Bushing tan δ and capacitance calculator
Get the dissipation factor and the capacitance from the bridge reading and compare them against the bushing nameplate values.
Bushing tan δ and capacitance, compared against the nameplate
Enter the bridge reading — voltage, current and losses — and get the dissipation factor and the capacitance. If you know the bushing's factory values, the tool also gives the C1 deviation and how far tan δ has risen, which is what actually decides.
Result
tan δ = P / (V × I) and C = I / (2πfV), from the bridge reading. Always test at the same voltage and record the temperature: without those two, the history is worthless.
Indicative result. The calculation runs in your browser and is not sent to any server. This website may contain errors or omissions: before making any decision about an installation, the information must be checked and validated by a qualified engineer. Legal notice and terms of use
What this calculator does
It turns the bridge reading — applied voltage, current and losses — into the two quantities that get interpreted, tan δ and capacitance, and compares them with the factory values stamped on the bushing nameplate.
Where the numbers come from
Real insulation is not a perfect capacitor: besides the capacitive current there is a component in phase with the voltage, and that is what turns into heat. The dissipation factor is the ratio of the two:
tan δ = P / (V × I)
and the capacitance follows from the capacitive current:
C = I / (2π f V)
with P in watts, V in volts and I in amperes. The calculator handles the units the instrument gives you (kV, mA and mW).
Why C1 governs over tan δ
In a condenser bushing — impregnated paper with interleaved conducting foils — the insulation is divided into layers in series. When one layer fails, the rest share the same voltage across less thickness: the capacitance rises in steps, and each step betrays a lost layer.
That step appears before tan δ moves appreciably, because tan δ responds to general degradation — moisture, ageing, contamination — and not to a localised defect. That is why the removal criterion is written on the C1 deviation and tan δ serves as confirmation.
| C1 deviation | What to do |
|---|---|
| Below 2% | Normal, measurement noise |
| 2% to 5% | Repeat and compare against the history |
| 5% to 10% | Investigate and shorten the test interval |
| Above 10% | Take the bushing out of service |
For tan δ, the classic alarm signal is doubling the factory value.
Worked example
Bushing with a nameplate C1 of 3,100 pF and a factory tan δ of 0.30%. Tested at 10 kV, the readings are 10 mA and 300 mW.
- tan δ = 0.3 / (10,000 × 0.01) = 0.30% — the same as the factory value
- C = 0.01 / (2π × 50 × 10,000) = 3,183 pF
- C1 deviation = +2.7% → repeat the measurement and look at the history
If the next campaign gave C1 = 3,400 pF (+9.7%) with tan δ still at 0.35%, it would be time to investigate: that step is what counts, however good the tan δ looks.
What this calculator does not do
It does not decide for you. It does not know the bushing type (OIP, RIP, RIS), which changes what counts as a normal tan δ; it applies no temperature correction factors, which depend on the insulation; and it does not interpret the test-tap measurement (C2), which has criteria of its own. Nor does it replace comparison with the other two phases and with the history, which is where the problem actually shows.
What measures it
The measurement is made with a power factor or tan δ bridge at 10 kV. In service, the alternative is continuous bushing monitoring, which watches leakage current and capacitance without taking the transformer out of service and flags a change between campaigns; for the oil, dissipation factor is measured with dedicated transformer test equipment. The diagnosis is completed by partial discharge detectors, which see what tan δ averages out.
Frequently asked questions
- Which warns first, capacitance or tan δ?
- Capacitance. A condenser bushing is built from paper layers with conducting foils between them; when one layer punctures, the rest are left in series across less thickness and the capacitance rises in a step. That step appears before tan δ moves, which is why the removal criterion is written on C1.
- What C1 deviation is worrying?
- The most widespread field criterion: below 2% is measurement noise; between 2% and 5% repeat and compare against the history; between 5% and 10% investigate and shorten the test interval; above 10% take the bushing out of service. What governs is always the trend against its own nameplate, not an absolute figure.
- Should tan δ be temperature corrected?
- With great care. Correction factors depend on the insulation type and the guides warn that applying them blindly can introduce more error than they remove. The practical approach is to test in similar conditions every time, record the temperature and compare measurements made at similar temperatures. The capacitance comparison does not have that problem.