CURRENT

1. What is trigatron gap? (A/M 2017)

A trigatron gap consists of a high voltage spherical electrode gap of suitable size,

an earthed main electrode of spherical shape, and a trigger electrode through the main


electrode. Trigatron gap is polarity sensitive and a proper polarity pulse should be applied

for correct operation

2. A 12 stage impulse generator has 0.12µF capacitor. The wave front and wave tail

resistances connected are 400Ω and 600Ω respectively. If the load capacitor is

800pF, find the front and tail time of the impulse wave produced. (A/M 2017)

The generator capacitance C1 = 0.12 / 12 = 0.01 µF

Load capacitance C2 = 0.0008 µF

Resistance R1 = 400Ω , R2 = 600Ω

Time to front

1 21 1

1 2

6 6

6

3( )

(0.01 10 0.0008 10 )3 400

(0.01 0.0008) 10

12.96

C Ct R

C C

s

2 1 2 1 2

6

0.7( )( )

0.7(400 600)(0.01 0.0008) 10

7.56

timeto tail t R R C C

s

3. How is circuit inductance controlled and minimized in impulse current generator?

(N/D 2015)

If the series resistance R\ is increased, the wave front oscillations are damped,

but the peak value of the voltage is also reduced. Thereby circuit inductance can be

controlled and minimized in the impulse current generator. 4. Mention the specification of impulse current as per Indian standards. (N/D 2015)

Standard impulse have a rise time of 0.5 µ s and 10 µ s and decay time of 50 % of

peak value and of the order of 30 to 200 µ s.

5. What is tesla coil? (N/D 2016)

The commonly used high frequency resonant transformer is the Tesla coil. It is a d only

tuned resonant circuit. The primary and secondary windings are wound on an insulated former

with no core and are immersed in oil.

6. What is deltatron circuit? (N/D 2016)

Deltatron is a combination of cockcroft Walton type voltage multiplier

with cascaded transformer.

7. What are the classification of high voltages?

High DC voltages

High ac voltages of power frequency

High ac voltage of high frequency

High transient or impulse voltage of very short duration such as lightening over

voltages

Transient voltage of longer duration such a switching surges.

8. Give few applications of high d.c voltage.

Electron microscope, X-ray tubes, Electrostic precipitators, Particles accelerator, Testing


Power apparatus

9. Draw the circuit diagram of a simple voltage doubler.

10. A cockcroft –walton type voltage multiplier has eight stages with capacitances, all

voltage is 125kV at a frequency of 150Hz. If the load current to be supplied is

5MA. Find the optimum no.of stages for minimum voltage regulation.

11. What is the use of voltage doubler circuit?

Both full wave and half wave rectifier circuits produce a DC voltage less than the

rectifier maximum voltage. When higher DC voltages are required a voltage doubler or

cascaded rectifies doubler circuit is used.

12. What is the difference between electromagnetic and electrostatic machines?

In electromagnetic machines, current carrying conductors are moved in a

magnetic field so that mechanical energy is converted into electrical energy,. In

electrostatic machines charged bodies moved in an electric field against an

electrostatic field in order that mechanical energy is converted into electrical

energy. 13. Specify the circuits producing switching surges.

Impulse generator circuit modified to give longer duration wave shapes.

Power transformer or testing transformers excited by DC voltages giving oscillatory

wave.


14. What are the uses of high frequency high voltages?

High frequency high voltages are required for rectifiers Dc power

supplies. Also for testing electrical apparatus for switching surges, high frequency high voltage

damped oscillations are needed which need high voltage high frequency transformer.

15. Give some circuits that produce impulse waves.

16. What are the advantages of high frequency transformers?

(i) The absence of iron core in transformers and hence to saving in cost and size.

(ii) Pure sine wave output.

(iii) flow build up of voltage over a few cycles and hence no damage due to

switching surges etc.

(iv) Uniform distribution of voltage across the winding coils due to subdivision of coil

stack in a number of units.

17. What is the need for generating high impulse current?

Lighting discharges involves high impulse voltage and high impulse current

on transmission line protection gears like surge diverters here to discharge the

lighting currents without damage . Therefore generation of impulse current wakeful of

high ministered (100 KVA) Peak find application in test work as well as in basic

research on non linear resister, electric arc studies & studies reaching to electric plasma

in high current discharge.

18. Define creeping distance.

It is the shortest distance on the contour of the external surface of the insulator unit.

19. What is a surge diverter?

It is a non-linear resistor in series with a spark gap kept at line terminals in the

substations.

20. Mention the problems associated with bifilar strip design.

The shunt suffers from stray inductance associated with resistance element and its

potential leads are linked to a small part of the magnetic flux generated by the

current that is measured.


UNIT – 4 MEASUREMENT OF HIGH VOLTAGE

1. Explain the basic principle of Hall generator. (A/M 2017)

If an electric current flows through a metal plate located in a magnetic field

perpendicular 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 and e.m.f. in the normal direction called the Hall voltage.

2. List some advantages of Faraday generator. (A/M 2017)

No electric connection between the source and the device.

No thermal problems even for large currents of several kA

No insulation problem arise for EHV system, because signal transmission is

through optical system.

3. What are the drawbacks of series resistance micro-ammeter technique in HVAC

measurements? (N/D 2015)

Resistance variation with temperature.

Residual inductance of the resistance gives rise to impedance different from its

ohmic resistance.

4. How the stray effect of capacitance potential divider is minimized for impulse

measurements?

The effect to residual and lead inductances becomes pronounced when fast rising

impulses of less than one microsecond are to be measured. The residual inductances

damp and slow down the fast rising pulses. Secondly, the layout of the test objects, the

impulse generator, and the ground leads also require special attention to minimize

recording errors.

