DEPARTMENT OF ELECTRICAL AND ELECTRONICS ENGINEERING
Subject name: High Voltage Engineering
1. State the different types of over voltages occurring in power system and
mention their magnitude.
i Switching an open ended line with
a )nfinite bus as source with trapped charges on line. ‐ .
b )nfinite bus as source with trapped charges on line. ‐ .
c Deenergizing an unfaulted line with a line to ground fault. – .
d Deenergizing an unfaulted line with a restrike in the circuit breaker – .
ii a Switching a kV line through an auto transformer kV / kV
b Switching a transformer terminated line. – .
c Series capacitor compensated line with % compensation. – .
d Series capacitor compensated line with shunt reactor compensation. ‐ .
iii (igh speed reclosing of the line after fault clearance. – .
2. Define indirect stroke.
)n indirect stroke a negative charge in a cloud cause bound positive charges on the conductor of a nearby transmission line.
3. What are the techniques to be adopted for controlling the switching over voltages?
a )nstallation of shunt reactors. b Use of pre insertion resistor. c Synchronous reclosing and simultaneous operation of CB at both end. d Use of surge arrester and elimination of trapped charges by line
shunting after opening by means of earthling switch. e Use of surge absorber or resistance switching.
4. What is a surge arrester?
Surge arrester is a revolutionary advanced surge protective device for power system. )t is constructed by a series connection of zinc oxide elements having a highly non linear resistance.
5. What are the causes of power frequency over voltages?
The causes for power frequency and harmonic over voltages in E(V and U(V systems are: Sudden load reflection loss of loads a Disconnection of inductive loads or connection of capacitive loads. b Ferranti effect. c Unsymmetrical faults. d Saturation in transformers, etc, e Tap charging operations.
6. 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. )nternal causes of over voltages are a Switching Transients b Arcing ground c )nsulation failure d Resonance External cause for over voltages are Lightning.
7. What are the different types of fault that may occur on power lines?
i Symmetrical faults ‐ Φ fault LLLG ii Unsymmetrical faults
a L‐G fault b L‐L fault c L‐L—G fault
8. Explain why a simple spark gap cannot offer full protection against over
voltages. There is no current limiting device provided so as to limit the current after spark over. (ence 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 characteristics depends on the atmospheric condition, polarity of wave and wave shape.
9. What is insulation co-ordination?
)nsulation co‐ordination means the correlation of the insulation of various equipments in a power system to be insulation of protective devices used for the protection of those equipments against over voltage.
10. Name the sources of switching surges.
a Opening and closing of switchgears. b )n 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. c (igh natural frequencies of the system. d Damped normal frequency voltage components. e Restriking and recovery voltage with successive reflected waves from terminations.
11. Define surge impedance of transmission line.
The characteristic impedance of a line is the surge impedance of transmission line. )t is given by Z = √R + jwL / G + jwC
12. Mention the different kinds of over voltages.
a Over voltage due to lightning. b Over voltage due to switching transients. c Over voltage due to arcing ground. d Over voltage due to insulation failure. e Over voltage due to resonance.
13. What is meant by switching surges? Mention the approximate magnitude of
switching surges and their frequency. The switching voltage surges, occur during and closing of unload E(V Ac lines breaking inductive loads, breaking capacitive loads etc. The switching voltage surges are of comparatively longer duration μs lower rate of rise. The peak value of switching surge is expressed in terms of switching over voltage factor. Switching surges can be of the order to . pu and will have the magnitudes of the order kV to kV.
UNIT II 1. Define Gas Law.
)t consists of many laws. The following laws are very important.
a Charle s law P α T Volume is constant b Boyle s law P α /V T is constant General equation: PV = nRT Where P‐pressure ; V‐Volume; n‐number of moles; R‐molar gas constant; T‐temperature.
2. What is Paschen’s law?
= This equation shows a relationship between V and pd , and implies that the breakdown voltage varies as a product pd varies knowing the nature of functions f and f we can rewrite this equation V = f pd Where P ‐ Gas pressure f ‐ function d ‐ the electrode of dp V ‐ Voltage
3. 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 SF gas or vacuum as insulating medium. SF circuit breakers are manufactured up to the higher transmission voltage of kV and current range of kA and kA. (owever it is an expensive gas and at the normal operating pressure of bar it condenses at temperatures lower than °C.
4. 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.
5. Define uniform and non uniform field and give examples of each.
)n uniform field the applied field remains constant across the gap. Example: The field between the two plane electrodes. )n non uniform field the applied field varies across the gap. The examples are coaxial cylinders, point‐ plane and the sphere plane gaps.
