Important Short Questions and Answers: Modelling and Performance of Transmission Lines

63.What is corona?

The phenomenon of violet glow, hissing noise and production of ozone gas in an overhead line is called corona.

64.Mention the factors affecting corona.

Atmosphere, conductor size, spacing between conductors and line voltage.

65.Define critical disruptive voltage.

It is defined as the minimum phase voltage at which corona occurs.

66.Define visual critical voltage.

It is the minimum phase voltage at which corona appears all along the line conductors.

67.State any two merits of corona.

Reduces the effects of transients produced by surges .System performance is improved.

68.Give the two demerits of corona.

The transmission efficiency is affected. Corrosion occurs.

69.Mention the methods of reducing corona effect.

By increasing the conductor size and conductor spacing.

70.Why ACSR conductors are used in lines?

If the size of the conductor is larger corona effects are reduced and reduces the proximity effect .Hence they are used in lines.

71.Define medium lines.

Lines having length between 60 and 150 km and line voltages between 20 and 100kv are called medium lines.

72.Define short lines.

Lines having lengths below 60km and voltages below 20kv are called short lines.

73.Explain the term voltage stability.

The ability of the system to maintain the voltage level within its acceptable limits is called as voltage stability.

74.Differentiate between voltage stability and rotor angle stability.

Voltage stability: -It means load stability. -It is mainly related to reactive power transfer. -Here problems arise mainly in the event of faults.

Rotor angle stability: -It means basically generator stability. -It is mainly interlinked to real power transfer. -Here problems arise during and after faults.

75.What is shunt compensation ?

Shunt compensation is the use of shunt capacitors and shunt reactors is the line to avoid voltage instability.

76.Define a synchronous compensator (condenser)?

Synchronous compensator is a synchronous motor with no mechanical output .When it is under exited it operates at lagging p.f (ie it delivers vars ) .Thus it operates both as a shunt capacitor and as a shunt reactor .

77.Mention any two advantages of series compensation

improves system stability .

 are generally self regulatory and requires little or no control equipment

78.            State any two comparisons between series compensation and shunt compensation

Series compensation is cheaper than SVS .

  Losses are lower than in SVS .

79.            Why series compensation is used in long series ?

to increase transmission capacity

to improve system stability .

to obtain correct load division between parallel circuits .

80.Mention any two advantages of SVS .(Static Var System )

Provides fast control over temporary over voltages. Provides a better control of voltage profile .

81.Mention the components used in series compensation ?

Series capacitors Spark gap protection Metal oxide resistor (for protection ) Current limiting reactor Bypass switch Control & protection equipment .

82.Mention the components used in static var compensator .

Saturated reactor Thyristor controlled reactor –fired capacitor (TCR –FC )

Thyristor switched capacitor – thyristor controlled reactor (TSC –TCR )

Harmonic filters.

83.Give any two reasons to minimize the reactive power transfer in lines .

leads to voltage stability.

results in greater real and reactive losses

84.List the four parameters by which the performance of transmission lines are governed.

Series resistance Series inductance Series capacitance Series conductance.

85.Define Ferranti effect .

Under no load or lightly loaded conditions ,the capacitance associated with the line generates more reactive power than the reactive power which is absorbed & hence thevoltage at the receiving end is found to be greater than at the sending end . This is called as Ferranti effect