High-Side Bushing Replacement
Lee County Electric Co-op
A planned one-day outage to replace three leaking high-side transformer bushings ran into a problem that a style-number match alone couldn’t prevent: the new bushings carried a corona ring that the older mounting structure wasn’t built to accept. Southern Switch identified the interference on-site, held while LCEC located compatible replacements, pre-tested the substitute units, installed them, and performed overall transformer testing before the unit returned to service.
Three leaking bushings.
One planned outage.
Oil leaking from a high-side transformer bushing is not a condition you run indefinitely. The leak starts at the gasket or the porcelain-to-flange interface and worsens under thermal cycling. Left alone, the oil level drops, the bushing’s internal insulation dries out, and you go from a maintenance item to a failure. Lee County Electric Cooperative had three high-side bushings in that condition. The right call was to pull the outage and replace them before one of them forced the issue.
LCEC scheduled the outage, sourced the replacement bushings from the original manufacturer using the existing style number, and called Southern Switch to execute. The plan was straightforward: pre-test the new bushings, pull the three old ones, install the new, test the transformer, return to service. One day.
Test before the outage,
not after.
Southern Switch pre-tested the incoming bushings before the outage day. Pre-testing new bushings is standard practice for anyone who has ever started an installation and found a defective unit in the box. A bushing with a cracked porcelain or a wet internal assembly is not something you want to discover after the transformer is already de-energized and the old bushings are on the ground.
Pre-test on new bushings includes power factor and capacitance measurements against the nameplate data. Capacitance outside of tolerance flags a manufacturing defect or shipping damage. Power factor elevated above the factory test report is a red flag on a new unit that should never have left the factory in that condition. Both tests run in minutes with a Doble M4100. Both tests are worth running.
Dielectric loss measured as a percentage. New bushings from the factory should test at or below the nameplate value, typically under 0.5% for oil-impregnated paper designs. An elevated reading on a new bushing suggests moisture contamination or a paper quality problem.
Capacitance checked against the factory nameplate. Acceptable deviation is typically ±10% from nameplate. A reading significantly outside that range on a new bushing is a defect, not a measurement error. Document the nameplate value and the test value both.
Style number matched.
Corona ring didn’t fit.
On outage day, Southern Switch pulled the three old bushings and prepared to install the new. The style number on the new units matched the existing hardware. The electrical pre-tests had passed. What nobody had verified was the physical geometry.
The new bushings had a corona ring. The older transformer’s mounting structure did not have clearance for it. Same manufacturer, same style number designation, but the current production run had incorporated a corona ring into the design — a modification that increases the effective diameter and changes the fit at the flange. The new bushings would not seat.
This is a known risk with bushing replacements on older equipment. Manufacturers periodically revise designs to improve corona performance or meet updated standards. The style number carries forward. The dimensions do not always.
A corona ring is a toroidal aluminum fitting added to the high-voltage end of a bushing to distribute the electric field uniformly and suppress corona discharge. At transmission voltages, sharp edges on the bushing terminal concentrate the field and produce corona, which causes radio frequency interference, audible noise, and over time, insulation degradation.
The problem in a replacement context: a corona ring adds significant diameter to the bushing assembly, particularly at the mounting flange area. A transformer designed for a bushing without a corona ring may not have the physical clearance in the turret, throat, or adjacent structure to accept one. The bushing won’t seat, and forcing it risks damaging both the bushing and the transformer.
Hold position. Find
the right parts. Finish the job.
With the transformer de-energized and the original bushings removed, there was no option to reschedule. LCEC needed to locate three compatible bushings — same electrical ratings, no corona ring — while the outage held. Southern Switch remained on-site.
LCEC found a set. Southern Switch pre-tested them on-site before installation: power factor and capacitance, same as the first set. Both checked out. The three bushings were installed, torqued to specification, and oil topped off.
With installation complete, Southern Switch performed overall transformer testing before LCEC returned the unit to service.
Repeated on the substitute bushings before installation. Pre-testing both sets on the same day meant the installed units were verified clean before they went into the transformer.
Winding-to-ground and winding-to-winding megohm readings and polarization index. Confirms the installation disturbed nothing in the transformer’s insulation system.
Ratio across all tap positions against nameplate. Confirms winding integrity was not disturbed during the bushing work and that the unit is correct before LCEC re-energizes.
Overall transformer power factor testing with the new bushings installed and wired. Establishes a post-replacement baseline and confirms the new bushings are performing within specification in-circuit.
A style number match is a starting point, not a finish line. Before the outage day, verify:
- →Flange bolt circle diameter and pattern against the existing hardware or drawings
- →Whether the new bushing includes a corona ring and whether the transformer throat and adjacent structure have clearance for it
- →Overall mounted length — if the transformer has limited clearance above the turret, a longer bushing assembly may conflict with bus or structure
- →Current revision of the manufacturer’s dimensional drawing against what the transformer was originally built for
Call (727) 789-0951 or send a request. We pre-test, we install, we do the post-work testing. Florida and the Southeast.