The Problem: Power Loss
When electricity travels through transmission lines, some of the electrical energy is inevitably lost as heat. This power loss is a fundamental issue in electrical engineering that directly impacts the efficiency and economics of power systems.
The power loss in transmission lines is governed by Joule’s Law:
Where:
- Ploss = Power lost as heat (watts)
- I = Current flowing through the conductor (amps)
- R = Resistance of the conductor (ohms)
This equation reveals a critical relationship: power loss is proportional to the square of the current. This means that if the current doubles, the power loss increases by a factor of four!
For transmission system operators, this presents a significant challenge. Power loss represents:
- Wasted energy that must be generated but not delivered to consumers
- Reduced efficiency of the entire power system
- Increased operational costs
- Environmental impact from burning additional fuel
The Solution: High Voltage
To understand how voltage helps solve the power loss problem, we need to consider the relationship between power, voltage, and current:
Where:
- P = Power transmitted (watts)
- V = Voltage (volts)
- I = Current (amps)
This fundamental equation shows that for a given amount of power (P), we can either:
- Transmit at high voltage and low current, or
- Transmit at low voltage and high current
Since power loss is proportional to the square of current (I²), the first option is clearly superior. By increasing the voltage, we can significantly reduce the current required to transmit the same amount of power, thereby dramatically reducing power losses.
For example, to transmit 1,000,000 watts (1 MW) of power:
- At 10,000 volts: Current = 100 amps, Loss = 100²R = 10,000R
- At 100,000 volts: Current = 10 amps, Loss = 10²R = 100R
In this example, increasing the voltage by a factor of 10 reduces the current by a factor of 10, but reduces power loss by a factor of 100!
The Result
The use of high voltage transmission results in several significant benefits:
- Transmission losses are typically kept below 5-7% of total power transmitted
- This makes long-distance power transmission economically viable
- Reduces the need for additional generation capacity to compensate for losses
- More electrical energy reaches the end consumer
- Reduced fuel consumption at power plants
- Lower carbon emissions per unit of electricity delivered
- Reduced conductor size requirements (lower current means thinner wires)
- Lower operational costs due to reduced losses
- More economical long-distance power transmission
Voltage Transformation
To make high-voltage transmission practical for end users, transformers are used throughout the power system:
- Located at power plants
- Increase voltage from generator levels (11-33 kV) to transmission levels (220-765 kV)
- Enable efficient long-distance transmission
- Located at substations near population centers
- Reduce transmission voltage to distribution levels (11-33 kV)
- Further step-down transformers reduce voltage to consumer levels (120/240 V)
Modern Transmission Systems
Today’s power transmission systems utilize extremely high voltages:
- Extra High Voltage (EHV): 220-765 kV
- Ultra High Voltage (UHV): 800-1,100 kV
These high voltage levels enable the transmission of massive amounts of power across continents with relatively low losses, making it possible to:
- Connect remote renewable energy sources to population centers
- Create wide-area power grids for improved reliability
- Share electricity between regions and countries
