Electricity and Magnetism

Mr D. Patterson

Outcomes• explain why electrical energy is transmitted as

AC at very high voltages, and describe and explain the impact on everyday life of electrical power generation and transmission—this will include applying the relationships:

Power Transmission

Power Transmission

Australian homes use 240 V

Power requirements for a city are in the order of thousands of MWP=VI

If power plants supplied 240 V, current requirements would be millions of amperes.

This needs a cable about 1 m in diameter

Can we make such a wire?• A 1m diameter power line, running even 10 m

along a highway would weigh 211 kg and cost $400 as of 2013.

• There is a better way

Power Transmission

Australian homes use 240 V

Power requirements must be met. P=VISupplying a larger voltage will mean less current is required

Power plants can produce much higher than 240 V, but not nearly high enough.

A step up transformer is used to up the voltage to a few hundred thousand volts for transmission. This requires AC, as DC does not work with transformers

Power lossThe amount of energy lost through the wires is also reduced when using high voltage, low current transmission

240 V OR 500, 000 V?

How much power loss?Try this calculation: City X needs 200 MW.1. How much power is lost through a power line of resistance 4.00 Ω when transmitted at 240 V?2. How much power is lost when it is transmitted at 500 kV?

Important: We do not usebecause V would be the voltage drop across the power line, not the voltage supplied to the city.

Power loss• The power lost through a transmission line is

proportional to current2

The voltage is stepped down in stages as the power grid splits up into suburbs, streets and then individual houses.

AC or DC transmission• Power is typically transmitted as AC because it

can easily be stepped up and down using transformers

AC or DC transmissionAC wastes more energy than DC at the “same voltage”

800 kV

The highest voltage is around 800 kV before wires will ionise the air around them causing a short circuit

A lot of the power is wasted at lower voltages of an AC transmission

The average voltage is called the RMS voltage

A DC transmission could transmit at the highest safe voltage all the time, which is more efficient than AC

DC voltage

AC or DC transmission

The world mainly uses AC because despite the energy loss draw backs, it is cheap to step up and step down.

As we find cheaper ways to step up and down DC, this may change in the future.