The National Fire Protection Association (NFPA) classifies diesel fuel as a Class II fuel. Class II fuels are not considered to be flammable liquids. They are, however, considered to be combustible liquids.
The installation of fuel supplies outdoors will be referenced to NFPA 30, Flammable and Combustible Liquids Code. Chapter 15 Outdoor Storage will give you the installation requirements. Chapter 21 and 22 will give the requirements for storage of the liquid in tanks above ground. If the building safety codes call for the generator to comply with NFPA 110, Standard for Emergency and Standby Power Systems, Section 7.9 Fuel System will give the requirements.
Refer to Table 22.4.2.1 of NFPA 30 for spacing requirements between tanks.
If the building safety codes call for the generator to comply with NFPA 110, Standard for Emergency and Standby Power Systems, Section 7.9 Fuel System will give the requirements.
To me if the design does not want the generator to have have exposure from a tank fire and a transformer fire, 15m should be the design separation between the fuel tank. transformer, and generator.
Additional References:
NFPA 30 FAQs – https://www.nfpa.org/Assets/files/AboutTheCodes/30/30_FAQs.pdf