Ohm's Law and BassLink

by John  L. Daly

Why don't electricity authorities bury long-distance electrical transmission lines underground?  They are unsightly, true.  But there is really no choice - and here's why.

Power is usually generated at long distances from the point of usage.  They are located at coal mines, dams, gas deposits, even wind farms in windswept areas.  The power has then to be sent mainly to the cities where the main users are, and sent there with a minimum loss of energy in the transmission wires, many of them running for hundreds of miles.  Every wire has a property called `resistance' and the longer the wire and the thinner the wire the more resistance there is.  Resistance is measured in a unit called `Ohms'.

The basic Ohm's Law states  Voltage = Current * Resistance,   (or V = I * R for short)

where voltage (V) is in Volts, current ( I ) is in Amps, and resistance (R) is in Ohms.
(  *  is the multiplication symbol)

Note: `Voltage' is analogous to pressure - it is electrical pressure.  Current is different in that it is a measurement of the actual flow of electricity through the wire.  It is comparable to plumbing where water pressure (voltage) and water flow (current) are quite different concepts.

When current passes through a wire, the electrical friction created between the current and the resistance of the wire results in heat being generated, and this heat represents a complete waste of valuable energy.  If the power was originally generated from coal or gas, it also represents emission of greenhouse gases for no gain at all.  So, heat in the transmission wires must be minimised.

Heat in a wire is given by this equation -    Power = Voltage * Current   (or  P = V * I )

where the power is expressed in Watts.

This means that P = ( I * R ) * I    (because ( I * R ) is another way of saying `V'.

The final heat loss is therefore  P = I 2 * R (the two I's are multiplied together to give I 2 )

In other words, the waste heat given off is proportional to the square of the current, all the more reason to keep the current to an absolute minimum.  But if we do that, how do we send useable power hundreds of miles if we insist on keeping the current so small?

Easy.  All we have to do is make sure the basic Ohm's Law equation ( V = I * R) stays in balance.  Since the resistance (R) is fairly fixed, then to make the Current ( I ) as small as possible, we have to make Voltage (V) as large as possible - tens of thousands of volts in fact.  In other words, we trade current for voltage.

The only way we can do that is to suspend the wires in the air, hanging them off high pylons with big ceramic insulators supporting the wires.  These insulators prevent the very high voltage from flashing over to the pylon structure and to ground.

If we put such cables underground, we would need an insulating coating all along the full length of the wire, not just every 200 metres or so as with pylons, and the underground wire  would have to withstand voltages of tens of thousands of volts.

That's neither practical or economic, so such cables must be sent overland on high pylons to get the full benefit of the low current/ high voltage combination.

With BassLink, much of the cable will be undersea and so the voltage will by necessity have to be kept lower and the current level higher in order to prevent flashover.  What BassLink does not need is to have to complete the last leg of its long journey underground in Victoria.  It would involve massive waste of energy, precious renewable energy at that (since most of the power will be from Tasmania to Victoria), and all because some people think the pylons would be unsightly travelling across some fairly ordinary country.  These same people see no problem with ugly wind turbines mounted on pristine coasts.

The bottom line is - where overhead transmission is possible, then avoidance of waste demands that  that option should be used.  Anyone objecting to them is demonstrating that they are not really serious about either conserving energy or reducing greenhouse gas emissions.