Why is Power Transmitted at High Voltage Instead of High Current?

Electrical Technology

Why is High Voltage Used for Power Transmission and Not Lower Voltage?

Transmitting electrical power at high voltage and low current is a more efficient and economical approach for long-distance power transmission, as it helps reduce resistive losses, minimizes the need for expensive conductors, and improves overall system efficiency. In the following article, we will explain why high voltage is chosen for power transmission instead of low voltage or high current, citing various valid reasons.

Power is transmitted at high voltage instead of high current for several practical and economic reasons. The power (P) in an electrical system is calculated using the formula:

$P = V I$

Where:

$P$ is electric power,
$V$ is voltage, and
$I$ is electric current.

From this formula, you can see that power is directly proportional to both voltage and current. However, when transmitting power over long distances, it is more efficient to use high voltage and low current for the following reasons:

Reduced I²R Losses:

- Power losses in a transmission line are directly proportional to the square of the current ( $I^{2}$ ) and the resistance ( $R$ ) of the line. High current results in higher I²R losses, leading to significant energy loss as heat dissipation.
- By increasing the voltage, you can reduce the current for a given amount of power. This reduction in current helps minimize resistive losses in the transmission lines, making the overall system more efficient.

Voltage Drop:

- Voltage drop along a transmission line occurs due to the resistance of the line. Higher current results in a larger voltage drop, which can lead to a drop in the quality and efficiency of power transmission.
- Transmitting power at high voltage helps to minimize the voltage drop, ensuring that the power delivered to the end-users is closer to the generated power.

Economical Use of Conductors:

- Conductors (wires) used in power transmission have a cost associated with their production and installation. Higher current requires larger, more expensive conductors to handle the increased load.
- The volume of the conductor is inversely proportional to the square of voltage (VcosΦ) when power ‘W,’ the length of the transmission line (L), resistivity of the conductor (ρ = ρ), and power loss in transmission lines are constant. This implies that the higher the voltage level, the lower the volume of conductor needed.
- By transmitting electric power at high voltage, the current is reduced, allowing the use of smaller size and less expensive conductors.

Transformer Efficiency:

- Transformers, which are used to step up or step down voltage, are more efficient when working with higher voltages. Higher voltage transmission reduces the number of times voltage needs to be transformed along the transmission path, minimizing losses.
- In this regard, choosing high voltage is the best option for smoothly managing the transmission and distribution of electrical energy in various applications.

Regulation Standards:

- National and international standards often dictate the maximum allowable voltage drop and minimum power factor for power transmission. Using high voltage allows utilities to meet these standards more effectively.
- The selection of high voltage should be kept within safe limits. If the voltage is increased beyond the suitable level, the overall cost will rise due to the need for additional insulation, transformers, switchgears, lightning arresters, poles and towers, conductors, insulators and other components and devices used in substations, switchyards, and power stations. Furthermore, it will increase the fault level, meaning that the likelihood of a fault occurring will be higher.

Good to Know:

Extra High Voltage (EHV) and Ultra High Voltage (UHV), ranging from 380 kV to 800 kV and beyond, are effectively used in overhead transmission lines for power transmission.
High Voltage (HV) up to 150 kV is employed in underground cables for both power transmission and distribution.

What Would Happen If We Were to Use High Current Instead of High Voltage for Power Transmission?

If we opt for high current instead of high voltage for power transmission and distribution, the cost of systems will increase, reducing the overall system efficiency.

While high current can be used for short-distance power transmission, it is not practical for long-distance transmission due to the increased power losses (I²R), higher costs, large size devices (large kVA rating of equipment i.e. alternator, transformer, switchgears and conductors, etc.) and safety concerns associated with elevated current levels.

Transmitting power at high current instead of high voltage will have the following impacts on the power system.