Three 85 MVA Solar GSUs,
Assembled and Tested
A Florida investor-owned utility built three new utility-scale solar sites in 2026, with about 170 MW of combined capacity. Each site needed a new 85 MVA Prolec generator step-up transformer assembled on its pad, processed, and tested before the OEM would approve energization. Southern Switch did all three, one after the other, from July through October 2026.
Three new GSUs,
one crew.
Each unit is an 85 MVA, three-winding Prolec GSU with a load tap changer. They came to site with the main tank sealed and the conservator, radiators, fans, bushings, turrets, and arresters crated. The utility and Southern Switch walked each site together and wrote a detailed work scope from that walk. All three units used the same scope.
From the field.
What Southern Switch
did on each unit.
- →Installed the conservator tank and its piping
- →Installed bushing turrets, bushings, and downpipes
- →Installed the radiators and fans, and wired the fans
- →Installed and wired the lightning arresters, PRDs, and protective relays
- →Pulled vacuum for 84 hours, as the Prolec installation manual specifies
- →Processed the oil and vacuum-filled the main tank
- →Filtered the oil and filled the LTC compartment
- →Power factor (C1 and C2) on each new bushing before installation
- →Winding power factor and bushing power factor after assembly
- →TTR on all LTC taps at the as-found DETC position, winding resistance, and excitation current
- →Core ground resistance and SFRA
- →Ratio and polarity on all bushing CTs, plus surge arrester tests
- →Functional check of all alarms
- →Vacuum logs, oil processing logs, and test files sent to the OEM for its warranty review
Why the dress-out
sets the unit’s life.
A new GSU is at its driest when it leaves the factory. Every hour a turret or a manhole is open on site, the paper insulation takes on moisture from the air. That moisture does not leave by itself. It raises the power factor, lowers the dielectric margin, and speeds up the aging of the paper for the life of the unit. The only fix in the field is time under vacuum, so we ran the full 84 hours the manual calls for on each unit, with no shortcuts to make a date.
The tests have a second purpose. A power factor test on each bushing before installation finds shipping damage while the bushing is still easy to return. The SFRA and power factor results after assembly become the baseline for the unit. When the GSU sees a through-fault or a move, the new trace is compared to this one. For that comparison to mean anything, the field setup must match the factory test setup terminal for terminal.
Working to the
OEM’s rules.
The OEM sent a field representative for three days at each unit to witness the bushing installation and the internal connections. Entry into the tank for the bushing connections is a permit-required confined space. Southern Switch wrote its own entry permit and rescue plan for each entry. After assembly, the utility sent our test data to the OEM, and the OEM compared it to its factory acceptance tests before it released each unit.
All three
approved to energize.
Three 85 MVA GSUs assembled, vacuum-processed, filled, and tested by one Southern Switch crew between July and October 2026. All three passed the OEM’s review and were approved for backfeed and energization.
Southern Switch assembles, vacuum-processes, fills, and tests new and relocated power transformers for utilities and solar builders in Florida and the Southeast. Our own crew, our own processing equipment. Send us the unit and the schedule.