How to Test Partial Discharge in 12 kV Epoxy Resin Busbar Bushings
Partial discharge (PD) testing is a critical quality and reliability check for 12 kV epoxy resin busbar bushings used in medium-voltage switchgear. Even when insulation meets power-frequency withstand requirements, localized defects such as voids, cracks, or interface gaps can lead to partial discharge, accelerating insulation aging and increasing the risk of in-service failure.
This article explains why PD testing matters, the standard test methods, and practical procedures for testing partial discharge in 12 kV epoxy resin busbar bushings.
1. Why Partial Discharge Testing Is Important
Epoxy resin bushings are solid insulation components. Unlike air-insulated parts, defects inside epoxy cannot be seen or corrected after manufacturing.
Partial discharge may occur due to:
Internal voids or porosity in epoxy resin
Poor bonding between epoxy and embedded conductors
Sharp edges or stress concentration points
Surface contamination or moisture ingress
Even low-level PD can cause:
Progressive insulation degradation
Treeing and erosion inside epoxy
Reduced service life
PD testing helps identify hidden insulation weaknesses before installation or commissioning.
2. Applicable Standards for PD Testing
PD testing of 12 kV epoxy resin bushings generally follows principles from:
IEC 60270 β Partial discharge measurement
IEC 62271-1 β High-voltage switchgear and controlgear
IEC 60060-1 β High-voltage test techniques
Manufacturers often define internal acceptance limits that are stricter than minimum IEC requirements.
3. Test Setup for 12 kV Epoxy Busbar Bushings
Basic Test Arrangement
A typical PD test setup includes:
High-voltage AC test transformer
Coupling capacitor
Measuring impedance (quadrupole)
PD detector and data acquisition system
Shielded test environment
The bushing under test is mounted in a configuration representative of actual installation whenever possible.
Grounding and Shielding
Proper grounding is essential to avoid external noise interference:
All non-energized conductive parts must be grounded
Test cables should be shielded
The test area should be free from corona sources
Background noise level should be verified before testing.
4. Test Voltage Levels for 12 kV Bushings
For 12 kV systems, the highest voltage for equipment (Um) is typically 12 kV or 12.5 kV.
Typical PD Test Voltage
PD measurement voltage: 1.2 Γ Um
Common test value: β 14β15 kV AC
The voltage is raised gradually to the test level and held long enough to observe stable PD behavior.
5. Partial Discharge Measurement Procedure
Step 1: Background Noise Measurement
Before energizing the test object:
Measure system noise
Ensure background PD level is sufficiently low (typically < 2β5 pC)
Step 2: Voltage Ramp-Up
Increase voltage smoothly from zero
Observe PD inception voltage (PDIV)
Note any sudden PD activity
A higher PDIV generally indicates better insulation quality.
Step 3: PD Measurement at Test Voltage
At the specified test voltage:
Measure apparent charge in pico-coulombs (pC)
Monitor PD magnitude and stability
Record phase-resolved PD patterns if available
Step 4: Voltage Reduction
Reduce voltage slowly and observe PD extinction voltage (PDEV). A significant gap between PDIV and PDEV may indicate internal defects.
6. Typical Acceptance Criteria
While exact limits depend on manufacturer and application, common acceptance values for 12 kV epoxy resin bushings are:
PD magnitude β€ 5β10 pC at test voltage
Stable PD behavior with no increasing trend
No sudden discharge bursts
High-quality APG-produced bushings often show PD levels below 5 pC.
7. Common Causes of Excessive PD
If PD exceeds acceptable limits, common causes include:
Internal voids from poor casting process
Inadequate pressure control during curing
Sharp conductor edges inside epoxy
Poor surface finish or contamination
Such defects are usually not repairable, and the bushing should be rejected.
8. Factory Test vs Field Test
Factory PD Testing
Controlled environment
Low noise level
More accurate PD measurement
Preferred for quality control
Field PD Testing
Influenced by environmental noise
Often limited to trend monitoring
Useful during commissioning or diagnostics
For epoxy resin bushings, factory PD testing provides the most reliable results.
9. Practical Recommendations
Always perform PD testing on critical 12 kV epoxy bushings
Combine PD testing with power-frequency withstand tests
Record PD patterns for future reference
Ensure test personnel are trained in noise discrimination
Proper PD testing significantly reduces early-life failures in medium-voltage switchgear.
10. Conclusion
Partial discharge testing is a vital quality assurance step for 12 kV epoxy resin busbar bushings. By following IEC-based procedures and applying appropriate acceptance criteria, manufacturers and users can detect internal insulation defects before service, ensuring long-term reliability and safe operation.
In modern medium-voltage systems, PD testing is not just a compliance requirementβit is a key tool for preventing costly insulation failures.












