Overview
During routine diagnostic testing, a Phase 2 bushing exhibited an abnormally low C1 capacitance. Although the dissipation factor (DF) values appeared normal, the low C1 reading indicated that the internal connection to the final aluminum foil layer—known as the test tap—had failed. This disconnection created a floating potential within the bushing, making it highly susceptible to internal arcing and partial discharge. Immediate replacement of the bushing was required to prevent potential explosive failure.
fig.1: An observation of the bushing test tap.
Observed Abnormal Results
The abnormal readings recorded during testing highlighted the internal disconnection. At a 10 kV test voltage, C1 capacitance for Phase 2 was significantly lower than expected, while C2 remained within normal ranges. The dissipation factor showed no obvious deviation, masking the underlying issue.
| Energize | V Test (kV) | C (pF) | Tgδ (%) | Temperature (°C) | Measured |
| 1U | 10 | 514.8 | 0.193 | 26 | C1 |
| 1U | 1 | 755.778 | 0.567 | 26 | C2 |
| 1V | 10 | 2.4 | 0.251 | 26 | C1 |
| 1V | 1 | 21.66 | 0.98 | 26 | C2 |
Comparison With Historical Data
Historical measurements revealed that the C1 and C2 values had previously been stable and within expected ranges. The stark difference between past and current results confirmed the test tap disconnection.
| Energize | V Test (kV) | C (pF) | Tgδ (%) | Temperature (°C) | Measured |
| 1U | 10 | 516.9 | 0.21 | 38 | C1 |
| 1U | 1 | 757.23 | 0.617 | 38 | C2 |
| 1V | 10 | 517.34 | 0.194 | 38 | C1 |
| 1V | 1 | 761.54 | 0.587 | 38 | C2 |
Diagnosis
The abnormally low C1 capacitance, combined with normal DF readings, pointed directly to an internal disconnection of the test tap. This defect created a floating potential in the final aluminum foil layer, elevating the risk of internal arcing and partial discharge.
Action Taken
Due to the critical nature of this failure, the affected bushing was immediately removed from service and replaced. Prompt intervention prevented a potential catastrophic failure that could have resulted from explosive internal arcing.
Conclusion
This case highlights the importance of detailed capacitance monitoring and comparison with historical baseline data. Even when dissipation factor readings appear normal, abnormal capacitance trends can reveal hidden defects such as internal test tap disconnection. Regular trend analysis ensures early detection and prevention of potentially dangerous failures in high-voltage equipment.
