Bushing failures are one of the leading causes of transformer outages. The failure mode is fast and often catastrophic. The warning signs, when you know how to read them, show up well before anything fails, and a simple power factor test with a tap measurement is usually enough to find a bushing that is headed toward trouble.
A transformer bushing is the insulated conductor that passes the energized winding lead through the grounded transformer tank wall to the outside terminal. On a high-voltage transformer, the bushing is doing serious work: holding off tens or hundreds of kilovolts across its length while carrying full load current through its center conductor.
Most high-voltage bushings used on power transformers are oil-impregnated paper (OIP) or resin-impregnated paper (RIP) condenser-type bushings. The insulation is built in concentric cylindrical layers around the conductor, with graded foil layers interspersed to control the radial and axial voltage distribution. That layered structure is what makes the bushing testable.
A condenser bushing has a test tap, usually a small threaded fitting near the flange, that provides access to the outermost foil layer. This lets you test the bushing insulation in two sections independently.
C1 is the main insulation: the capacitance from the center conductor to the test tap. This is the primary insulation section, the one holding off most of the voltage. It contains all of the inner foil layers. A problem in C1, moisture ingress, void formation, delamination between paper layers, shows up as an elevated power factor here.
C2 is the outer section: the capacitance from the test tap to ground (the flange). This section includes the bottom skirt insulation and the oil space between the bushing and the transformer cover. C2 is more exposed to contamination from the outside. A dirty or wet porcelain surface, tracking on the bottom skirt, or a compromised gasket shows up in C2 before it shows up anywhere else.
Testing both sections separately is what makes bushing power factor testing useful. If you only run the winding tests during a transformer outage and see elevated results on CH or CL, you cannot tell from those readings alone whether the problem is in the winding insulation or a bushing. Running the bushing tap tests resolves that ambiguity quickly.
New OIP bushings typically come in below 0.5% power factor on C1. Published caution levels vary by manufacturer and standard, but 1.0% on C1 is a widely used threshold for investigation, and anything above 2.0% on an in-service bushing is serious. RIP bushings generally run lower, often below 0.3%, and show less temperature sensitivity.
C2 power factor is less critical as an absolute number because the outer section is more variable. What matters with C2 is the trend and whether the value is consistent with the physical condition of the bushing exterior. A C2 reading that is climbing faster than C1 points to an external contamination or gasket problem, not internal insulation breakdown.
Capacitance values matter as much as power factor. The capacitance of C1 should be stable over time. If capacitance has shifted more than 5% from factory nameplate, a layer of the bushing insulation has either shorted or opened, both of which are serious. A bushing that tests fine on power factor but shows a capacitance deviation is still a bushing that needs to come out.
OIP bushing power factor is temperature-sensitive in the same way transformer winding insulation is. Correct test results to 20°C before comparing them to previous readings or published limits. Bushing oil temperature, not ambient air, is the right reference. On a transformer that has been de-energized for hours, bushing temperature will be close to oil temperature by the time you run the tests.
Humidity affects the test more for bushings than for winding tests because the porcelain exterior is exposed to ambient conditions. High humidity can cause surface leakage current on the porcelain that inflates the apparent C2 result. If you are testing on a humid day and C2 looks high but C1 is clean, dry the bushing surface before concluding there is an internal problem.
Moisture is the dominant failure driver. OIP bushings contain oil, and the oil level changes with temperature cycling. Over years of service, small amounts of moisture can enter through degraded gaskets or through the oil preservation system. Once moisture is in the oil space, it migrates into the paper and raises power factor. Wet paper is also mechanically weaker and more vulnerable to partial discharge.
Partial discharge is the failure mode that follows moisture. Voids or delaminated layers in the insulation become sites for low-level discharge activity. PD gradually carbonizes the paper, opens more voids, and accelerates the process. A bushing with active partial discharge can go from elevated-but-stable power factor to catastrophic failure in months, or faster under overload conditions.
External contamination is the other path to failure. Pollution, salt deposit, bird droppings, and industrial fallout on the porcelain create conducting paths that can cause flashover at voltages the bushing normally handles without issue. Regular visual inspection catches this before power factor testing is even necessary.
A bushing with C1 power factor above 1.0%, a rising trend over two test cycles, or a capacitance deviation beyond 5% of nameplate should come out at the next planned outage. If C1 is above 2.0% or the capacitance shift is significant, do not wait for a scheduled outage.
Replacing a bushing is a straightforward repair compared to what a failed bushing does to a transformer. A bushing that flashes over internally often causes a tank rupture or fire. The repair cost for the bushing itself is a small fraction of the cost to repair or replace the transformer after an internal fault, and a fraction again of the cost of an unplanned outage on a critical substation.
The test data is the justification for the outage. A well-documented trend with corrected power factor values and capacitance readings gives the engineer the evidence needed to schedule the replacement before the transformer owner needs to be convinced by a failure.
Our field crews run bushing power factor and capacitance tests on power transformers across Florida and the Southeast. Send us your equipment details and we’ll respond within one business day.