The National Grid relies on transformers. There are at least five transformers between a power station and a domestic user.

This is because we use high voltage for carrying the electric current over large distances and we use small voltages in our homes. The high transmission voltages are to reduce large losses from joule heating in the transmission cables. And the low voltages in the home are for reasons of safety.

If we use a high transmission voltage, then the current that needs to flow to the town will be less (and still deliver the same power). A smaller current causes less heating in the transmission cables. So there is less waste.

The table below shows the calculations.

	Quantity Formula used Low V High V Power requirement of town (W) Pout 100, 000, 000 Supply voltage at the town (V) Vout 20, 000 400,000 Current needed to provide the power (A) Pout = Vout x I 5, 000 250 Joule heating in power cables (W) Pwaste = I2R 75,000,000 187,500 Power that the power station needs to supply (W) Pin = Pwaste + Pout 175, 000, 000 100,187,500 Efficiency of system (%) eff = (100 * Pout) ÷ Pin 57.1 99.8
	Table 4. Worked examples for a low transmission voltage (20 kV) and a high voltage (400 kV). Resistance of cable is taken to be 3W.

Notice that when the transmission voltage is 20 times bigger, the current needed is reduced by a factor of 20 as well. However, the wasted power is reduced by a factor of 400 (= 20²). This is because the joule heating depends on the square of the current.

Most transformers in the system were installed during the 1960s when demand for electricity increased rapidly. The efficiency of many of these 40 year old transformers is poor compared to the best practice of today. In fact, relatively poor transformers are still being installed so the system isn't necessarily improving with new transformers.

It is therefore desirable to manufacture and install more efficient transformers.

Losses occur:

• in the primary coil and secondary coil (because of their electrical resistance)
• in the core (because energy is lost as the magnetic field changes, and because small currents are induced in the core, and these waste energy).

The core losses are minimised by using high quality steel for the core.

The losses in the coils are known as load losses. They increase with the square of the current being taken from the transformer. Load losses are minimised by using copper windings for low resistance and by using appropriate construction techniques to reduce eddy current losses.