Role of Transformer in Transmission System : Technical Aspects

Why we need high voltage in transmission system ? We have discussed Why we need Transmission System and in current article we'll discuss why a very high voltage is chosen for transmission of electric power transfer from generating point to load centers ?

Role of Transformer in Transmission System

Transformer are used to raise the level of voltage that is generated at generated point and transformers are also used to reduce the level of voltage near load centers as discussed in Basics of Transformers and Working of Transformer.

Why High Voltage is Used in Transmission System

Why very high voltage is chosen for transmission ? This is the question that struck into the mind of not only an electrical engineering student but also the mind of a common person. Here are some technical reasons why we choose very high voltage for transmission ?
Following are the benefits of using very high voltage for transmission :

• Small Conductor Size:
  Let us consider we have to transmit a power of W watts over a distance L. Now consider two situations
  a) Power is being transmitted in line at V volts
  a) Power is being transmitted in line at 2V volts
  We know that P=VI cos Φ,
  Where
  P=Power in the system
  V=Voltage of the line
  I=Current in the line
  CosΦ=Power factor of line under consideration [ Considering it constant/same in both cases ]
  in first case I=P/V=k(say) but for second case I=P/2v=k/2.
  Hence double the voltage level half will be the current in the line.
  We know that for a small current small area of cross section of the conductor is required as transmission lines are designed at a constant current density (Current/Area). This shows that increase in transmission voltage will result in reduction of conductor area and hence conductor weight. Thus an increase in voltage level will result in saving of conductor cost.
• Reduction In Copper Losses :
  We know that Copper Losses, P_Cu = I²R
  Where, P_Cu = Copper loss in transmission Line
  I=Current in Line
  and R= Resistance of Line.
  Now resistance of line is constant (once installed) so if we increase voltage level then this will reduce the copper losses in the transmission line which further result in saving of power and ultimately saving of money.
• Better Voltage Regulation :
  Voltage Regulation is given by the formula,
  V_Reg=(V_nl-V_fl)/V_fl
  Where V_nl= Voltage at no load
  V_fl= Voltage at full load
  For a transmission line V_fl=V_nl-V_Drop
  V_Drop being drop at the transmission line and this is given by V_Drop=I*R, now if I is reduced in line then Voltage regulation will be reduced and operation of machines and line will be good.

If we can generate power at high voltage and use it at high voltage level

Now the question arises, If there are many benefits of increasing voltage in transmission line, then why we use transformers, why shouldn't we generate and supply directly at high voltage level ? The answer to this question is that, It is not possible due to insulation problems in generators and also due to safety reasons this is not practical. Also our systems are designed for a low voltage so it can not be generated or consumed at very high level.

What are limits to Voltage Level in Transmission System

As I said the insulation is a big issue. We use transformers which has to withstand the voltage in the line but there is only a limited voltage for which insulation can be provided. Further if we increase voltage then for very high voltage we'll require high steel tower and at a point cost of installing line will be more then cost saved due to high voltage. Hence these things limit the voltage level in transmission line.