Overview
During a routine inspection of a recently re-installed bushing using CAPTAN12, a sudden spike in the Dissipation Factor (DF) was observed. This unexpected increase initially raised concerns about potential insulation degradation.
fig.1: CAPTAN12 in action.
Diagnostic Investigation
To determine the root cause, diagnostic measurements were performed, including a C2 test (insulation from test tab to ground). The investigation revealed that the flange ground connection had been overlooked during mechanical installation.
Once the flange was properly grounded, stray currents were eliminated, and DF values returned to reference levels. This confirmed that the issue was installation-related rather than a failure of the oil-impregnated paper (OIP) insulation.
Historical Data Comparison
The table below highlights that phase 1W showed a significant DF increase due to the missing ground connection, while other phases remained relatively stable.
Table 1. Historical Comparison of Bushing Dissipation Factor Measurements (2022 vs. 2024)
| Phase | Voltage | Year | Capacitance | Dissipation Factor (%DF) |
| 1U | 10 kV | 2022 | 296 pF | 0.24% |
| 1U | 10 kV | 2024 | 296 pF | 0.33% |
| 1V | 10 kV | 2022 | 296 pF | 0.24% |
| 1V | 10 kV | 2024 | 296 pF | 0.31% |
| 1W | 10 kV | 2022 | 296 pF | 0.25% |
| 1W | 10 kV | 2024 | 296 pF | 1.43% |
Analysis
The spike in DF was traced entirely to a mechanical oversight during installation, not insulation failure. This underscores the importance of proper grounding and careful mechanical assembly.
fig.2: A closer look on the flange.
fig.3: flange ground connection.
Key Takeaways
- Even minor mechanical oversights, such as forgetting to ground a flange, can lead to misleading DF readings.
- Comparing current measurements with historical data helps differentiate between installation issues and genuine insulation degradation.
- Proper installation checks prevent unnecessary alarms, downtime, and costly corrective actions.
