Effects On Transmission Lines




In DC system, there is an equal distribution of current in the cross section of the conductor.Because the frequency of DC system is 0Hz. Therefore, there is no skin effectpresent in dc systems.

But in case of ac system, the frequency is alternating in nature. Due to this the current tries to flow near the surface of conductor.This current flowing near the surface of the conductor is known as skin effect.

Figure 1

For example, consider a solid round conductor carrying current as shown in fig 1.

  • Each strand will be having some inductance.
  • Higher inductance is present at the centre of the conductor.
  • High inductancealso means higher reactance and therefore, the inner strands will oppose the flow of current.
  • As the outer strands have less reactance, therefore, the current tries to flow through the skin of conductor and as a result all the current tries to concentrate near the surface/skin of conductor.

Skin effect depends upon:-

  1. Type of conductor material
  2. Frequency
  3. Diameter of conductor
  4. Shape of conductor.

Where the skin effect can be found?

  • In transmission lines.

Proximity effect

Proximity effect occurs when there is current carrying conductor which is placed near another current carrying conductor.

Figure 2

Proximity effect is very similar to the skin effect. The skin effect is said to take place when there is no other current carrying conductor placed in its vicinity.

  • When 2 conductors carry current in the same direction then the current density is more in the remote areas.
  • Thus,the fluxes in the adjacent areas cancel out.
Figure 3
  • When 2 conductors carry current in opposite direction, the current density is more in the adjacent areas because the fluxes get added of the individual conductor (shown in fig 3).

“Can Proximity effect also occur within electrical cables?”

  • For example, if the conductors are a pair of audio speaker wires, their currents have opposite direction, and currents will favorably flow along the sides of the wires that are facing each other. The AC resistance of the wires will change (slightly) along with the frequency of the audio signal, though for any frequency, the amplitude of the current will still be linearly proportional to the voltage. Some believe that this will potentially introduce distortion and degrade stereo imaging. However, it can be shown that, for reasonable conductor sizes, spacing, and length, this effect has an insignificant influence on audio quality.

Ferranti Effect

This effect occurs in ac system only and not in dc.
In this effect, we find that at light loads or at no load the sending end voltage is less than the

Figure 4

receiving end voltage due to the high capacitance of long transmission lines.

This rise in voltage at the receiving end voltage is the Ferranti effect.This voltage rise is because the charging current leads the receiving end voltage by 900. The charging current causes a resistive drop in the phase.

Compensation for Ferranti effect

Shunt Reactorcompensation at the receiving end might help to reduce the effect of Ferranti Effect. Shunt Reactor absorbs the excess reactive power generation during no load / light load condition and thus helps in stabilizing the voltage of Transmission Line.

But for longer lines of about 400 km and above compensation has to be provided somewhere in the middle of the line in addition to the receiving end. For very long line compensation is needed after every 250 to 300 km.

In AC power distribution networks,

  • Power factor is a term standing for real power versus apparent power.
  • Real power is the power in the transmission line that produces the working power to appliances on the receiving end.

When compensating for Ferranti effect, the real power needs to be differentiated from apparent power, and the requisite amount of step-down of power should be applied through shunt reactors to provide the counter effect to build-up of voltage.

Additionally, line engineers can use the electronic switching on and off of capacitors to make adjustments as needed when monitoring informs of Ferranti effect overloads.



Adapted from Electrical Power Systems by Sunil S Rao

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