EE1402(Unit V)

Unit V


1. Define type test.
They are intended to prove or check the design features and the quality. They are done
on samples when new designs or design changes are introduced.

2. Define routine test.
They are intended to check the quality of an individual test piece. They are done to
ensure the reliability of an individual test object and quality and consistency of materials
used in their manufacture.

3. Define withstand test voltage.
The voltage which has to be applied to a test object under specified conditions in a
Withstand test is called withstand voltage.

4. Define flashover voltage.
The voltage that causes a flashover at each of its applications under specified conditions
when applied to test objects as specified is called flashover voltage.

5. Name the tests conducted on surge diverters.
Power frequency spark over test, Hundred percent impulse spark over test, Front of
Wave sparkover test and residual voltage test.
6. Give any two classification of test on cables.
Partial discharge test and impulse withstand voltage test.

7. Explain the porosity test on insulators.
The insulator is broken and immersed in a 0.5 % alcohol solution under a pressure of
13800 KN/sq.m for 24 hours. The broken insulator is taken out and further broken. It
Should not show any sign of impregnation.

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

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

10. What is the importance of radio interference voltage measurements for extra high
Voltage power apparatus?
In the power apparatus it produces unwanted electrical signals in radio and high
frequency ranges.It is important to see that the noise generated should be reduced.
For this purpose this measurement is important.

11. Name the test conducted on bushings.
Power frequency test , impulse voltage test and thermal test.


12. Define partial discharge.
An electrical discharge that only partially bridges the dielectric between the
Conductors. Examples are surface discharge, internal discharge

.
13. What is the function of surge arrester?
They are capable of discharging 10 to 20 KA of long duration surges and 100 to
250 KA of short duration surge currents.

14. State the consequences of RIV.
When the noise meter measurements are stated the information regarding the
Specification of meters used in the band pass characteristics and the open circuit the
detector characteristics has to be mentioned.

15. What do you mean by radio interference?
The power apparatus produces unwanted signals in the radio and high frequency
ranges. These are called radio interference.

16. Define an isolator.
It is a disconnector or a mechanical switching device which provides in the open
position an isolating distance in accordance with special requirements.

17. Define a circuit breaker.
It is a switch which automatically interrupts the circuit when a critical current or
voltage rating is exceeded.

18. Define 50% flashover voltage
It is the voltage which has the 50 % flashover when applied to test object.


19. Define 100 % flash over voltage
The voltage that causes a flashover at each of its application under specified conditions
when applied to test objects as specified.

20. State 2 standard tests to be conducted on HV Transformers.
Induced overvoltage test and partial discharge test

21. What does the selection of BIL level for lines depend?
Atmospheric conditions, lightning activity, insulation pollution and acceptable outage of
the line.

EE1402(UNIT-IV)

UNIT-IV
1. What are the factors that affect the spark over voltage of sphere gap?
• Nearby earthed objects
• Atmospheric conditions and humidity
• Irradiation
• Polarity and rise time of voltage waveforms
2. State the measurement techniques that is suitable for measuring A.C. high frequency
Voltages and impulse.
• Potential divider with CRO (resistive (or) capacitive dividers).
• Peak voltmeters.
• Sphere gaps.
3. What are the methods that are used to measure D.C. voltages?
• Series resistance micro ammeter.
• Resistance potential divider.
• Generating voltmeter.
• Sphere and other spark gaps.
4. What are the drawbacks in the series resistance with micro ammeter method?
• Power dissipation and source loading.
• Temperature effects and long time stability.
• Voltage dependence or resistive elements.
• Sensitivity to mechanical stresses.
5. State the advantages and limitations of generating voltmeters?
Advantages:
• No source loading by the meter.
• No direct connection to high voltage electrodes.
• Scale is linear and extension of range is easy.
• Convenient instrument.
Disadvantages:
• They require calibration
• Careful construction is needed and it is cumbersome instrument requiring an auxiliary drive.
• Disturbance in position and mounting of the electrodes makes the calibration invalid.


