Power Systems - short answer questions (Winter 2021)


 Answer the following in brief:

What do you understand by Skin Effect?

  • Skin effect is a tendency for alternating current (AC) to flow mostly near the outer surface of an electrical conductor, such as metal wire. 
  • The effect becomes more and more apparent as the frequency increases.  
  • This phenomenon restricts the current to a small part of the total cross-sectional area and so has the effect of increasing the resistance of the conductor.

Discuss the Proximity Effect.

  • When the conductors carry the high alternating voltage, then the currents are non-uniformly distributed on the cross-section area of the conductor. This effect is called proximity effect. 
  • The proximity effect results in the increment of the apparent resistance of the conductor due to the presence of the other conductors carrying current in its vicinity.

Briefly discuss electromagnetic and electrostatic interference with communication lines.

  • The "Electromagnetic effect" produces currents due to induction resulting its superposition on speech current of communication line causing distortion.
  • The "electrostatic effect" raises the potential of the communication circuit as a whole and thereby posing a threat of damage to the communication equipments.

How can be power factor be improved?

  • Use Of Static Capacitors. As we all know, most of the time load is inductive, which take lagging current and decreases the power factor. Therefore, to improve power factor, capacitors are installed in parallel with such loads. 
  • Use Of Synchronous Condenser. Make use of a three-phase synchronous motor that has the potential to run on no-load. This complete setup is known as synchronous condenser which can operate on unity, lagging and leading power factor rating. 
  • Use Of Phase Advancer. Phase advancer is the most effective method for correcting the power factor as it uses an AC exciter for the correction aspects. It is connected to the motor’s rotor circuit to enhance the ampere-turns. It can help obtain a leading power factor as well, with higher ampere-turns.

Discuss the advantages of Neutral Grounding.

  • Voltages of the phases are limited to phase to ground voltages.
  • The high voltages due to arcing grounds or transient line to ground faults are eliminated.
  • Sensitive protective relays against line to ground faults can be used. The over-voltages due to lightning are discharged to ground, otherwise there will be positive reflection at the isolated neutral of the system.

What are the advantages of bundled conductors in transmission lines?

  • Reduced reactance.
  • Reduced voltage gradient.
  • Reduced corona loss.
  • Reduced radio interference.
  • Reduced surge impedance.

What is the significance of Power Angle?

Power Angle Curve tells us about the electrical power output of a synchronous machine when power angle δ is varied. It can be seen from this curve that as we increase δ from 0 to 90°, the output increases sinusoidally.

Define Propagation Constant.

Propagation constant is a measure of changes in a sinusoidal electromagnetic wave in terms of amplitude and phase, while propagating through a medium. This can be a transmission line or free space.

Propagation constant (γ) = α + jβ

α = Attenuation constant
β = Phase constant

List out two methods of grading of Cables.

  • Capacitance Grading: Capacitance grading is done by employing various layers of different dielectrics having different permittivity’s between the core and the sheath
  • Inter-sheath Grading: In this method, instead of using various dielectrics and having a composite dielectric, we use a homogeneous dielectric material

Write down empirical formula of calculating Corona Loss.

Corona loss in kW/km/phase is given by
{P_c} = 243.5\frac{{(f + 25)}}{\delta }\sqrt {\frac{r}{D}} {(V - {V_d})^2}x{10^{ - 5}} 
V = Phase voltage in kV (rms value)
Vd = Disruptive critical voltage in kV (rms value)
r = radius of conductor in metres
D = Spacing (or equivalent spacing) between conductors in metres
f = system frequency

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