There are a number of environmental advantages by transmitting power with HVDC or HVDC Light.
Positive effects on the power systems
Many HVDC transmissions have been built to interconnect different power systems by overhead lines or cables. By means of these links the existing generating plants in the networks more effectively so that the building of new power stations can be deferred. This makes economic sense, but it is also good for the environment. There is an obvious environmental benefit by not having to build a new power station, but there are even greater environmental gains in the operation of the interconnected power system by using the available generating plants more efficiently. The greatest environmental benefit is obtained by linking a system, which has much hydro generation to a system with predominantly thermal generation. This has the benefit of saving thermal generation ( predominately at peak demand ) by hydro generation. Also the thermal generation can be run more efficiently at constant output and does not have to follow the load variations. This can be done easily with the hydro generation.
Read what the owners of the SwePol link say about the environmental benefits.
Link to: The Scandinavia - Northern Europe interconnections
Reduced ROW (right of way) for a DC line.
One bipolar HVDC overhead line can be compared to a double circuit AC line from reliability point of view. Therefore a single HVDC line with two conductor bundles has less environmental impact than a double circuit AC line with six conductor bundles - it requires less space and has less visual impact.
Link to: Overhead HVDC lines
Minimum environmental impact with HVDC Light.
The HVDC Light technology has made it possible to use extruded polymer cables for DC. This has made the use of land cables an interesting alterative over traditional overhead lines in the 50 - 550 MW range for rather long distances ( Gotland HVDC Light, 70 km; Murraylink , 180 km).