This lessened need for generation and spinning reserves can lead to a reduction in greenhouse gas emissions. Higher capacity transmission systems, including HVDC, could also increase the availability and economic viability of renewable resources that are remote from load centers. For purposes of discussion, the transmission system will be defined as including lines rated at 69 kV and above. As AC power is delivered on a transmission line, the electricity tends to travel through the outer portion of the conductor, not evenly over the cross section of the conductor. DC power delivery produces just the opposite effect.
Electricity travels more evenly over the entire cross section of the conductor.
A typical HVDC line design can have less than 50 percent of the losses associated with an AC line of the same power transfer capability. The per mile construction costs of the HVDC line are also considerably less. However, the fixed terminal costs for HVDC equipment preclude the use of HVDC except for very long lines or other special situations. Applications of HVDC can be for new lines or conversion of existing AC lines. Annual power system losses in the United States amount to billions of kWh.