6. State the principle of operation of electrostatic voltmeters.
When one electrode is free to move, the force on the plate can be measured by controlling it by a spring or balancing it with a counter weight. The force is proportional to the square of the applied voltage, the measurement can be made for D.C or A.C.
F= abs (½ V2 δC/δS)
7. What are the elements that mainly constitute errors in potential dividers for impulse voltage measurement ?
• Residual inductance in the elements.
• Stray capacitance occurring.
i. Between elements.
ii. From sections and terminals of the elements to ground.
iii. From the high voltage lead to the elements or sections.
• The impedance error.
i. Connecting leads between the divider and the test objects
ii. Ground return leads and extraneous current in ground leads.
• Parasitic oscillations due to lead and cable inductances and capacitance of high voltage terminal ground.


8. State the measurement techniques that is suitable for measuring A.C. high frequency
Voltages and impulse
• Potential divider with CRO (resistive (or) capacitive dividers).
• Peak voltmeters.
• Sphere gaps.


9. Why capacitance voltage divider preferred for high voltage ac measurement?
It is used to eliminate the errors due to the hamonics with the help of an Electro static voltmeter(ESV) or high impedance voltmeter.

EE1402(GENERATION OF HIGH VOLTAGES AND CURRENTS)

UNIT-III

GENERATION OF HIGH VOLTAGES AND CURRENTS

1.What are the methods that are available to generate high DC voltages?

• Rectifiers
• Voltage doubler circuit
• Cascaded Voltage doubler circuit
• Voltage multiplier circuit

2.State some applications of vande Graff generator.

• It is useful for very high voltage and low current applications.
• The output voltage is easily controlled by controlling the corona source voltage and the rate of charging.
• The voltage can be stabilized to 0.01%.
• It is flexible and precise machine for voltage control.

3.What are the drawbacks of cascaded voltage doubler circuit that led to the development of voltage multiplier circuits?

• It requires suitable isolating transformers.
• The filament transformers have to be suitably designed and insulated.
• All the cathodes will not be at the same potential from ground.

o The arrangement becomes cumbersome if more than 4V is needed with cascaded steps.

4.How are capacitances connected in an impulse current generator?

A bank of capacitors connected in parallel are charged to a specified value and are discharged through a series R-L circuits.

5.What are the general methods available for generation of A.C. voltages?

· Cascaded transformer

· Tesla coil

· Resonant transformer

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

They are required for rectifier dc power supplies. Also for testing electrical

apparatus for switching surges it is used.

7. Mention the advantages of high frequency transformers.

Saving in cost and size, pure sine wave output , uniform distribution of

voltage across

the winding coils due to subdivision of coil stack into number of units.

8. How are impulse waves specified?

By defining the rise of front time, fall time to 50% peak value and the

value of the peak voltage.

9. State the advantages of resonant transformer and high frequency Transformer.

· It gives an output of pure sine wave.

· Power requirements are less.

· No high power arcing and heavy current surges occur if the test object fails, as resonance ceases at the failure of the test object.

· Cascading is possible for very high voltages.

· Simple and compact.

· No repeated flashovers occur in case of partial failures of the test object and insulation recovery.

10. State the principle of operation of Marx circuit for impulse voltage generation.

For producing very high voltages a bank of capacitors are charged in parallel and then

discharged in series.

11. What are the components of impulse generators?

· Capacitance of the generator

· Charging resistor

· Spark gap

· Wave shaping resistors

· Test object

12.What is a tesla coil?

The high frequency resonant transformer is called as a tesla coil, which is doubly tuned resonant winding.

13. What are the advantages of using Tesla coil for high AC voltage generation?

· The absence of iron core in transformers and hence saving in cost and size.

· Pure sine wave output.

· Slow build-up of voltage over a few cycles and hence no damage due to switching surges.

· Uniform distribution of voltage across the winding coils due to subdivision of coil stack into a number of units.

14. What are the advantages of parallel resonance test system over series

resonance test System?

The advantage of parallel resonant circuit is

· More stable output voltage can be obtained along with a high rate of rise of test voltage

· Independent of the degree of tuning and the Q factor.

· Single unit resonant test systems are built for output voltages up to 500KV.

· Cascaded units for outputs up to 3000KV 50/60 Hz are available.

15. What are the drawbacks of RLC type impulse generator?

· The circuit occupies considerable space.

· The effect of inductance is to cause oscillations in the wave front and in the wave tail portions.

