#insulationreliability

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 – Partial discharge measurement

IEC 62271-1 – High-voltage switchgear and controlgear

IEC 60060-1 – 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 × Um

Common test value: ≈ 14–15 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 < 2–5 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 ≤ 5–10 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.