Power transmission systems | High voltage vs High current

I remember my first year studying electrical engineering and I got to know that bulk electrical power is transmitted as High voltage Low current, since the product of current and voltage gives power (P = IV) so whether it is high current-low voltage or high voltage-low current or same quantity of current and voltage, as long as their product gives the desirable power, all is fine. It sounded like a good argument but, practically power is transmitted at high voltage-low current. Why is like that and not the other way around?

^{High voltage transmission lines. from maxpixel under the cco license}

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ELECTRICITY BASICS

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First we need to understand some basics about electricity and electric power transmission. Like I said earlier, power can consist of voltage and current, and it could also be a relationship between current and resistance, so that the power will be a product of the square of the current and the resistance to the flow of current (P = I2R). Current is the flow of electric charge, Voltage is the potential energy per unit charge, or simply put is the force that pushes current to flow in a circuit, Resistance is the opposition to the flow of electric current. Something interesting is that when current is made to flow through a conductor or cable, with a finite resistance, there will be loss along the conductor which will be both voltage drop (product of current and resistance, V = IR) and power loss (product of the square of current and resistance, P = I2R). The magnitude of the resistance a cable can offer to current flow will depend on:

• the resistivity of the material the cable is made of (copper or aluminum);
• the length of the conductor;
• the diameter (cross-sectional area).

There exist a relationship between these properties that affect the resistance of a conductor. The product of the resistivity ρ, and length l, is divided by the cross-sectional area (ρl/A). This means that if the resistivity of the cable increases, the resistance will increase correspondingly, also if the length of the cable increases, the resistance of the cable will also increase, but the same will not be said of the cross-sectional area because if it increases (if we make the cable bigger) the resistance of the cable will reduce, and this is simply because more current will pass through without opposition. This relationship will be very important throughout the article.
Now, after understanding these basics we can move on to understand why we have to transmit power at high voltage.

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Why High Voltage

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To reduce the volume and weight of the conductor

When electric energy is generated at the generating station, which is usually a lot of distance from where it is needed, and as such long length of cables will be required to transmit the electric power. So in order to reduce the volume of the entire cable, the cross-sectional area has to be reduced. How can this be achieved? By increasing the voltage to a sufficiently high value. The relationships described earlier will be used to demonstrate how it is possible. This will reduce the cost of transmission lines.

Reduction in line losses

Resistance increases as the length of the cable increases, as it has been established. For transmission lines, the length of the cable required is a lot of kilometers, which means the resistance of the cable too will be very high. If power is transmitted over that long distance with increased value of current, possibility is that the power will all be lost to heat due to very high resistance and as such may not get to its destination.
We know that (R= ρl/A) so since the length required is very much, the resistance of the cable will increase greatly. Also we know that power loss, W = I2R so if we transmit at high current over a long distance, the loss will very great. But a sufficient decrease in the current present in the transmission will reduce the losses on the line. To maintain the same electric power required for the transmission, the voltage has to be raised corresponding to the decrease in the current. Hence, high voltage therefore decreases the loss on the line.

Increase in Transmission Efficiency

Transmission efficiency is measured as a percentage of input power that reaches the output. A transmission line is said to be efficient, if its efficiency is high enough. Realistically, its efficiency cannot be 100%, as this will mean there is no loss along the line, which is practically impossible. Input power = output power + power losses This means that, the higher the losses, the lower the output power (power at the receiving end). Transmission efficiency = (power output / power input) X 100% The mathematical analysis shows that high voltage transmission helps to increase the transmission efficiency.

Improved Voltage Regulation

If power is transmitted at high voltage, current will be low and this means that there will be lesser voltage drops along the line, and the voltage regulation will be increased. But then what is voltage regulation? Voltage Regulation is defined here as, the drop in receiving end voltage from no load to full load expressed as percentage of Receiving end voltage.
It is expressed in percentage as ( Vs-Vr)/Vr X 100
Where

Vs is the voltage at the sending end
Vr is the voltage at the receiving end.
At no load, the receiving end voltage is the same as the sending end voltage since there is no loss along the line.
At full load the receiving end voltage is less than the sending end voltage.
The difference in full load and no load voltage with respect to the receiving end voltage is the voltage regulation. This means that when the percentage regulation is low, it is better for the transmission line, and this is only possible if the voltage drops along the line is low. The voltage drop can only be low if the transmission current is low as well.

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The high voltage transmission obviously has proven to be the realistic method for bulk electric power transmission. It however has its own inherent limitations, but do not out-weigh the advantages. It is important to also consider this limitation so as to learn.

Limitations of High Voltage Transmission

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• Insulation Requirement

Since the voltage on the line is very high, the insulation between the cables also increases as well as the insulation between the conductors and the earthed towers or transmission towers must also increase, and that means more cost on the line.

• Tower Height
  Due to the high voltage across the line, there is a standard distance the conductors must maintain from the ground to ensure safety of humans and animals. This mean that the transmission towers must be sufficiently high to achieve that. High towers mean more cost.

• Corona loss and Radio Interference
  If you do not understand the concept of corona, just check through my previous post. Very high voltages causes corona losses and can generate signals whose frequencies can cause radio interference or Electromagnetic interference.

• Transformer Requirement
  Bigger transformers will be required to step up to a high voltage at the generating station and also to step the voltage down at the receiving stations. Cost of incurring and maintaining these transformers is another disadvantage.
  
  High voltage transformer from wikimedia commons

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Regardless of the limitations, it is only realistic to transmit bulk electric power over a long distance at High voltage.

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CONCLUSION

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As a student, this part of electrical engineering was interesting to learn, and am always eager to teach others when the need arise! I do hope that you have learnt something new or reminded yourself of past memories. Drop your comments and let us relate.

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References Used

1. High voltage Transmission Advantages and disadvantages
2. Advantages of very high voltages for transmission Purpose
3. Voltage Regulation and Efficiency of Transmission Line
4. Limitations of EHV Transmission

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