Why Electric Power is Transmitted at High Voltage?

Why Electric Power is Transmitted at High Voltage

The demand for electric power has increased tremendously during the past few decades. And it results in increasing the generation and transmission capacity of electric power.

The capacity of a transmission line increases by increasing the voltage level at which electric power is transmitted.

Up to the 1970s, the electric power was transmitted in AC high voltage (up to 220 kV). In the present state of the power system, the power is transferred at high voltage (HV) (up to 300 kV) to extra-high voltage (EHV) (300 to 765 kV) and ultra-high voltage (UHV) (above 765 kV).

In the transmission system, HVDC transmission lines are also used.

But the question is, “Why Electric Power is Transmitted at High Voltage?”

The answer to this question is the advantages of high voltage in a transmission system.

Advantages of High voltage Transmission System

The advantages of transmission system are briefly explained as below.

Reduction in Cross-section Area of Conductor

AC power is given as;

    \[ P = \sqrt{3} V I cos \phi \]

Line current I;

    \[ I = \frac{P}{\sqrt{3} V cos \phi } \]

Resistance of conductor is defined as;

    \[ R = \rho \frac{l}{a} \]

When the current passes through the conductor, a copper loss is produced. And the equation of copper loss is;

    \[ W_C = 3 I^2 R \]

Now, put the value of line current and resistance in the above equation;

    \[ W_C = 3 (\frac{P}{\sqrt{3} V cos \phi }) (\rho \frac{l}{a}) \]

    \[ W_C = \frac{P^2 \rho l}{V_2 cos^2 \phi a} \]

Let’s subject the area of conductor;

    \[ a = \frac{P^2 \rho l}{V_2 cos^2 \phi W_C} \]

From the above equation, we can conclude that the cross-section area of a conductor is inversely proportional to line voltage (keeping other parameters constant).

Therefore, if we increase the transmitting voltage level, the area of a conductor decreases.

Increase Transmission Efficiency

    \[ I = \frac{P}{\sqrt{3} V cos \phi} \]

If line voltage increases, the current magnitude decreases to transmit the same amount of power at the same power factor.

Hence, this results in a reduction of line loss (3I2R) (Copper loss) and increases the efficiency of a transmission system.

Better Voltage Regulation

As explained above, the transmitted voltage increases, the current magnitude decreases, resulting in a reduction of voltage drop (IR+jIXL) and further improving the voltage regulation.

Increase in Transmission Capacity of Line

Power transferred by a transmission line is given as;

    \[ P = \frac{|V_S||V_R| sin \delta}{X} \]

Where,

VS = Sending end voltage
VR = Receiving end voltage
δ = Load angle
X = Line reactance

For better regulation of line, we assume;

    \[ V_S \approx V_R \]

    \[ \therefore P = \frac{V_S^2}{X} sin \delta \]

    \[ \therefore P \propto V_S^2 \]

So, the transmitted capacity of the line is directly proportional to the square of sending end voltage. Thus, the power delivered capacity can be increased by increasing the sending end voltage.

Increase of Surge Impedance Loading (SIL)

Surge impedance loading is the method of expressing the load-carrying capability of a line when it is terminated by a load equal to the surge impedance loading.

The equation of surge impedance loading is given by;

    \[ Z_C = \sqrt{\frac{L}{C}} \]

Where L and C is line inductance and capacitance, respectively

The surge impedance loading for a transmission line is expressed as \frac{3V^2}{Z_C} (V line to neutral voltage).

Therefore, the SIL increases by increasing the voltage level. So, the power transfer capability of the line increases with an increase in voltage level.

Disadvantages Associated with EHV Transmission Line

There are many advantages of high voltage in a transmission line. But there are some disadvantages also, that are listed below.

  • An increase in line voltage also increases the cost of the tower, insulation, and terminal equipment. These costs are directly proportional to the transmission line voltage.
Transmission cost vs voltage level
Transmission cost vs voltage level

The above graph represents the saving of the cost of energy loss and increases in transmission cost due to an increase in line voltage. Hence, the economic voltage level is selected for a particular transmission line distance.

  • The number of insulators in the transmission system increases by increasing the voltage level. And also, the insulation of other equipment that is connected with the high voltage line has to be increased. This will increase the cost of the entire system.
  • Increasing voltage levels complicate the design of switches, circuit breakers, and instrument transformers (CT and PT).
  • In order to provide sufficient clearance between earth and conductor, the height of the transmission tower increases.
  • Due to the high voltage in a transmission system, the corona loss increases. But, by using bundled conductor, it can be reduced.

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