It depends on the design of specific vendor. I have been recently involved in a 50 MVAR STATCOM from one of the reputed supplier. Including the layout, the engineering will be by the STATCOM supplier. Normally, in the converter room there is no major flammable material. The fire risk due to the electrical insulation failure and arcing will be generally mitigated by using the differential (zone type) protection. The relay is expected to detect the fault with in 30 to 50 ms.
Thus generally, the standard fire protection system commensurate with the substation equipments should suffice.