5. What are the different types of resistive shunt used for impulse and frequency

measurements? (A/M 2017)

Bifilar flat strip design

Co-axial tube or park‟s shunt design

Co-axial squirrel cage design.

6. What are the problems associated with measurement of very high impulse voltages?

(A/M 2017)

Large power dissipation

Large leakage current

Limitation of voltage stress per unit length

Change in resistance due to temperature variations, etc.

7. What are the advantages of generating voltmeters? (N/D 2016)

No source loading by the meter.

No direct connection to high voltage electrode.

Scale is linear and extension of the range is easy.


A very convenient instrument for electrostatic devices like vandal Graff

generator.

8. Name few methods of measuring high d.c voltages.

Series resistance micro ammeter.

Resistance potential divider.

Generating voltmeter.

Sphere gaps.

9. What are limitations in the series resistance ammeter?

Power dissipation and source loading.

Temperature effects and long-time stability

Voltage dependence or resistive elements

Sensitivity or mechanical stresses.

10. What is CVT? And state its advantages.

Capacitance divider with a suitable isolating, potential transformer turned for

resonance can be used for voltage measurement. These are called CVT (Capacitance

Voltage Transformer).

Capacitance divider requires high impedance meters like electrostatic voltmeter

for measurement where as a CVT requires low impedance device like watt meter pressure

coil for measurement.

11. What is Rogowski coil?

Rogowski coil also known as magnetic potentiometer is a toroidal shape coil

wound on a non-magnetic former and is placed coaxially surrounding a current carrying

conductor for measurement of high impulse current.

12. Name the various factors influencing the spark over voltage of sphere gaps.

Nearby earthed objects.

Atmospheric conditions and humidity.

Irradiation.

Polarity and rise time of voltage waveforms.

13. What are the general methods used for measurement of high frequency and impulse

currents? The general methods used for measurement of high Frequency and impulse currents are

Resistive shunts

Rogowski coil and magnetic links

Hall generators

Faraday generator.

14. Why inductors are normally not used as voltage dividing elements?

Inductors are normally not used as voltage dividing elements as pure induction of

proper magnitudes without stray capacitance cannot we built.

Inductors would form oscillatory circuit with the inherent capacitance of the test

object hence leading to inaccuracy in measurement.


15. Define “Skin depth” as applied to resistive shunts.

Skin depth „d‟ is defined as the distance or depth from the surface at which the

magnetic field intensity is reduced to 1/2.718 times of the surface value for a given

frequency.

16. What is a delay cable?

The delay cable is a screened lead used to transmit samples of the impulse voltage

wave tapped across the low voltage CRO. The delay cable is essentially used to delay the

arrival of the sample wave at the CRO until the electron beam is in a position to start

tracing.

17. What are the requirements of a sphere gap for measurement of high voltage?

Both sphere should be arranged symmetrically at high

With one sphere grounded.

Two spheres must be of identical in size and shape.

The spacing should be proper.

Their surface must be smooth & with uniform curvature.

18. Give the expression for velocity of impulse signal in a impulse measurement system

using coaxial signal cable.

r

CV

llr

When C = 3 x 10 m/s: Velocity of light.

r = Relative permittivity

r = Relative permeability

19. What are the requirements of an oscillograph for impulse and high Frequency

measurements in high voltage test circuit?

When the test voltage is of less magnitude & of short duration it is easy to

measure by CRO.

It is easy to record / captures a rapidly changing signal.

20. Why are capacitance voltage dividers preferred for high a.c voltage (HVAC)

measurement?

These are portable

The errors due to harmonic voltage can we avoided by use of capacitive voltage

divider.

UNIT – 5 HIGH VOLTAGE TESTING & INSULATION COORDINATION

1. What are the different testes conducted on insulators? (A/M 2017)

The power frequency tests

Impulse tests

2. What are type and routine tests? (A/M 2017)

Type Test :Type tests are intendedto prove the design features and quality.

Type tests are done on samples when new designs or design changes are introduced.


Routine Test :The routine tests are intended to checkthe quality of the individual test

piece.

Routine tests are doneon each and every finished piece of equipment.

3. Distinguished between flash over and puncture. (N/D 2015)

Flash over :When a discharge takes place between two electrodes in a gas or a liquid

or over a sold surface, it is called flashover.

Puncture :The discharge taking place through a solid insulation is called puncture.

This create a permanent loss of strength.

4. Define safety margin as applied to insulation co-ordination. (N/D 2015)

The difference between the two volt ampere curves corresponding to protection

devices and the device to be protected, provides the safety margin for the insulation

system.

5. List out the various electrical tests to be carried out for bushings. (A/M 2017)

Power frequency tests

Impulse tests

Thermal tests

6. Define : Air density correction factor. (A/M 2017)

The electrical characteristics of the insulators and other apparatus are normally

referred to the reference atmospheric conditions. According to Indian Standard

Specification,

Temperature : 27o C

Pressure : 1013 millibars (or 760 torr)

Absolute humidity : 17g/m3

7. Define 50 % flash over voltage. (N/D 2016)

The voltage which has a probability of 50% flash over, when applied to a test object.

This is normally applied in impulse test in which the loss of insulation strength is

temporary.

8. What are the testes needed to be conducted on power transformers? (N/D 2016)

Induced over voltage test

Partial discharge test

Impulse testing.

9. Define disruptive discharge voltage.

Disruptive discharge voltage is defined as the voltage which produces the loss of

dielectric strength of an insulation. It is that voltage at which electrical stress in the

insulation causes a failure which induces the collapse of voltages passes of current.