6. Define the following as applied to high voltage breakdown.
a )nternal and External insulation b Flashover a )nternal and External insulation
Disruptive discharge voltage produces the loss of dielectric strength of insulation. )t is the voltage at which the electrical stress in the insulation causes a failure which includes the collapse of voltage and passage of current. b Flashover When discharge takes place between two electrodes in gas or liquid over a solid surface on air it is called flashover.
7. Define the following as applied to disruptive voltage.
a Flashover voltage b Spark over voltage a Flashover
When discharge takes place between two electrodes in gas or liquid over a solid surface on air it is called flashover. b Spark over voltage
The voltage between two spheres on sphere gap is raised till a spark passes between two spheres. The value voltage of spark over depends upon the dielectric strength of air, size of sphere and distance between the sphere and other factors.
8. 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 = defines the condition for the beginning of spark and is known Townsend criterion for spark formation or breakdown.
9. What are the different theories related with liquid dielectric breakdown?
The first theory is extension of gaseous breakdown based on the avalanche ionization of atoms caused electron collision in the applied field. The second theory is based on the fact the presence of foreign particles in liquid insulation is polarizable and is of higher permittivity than the liquid.
10. 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 more 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
11. What are Meta stable atoms? How they are ionizing the gaseous dielectric
medium? A Meta stable atom or a molecule is an excited particle whose life time is very large ‐ sec compared to the life time of an ordinary particle. Meta stables have a relatively high potential energy and are therefore able to ionize neutral particles.
12. Define formative time lag.
After the appearance of the electron a time t f is required for the ionization processes to develop fully to cause the breakdown of the gap and this time is called formative time lag. 13. What is composite dielectric?
)t is difficult to imagine complete insulation system in electrical equipment which does not consist of more than one type of insulation. )f 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 SF in parallel with solid insulation or in series with one another. Such insulation systems are called composite dielectrics.
1. Give some uses of HVDC.
Generation of high d.c voltages is mainly required in research work in the areas of pure and applied physics. Sometimes high direct voltages are needed in insulation tests on cables and capacitors. )mpulse generator charging units also require (VDC of about to kV. Normally for the generation of dc voltages of up to and the output currents are about mA.
2. What are the applications of impulse current wave forms of high magnitude?
Generation of impulse current waveforms of high magnitude kA peak find application in test work as well as in basic research or non linear resistors, electric arc studies relating to electric plasmas in high current discharges.
3. How are capacitances connected in an impulse current generator?
For producing impulse currents of large value a bank of capacitors are connected in parallel and are charged to a specified value and are discharged through a series R.L circuit.
4. What type of wave form will be available in impulse current generator output?
Damped wave or damped oscillatory wave.
5. Draw a circuit diagram of simple voltage doublers.
6. Write the expressions to find the optimum number of stages and % ripple in a
multiplier circuits. Optimum number of stage = √ V
) = load current and opt % ripple voltage = [n) / f
7. Draw a simple Tesla Coil equivalent circuit for generation of high frequency
A.C high voltage.
8. Write an expression to find the % ripple and % voltage regulation in a multi
voltage multiplier circuit. % ripple = ) n n+
% regulation = ) / f
[ / n +n ‐n/ ]
9. Explain the superiority of cascaded transformer over two winding
transformer. a Natural cooling is sufficient. b Transformers are compact in size, so the transportation and assembly is easy. c Construction is identical. d Three phase connection in star or delta is possible.
10. An impulse generator has 10 stages with capacitors rated 0.15 μF and 150 kV
per stage. The load capacitor is 1000 pf. Find the front and tail resistance to produce an impulse of 1.2 / 50 μs (approximate formula). G.D Number of stage = solution C = Generator capacitance = [ . / ] = . μF C = Load capacitance = [ / ] = ‐ F To find Front resistance Front time = . μ sec t = * time constant when neglecting R t = * R [C C /C + C ] R = . Ω Tail time t = μ sec t = . R + R C + C R = . Ω
11. Define the specification of impulse voltage as per Indian standard.
Standard impulse have a rise time of . μ s and μ s and decay time of % of peak value and of the order of to μ s.
12. What is the need for generating impulse currents?
Generation of impulse current waveforms of high magnitude find application in testing work as well as in basic research in nonlinear resistors electric arc studies and studies relating to electric plasmas in high current discharges.
13. What is a tesla coil?
Tesla coil is a high frequency resonant transformer. )t is a doubly tuned resonant circuit as shown in the following figure.
