State the advantages of high-voltage transmission

4.5.6 The Transformer

A transformer is a device that changes the voltage of an alternating current (AC) without changing its frequency. It works on the principle of electromagnetic induction: a changing magnetic field in the primary coil induces a voltage in the secondary coil. The ratio of the number of turns in the secondary coil to the primary coil determines the voltage change: $$\frac{V_s}{V_p} = \frac{N_s}{N_p}.$$ Because power (ignoring small losses) is conserved, $P = V\,I$, a higher voltage means a lower current for the same power. This is the key to efficient long‑distance power transmission.

⚡ Advantages of High‑Voltage Transmission 🚀

• Reduced I²R losses: Power loss in a conductor is $P_{\text{loss}} = I^2 R$. By increasing voltage and reducing current, the loss drops dramatically. For example, transmitting 100 MW at 400 kV uses only 250 A, whereas at 20 kV it would require 5 kA!
• Smaller conductors: Lower current allows the use of thinner wires or less copper, saving material costs and making the lines lighter and easier to install.
• Long‑distance capability: High voltage keeps the power loss per kilometre low, enabling electricity to travel from remote power stations (e.g., hydro plants in the mountains) to cities far away.
• Better voltage regulation: Transformers can step down the voltage at the consumer end to safe levels, ensuring consistent supply even when the grid experiences fluctuations.
• Enhanced reliability: With lower currents, the risk of overheating and fire is reduced, making the grid safer.

🔌 Water‑Pipe Analogy

Think of the power grid like a water supply system. If you want to deliver a large amount of water (power) over a long distance, you use a big pipe (high voltage) to reduce friction losses. A smaller pipe (low voltage) would cause a lot of pressure drop (I²R loss), just like a high current would heat up the wires. At the destination, you use a valve (transformer) to reduce the pressure to a safe level for your tap.

📚 Exam Tip Box

1. Remember the key equation: $P = V \times I$. High $V$ → low $I$ → low $I^2R$ loss.
2. Use the water‑pipe analogy to explain why high voltage is better for long distances.
3. When answering “advantages of high‑voltage transmission”, list at least three points and give a brief example for each.
4. Draw a simple diagram of a transformer with primary and secondary coils labelled, and note the turn ratio.
5. Practice converting between units: kV, MW, A, and calculate $I$ for a given $V$ and $P$.

📊 Comparison Table