SMUD averted a major outage by using the largest mobile transformer built with high temperature insulation

This is, so far, the largest transportable (mobile) liquid-filled auto three-phase power transformer with a high temperature insulation used successfully for a utility emergency application in the USA. In order to obtain the 100 MVA capacity, for this transformer, without exceeding certain size and weight limits for transportability, the transformer has to be designed with a 95°C average winding temperature rise and a high temperature solid insulation capable of withstanding the high temperature continuously, The high temperature solid insulation of the transformer is composed of the `Nomex´ insulation adjacent to the conductor and the remaining insulation for the transformer is cellulose boards for winding cylinders, end blocks, etc, known as a hybrid insulation system. In accordance with the information obtained from the mobile transformer manufacturers, the upper limit of the size for a mobile unit typically manufactured in this country was 60 MVA before this 100 MVA unit was built. The Sacramento Municipal Utility District (SMUD) averted a major outage of about twelve months due to the failure of a transmission substation transformer, by moving this ready-to-use transformer quickly to the failure location and replacing the failed unit to restore power in a matter of weeks. The transformer performed well as designed during its use in this application without exceeding any technical specification limit. The generated dissolved gases, as anticipated for the high temperature operation, are higher than those of a conventional transformer