14. What are the factors influencing the measurements using sphere gap?
The factors influencing the measurements using sphere gap a )nfluence of nearly earthed objects. b )nfluence of humidity. c )nfluence of dust particles
15. Define the front and tail times of an impulse wave.
Referring to wave shape the peak value A is fixed and referred to as % value. The points corresponding to % and % of peak values are located in the front time portion points C & D. The line joining these points is extended to cut the time axis at . . (ere . is taken as the virtual origin. . times the interval between times t and t corresponding to points C D is defined as front time . [o t=o t ]. The point E is located on the wave tail corresponding to % of peak value and ot is defined as the tail time.
1. What are the general methods used for measurement of high frequency and impulse currents?
The following methods used for measurement of high frequency and impulse currents a Potential divider with a cathode ray oscilloscope. b Peak voltmeters. c Sphere gaps
2. What is the high voltage d.c measurement techniques used?
The following techniques are used for measurement of high voltage dc. a Series resistance micro ammeter b Resistance potential‐divider c Generating voltmeter d Sphere and other spark gaps
3. For what measurement are Hall generators normally used?
These are used for measurement of high direct currents. )t can be used for measurement of unidirectional a.c impulse current also.
4. What type of measuring devices are preferred for measurement of Impulse
short duration? Park s tubular shunt or coaxial tube is preferred for measurement of impulse current of short duration. 5. Draw the simple circuit of peak reading voltmeter and its equivalent.
6. List the factors that are influencing the peak voltage measurement using
sphere gap. The following are the factors a Nearby earthed objects b Atmospheric conditions and humidity c Radiation d Polarity and rise time voltage of waveforms.
7. What are the advantages of CVT measurement in HVAC?
a Simple design and easy installation b Can be used both as a voltage measuring device for meter and relaying purpose and as a coupling condenser for power line carrier communication and relaying c Frequency independent voltage distribution along elements as against conventional magnetic potential transformer which require additional insulation design d Provides isolation between the high voltage terminal and low voltage testing.
8. Why do we resort to statistical approach during breakdown due to impulse
voltage? The test is done with the specified voltage usually the probability failure is % and % failure values or % and % value since it is difficult to adjust the test for the exact % flashover values. The average value of upper and lower limit is taken.
9. State the demerits of CVT measurement for HVAC measurements.
a Voltage ratio is susceptible to temperature variations. b )n the presence of capacitance and choke, the problem of ferro‐resonance occurs in power system.
10. What are high current shunts mention their design criterion?
The most common method employed for high impulse current is a low ohmic pure resistive shunt. The resistance shunt is usually designed in the following manner to reduce the stray effects. a Bifilar flat strip design b Coaxial cable or parts shunt design c Coaxial squirrel cage design.
11. Why are capacitive voltage dividers preferred for AC high voltage measurements?
The high voltage dividers are preferred for high ac voltage measurement because the capacitance ratio is independent of frequency. 12. Calculate the correction factors for atmospheric conditions, if the laboratory
temperatures is 37°C, the atmospheric pressure is 750 mm Hg and the wet
bulb temperature is 27°C.
millibar = mm mm = / * = millibar Correction factor =
13. Explain the merits and demerits of analog and digital techniques used for high
voltage measurements. )nformation storage is easy that is accomplished by special switching circuit that latches on information and holds it for as long as necessary. Accuracy and precision are greater. Digital system can handle as many digits of precision as you need simply by adding more switching circuits. Analog system is cheaper and involves less complex circuitry.
14. What are the general methods used for measurement high frequency and impulse currents?
The following methods are used a Resistive shunts b Magnetic potentiometer or Rogowski coils c Magnetic links d (all Effect generator.
1. What are tests conducted on insulators?
i) Power frequency voltage test ii) )mpulse voltage test
Power frequency test can be classified into
a Dry and wet flashover tests b Wet and dry withstand tests one minute )mpulse tests can be classified into
a )mpulse withstand voltage test b )mpulse flashover test c Pollution testing.
2. What are test conducted on Bushings?
i Power frequency voltage test ii )mpulse voltage test
Power frequency test can be classified into a Power factor voltage test b )nternal or partial discharge test c Momentary withstand test at power frequency d One minute wet withstand test at power frequency test e Visible discharge test at power frequency )mpulse voltage tests can be classified into a Full wave withstand test b Chopped wave withstand and switching surge test Temperature rise and thermal stability tests are also used for testing of bushings.
3. Define withstand voltage.
The voltage which has to be applied to attest object under specified condition in a withstand test is called the withstand voltage.
