I thought the loss on long distance high voltage lines was more on the order of 10 or 20%. Is that wrong?
The electrical loss of electricity leaving the power plant, through the step-up transformers, across the high voltage transmission line, through the local substation including step-down transformer, across local distribution lines, through the end transformer to the customer meter totals 6~7% typically. The transmission line alone is a fraction of that total.
High Voltage DC lines have even less losses, but the stations of HVAC-HVDC are very expensive and this set up is normally only used to move power either through long distances without intermediate drop off, or systems isolated by frequency.
High Voltage transmission lines are used because as the voltage increases, for the same amount of power flow, current decreases. Losses are impedance (resistance plus reactance) times the current squared. If you double the voltage, halving the current, the losses are quartered. This why we use voltage up to 765kV in the US for long distances.