16. State the principle of operation of Marx circuit for impulse voltage generation.

For producing very high voltages a bank of capacitors are charged in parallel and then Discharged in series.

17. What are the components of a multistage impulse generator?

Dc charging unit, charging resistors, generator capacitors and spark gaps , wave

shaping resistors and capacitors , triggering system , voltage dividers and gasinsulated impulse generators.

18. Define the duration of the wave.

It is defined as the total time of the wave during which the current is at least 10% of its

peak value.

19. How are impulse currents of large value produced?

A bank of capacitors connected in parallel are charged to a specified value and are

discharged through a series R-L circuit.

20. How will you generate rectangular current pulses with high magnitudes?

They are generated by discharging a pulse network.

21. Mention the advantage of trigatron gap?

It requires much smaller voltage for operation compared to the three electrode gap.

22. What are the components of a trigatron gap?

It consists of a high voltage spherical electrode of suitable size, an earthed

main electrode of spherical shape and a trigger electrode through the main electrode.

PART-B

1. Explain about : - (16)

a) Voltage doubler circuit .

b) Voltage multiplier circuit (Cock-Craft Walton)

2. Explain about :

a) Deltetron circuit. –(8)

b) Van-de-Graft generator. –(8)

c) Electrostatic generator. –(8)

3. Explain the generation of High alternating voltages using Cascaded transformers?
4. Explain the generation of High frequency AC high voltages?- (8)
5. Explain the Multistage impulse generator (Marx-circuit) ? - (8)
6. Explain the impulse current generator? –(8)

7. An impulse generator has 12 capacitors of 0.12 uj% and 200 kV rating. The wave front

and wave tail resistances are 1.25 IcQ and 4 kQ respectively. If the load capacitance

including that of the test object is 10 000 pF, find the wave from and wave tail times

and the peak voltage of impulse wave produced

8. 8-stage impulse generator has 0.12 \iF capacitors rated for 167 kV. What is its

maximum discharge energy ? If it has to produce a 1/50 |is waveform across a load

capacitor of 15,000 pF, find the values of the wave front and wave tail resistances.

9. Calculate the peak current and waveshape of the output current of the following

generator. Total capacitance of the generator is 53 M-F. The charging voltage is 200

kV. The circuit inductance is 1.47 mH, and the dynamic resistance of the test object

is 0.051 ohms.

EE1402(unit II)

UNIT-II

1. What are the advantages of HVDC over HVAC?
HVDC transmission has the following advantages over HVAC
• Skin effect is absent in HVDC
• Corona is absent
• Power loss is minimum
• Number of conductors required for transmission is less in HVDC than HVAC
• Tie between two stations is very difficult in HVAC .But in HVDC it is very easy
2. State few high voltage applications.
• Electric Traction
• Particle accelerator
• Biomedical applications
• Lightning arrestors

3. Define dielectric strength. What are the types of Dielectrics? State few examples.
Dielectric strength is defined as the maximum applied voltage that a dielectric can withstand
Without causing breakdown. Types of Dielectrics and examples
• Solid dielectric - Mica ,ceramic,Plastic,PVC
• Liquid dielectric - transformer oil ,Askarels
• Gaseous dielectric - Air, oxygen,SF6,Freon

4. What are electronegative gases? Give some examples.
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

5. Define Paschen’s Law for breakdown?
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)

6. What are the discharges that occur after breakdown of gaseous dielectric?
The discharges that occur after breakdown of gaseous dielectric are
• Glow discharge
• Arc discharge

7. Why SF6 is most widely used as gaseous dielectric?
SF6 is most widely used as gaseous dielectric because it has
• High dielectric strength
• Low liquification temperature
• Good arc quenching property
• Good heat transfer
8. What is corona? What are the effects of corona?
Corona is a phenomena associated with non uniform field. When ac voltage is applied between the conductors, under normal voltage the air between the conductor remains as such and there will be no change in it. But when the voltage is increased beyond certain voltage called critical disruptive voltage, then the air gets ionized and blue violet luminent glow appears all along the conductor called corona.
Effects of corona:
• Hissing noise
• Air surrounding the conductor gets ionized resulting in the formation of ozone
• Radio interference voltage affects the communication lines
• Deteriation of insulating property of conductors

9. What are the practical considerations for selecting gases as dielectrics?
The practical considerations for selecting gases as dielectrics are
• High dielectric strength
• Low liquification temperature
• Good thermal stability
• Good heat transfer
• Available at moderate cost
• Chemical inactivity towards the material of construction
• Physiological inertness of gas

10. Define Townsend’s Primary ionization coefficient.
Townsends Primary ionization coefficient α is defined as the number of ionizing collision
made by the free electron per centimeter travel in the direction of applied electric field.


