[LINK] transmission losses (was Re: Past the Black Stump - Was Hot Rocks

[Greg Taylor]

On 2011/04/05 11:09 AM, Chris Maltby wrote:
> On Tue, Apr 05, 2011 at 12:38:53AM +1000, Greg Taylor wrote:
>> A (simplistic) back of the envelope calculation:
>>
>> Power loss P =  I^2 * R   (Joule Effect)
>>     where I = current, and R=resistance
>>
>> For a 2MW plant and a transmission line voltage of 220kV, we have:
>> Current I = power/voltage = 2000000/220000 = 9.09A
>>
>> For power loss to be 2kW over a distance of 1km, resistance would have
>> to be:
>> R = P/(I^2) = 2000/(9.09*9.09) = 24.2 ohms per km.
>>
>> That is about the resistance of 18AWG (1mm diameter) copper wire at 25C
>> (1). Transmission wires are much thicker than that, and therefore of
>> much lower resistance, e.g. 0 AWG copper (9.5mm diam) has a resistance
>> of 0.329 ohm/km.
>>
>> Your point about transmission line length being a factor with this
>> technology is well taken, but your loss figure of 2kW/km seems to be
>> overstated.
>>
>> Greg
>>
>> (1) Source:
>> Wire Resistance and Voltage Drop Calculator
>> http://www.stealth316.com/2-wire-resistance.htm
> It's a long time since I was required to study some of this stuff but I
> think there are factors relating to it being three phase AC transmission
> that also need to be included. The 220kV may be the inter-phase voltage
> not the phase-neutral voltage which would increase the current and the
> losses from your calculation. But the fact that the power is to be
> transmitted over three conductors further reduces the transmission
> losses a lot, though there are many complicating factors including
> out-of-phase issues for the current and voltage.
>
> Also 220kV is a lowish transmission voltage.  The choice of transmission
> voltage is a trade off between ongoing losses and upfront construction
> costs. That balance would be affected by the floor price of electric
> power with increases in that price tending to favour more efficient
> transmission voltages. Also, 2MW is a lowish power for long distance
> transmission.  It would probably make more sense to use that power
> closer to the point of generation...
>
> Chris
>

These are valid points. The 2MW figure came from Tom's initial 
information, which he now says should have been 50MW. Even a 50MW plant 
is clearly only a demonstration project to prove the power generation 
technology. There would be no point in connecting such a small plant to 
the national grid over long distances because of the transmission line 
cost. A commercial plant would probably need to be 1000MW or more to be 
viable.

I'm sure the backers of these projects have done their sums and figured 
out whether a commercial-scale plant in these remote locations is feasible.

Greg