Transmission lines are the long-distance highways that move electricity from renewable energy machines, such as wind turbines, solar farms, and hydroelectric dams, to towns and cities. They matter because the best renewable resources are often far from where people use the most power. High-voltage transmission makes it possible to deliver large amounts of energy across hundreds of kilometers.
Without these lines, much more clean energy would be wasted or trapped near its source.
A transmission line carries electric power using voltage and current, with power given by P = VI. For the same power, raising the voltage lowers the current, which greatly reduces heating losses in the wires because P_loss = I^2R. Step-up transformers raise the voltage near the power plant, and step-down transformers lower it near homes and businesses.
The grid uses towers, conductors, insulators, substations, and control systems to keep electricity moving safely and reliably.
Key Facts
- Electric power is P = VI, where P is power, V is voltage, and I is current.
- Heating loss in a transmission line is P_loss = I^2R, so lowering current reduces wasted energy.
- For a fixed power, increasing voltage decreases current because I = P/V.
- Step-up transformers raise voltage for long-distance transmission, while step-down transformers lower voltage for local use.
- High-voltage transmission lines often operate from about 69 kV to 765 kV, depending on distance and grid design.
- Alternating current is commonly used because transformers can change AC voltage efficiently.
Vocabulary
- Transmission line
- A transmission line is a set of large conductors that carries electric power over long distances at high voltage.
- Voltage
- Voltage is the electric potential difference that pushes electric charge through a circuit.
- Current
- Current is the rate at which electric charge flows through a conductor.
- Transformer
- A transformer is a device that changes AC voltage up or down using electromagnetic induction.
- Substation
- A substation is a grid facility that switches, controls, and changes voltage levels in electric power systems.
Common Mistakes to Avoid
- Thinking high voltage means the wire must carry more current. For the same power, higher voltage actually means lower current because I = P/V.
- Ignoring line resistance in power delivery. Real wires have resistance, so some energy is converted to heat according to P_loss = I^2R.
- Using household voltage to describe transmission voltage. Homes may use about 120 V or 230 V, but transmission lines use thousands to hundreds of thousands of volts.
- Forgetting the role of transformers. Renewable generators and city loads need different voltage levels, so transformers are essential for efficient and safe delivery.
Practice Questions
- 1 A wind farm sends 100 MW of power through a 200 kV transmission line. What current flows in the line? Use I = P/V.
- 2 A transmission line has resistance 5 ohms and carries 300 A. How much power is lost as heat in the line? Use P_loss = I^2R.
- 3 Explain why a grid operator raises the voltage before sending renewable electricity a long distance, then lowers it before it reaches homes.