10. What are the different electrical tests done on isolator’s circuit breakers?

The dielectric testy or Overvoltage test.

The temperature rise tests.

The mechanical test.


The short circuit test.

11. Draw the direct testing diagram of a short circuit testing laboratory.

12. What are the different tests conducted on cables?

Mechanical tests like bending test dripping & drainage test and Fire resistant test &

corrosion test.

Thermal duty test.

Dielectric power Factor test.

Impulse withstand voltage test.

Partial discharge test.

Life expectancy test.

13. Name few test done on surge diverters.

Power Frequency spark over test.

Hundred Percent standard impulse spark over test.

Front of wave spark over test.

Residual voltage test.

Long duration impulse current test.

Operating duty tests.

14. Define RIV and RI

The noise generated in the ratio Frequency band as a result of Corona or Partial discharge

in high voltage power apparatus can be measured.

By the radio frequency line to ground voltage known as the radio influence voltage or

RIV and

As an interfering field by means of an antenna known as the radiated radio interference

voltage or RI.

15. What is the purpose of conducting impulse test on transformers?

The purpose of the impulse tests is to determine the ability of the insulation of the

transformers to withstand the transient voltage due to lightning etc.


16. Under what test results a surge diverter is said to have passed the high current

impulse test?

The power frequency spark over voltage before and after the test does not differ by more

than 10%

The voltage and current wave forms of the diverter do not differ significantly in the two

applications.

The non-linear resistance element in the diverter do not show any sign of puncture or

external flashover.

17. What do you mean by withstand Level of Apparatus / Equipment? The value of standard test wave (Power Frequency / or impulse) which the

Equipment / Apparatus is assigned to withstand under specified test conditions as per IS:

2099 :& IS : 731.

18. Why temperature rise and thermal stability tests are conducted on bushings?

The purpose of these tests are to ensure that the bushing in service for long does not

have an excessive temperature rise and also does not go in the “thermal runaway”

condition of the insulation used.

19. State the advantages of testing the circuit breaker in the network directly.

The circuit breaker is tested under actual conditions like those that occur in a given

network.

Special occasions like breaking of charging currents of long lines. Very short line

faults, interruption of small inductive currents, etc., can be tested by testing only.

20. Distinguish between functions of isolator and circuit breaker.

Isolator: Isolator is a mechanical switching device which is capable of opening and closing

a circuit with a small current.

Circuit breaker: Circuit breaker is a automatic switching device which is capable of opening

and closing a circuit with high current.

Vel Tech High Tech Dr.Rangarajan Dr.Sakunthala Engg. College EE6701 – High Voltage Engineering

Prepared by Prabaakaran K AP/EEE Page No.1

UNIT 2 - DIELECTRIC BREAKDOWN

1. What are the properties required for gaseous di-electric? (A/M 2017)

High dielectric strength

High thermal stability

Non – flammability

Low toxicity

Low boiling point

Good heat transfer properties and

Low cost

2. What are commercial liquid dielectric. (A/M 2017)

The commercial liquids, oil are not chemically pure, normally consist s of mixture of organic

molecules.

3. Name the secondary ionization process involves in gaseous dielectric breakdown? (N/D 2015)

Electron emission due to Positive ion impact

Electron emission due to Photon

Electron emission due to Meta stable and neutral atoms

4. What is electro negative gas? Give its significance. (N/D 2017,2015)

An electro negative gas is an electron attached gas in which long lived negative ions

(Parent / fragment) are produced.

Significant : Direct attached and dissociative attachments.

5. Define Paschen’s Law. (A/M 2018, 2017)

It is defined as the break down voltage which is directly proportional to the product of gas pressure and

the gap distance between them.

V = f (Pd)

6. Define Townsends first & second ionization coefficient. (A/M 2017)

Townsend‟s first ionization coefficient is the average number of ionizing collisions made by an

electron per centimeter travel in the direction of the field.

The Townsend‟s secondary ionization coefficient is defined as the net number of secondary

electrons produced per incident positive ion, photon, excited particle or meta stable particle.

7. What is ionization by collision? (N/D 2016) In the process of ionization by collisio n, a free electron collides wit h a neutral gas molecule and gives rise to a new electron and a positive ion.

8. Define Gas Law. (N/D 2016) It consists of many laws. The following laws are very important. a)Charle‟s law P a T Volume is constant b)Boyle‟s law P a 1/V T is constant General equation: PV = nRT

Where

P-pressure ; V-Volume; n-number of moles; R-molar gas constant; T-temperature.



9. Which insulation is used in high voltage circuit breakers of large power rating?

Now a day‟s most of all circuit breakers that are in operation use SF6 gas or vacuum as insulating medium.

SF6 circuit breakers are manufactured up to the higher transmission voltage of 800 kV and current range of

63 kA and 80 kA. However it is an expensive gas and at the normal operating pressure of 6 bar it condenses

at temperatures lower than 20°C.

10. Define uniform and non uniform field and give examples of each. A/M 2018

In uniform field the applied field remains constant across the gap.

Example: The field between the two plane electrodes.

In non uniform field the applied field varies across the gap.

The examples are coaxial cylinders, point- plane and the sphereplane gaps.

11. What is Time lag in the breakdown of dielectrics? The time that elapses between the application of the voltage to a gap sufficient to cause breakdown and actual breakdown is called time lag.

12. What is meant by Townsend discharge and explain its main feature? When the voltage between anode and cathode is increased the current in the anode equals the current in the external circuit. Therefore the current becomes infinitely large under the above mentioned condition but practically it is limited by the resistance of external circuit and practically by the voltage drop in the arc. The condition Ve

αc = 1 defines the condition for the beginning of spark and is known Townsend criterion for spark

formation or breakdown.