11. What are the different time lags that occur during breakdown?
Time difference between the application of sufficient voltage to cause breakdown and the real occurrence of breakdown itself is called time lags.. The different time lags that occur during breakdown are
• Statistical Timelag
• Formative Timelag
12. What are electronegative gases? Give some examples.
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

13. Define Paschen’s Law for breakdown?
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)

14. What are the discharges that occur after breakdown of gaseous dielectric?
The discharges that occur after breakdown of gaseous dielectric are
• Glow discharge
• Arc discharge

15. Why SF6 is most widely used as gaseous dielectric.
SF6 is most widely used as gaseous dielectric because it has
• High dielectric strength
• Low liquification temperature
• Good arc quenching property
• Good heat transfer

16. Define an attachment coefficient.
An attachment co-efficient is defined as the number of attaching collusions made by one electron drifting one centimeter in the direction of the field.

17. What is a statistical time lag of the gap?
The time which lapses 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.

18. Mention the different mechanisms for the breakdown in vacuum.
• Partial exchange mechanism,
• Field emission mechanism and
• Clump theory.

19. Write the two mechanisms used in field emission theory?
• Anode heating
• Cathode heating mechanisms.

20. What are the classifications of vacuum?
• High vacuum
• Very high vacuum and
• Ultra high vacuum

21. What are liquid dielectrics?
Liquid dielectrics are normally mixtures of hydrocarbons and are weakly polarized.

22. Mention some of the applications of liquid dielectrics.
They are used as impregnants in high voltage cables and capacitors, and for filling up of transformers, circuit breakers .They are also used as heat transfer agents in transformers and as arc quenching media in circuit breakers.

23. Name some examples of liquid dielectrics.
Petroleum oils, Synthetic hydrocarbons, halogenated hydrocarbons, silicone oils and fluorinated hydrocarbons.

24. What are pure liquid ? Give examples.
They 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, n-heptanes and other paraffin hydrocarbons.

25. Mention the impurities added in liquid dielectrics.
Dust, moisture, dissolved gases and ionic impurities.

26. What are the different types of solid insulating materials?
Organic materials : paper, wood and rubber
Inorganic materials: Mica, glass and porcelain
Synthetic polymers : Perspex lex, pvc, epoxy resins

27. State the properties of good dielectrics
Low dielectric loss, high mechanical strength, should be free from gaseous inclusions and moisture and be resistant to thermal and chemical deterioration.

28. Name the two types of intrinsic breakdown mechanisms.
• Electronic breakdown .
• Avalanche or streamer breakdown

EE1402(OVER VOLTAGES IN ELECTRICAL POWER SYSTEMS)

PART- A

UNIT-I

OVER VOLTAGES IN ELECTRICAL POWER SYSTEMS

1. What are the 2 types of over voltages?

1. Lightning over voltages,

2. Switching over voltages.

2. Explain the various regions of the cloud.

The upper regions of the cloud are positively charged, whereas the lower region and the base are predominantly negative except the local region near the base and the head which is possible.

3. Mention the different theories of charge formation.

Simpson’s theory, Reynold’s theory and Mason’s theory

4. What does a thunder cloud consist?

A thunder cloud consists of super cooled water droplets moving upwards and large hailstones moving downwards.

5. Mention the requirements for the thunder clouds and charge formation of air currents.

Moisture and specific temperature range

6. What is back flashover?

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

7. State the parameters and the characteristics of the lightning strokes.

Amplitude of the current, the rate of rise, the probability distribution of them and the waveshapes of the lightning voltages and currents.

8. Define Isokeraunic level or thunderstorm days.

It is defined as the number of days in a year when the thunder is heard or recorded in a particular location. Often it does not distinguish between the ground strokes and the cloud-to-cloud strokes.

