What are the Limitation of EHV Transmission line?

What are the Limitation of EHV Transmission line?

Right Answer is:

All of the above

SOLUTION

Limitations of EHV line

As the line voltage is increased, we encounter some problems which are discussed as follows:

  1. There is more corona loss and radio interference in EHV transmission.
  2. For EHV transmission lines, very heavy supporting structures are required, and hence their erection is difficult.
  3. The insulation level depends on the magnitude of the voltage. Hence, in EHV transmission more insulation is required.
  4. The transmission distance in the EHV AC system is limited and hence. the stability is less. However, this limitation can be overcome by the use of series capacitors or shunt reactors.
  5. Loading of the overhead line conductor is independent of thermal conditions. Although the size of the conductor in the overhead line is determined on the basis of the thermal considerations (continuous and short-term current-carrying capacity), in underground cables, both the thermal and charging current limits are very important, and hence this is the main reason for not using the underground cable for EHV transmission. Moreover, the underground cables are more costly compared to the overhead lines.
  6. The receiving-end voltage of the line is higher than the sending end voltage due to the Ferranti effect. This increment is of the order of 1.5% for 160 km, 13% for 500, km and 100% for 960 km. When the load on the line is suddenly switched off, the sending-end voltage is also increased. Special care has to be taken from these aspects. However, the increase in voltage due to the Ferranti effect can be controlled by using shunt reactors at the load.
  7. SIL is also known as a natural load. When the line is operated on a natural load, the voltage along the entire line is the same. But in practice, a line is not operated on natural load because of the voltage drop due to the inductance and capacitance of the line. This drop does not impose any restriction on the distance over which the power is to be transferred but it fixes the voltage that can be used for transmission. vii) The mechanical design of the tower, its dimensions, phase spacing, sub-conductor spacing, conductor height, etc., are required to be modified for EHV transmission. These modifications affect the electrical design, such as voltage gradient.
  8. The mechanical design of the tower, its dimensions, phase spacing, sub-conductor spacing, conductor height, etc., are required to be modified for EHV transmission. These modifications affect the electrical design, such as voltage gradient.
  9. The electrostatic and electromagnetic fields produced by the EHV transmission line induce currents and voltages in animals, birds, and human beings. Hence, EHV lines should be designed that the maximum electrostatic field gradient does not exceed 9 kV/m at mid-span under the line near the ground level. Moreover, a current of 10-100 mA/m2 causes minor biological effects but beyond 100 mA/m2 it results in health hazards.
  10. EHV transmission and substations produce audible noise due to corona, the humming of transformers, cooling system, and mechanical and electrical auxiliaries. The audible noise (AN) level for lines below 500 kV does not exceed the permissible noise level but above 500 kV, the line must be designed such that the AN level remains within the permissible limits.
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