13. Distinguish between insulators and dielectrics and give examples for each. A dielectric is a non conducting substance ie, an insulator. Although the dielectric and insulator are generally considered synonymous the term dielectric is more often used to describe the material where the dielectric polarization is important like the insulating material between the metallic plates of a capacitor while insulator is m ore often used when the material is being used to prevent a current flow across it.

Examples of insulators: Glass and porcelain Examples of dielectric: Paper and Mica

14. What is composite dielectric? It is difficult to imagine complete insulation system in electrical equipment which does not consist of more than one type of insulation. If insulation as a whole is considered, it will be found that more than one insulating material is used. These different materials can be in parallel with each other such as air or SF6 in parallel with solid insulation or in series with one another. Such insulation systems are called composite dielectrics.

15. Name some of the gaseous insulating media.

Nitrogen(N2)

Carbondioxide(CO2)

Freon(CCl2F2)

Sulphur Hexa fluride(SF6)

16. List out the factors that affect ionization.

Humidity, Pressure, Temperature, nature of applied voltage, imperfections into dielectric electrode field

configuration, nature of electrode surface initial condition of particles.



17. What are the drawbacks of Townsend mechanism?

The current growth occurs due to result of ionization process only. But in practice

breakdown voltage were found to depend on the gas pressure and the geometry of the gap.

The mechanism predicts time lags up the order for 10-5 s, while in actual practice breakdown

was observed to occur at very short times of the order of

10-8s.

This predict a very differed from of discharge in a actual practice discharges were found to the

filamentary and irregular.

18. State the preferred properties of a gaseous dielectric for high voltage applications.

High dielectric Strength

Thermal stabilit y and chemical inactivity towards material of construction.

Non-flammability and physiological inertness.

Low temperature of condensation.

Good heat transfer

Ready availability at moderate cost

19. What are the different mechanisms for breakdown in vacuum?

Particle exchange mechanism

Field emission mechanism

Clump Theory. (Mechanism)

20. Give some examples of liquid dielectrics.

Petroleum oils (transformer) are the most commonly used liquid dielectrics. Synthetic hydrocarbons and

halogenated hydrocarbons are also used for certain applications. For very high temperature application,

silicone oils and fluorinated hydrocarbons are employed.

PART – B

1. Explain the breakdown mechanism involving in solid dielectrics breakdown. A/M 2018

Electronic Breakdown :



Electromechanical Breakdown



2. Explain the townsends criterion for a spark and list out problems caused by corona discharge.

A/M 2018



3. Explain in details about the various mechanisms of breakdown in vacuum. N/D 2017

In the Townsend type of discharge in a gas described earlier, electrons get

multiplied due to various ionization processes and an electron avalanche is formed. In a high

vacuum, even if the electrodes are separated by, say, a few centimeters, an electron crosses

the gap without encountering any collisions. Therefore, the current growth prior to

breakdown cannot be due to the formation of electron avalanches. However, of a gas is

liberated in the vacuum gap, then, breakdown mechanisms in high vacuum aim at

establishing the way in which the liberation of gas can be brought about in a vacuum gap.

During the last 70 years or so, many different mechanisms for breakdown in vacuum

have been proposed. These can be broadly divided into three categories

(a) Particle exchange mechanism

(b) Field emission mechanism

(c) Clump theory

(a) Particle Exchange Mechanism:

In this mechanism it is assumed that a charged particle would be emitted from

one electrode under the action of the high electric field, and when it impinges on the other

electrode, it liberties oppositely charged particles. These particles are accelerated by the



applied voltage back to the first electrode where they release more of the original type of

particles. When this process becomes cumulative, a chain reaction occurs which leads to the

breakdown of the gap.

The particle –exchange mechanism involves electrons, positive ions, photons and the

absorbed gases at the electrode surfaces. Qualitatively, an electron present in the vacuum

gap is accelerated towards the anode, and on impact releases A positive ions and C photons.

These positive ions are accelerated towards the cathode, and on impact each positive ion

liberates B electrons and each photon liberates D electron. This is shown schematically

in Fig 2.24. The breakdown will occur if the coefficients of production of secondary

electrons exceed unity.

Mathematically, the condition for breakdown can be written as

(AB+CD)>1

Later, Trump and Van de Graff measured these coefficients and showed that they

were too small for this process to take place. Accordingly, this theory was modified to

allow for the presence of negative ions and the criterion for breakdown then becomes

(AB+EF)>1

Where A and B are the same as before and E and F represent the coefficients for

negative and positive ion

liberation by positive and negative ions. It was experimentally found that the values

of the product EF were close enough to unity for copper, aluminum and stainless steel

electrodes to make this mechanism applicable at voltages above 250kV.

b) Field Emission Theory

(i)Anode Heating Mechanism

This theory postulates that electron produced at small micro-projections on the

cathode due to field emission bombard the anode causing a local rise in temperature and

release gases and vapors into the vacuum gap. These electrons ionize the atoms of the gas



and produce positive ions. These positive ions arrive at the cathode, increase the primary

electron emission due to space charge formation and produce secondary electrons by

bombarding the surface. The process continues until a sufficient number of electrons are

produced to give rise to breakdown, as in the case of a low pressure Townsend type gas

discharge. This is shown schematically in Fig 2.25

(ii) Cathode Heating Mechanism

This mechanism postulates that near the breakdown voltages of the gap, sharp

points on the cathode surface are responsible for the existence of the pre-breakdown

current, which is generated according to the field emission process described below.

