Why do power companies transmit three-phase power at very high voltages?

Transmitting three-phase power at very high voltages is primarily done to minimize power losses. When electricity is transmitted over long distances, power losses occur due to the resistance of the transmission lines, resulting in heat generation and energy loss. By increasing the voltage, the current can be reduced for the same amount of power transmitted, as power is the product of voltage and current (P = VI). This reduction in current leads to a significant decrease in the I²R losses (where 'I' is current and 'R' is resistance) associated with the transmission lines. Consequently, high-voltage transmission is an efficient way to deliver electricity over long distances while ensuring that most of the generated power reaches its destination.

While installation costs, safety, and consumer demand may have some relevance in the overall operation of power systems, the primary engineering rationale for high-voltage transmission is to effectively reduce power losses, making this the most appropriate choice.