Described embodiments include a device, a system, and a method. A described device includes a closed-cycle heat transfer device. The device includes a thermal management system adapted for dissipating heat generated by transmission lines of a power transmission system in use to transport electric power from one place to another. The system includes a heat-dissipation device and a transmission-line temperature manager. The heat-dissipation device includes a heat-acquisition element configured to absorb heat from a portion of a live transmission line of the power transmission system. The heat-dissipation device includes an assembly of one or more controllable fins thermally coupled to the heat-acquisition element and configured to dissipate at least a portion of the heat absorbed by the heat-acquisition. The transmission-line temperature manager is configured to control an aspect of the assembly of one or more controllable fins.