This current causes resistive heating at the tip of a point and when a critical current

density is reached, the tip melts and explodes, thus initiating vacuum discharge. This

mechanism is called field emission as shown schematically in fig. 2.26. Thus, the initiation

of breakdown depends on the conditions and the properties of the cathode surface.

Experimental evidence shows that breakdown takes place by this process when the

effective cathode electric field is of the order of

106 to 107 V/cm.

(c) Clump Mechanism

Basically this theory has been developed on the following assumptions (Fig. 2.27):

(i) A loosely bound particle (clump) exists on one of the electrode surfaces

(ii) On the application of a high voltage, this particle gets charged, subsequently

get s charged, subsequently gets detached from the mother electrode, and is accelerated across the gap.

(iii) The breakdown occurs due to a discharge in the vapour or gas released by the

impact of the particle at the target electrode.



Cranberg was the first to propose this theory. He initially assumed that breakdown

will occur when the energy per unit area, W, delivered to the target electrode by a clump

exceeds a value C’, a constant, characteristic of a given pair of electrodes. The quantity

W is the product of gap voltage (V) and the charge density on the clump. The latter is

proportional to the electric field E at the electrode of origin. The criterion for breakdown,

therefore, is

VE = C’

In case of parallel plane electrodes that field E = V/d, where d is the distance between

the electrodes. So the generalized criterion for breakdown becomes

V = (Cd)1/2 (2.35)

Where C is another constant involving C’ and the electrode surface conditions.

Cranberg presented a summary of the experimental results which satisfied this

breakdown criterion with reasonable accuracy. He stated that the origin of the clump was

the cathode and obtained a value for the constant C as 60 x 1010 V2/cm (for iron particles).

However the equation was later modified as V = C d, where varies between 0.2 and 1.2

depending on the gap length and the electrode material, with a maximum at 0.6. The

dependence of V on the electrode material, comes from the observations of markings on the

electrode surfaces. Craters were observed on the anode and melted regions on the cathode or

vice-versa after a single breakdown.


Prepared by Prabaakaran K AP/EEE Page

No.10

4. Explain the various theories of breakdown mechanism of commercial liquid dielectrics. N/D 2017

The breakdown mechanism is commercial liquids is dependent, as seen above, on

several factors, such as, the nature and condition of the electrodes, the physical properties of the

liquid, and the impurities and gases present in the liquid. Several theories have been proposed to

explain the breakdown in liquids, and they are classified as follows:

a) Suspended Particle Mechanism

b) Cavitation and Bubble Mechanism

(c) Stressed Oil Volume Mechanism

Suspended Particle Theory

In commercial liquids, the presence of solid impurities cannot be avoided. These impurities

will be present as fibres or as dispersed solid particles. The permittivity of these particles ( 2) will

be different from the permittivity of the liquid (1). If we consider these impurities to be spherical

particles of radius r, and if the applied field is E, then the particles experience a force F, where

This force is directed towards areas of maximum stress, if 2>1, for example, in the case of the

presence of solid particles like paper in the liquid. On the other hand, if only gas bubbles are

present in the liquid i.e. 2<1, the force will be in the direction of areas of lower stress. If the

voltage is continuously applied (d.c.) or the duration of the voltage is long (a.c.). then this force

drives the particles towards the areas of maximum stress. If the number of particles present are

large, they becomes aligned due to these forces, and thus form a stable chain bridging the

electrode gap causing a breakdown between the electrodes.

If there is only a single conducting particle between the electrodes, it will give rise to local field

enhancement depending on its shape. If this field exceeds the breakdown strength of the liquid,

local breakdown will occur near the particle, and this will result in the formation of gas bubbles

which may lead to the breakdown on the liquid.

The values of the breakdown strength of liquids containing solid impurities was found to be

much less than the values for pure liquids. The impurity particles reduce the breakdown

strength, and it was also observed that the larger the size of the particles the lower were the

breakdown strengths.

b. Cavitation and the Bubble Theory

It was experimentally observed that in many liquids, the breakdown strength depends strongly on the applied hydrostatic pressure, suggesting that a change of phase of the medium is involved in the breakdown process, which in other words means that a kind of vapour bubble formed is responsible for breakdown. The following processes have been suggested to be responsible for the formation of the vapour bubbles:

(a) Gas pockets at the surfaces of the electrodes:

(b) Electrostatic repulsive forces between space charges which may be

sufficient to overcome the surface tension;



No.11

(c) Gaseous products due to the dissociation of liquid molecules by electron collisions;

and

(d) Vapourization of the liquid by corona type discharge from sharp points

and irregularities on the electrode surfaces.

Once a bubble is formed it will elongate in the direction of the electric field under the

influence of electrostatic forces. The volume of the bubble remains constant during

elongation. Breakdown occurs when the voltage drop along the length of the bubble

becomes equal to the minimum value on the Paschen’s curve (see Chapter 2, sec. 2.10) for the

gas in the bubble. The breakdown field is given as

5. Obtain the relationship between sparking potential and pd values Paschen’s Law. A/M 2017



No.12

6. Explain the breakdown mechanism involving in composite solid dielectrics breakdown. A/M 2016



No.13

UNIT – 3 GENERATION OF HIGH VOLTAGE & HIGH CURRENT

1. What is trigatron gap? (A/M 2017)

A trigatron gap consists of a high voltage spherical electrode gap of suitable size, an earthed main electrode

of spherical shape, and a trigger electrode through the main electrode. Trigatron gap is polarity sensitive

and a proper polarity pulse should be applied for correct operation.

