Partial discharge represents localized electrical breakdown within insulation that doesn't completely bridge conductors. Solar cables experiencing partial discharge face progressive insulation degradation eventually leading to complete failure. Understanding detection methods and prevention strategies ensures long-term cable reliability in photovoltaic systems.
Understanding Partial Discharge
Mechanism: Partial discharge occurs when electric field intensity exceeds insulation breakdown strength at localized weak points—typically voids, contaminants, or manufacturing defects. These discharge events generate small current pulses, electromagnetic radiation, and localized heating that gradually erode surrounding insulation.
In DC solar systems, partial discharge behavior differs from AC applications. DC voltage creates charge accumulation at dielectric interfaces, potentially causing different discharge patterns than AC systems experience.
Progressive Damage: Each discharge event damages insulation through electrical and thermal stress. Over time, discharge activity enlarges defect sites, increasing discharge magnitude and frequency. This progressive degradation eventually creates complete insulation failure if unaddressed.
Causes in Solar Cables
Manufacturing Defects: Voids or contaminants within insulation create electric field concentrations where partial discharge initiates. Quality manufacturing processes minimizing voids and contamination prevent these inherent weak points.
Cross-linked insulation with inadequate cross-linking density may contain regions with reduced dielectric strength susceptible to partial discharge initiation.
Installation Damage: Mechanical stress during installation can create micro-cracks in insulation. These defects provide sites where moisture penetrates and partial discharge initiates when voltage applies.
Excessive bending, pulling tension, or impact during installation creates internal insulation damage that may not be immediately visible but provides partial discharge initiation sites.
Environmental Factors: Moisture ingress through damaged cable jackets or improperly sealed connections introduces water into insulation. Water-filled voids exhibit lower breakdown strength than dry insulation, enabling partial discharge at operating voltages.
UV degradation of cable jackets allows moisture penetration reaching insulation layers, creating conditions for partial discharge development.
Detection Methods
Electrical Detection: Specialized equipment detects electrical pulses generated by partial discharge events. High-frequency current transformers or capacitive sensors capture discharge signals, with analysis systems quantifying discharge magnitude and frequency.
Testing typically applies voltage exceeding operating levels while monitoring for discharge activity. Inception voltage—the voltage at which discharge begins—indicates insulation quality margins.
Acoustic Detection: Partial discharge generates ultrasonic acoustic emissions. Acoustic sensors detect these signals, enabling discharge location identification in installed systems. This non-invasive method suits field testing of operating installations.
Chemical Detection: Discharge activity produces ozone and other chemical byproducts. While less common for cable testing, chemical detection methods can identify discharge presence in enclosed systems.
Testing Standards
IEC 60270: IEC 60270 establishes partial discharge measurement procedures including calibration methods, sensitivity requirements, and result interpretation. While primarily addressing AC systems, principles apply to DC cable testing with appropriate modifications.
Qualification Testing: Cable qualification testing may include partial discharge verification ensuring design provides adequate margins against discharge inception at operating voltages. Testing occurs at elevated voltage levels verifying no discharge activity at specified multiples of rated voltage.
Prevention Strategies
Manufacturing Quality: High-quality extrusion processes minimize void formation in insulation. Careful material handling prevents contamination. Proper cross-linking ensures uniform insulation density throughout cable construction.
KUKA CABLE's manufacturing processes emphasize void-free insulation production through controlled extrusion parameters and comprehensive quality monitoring.
Insulation Thickness: Adequate insulation thickness reduces electric field intensity, maintaining stress levels well below material breakdown strength. PV cables designed with appropriate safety margins avoid field intensities approaching partial discharge inception levels.
Material Selection: High-quality cross-linked polyethylene exhibits excellent dielectric properties with high breakdown strength and minimal defect formation. Material purity and proper formulation prevent weak points susceptible to partial discharge.
Installation Practices: Following manufacturer installation guidelines prevents mechanical damage creating discharge initiation sites. Proper bend radius, pulling tension limits, and avoiding impact stress preserve insulation integrity.
Field Monitoring
Periodic Testing: Established installations may benefit from periodic partial discharge testing, particularly for critical systems or installations experiencing performance issues. Testing identifies developing problems before complete failures occur.
Diagnostic Indicators: Unexpected insulation resistance degradation may indicate partial discharge activity. Declining insulation resistance warrants investigation including potential partial discharge testing.
Visual Inspection: Surface tracking or discoloration at cable terminations or damage sites may indicate discharge activity. Regular visual inspection identifies these warning signs enabling corrective action.
System Voltage Considerations
Higher Voltage Risk: 1500V DC systems experience higher electric field intensity than lower voltage installations. This increased stress makes proper insulation design and manufacturing quality more critical for preventing partial discharge.
Transient Overvoltage: Lightning strikes or switching events create voltage transients potentially exceeding steady-state design levels. These transients can initiate partial discharge in cables with marginal insulation quality, emphasizing importance of adequate design margins.
Quality Assurance
Production Testing: While routine partial discharge testing of every cable during production is uncommon due to testing complexity, manufacturers conduct periodic qualification testing verifying production batches maintain partial discharge resistance.
Material Qualification: New insulation formulations undergo comprehensive partial discharge testing during development, ensuring materials meet requirements before production implementation.
Conclusion
Partial discharge represents a serious threat to solar cable long-term reliability. Prevention through quality manufacturing, proper material selection, adequate insulation thickness, and careful installation practices provides more effective protection than detection and remediation after discharge activity begins.
KUKA CABLE's emphasis on manufacturing quality and material excellence ensures cables provide the dielectric integrity necessary to prevent partial discharge throughout 25+ year solar system lifetimes.
Contact KUKA CABLE technical team for information about partial discharge testing and prevention in solar cable manufacturing.