9. State the factors influence the lightning induced voltages on transmission lines.

• The ground conductivity,
• The leader stroke current and
• The corona.

10. What are the sources (or) origin of switching surges?

* Opening and closing of switchgears.

* High natural frequencies of the system.

* Damped normal frequency voltage components.

* In circuit breaker operation, switching surges with a high rate of rise of voltage may cause repeated restriking of the arc between the contacts of the C.B.

11. What are the characteristics of switching surges?

* Deenergizing of lines, cables and shunt capacitor banks etc.

* Disconnection of unloaded transformers, reactors etc.

* Opening and closing of protective devices connected to lines and reactive loads.

* Switch off the loads suddenly.

* Clearing of the faults.

12. What are the causes of switching over voltages in EHV and HV system?

* Interruption of low inductive currents by high speed circuit breakers.

* Interruption of low capacitve currents by switching off the unloaded lines.

* Ferro-resonance condition.

* Energization of long EHV or UHV lines.

* Interruption of fault cuurent when the fault is cleared.

13. How to control the over voltages due to switching and power frequency?

* Inserting resistors

* Phase controlled closing of C.B. with proper sensors

* Drain the trapped charges before reclosing of the lines

* Using shunt reactors

* By using lightning arresters or surge diverters

14. When over voltages are generated in EHV system?

Over voltages are generated in EHV systems when there is a sudden release of internal energy stored either in the electrostatic form or in the electromagnetic form.

15. What are the causes for power frequency and its harmonic over voltages?

• Sudden loss of loads,
• Disconnection of inductive loads or connection of capacitive loads,
• Ferranti effect,
• Unsymmetrical faults and saturation in transformers etc.

16.What is ground 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.

17. What is the use of ground wire?

It shields the transmission line conductor from induced charges,from clouds as well as from a lightning discharge.

18. What is an expulsion gap?

Expulsion gap is a device which consists of a spark gap together with an arc quenching device which extinguishes the current arc when the gap breaks over due to overvoltage.

19. Mention the parts of an expulsion gap.

It consists of a rod gap in air in air in series with a second gap enclosed within a fibre tube.

20. What is a protector tube?

It is a device which consists of a rod or spark gap in air formed by the line conductor and its high voltage terminal.It is mounted underneath the line conductor on a tower.

21. How are the insulation level and the protective safety margin arrived?

Selecting the risk of failure,the statistical safety factor and by firing the withstand level of any equipment or apparatus corresponding to 90% or 95% of the withstand voltage.

22. Define Basic Impulse Level.

It is defined as the minimum insulation impulse withstand voltage of any power equipment or apparatus. The BIL of a power system is usually chosen as 25% to 30% more than the protective level offered by the protective devices.

23. Mention the various insulation levels in a substation.

The busbar insulation is the highest to ensure the continuity of supply in a substation. The circuit breakers, isolators, instrument and relay transformers are given the next lower limit level .The power transformers are the costliest and sensitive device and the insulation level for it is the lowest.

24. What is the use of ground rod?

Ground rods are used to reduce the tower footing resistance. These are buried into the ground surrounding the tower structure.These rods are a number of rods about 15mm diameter and 3m long driven into the ground

25. What are surge arresters?

Surge arresters are non-linear resistors in series with spark gaps which act as fast switches. Surge diverters or lightning arresters are devices used at sub-stations 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 sub-station.

26. State few high voltage applications.

• Electric Traction
• Particle accelerator
• Biomedical applications
• Lightning arrestors

UNIT 1:

PART-B

1. Explain the lightning phenomena (or) charge formation in clouds? – (8)
2. Explain the Mechanism of lightning strokes? –(8)
3. Derive the expression for the transmission lline (or) derive the telegraphic equation?- (10)
4. What is meant by switching surge, how it originates and explain their characteristics?-(16)
5. How will you control the over voltages due to switching?-(8)
6. Explain the protection of transmission line against over voltage?-(16)
7. Explain the principles of insulation coordination of HV and EHVpower system?
8. Explain about the surge arresters and how will you select the surge arrester?-(8)
9. Explain the reflection of transmission waves at transient point?-(8)
10. Explain about the successive reflection and lattice diagram?-(10)