2. A 12 stage impulse generator has 0.12µF capacitor. The wave front and wave tail resistances

connected are 400Ω and 600Ω respectively. If the load capacitor is 800pF, find the front and tail

time of the impulse wave produced. (A/M 2017)

The generator capacitance C1 = 0.12 / 12 = 0.01 µF

Load capacitance C2 = 0.0008 µF

Resistance R1 = 400Ω , R2 = 600Ω

Time to front

1 21 1

1 2

6 6

6

3( )

(0.01 10 0.0008 10 )3 400

(0.01 0.0008) 10

12.96

C Ct R

C C

s

2 1 2 1 2

6

0.7( )( )

0.7(400 600)(0.01 0.0008) 10

7.56

timetotail t R R C C

s

3. How is circuit inductance controlled and minimized in impulse current generator? (N/D 2015)

If the series resistance R\ is increased, the wave front oscillations are damped,

but the peak value of the voltage is also reduced. Thereby circuit inductance can be controlled and

minimized in the impulse current generator.

4. Mention the specification of impulse current as per Indian standards. (N/D 2015)

Standard impulse have a rise time of 0.5 µ s and 10 µ s and decay time of 50 % of peak value and of the

order of 30 to 200 µ s.



No.14

5. What is tesla coil? (A/M 2018 ,N/D 2016)

The commonly used high frequency resonant transformer is the Tesla coil. It is a d only tuned resonant

circuit. The primary and secondary windings are wound on an insulated former with no core and are

immersed in oil.

6. What is deltatron circuit? (N/D 2016)

Deltatron is a combination of cockcroft Walton type voltage multiplier with cascaded

transformer.

7. What are the classification of high voltages?

High DC voltages

High ac voltages of power frequency

High ac voltage of high frequency

High transient or impulse voltage of very short duration such as lightening over voltages

Transient voltage of longer duration such a switching surges.

8. Give few applications of high d.c voltage.

Electron microscope, X-ray tubes, Electrostic precipitators, Particles accelerator, Testing Power apparatus

9. Draw the circuit diagram of a simple voltage doubler.

10. A cockcroft –walton type voltage multiplier has eight stages with capacitances, all voltage is

125kV at a frequency of 150Hz. If the load current to be supplied is 5MA. Find the optimum

no.of stages for minimum voltage regulation.

11. What is the use of voltage doubler circuit? Both full wave and half wave rectifier circuits produce a DC voltage less than the rectifier maximum



No.15

voltage. When higher DC voltages are required a voltage doubler or cascaded rectifies doubler circuit

is used.

12. What is the difference between electromagnetic and electrostatic machines?

In electromagnetic machines, current carrying conductors are moved in a magnetic field so that mechanical energy is converted into electrical energy,. In electrostatic machines

charged bodies moved in an electric field against an electrostatic field in order that mechanical

energy is converted into electrical energy.

13. Specify the circuits producing switching surges.

Impulse generator circuit modified to give lo nger duration wave shapes.

Power transformer or testing transformers excited by DC voltages giving oscillatory wave.

14. What are the uses of high frequency high voltages?

High frequency high voltages are required for rectifiers Dc power supplies. Also for testing electrical

apparatus for switching surges, high frequency high voltage damped oscillations are needed which need high

voltage high frequency transformer.

15. Give some circuits that produce impulse waves.

16. What are the advantages of high frequency transformers?

(i) The absence of iron core in transformers and hence to saving in cost and size. (ii) Pure sine wave output.

(iii) flow build up of voltage over a few cycles and hence no damage due to switching surges

etc.

(iv) Uniform distribut ion of voltage across the winding coils due to subdivision of coil stack in a number of units.

17. What is the need for generating high impulse current?

Lighting discharges involves high impulse voltage and high impulse current on transmission

line protection gears like surge diverters here to discharge the lighting currents without damage .

Therefore generation of impulse current wakeful of high ministered (100 KVA) Peak find application

in test work as well as in basic research on non linear resister, electric arc studies & studies reaching

to electric plasma in high current discharge.



No.16

18. Define creeping distance.

It is the shortest distance on the contour of the external surface of the insulator unit.

19. What is a surge diverter? It is a non-linear resistor in series with a spark gap kept at line terminals in the substations.

20. Mention the problems associated with bifilar strip design. The shunt suffers from stray inductance associated with resistance element and its potential leads are

linked to a small part of the magnetic flux generated by the current that is measured.

PART – B

1. Describe the construction and working principle of Van de Graff generator with a neat sketch.

A/M 2018



No.17

2. What is Tesla coil? How is damped high frequency oscillations obtained from a Tesla Coil?

N/D 2017



No.18

3. Explain in detail about tripping & control of impulse generator. N/D 2016



No.19



No.20

4. Explain with neat sketches cockroft-walton voltage multiplier circuit. Explain clearly its operation

when the circuit is i unloaded and ii loaded. A/M 2017



No.21



No.22

5. Describe the construction and principle of operation of a multistage Marx Generator. A/M 2018



No.23



No.24



No.25

6. A cockcroft Walton type voltage multiplier has eight stages with capacitances, all equal to 0.05µF.

The supply transformer secondary voltage is 125kV at a frequency of 150Hz. If the load current to

be supplied is 5mA. Find i the % ripple, ii the regulation. A/M 2016



No.26



UNIT – 4 MEASUREMENT OF HIGH VOLTAGE

1. How the stray effect can be reduced in resistive shunt type measurement? (A/M 2018)

It can be reduced by employing low ohmic pure resistive shunt element in circuit. The current through

resistive element R produces a voltage drop v(t) = i(t) R.

2. What are the advantages of CVT measurement in HVAC? (N/D 2017)

Simple design and easy installation

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.