4. Define impulse voltage.
)mpulse voltage is characterized by polarity. Peak value, time to front and time to half the peak value after the peak. The time to front is defined as . times to time between and % of the peak value in the rising portion of the wave. According to [s : ] standard impulse is defined as one with t
= . μs, t
= μs called . μs wave . The tolerances allowed are ± % on the peak value, ± % in the front time t
, and ± % in the tail time t
5. Differentiate type test and routine test.
Routine test is not to flash over the insulator but rather to detect existing fault before the insulator is put in service .Type test cover those features that are important to the purchaser such as dry and wet flash over, puncture voltage, tensile strength.
6. Define Disruptive discharge voltage.
This is defined as the voltage which produces the loss of dielectric strength of an insulation. )t is that voltage at which the electrical stress in the insulation causes a failure which includes the collapses of voltage and passage of current.
7. What are the atmosphere correction factor and mention their influence in
high voltage testing. Actual air density during measurement differs due to temperature and pressure variation. Spark over voltage depends upon the air density factor
Spark over voltage V = KV Where ,K = Correction factor related to air density factor d V = Spark over voltage under standard temperature and pressure.
d = P/P [ + t / + T ] Where K is a function of air density factor d P = air pressure at test condition P = air pressure at standard condition t = temperature at standard condition T = temperature at test condition
8. Explain the role of Bureau of Indian standards in high voltage testing.
All the E(V equipment voltage are governed by )ndian standard specification issued by Bureau of )ndian standards in our country. For different transmission voltages the test voltages are given in a tubular form.
9. Define insulation co- ordination.
)nsulation co‐ordination of suitable values for the insulation levels of the various components in any electrical system and their management in a rational manner is called insulation co‐ordination )nsulation co‐ordination of the insulation of the electrical equipment and circuits with the characteristics of the protective devices in such a manner that the insulation is well protected from the excessive over voltages.
10. What is the significance of impulse tests?
)mpulse test are done on insulator to ensure reliability of individual test objects and quality and consistency of material used in their manufacture and done on transformers is to determine the ability of the insulation of transformers to withstand the transient voltages due to lightning.
11. Name the different types of tests conducted on high voltage apparatus.
Dry ‐Wet flashover test and Dry ‐Wet withstands test are the two types of tests conducted on high voltage apparatus.
12. Explain the reasons for conducting wet tests on high voltage apparatus and
give the specifications of the water used for wet tests. Wet tests are to provide a criterion based on experience that a satisfactory service operation will be obtained. Conductivity of water microsiemens ± % Water temperature ambient ± %
13. Define creepage distance.
)t is the shortest distance of contour of the external surface of the insulator unit or between two metal fittings on the insulator.
14. Give the Indian standard reference atmospheric conditions for high voltage
testing. a Temperature ‐ °C b Pressure ‐ millibars or torr c Absolute humidity – gm/m
15. Define safety margin as applied to be insulation co-ordination.
Safety margin applied to insulation co‐ordination is the basic insulation level of equipment is defined as the peak voltage of . / μs wave which does not cause insulation to fail. The definition of a surge . / μs indicates a surge which reaches its peak value in . μs and decays to half the peak in μs.
Unit - I
. Draw the cross sectional view of non linear resistor lightning arrester valve type and explain its operation in detail and its V‐) characteristics.
. Discuss mechanism of lightning stroke and over voltages on transmission lines and give its mathematical model.
. Explain Townsend s first and second ionization process and derive the equation. . Explain different theories of charge formation in clouds. . What are the methods to control over voltages?
Unit - II
. State the criteria for sparking potential and hence obtain the relation between sparking potential and PD values Paschens law . Discuss nature of variation of sparking potential with PD values.
. Explain breakdown mechanism on solid dielectric. . Explain breakdown mechanism on vacuum dielectric. . Explain breakdown in non uniform fields and corona discharge.
Unit - III
. starting from basic marx circuit develop the circuit of modern multistage impulse
generator and explain its operation. . Discuss significance of various parameters. . Explain vandegraff generator. . Explain cascade transformer connections and resonant transformer.
Unit - IV
. Explain measurements of very high voltages using sphere gaps. Mention merits and de merits of using sphere gaps.
. Explain electro static voltmeter for (VAC measurements. Give its advantages and disadvantages.
. Explain generating voltmeter for measuring high DC voltage. . Explain Capacitance voltage transformer. . Explain hall generator.
. Explain testing of transformer. . Explain testing of insulator. . Explain testing of isolators and circuit breakers.