Electrical Power Transmission: Why the Grid Uses High Voltage

Getting Electricity from the Power Station to Your Home

Power stations are often far from the cities that use their electricity. The hydroelectric dams at Kainji and Jebba are on the River Niger, hundreds of kilometres from Lagos. Sending large amounts of power over such distances through ordinary cables would waste most of it as heat. The solution is to transmit electricity at very high voltages and step the voltage down near the users. This tutorial explains electrical power transmission: why power is lost in cables, why high voltage reduces the loss, the role of transformers, how the national grid is organised from generation to distribution, and why the grid uses alternating current. It builds on the tutorials on current electricity and electromagnetic induction.

Why Power Is Lost in Transmission Lines

Transmission lines are long, so even thick aluminium conductors have resistance R. When a current I flows through them, power is lost as heat:

P(loss) = I²R.

The loss depends on the square of the current. Halving the current reduces the loss to a quarter; dividing the current by 10 reduces the loss a hundredfold. So the key to efficient transmission is to keep the current small.

Why High Voltage Reduces Losses

The power a station sends out is P = VI. For a fixed power P, the current is I = P/V. Raising the transmission voltage lowers the current in proportion. Substituting into the loss formula:

P(loss) = (P/V)² R.

So the power loss is inversely proportional to the square of the transmission voltage. Increasing the voltage 10 times reduces the loss 100 times.

Example 1. A power station sends 100 MW along transmission lines with a total resistance of 10 Ω. Compare the losses if it transmits at 132 kV and at 330 kV.

At 132 kV: I = 100 × 10⁶ / 132 × 10³ ≈ 758 A. Loss = 758² × 10 ≈ 5.7 × 10⁶ W = 5.7 MW, about 5.7% of the power.
At 330 kV: I = 100 × 10⁶ / 330 × 10³ ≈ 303 A. Loss = 303² × 10 ≈ 0.92 × 10⁶ W = 0.92 MW, under 1%.

At the generator voltage of around 11 kV, the current would be about 9,100 A and the calculated "loss" would be larger than the power being sent, which simply means it cannot be done: the voltage drop along the line (IR ≈ 91,000 V) would exceed the supply voltage. High-voltage transmission is not just more efficient; it is what makes long-distance transmission possible at all.

A common mistake. Students sometimes use P = V²/R to argue that a higher voltage gives a higher loss. That formula needs the voltage across the resistance of the line (the voltage drop IR), not the transmission voltage between the line and earth. Always calculate the current first, then use I²R.

Thicker Cables and Other Ways to Cut Losses

Losses can also be reduced by lowering R: using thicker conductors or materials with lower resistivity. But thick cables are heavy and expensive, and need stronger pylons. Overhead lines therefore use aluminium (light and a good conductor) wound around a steel core for strength. For long-distance power transmission, raising the voltage is far cheaper than making cables much thicker, though very high voltages need tall pylons, long insulators and wide clearance for safety.

Transformers: Changing the Voltage

A transformer has two coils wound on a shared soft iron core. An alternating current in the primary coil produces a changing magnetic flux in the core, which induces an alternating e.m.f. in the secondary coil. For an ideal transformer:

Vs / Vp = Ns / Np and, since no power is lost, Vp Ip = Vs Is.

A step-up transformer (more turns on the secondary) raises the voltage and lowers the current.
A step-down transformer (fewer turns on the secondary) lowers the voltage and raises the current.

Example 2. A generator produces 16 kV. What turns ratio is needed to step this up to 330 kV for transmission?

Ns/Np = 330/16 ≈ 20.6. The secondary needs about 21 times as many turns as the primary. The current in the line is then about 1/21 of the generator current.

Real transformers are very efficient, often more than 99% for large units, but they do lose some energy through:

resistance of the copper windings (reduced with thick wire);
eddy currents in the core (reduced by making the core from thin, insulated, laminated sheets);
hysteresis, the energy needed to keep re-magnetising the core (reduced by using soft iron or special silicon steel);
flux leakage, where some flux from the primary misses the secondary (reduced by winding the coils on top of each other).

Power Transmission on the National Grid

In Nigeria, electricity flows through these stages:

Generation: gas-fired, hydroelectric and (increasingly) solar plants generate a.c. at around 11 to 25 kV.
Step-up: transformers at the power station raise this to 330 kV or 132 kV.
Transmission: the Transmission Company of Nigeria (TCN) carries power across the country on high-voltage lines supported by pylons.
Transmission substations: transformers step the voltage down to 132 kV and then 33 kV.
Distribution: distribution companies (DisCos) carry power at 33 kV and 11 kV to local areas, where transformers on poles or in small substations step it down to 415 V between phases, which gives 230 V for homes.

The many generators across the country are connected together and all run in step at 50 Hz, so that power can flow wherever it is needed and one failure doesn't black out a whole region. When demand suddenly exceeds supply, the frequency falls; if it falls too far, protective systems disconnect parts of the network, which is one cause of widespread grid collapses.

Why the Grid Uses Alternating Current

Transformers only work with alternating current, because they need a changing magnetic flux to induce a voltage. A.c. can therefore be stepped up and down easily and cheaply, which was the main reason it won over d.c. for power systems in the late 1800s.

However, very long-distance links and undersea cables sometimes use high-voltage direct current (HVDC). Modern power electronics can now convert between a.c. and d.c. efficiently, and HVDC avoids some a.c. losses over very long distances and allows grids running at different frequencies to be linked.

Safety and the Environment

High-voltage lines can cause arcing across air gaps, so nobody should fly kites, climb pylons or build close to transmission lines.
Step-down transformers near homes must be fenced and maintained; vandalism and illegal connections cause fires and electrocutions.
Transmission losses mean more fuel must be burned for the same useful energy, so efficient grids also cut costs and emissions.

Summary

In power transmission, the power lost in cables is I²R, so it depends on the square of the current.
For a fixed power P = VI, raising V lowers I; the loss is proportional to 1/V².
Transformers change voltage using Vs/Vp = Ns/Np and work only with a.c.
The grid steps power up to 330 kV for transmission and down through 33 kV and 11 kV to 230 V for homes.
Laminated cores and thick windings make transformers more than 99% efficient.