Frequency independent voltage distribution along elements as against conventional magnetic

potential transformers which require additional insulation design against surges.

Provides isolation between the high voltage terminal and low voltage metering.

3. Explain the basic principle of Hall generator. (A/M 2018, 2017)

If an electric current flows through a metal plate located in a magnetic field perpendicular 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 and e.m.f. in the normal direction

called the Hall voltage.

4. List some advantages of Faraday generator. (A/M 2017)

No electric connection between the source and the device.

No thermal problems even for large currents of several kA

No insulation problem arise for EHV system, because signal transmission is through optical

system.

5. What are the drawbacks of series resistance micro-ammeter technique in HVAC measurements?

(N/D 2015)

Resistance variation with temperature.

Residual inductance of the resistance gives rise to impedance different from its ohmic resistance.

6. How the stray effect of capacitance potential divider is minimized for impulse measurements?

The effect to residual and lead inductances becomes pronounced when fast rising impulses of less than

one microsecond are to be measured. The residual inductances damp and slow down the fast rising pulses.

Secondly, the layout of the test objects, the impulse generator, and the ground leads also require special

attention to minimize recording errors.

7. What are the different types of resistive shunt used for impulse and frequency measurements? (N/D

2017, A/M 2017)

Bifilar flat strip design

Co-axial tube or park’s shunt design

Co-axial squirrel cage design.

8. What are the problems associated with measurement of very high impulse voltages? (A/M 2017)

Large power dissipation

Large leakage current

Limitation of voltage stress per unit length

Change in resistance due to temperature variations, etc.

9. What are the advantages of generating voltmeters? (N/D 2016)

No source loading by the meter.

No direct connection to high voltage electrode.

Scale is linear and extension of the range is easy and



A very convenient instrument for electrostatic devices like vandal Graff generator.

10. Name few methods of measuring high d.c voltages.

Series resistance micro ammeter.

Resistance potential divider.

Generating voltmeter.

Sphere gaps.

11. What are limitations in the series resistance ammeter?

Power dissipation and source loading.

Temperature effects and long-time stability

Voltage dependence or resistive elements

Sensitivity or mechanical stresses.

12. What is CVT? And state its advantages.

Capacitance divider with a suitable isolating, potential transformer turned for resonance can be

used for voltage measurement. These are called CVT (Capacitance Voltage Transformer).

Capacitance divider requires high impedance meters like electrostatic voltmeter for measurement

where as a CVT requires low impedance device like watt meter pressure coil for measurement.

13. What is Rogowski coil?

Rogowski coil also known as magnetic potentiometer is a toroidal shape coil wound on a non-

magnetic former and is placed coaxially surrounding a current carrying conductor for measurement of

high impulse current.

14. Name the various factors influencing the spark over voltage of sphere gaps.

Nearby earthed objects.

Atmospheric conditions and humidity.

Irradiation.

Polarity and rise time of voltage waveforms.

15. What are the general methods used for measurement of high frequency and impulse currents?

The general methods used for measurement of high Frequency and impulse currents are

Resistive shunts

Rogowski coil and magnetic links

Hall generators

Faraday generator.

16. Why inductors are normally not used as voltage dividing elements?

Inductors are normally not used as voltage dividing elements as pure induction of proper

magnitudes without stray capacitance cannot we built.

Inductors would form oscillatory circuit with the inherent capacitance of the test object hence

leading to inaccuracy in measurement.

17. Define “Skin depth” as applied to resistive shunts.

Skin depth ‘d’ is defined as the distance or depth from the surface at which the magnetic field

intensity is reduced to 1/2.718 times of the surface value for a given frequency.

18. What is a delay cable?

The delay cable is a screened lead used to transmit samples of the impulse voltage wave tapped

across the low voltage CRO. The delay cable is essentially used to delay the arrival of the sample wave

at the CRO until the electron beam is in a position to start tracing.



19. What are the requirements of a sphere gap for measurement of high voltage?

Both sphere should be arranged symmetrically at high

With one sphere grounded.

Two spheres must be of identical in size and shape.

The spacing should be proper.

Their surface must be smooth & with uniform curvature.

20. Give the expression for velocity of impulse signal in a impulse measurement system using coaxial

signal cable.

r

CV

llr

When C = 3 x 10 m/s: Velocity of light.

r = Relative permittivity

r = Relative permeability

21. What are the requirements of an oscillograph for impulse and high Frequency measurements in

high voltage test circuit?

When the test voltage is of less magnitude & of short duration it is easy to measure by CRO.

It is easy to record / captures a rapidly changing signal.

22. Why are capacitance voltage dividers preferred for high a.c voltage (HVAC) measurement?

These are portable

The errors due to harmonic voltage can we avoided by use of capacitive voltage divider.

PART – B

1. Explain any two methods to measure high impulse current. (A/M 2018)



2. Explain the construction features and operation of generating type voltmeter. (N/D 2017)



3. Explain the operation of Electrostatic voltmeter with neat sketch and give its advantages and

limitations. (N/D 2017)



4. Explain the sphere gap measurement for peak voltage. (A/M 2017)



5. A coaxial shunt is to be designed to measure an impulse current of 50kA. If the bandwidth of the

shunt is to be at least 10MHz and if the voltage drop across the shunt should not exceed 50V, find

the ohmic value of the shunt and its dimensions. (A/M 2017)



UNIT – 5 HIGH VOLTAGE TESTING & INSULATION COORDINATION

1. What is meant by insulation co-ordination? A/M 2018, N/D 2017

In a few more words, Insulation Coordination is a series of steps used to select the dielectric strength of

equipment in relation to the operating voltages and transient overvoltages which can appear on the system

for which the equipment is intended.

