svcet · 2018. 12. 18. · ee6002 power system transients page 6 of 21. svcet sri vidya college of...

Unit IV

Traveling waves on transmission line

1. What is Velocity of Wave Propagation?

The velocity of propagation of any electromagneticdisturbance in air is the sameas that of light, namely 2.998*1010 cm per sec. The only difference for transmission lineis that the conductor provides a guide.

In terms of line constants,

Velocity=1/1/√=3*1010 cm per sec.

2. Draw the lumped equivalent of transmission line.

3. What are called lumped parameters? (Nov/Dec 2010)The lumped parameters are transmission line(i) Resistance (ii) Inductance(iii) Capacitance

4. Write voltage and current wave equation of a transmission lineVoltage wave equation =Current Wave Equation

=5. What is reflection and refraction of travelling waves?

Reflection involves a change in direction of waves when they bounce off abarrier; refraction of wave involves a change in direction of wave as they pass from onemedium to another, and diffraction involves a change in direction of wave as they passthrough an opening or around a barrier in their path.

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SRI VIDYA COLLEGE OF ENGG & TECH QUESTION BANK UNITIV

EE6002 POWER SYSTEM TRANSIENTS of 21

6. What is step response of traveling waves?The unique characteristic of the short circuit is that is impossible to develop

any voltage across it. Thus when a travelling wave of voltage reaches a short circuitthe reflected voltage wave must precisely cancel out the incident wave so that therefracted wave is zero.

If the incident voltage wave is V1 and the incident wave I1, The reflectedvoltage and current wave will be –V1 and +I2.

7. What is attenuation and distortion of travelling waves?In general in addition to the effects of reflection at transition points,

travelling waves are also subjected to both attenuation (decrease in magnitude) anddistortion (change is shape) as they propagate along the transmission line. They arecaused by losses in the energy of the wave due to resistance, leakage, dielectric, andcorona.

Corona is the main cause of attenuation at high voltages. It reduces themagnetic and the steepness of the wave fronts within a few miles to a safe voltage.

8. What is lattice Diagram? (Nov/Dec 2011)The lattice diagram determines the voltage at a given point and time in a

transmission system. It is a useful aid to keep track of travelling voltage wave orcurrent wave as it reflects back and forth from the ends of the line.

9. What are the properties of bewley’s lattice diagram?(May/June 2012)1) All waves travel downward, because time always increases.2) The position of any wave at any time can be deduced directly from the

diagram.3) The total potential at any point at any instant of time is the superposition of all

the waves which have arrived at that point up until the instant of time,displaced in position from each other by intervals equal to the difference intheir time of arrival.

4) The history and origin of the wave is easily traced. It is possible to find whereit comes from and just what other waves went into its composition.

5) Attenuation is included, so that the wave arriving at the far end of a linecorresponds to the value entering to the value entering multiplied by theattenuation factor of the line.

10. What is attenuation factor ‘k’?It is also common for an attenuation factor ‘k’ to be defined corresponding

to the length of a particular line.i.e k=e-αL for a line of length L.

11. Define propagation constant ‘r’?

The propagation constants of a line γ of a line of series impedance ‘Z’ and

shunt admittance ‘Y’ per unit length is given by

γ = √ . = ( + )( + )

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12. What is natural frequency?The natural frequency is the frequency at which a system when it is

disturbed.Systems have inserted and elastic characteristics which make them want to oscillate orvibrate at certain frequencies. These specific frequencies are called naturalfrequencies.

Wn=

13. Why natural frequencies are important?The natural frequencies are important for many reasons:

1. All things in the universe have a natural frequency, and many things havemore than one.

2. If you will know an object’s natural frequency, you know how it will vibrate.3. If you know how an object vibrates, you know what kinds of wave it will

create.4. If you know to make specific kinds of waves, you need to create objects with

natural frequencies that match the waves you want.

14. What is standing wave? When it occurs?(May/June 2011)Although waves usually travel, it is possible to make a wave stay in one

place. A wave that is trapped in one spot is called a standing wave. Standing waveoccurs at frequencies that are multiple of the fundamental, which is the naturalfrequency of the system.

15. How does the over voltage surge affect the power system?The over voltage of the power system leads to insulation breakdown of the

equipments. It causes the line insulation to flash over and may also damage the nearbytransformer, generators and the equipment connected to the line.

