Introduction The recommendations presented in this guide apply primarily to modern large oilimmersed power transformers with high-voltage windings rated 345 kV and above. This guide is a supplement to ANSI/IEEE C57.12.11- 1980@ Guide for Installation of Oil-Immersed Transformers [ l ] @1 which is basic to transformer installation procedures. It is assumed that the reader is familiar with the recommendations of ANSI/IEEE C57.12.11-1980. The higher dielectric stresses required for extra-high-voltage (EHV) transformers@ combined with thermal problems arising from their usually greater capacity@ demand a high degree of care during installation - much greater than transformers in the ratings covered by ANSI/ IEEE C57.12.11-1980. Additional problems arise from their large size which may@ in the future@ require site assembly of major components. The relatively high investment in EHV transformers and their importance to the power system demonstrates the need for rigorous refinements in transformer preparation. Generally@ the user must conform to the transformer manufacturer's minimum recommendations in order to obtain a warranty. All manufacturers require greater care in preparation of EHV transformers than for those of lower voltage. All manufacturers require some degree of vacuum treatment before filling with oil@ specify the vacuum to be maintained during oil filling@ and recommend limits on residual moisture and gas content of oil and dielectric strength of oil after filling. Limiting values for each transformer will be supplied by the manufacturer. Only the general order of magnitude of limits and the means required to reach these values are covered in this guide. 1 Numbers in brackets correspond to those of the references in Section 10.