2. What are the different testes conducted on insulators? (A/M 2017)

The power frequency tests

Impulse tests

3. What are type and routine tests? (A/M 2017)

Type Test :Type tests are intendedto prove the design features and quality.

Type tests are done on samples when new designs or design changes are introduced.

Routine Test :The routine tests are intended to checkthe quality of the individual test piece.

Routine tests are doneon each and every finished piece of equipment.

4. Distinguished between flash over and puncture. (N/D 2015)

Flash over :When a discharge takes place between two electrodes in a gas or a liquid or over a sold

surface, it is called flashover.

Puncture :The discharge taking place through a solid insulation is called puncture. This create a

permanent loss of strength.

5. Define safety margin as applied to insulation co-ordination. (A/M 2018, N/D 2015)

The difference between the two volt ampere curves corresponding to protection devices and the

device to be protected, provides the safety margin for the insulation system.

6. List out the various electrical tests to be carried out for bushings. (A/M 2017)

Power frequency tests

Impulse tests

Thermal tests

7. Define : Air density correction factor. (A/M 2017)

The electrical characteristics of the insulators and other apparatus are normally referred to the

reference atmospheric conditions. According to Indian Standard Specification,

Temperature : 27o C

Pressure : 1013 millibars (or 760 torr)

Absolute humidity : 17g/m3

8. Define 50 % flash over voltage. (N/D 2016)

The voltage which has a probability of 50% flash over, when applied to a test object.

This is normally applied in impulse test in which the loss of insulation strength is temporary.

9. What are the testes needed to be conducted on power transformers? (N/D 2016)

Induced over voltage test

Partial discharge test

Impulse testing.

10. Define disruptive discharge voltage. N/D 2017

Disruptive discharge voltage is defined as the voltage which produces the loss of dielectric strength of an

insulation. It is that voltage at which electrical stress in the insulation causes a failure which induces the

collapse of voltages passes of current.



11. What are the different electrical tests done on isolator’s circuit breakers?

The dielectric testy or Overvoltage test.

The temperature rise tests.

The mechanical test.

The short circuit test.

12. Draw the direct testing diagram of a short circuit testing laboratory.

13. What are the different tests conducted on cables?

Mechanical tests like bending test dripping & drainage test and Fire resistant test & corrosion test.

Thermal duty test.

Dielectric power Factor test.

Impulse withstand voltage test.

Partial discharge test.

Life expectancy test.

14. Name few test done on surge diverters.

Power Frequency spark over test.

Hundred Percent standard impulse spark over test.

Front of wave spark over test.

Residual voltage test.

Long duration impulse current test.

Operating duty tests.

15. Define RIV and RI

The noise generated in the ratio Frequency band as a result of Corona or Partial discharge in high voltage

power apparatus can be measured.

By the radio frequency line to ground voltage known as the radio influence voltage or RIV and

As an interfering field by means of an antenna known as the radiated radio interference voltage or RI.

16. What is the purpose of conducting impulse test on transformers?

The purpose of the impulse tests is to determine the ability of the insulation of the transformers to

withstand the transient voltage due to lightning etc.



17. Under what test results a surge diverter is said to have passed the high current impulse test?

The power frequency spark over voltage before and after the test does not differ by more than 10%

The voltage and current wave forms of the diverter do not differ significantly in the two applications.

The non-linear resistance element in the diverter do not show any sign of puncture or external flashover.

18. What do you mean by withstand Level of Apparatus / Equipment?

The value of standard test wave (Power Frequency / or impulse) which the Equipment / Apparatus

is assigned to withstand under specified test conditions as per IS: 2099 :& IS : 731.

19. Why temperature rise and thermal stability tests are conducted on bushings?

The purpose of these tests are to ensure that the bushing in service for long does not have an excessive

temperature rise and also does not go in the “thermal runaway” condition of the insulation used.

20. State the advantages of testing the circuit breaker in the network directly.

The circuit breaker is tested under actual conditions like those that occur in a given network.

Special occasions like breaking of charging currents of long lines. Very short line faults, interruption

of small inductive currents, etc., can be tested by testing only.

21. Distinguish between functions of isolator and circuit breaker.

Isolator: Isolator is a mechanical switching device which is capable of opening and closing a circuit with a

small current.

Circuit breaker: Circuit breaker is a automatic switching device which is capable of opening and closing a

circuit with high current.

PART – B

1. Explain the impulse testing procedure for insulators. A/M 2018



2. Explain the different high voltage tests conducted on bushings. A/M 2018



3. Explain the direct and synthetic testing of isolators and circuit breakers in details. N/D 2017



4. Explain in detail about insulation coordination. N/D 2017

A gradation of system insulation and protective device operation is to be followed.

Substations contain transformers and switchgear with non-self-restoring insulation should be protected

against flashover

For other apparatus which contain self-restoring insulation may be allowed to flashover.

Lightning impulse withstand level known as Basic Insulation Level (BIL).Various equipment and their

component parts should have their BIL above the system protective level by a margin which is

determined with respect to air insulation.

For higher system voltages, switching surges are of higher magnitude compared to the lightning over

voltages.

The flashover voltage of a protective device is chosen such that it will not operate for switching

overvoltage and other power frequency and its harmonic overvoltages. BIL has to higher.

For EHV systems, Switching Impulse Level(SIL) should be assigned to each protective device.

5. Explain the impulse testing of high voltage transformer. What is the procedure adopted for

locating the failure? A/M 2017

unit 1 over voltage in electrical power systems 1. … · 2019-11-22 · or due to initiation or...

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