Three phase rectifier heatsink
300W and 0.9K/W already gives 270K temperature difference.

Given the most power semiconductors are specified for 125°C to 150°C junction temperature and ambient temperature may reach 30°C at most locations, your headroom is at most 120K, not 270K.

Recalculate your heatsink.

Oh, and if you had to dissipate 1.8kW, think of a hair dryer and the storm it creates.