16. What do you mean by travelling waves?Any disturbance on a transmission line (or) system such as sudden opening or closingof line, a short circuit or a fault results in the development of over voltages or overcurrent at that point. This disturbance propagates as a travelling wave to the ends ofthe line (or) to a termination, such as a substation. Usually these travelling waves arehigh frequency disturbances and travel as waves. They may be reflected, transmitted,attenuated or distorted during propagation until the energy is absorbed.

17. What are the damages caused by the travelling waves?a. The high peak (or) crest voltage of the surge may cause flashover in the internal

winding thereby spoil the winding insulation.b. The steep wave front of the surge may cause internal flashover between inter turns

of the transformer.c. The high peak voltage of the surge may cause external flashover between the

terminals of the electrical equipment which may result in damage to insulators.d. The steep wave front resulting into resonance and high voltages may cause

internal or external flashover of an unpredictable nature causing building up to theoscillation in the electrical apparatus.

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18. What is surge impedance of a line and why is it also called the naturalimpedance?The ratio of voltage to current which has the dimensions of impedance is called assurge impedance of the line.= = = ( )It is also called the natural impedance because this impedance does not depend onload impedance but depends only on the constants. The value of this impedance is400Ω to 600Ω for an overhead line and 40Ω to 60Ω for a cable.

19. What is the application of Bewley’s Lattice diagram?With the use of Bewley’s Lattice diagramone can know at a glance the position anddirection of motion of every incident, reflected and transmitted wave on the system atevery instant of time.

20. What are the specifications of a travelling wave?(Nov/Dec, April/May 2011)A travelling wave is characterized by four specifications.a. Crest of a waveb. Front of a wavec. Tail of a waved. Polarity

21. Define crest and front of a travelling wave.The crest of the wave is the maximum amplitude of the wave and is usually expressedin KV (or) KAThe front of the wave is the portion of the wave before crest and is expressed in timefrom beginning of the wave to the crest value in ms (or) µs.

22. Define tail and polarity of a wave.Tail of the wave is the portion beyond the crest. It is expressed in time (µs) frombeginning of the wave to the point where the wave has reduced to 50% of its value atcrest.Polarity is polarity of crest voltage or current. A positive wave of 500kV crest, 1µsfront time and 25µs tail time will be represented as +500/1.0/25.0

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23. Why step waves are considered to be dangerous to the apparatus?The simplest and most commonly used representation is the infinite rectangular orstep wave.Such a wave jumps suddenly from zero to full value and is maintained at that valuethereafter.

As this wave has front causing maximum gradients and sustained tail producingmaximum oscillations in machine windings it is most dangerous toapparatus/equipment. Hence the analysis based on it is liable to error on the safer side.

24. Write the expression for reflection coefficient and refraction coefficient.(Nov/Dec2011)

Reflection coefficient: =Refraction coefficient: =Where ZA, ZB are the characteristics impedances of the lines.

25. What is the effect of shunt capacitance at the terminal of a transmission lines?The effect of shunt capacitance at the terminal of a transmission line is to cause thevoltage at the terminal is to rise to full value gradually instead of abruptly (i.e.,) tocause flattering of the wave front, which reduces the stress on the line end windings ofa transformer connected to the lines.

26. What are the design principles observed in the lattice diagram?a. All waves travel downhill (i.e.,) into positive time.b. The position of the wave at any instant is given by means of the time scale at the

left of the lattice diagram.c. The total potential at any instant of time is the superposition of all the waves

which arrive at that point until that instant of time, displaced in position from eachother by time intervals equal to the time difference of their arrival.

d. Attenuation is included so that the amount by which the wave is reduced is takencare.

e. The previous history of the wave, if derived can be traced. If the computation is tobe carried out at a point where the operations cannot be directly placed on thelattice diagram, the arms can be numbered and the quality can be tabulated andcomputed.

27. Define Standing wave Ratio (SWR)It is the ratio of the amplitude of a partial standing wave at an antinode (maximum) tothe amplitude at an adjacent node (minimum), in an electrical transmission line.The SWR is usually defined as a voltage ratio called the VSWR, for voltage standingwave ratio.

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28. What is meant by insulation co-ordination? (Nov/Dec 2011)Insulation coordination means the correlation of the insulation of the variousequipments in a power system to the insulation of the protective devices used for theprotection of those equipments against over voltages.

29. Sketch the Bewley’s Lattice diagram.(May/June 2011)

Where,

T = the time travel of the wave from the source to the open end

a = attenuation over one line length which the wave suffers.SVCET



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svcet · 2018. 12. 18. · ee6002 power system transients page 6 of 21. svcet sri vidya college of...

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