In the 1# transformer renovation project at Erjiegou Substation, a 66kV power transformer was installed on site. Before installation, all transformer bushings were subjected to dielectric loss testing (tan delta test) and capacitance measurement to ensure that their insulation performance met operational requirements.
This case study presents the full process of transformer bushing testing and demonstrates how dielectric loss testing can detect hidden insulation defects before installation.
1.Project Background
Transformer bushings are critical external insulation components in high-voltage equipment. Their insulation performance directly affects the safety and reliability of transformer operation.
In 66kV and higher voltage levels, capacitor-type bushings are widely used. These bushings consist of multiple capacitor layers. If internal insulation deteriorates due to moisture, aging, or defects, it will lead to increased tan delta values and abnormal capacitance.
Therefore, transformer bushing tan delta testing and capacitance testing are essential procedures before installation.
2.Testing Equipment and Method
The on-site testing was carried out using a dielectric loss tester manufactured by Baoding Qilida Electric Technology Co., Ltd., model QLDJS-DA.
The testing procedure was as follows.
First, the transformer bushings were lifted and positioned horizontally.
Second, the ungrounded specimen test method was applied.
Third, a 10kV AC test voltage was applied.
Finally, dielectric loss and capacitance measurements were performed on the fiberglass capacitor-type bushings.
3.Test Results Analysis
3.1 Abnormal Condition of Phase A Bushing
The measured dielectric loss was 0.552 percent.
The measured capacitance was 445.8 pF.
According to the nameplate data, the rated capacitance was 407 pF and the dielectric loss was 0.31 percent.
The capacitance deviation reached 9.53 percent, which exceeded the acceptable limit.
It was determined that internal insulation failure had occurred, likely due to capacitor layer breakdown or bridging.
3.2 Normal Condition of Phase B, C, and O Bushings
The dielectric loss values were within acceptable limits.
The capacitance values were consistent with factory test data.
All results complied with IEC 60137 standards.
4.Corrective Action and Solution
The defective A-phase bushing was removed and replaced.
After replacement, testing was repeated and results met all standards.
5.Transformer Installation and Commissioning
After all bushings passed testing, installation was completed.
The transformer then underwent commissioning tests and was successfully energized.
6.Importance of Tan Delta Testing
Tan delta testing can effectively detect insulation problems such as moisture ingress, capacitor layer damage, internal breakdown, and manufacturing defects.
7.Conclusion
This case confirms that dielectric loss testing and capacitance measurement are essential for ensuring safe and reliable operation of high-voltage equipment.
