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

[Chris Maltby]

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