Bushing Power Factor and Capacitance Testing: Method and Evaluation

What it is

A bushing power factor and capacitance test checks the condition of the insulation inside the high-voltage bushings that carry a conductor through the wall of a transformer tank, a circuit breaker tank, a reactor or a regulator. An alternating test voltage is applied to the bushing conductor and the instrument measures two things: the dielectric loss of the insulation, reported as power factor or dissipation factor, and the capacitance of the insulation. Most bushings of higher voltage ratings are built as a capacitor, with the central conductor wrapped in layers of paper or resin-bonded material separated by thin conducting foils, and they carry a test tap or potential tap near the base that gives access to the outer layer. That construction makes two separate readings possible on a single bushing, one for the main insulation and one for the tap insulation. The test is run on new bushings as received, on bushings installed in equipment, and on a recurring schedule for equipment in service. It complements the whole-unit insulation tests, because bushings are small in volume and a problem in one can be diluted in a measurement of the entire transformer.

Why it is done

Bushings are among the most common sources of serious transformer and breaker failures. Moisture that enters through a damaged gasket or a cracked porcelain, contamination of the oil or compound, partial breakdown between foil layers and thermal aging all weaken the insulation gradually. As the losses rise the bushing heats itself, which accelerates the damage, and the end of that process can be a violent failure that bursts the porcelain, starts a fire in the transformer and puts nearby people at risk. A bushing failure also takes the transformer out of service for a long and expensive repair. The advantage of the power factor and capacitance test is that it responds early. Loss rises when moisture or contamination appears, long before an insulation resistance reading would move, and capacitance changes when layers short out. Running the test at acceptance proves that the bushing survived shipping and storage, and running it periodically builds a record that makes a slow change visible and gives time to plan a replacement.

The principle

A capacitor bushing behaves like a series stack of capacitors, one for each insulating layer. With an alternating voltage applied, a nearly perfect stack draws a current that leads the voltage by almost a quarter of a cycle, and the small component of current in phase with the voltage represents the loss in the insulation. The ratio of the in-phase part to the total is the power factor, and for small values it is practically the same as the dissipation factor. Clean, dry insulation has very low loss. Moisture, carbon from partial discharge, contaminated oil and aged paper all add in-phase current, so the reading rises. Capacitance is set by geometry and by the permittivity of the materials. When one layer is shorted by a breakdown, the stack has fewer series capacitors left, and the total capacitance goes up by an amount related to the layer count. Because capacitance is compared with the nameplate value, even a modest change deserves attention. Temperature changes both quantities, so results are corrected to a reference temperature, and surface leakage over the porcelain is kept out of the measurement by guarding or by cleaning and drying the surface.

How it is performed

The equipment is de-energized, isolated from every source and grounded under the owner's procedures. Conductors, buswork and arresters are disconnected from the bushing terminals so that only the bushing and its host are measured, and the porcelain is cleaned and dried because surface leakage corrupts the reading. A power factor test set is connected with the high-voltage lead on the bushing conductor and the measuring lead on the test tap where the bushing has one. In the ungrounded specimen configuration the measuring lead reads only the current through the main insulation, with the tap held at the measuring circuit. In the grounded configuration on a tap-equipped bushing, the tap is grounded and the measurement covers the tap insulation as well. For bushings without a tap, a hot-collar test applies voltage to a conductive collar clamped around the porcelain petticoat and measures the response of the insulation below it. The tap cover is removed to reach the tap, and the tap is restored and grounded afterward, because an open tap can cause a failure in service. The applied voltage is selected within the limits for the bushing and the tap, never by guess. Temperature of the bushing, ambient conditions and humidity are noted, and the same method and instrument are used on repeat tests so that results can be trended.

What is recorded

The record notes the bushing identification and position on the equipment, the configuration of each measurement, the measured capacitance and the power factor or dissipation factor for the main insulation and for the tap where it was measured, the applied voltage and frequency, the temperature of the bushing and the surroundings, the humidity, and the identity of the instrument with its calibration status. The surface condition and any cleaning performed are noted, as is the as-found state of the tap cover. These details matter because the reading depends heavily on conditions, and later reviewers need to separate a real change in the bushing from a difference caused by temperature, surface leakage or the way the test was set up.

How results are judged

No single reading decides whether a bushing is acceptable, and this page gives no limits. The published criteria take several shapes that are used together. First, the measured power factor is compared with the value on the bushing nameplate or in the manufacturer's data, after correction of the reading to the reference temperature. Second, the measured capacitance is compared with the nameplate value, because a shift signals shorted layers. Third, the three phases of the same equipment, or sister bushings of the same type, are compared against one another, since an outlier points to a local problem. Fourth, results are trended against earlier tests on the same bushing, where a steady rise matters even if the numbers still look acceptable. Fifth, the tap measurement is judged separately from the main insulation. Published guidance sets thresholds at which a bushing is investigated or replaced. ANSI/NETA ATS-2025 covers acceptance and ANSI/NETA MTS-2023 covers maintenance testing of bushings; refer to your own licensed copies of the standards for the criteria, and to the manufacturer for bushing-specific data.

Variations

At acceptance the test establishes a baseline and confirms that the bushing as shipped matches its nameplate. In maintenance the interest lies in change since the last test, so a repeatable setup matters more than any single reading. The construction of the bushing changes the details: oil-impregnated paper, resin-bonded paper and resin-impregnated paper designs differ in how they respond to moisture and temperature, and solid bushings without a tap rely on the hot-collar method. On transformers, bushings are often tested together with the winding insulation using the same instrument, while on oil and gas circuit breakers the bushings may be tested while mounted in the breaker, with the contacts in a defined position. On regulators and reactors the same approaches apply, scaled to the equipment. Applying the test to the whole unit does not replace the bushing test, because the losses of a large winding can hide a small but dangerous bushing problem.