<P> HVDC links can be used to control problems in the grid with AC electricity flow . The power transmitted by an AC line increases as the phase angle between source end voltage and destination ends increases, but too large a phase angle will allow the systems at either end of the line to fall out of step . Since the power flow in a DC link is controlled independently of the phases of the AC networks at either end of the link, this phase angle limit does not exist, and a DC link is always able to transfer its full rated power . A DC link therefore stabilizes the AC grid at either end, since power flow and phase angle can then be controlled independently . </P> <P> As an example, to adjust the flow of AC power on a hypothetical line between Seattle and Boston would require adjustment of the relative phase of the two regional electrical grids . This is an everyday occurrence in AC systems, but one that can become disrupted when AC system components fail and place unexpected loads on the remaining working grid system . With an HVDC line instead, such an interconnection would: </P> <Ol> <Li> Convert AC in Seattle into HVDC; </Li> <Li> Use HVDC for the 3,000 miles of cross-country transmission; and </Li> <Li> Convert the HVDC to locally synchronized AC in Boston, </Li> </Ol> <Li> Convert AC in Seattle into HVDC; </Li>

Does the size of wires and cables used matter in energy transmission