Abstract: A system that controls nitrogen pressure in the ullage of a power transformer that has its windings submerged in oil. The pressure is controlled in a narrow range of approximately 0.5 psi to approximately 2.0 psi. A nitrogen generator supplies the nitrogen to a reservoir from which it is distributed to the ullage as well as to accessories such as a load tap changer or a control box. A temperature regulator is provided for substation installations that are located in climates with wide ambient temperature variations to control the pressure of the generated nitrogen in an acceptable range.