Partial discharge is a localized electrical breakdown inside cable insulation that fails to fully connect conductors. Unlike a short circuit that causes immediate outage, partial discharge releases tiny sparks continuously. These micro-discharges gradually erode XLPE insulation, expand voids and cracks, and eventually trigger cable failure. It is one of the hidden early defects for medium and high voltage cable systems.
Partial discharge mostly develops inside insulation voids, at joint interfaces or contaminated surfaces. When the local electric field exceeds the dielectric strength of the material, small discharge pulses appear. Each pulse is short and low-energy, which makes it hard to discover through regular visual inspection. Without timely intervention, partial discharge will accelerate insulation aging, form electrical trees, and lead to sudden cable breakdown during operation.
Main Causes of Cable Partial Discharge
Manufacturing defects are a primary source. Tiny air voids left inside insulation during extrusion, uneven material blending or impurities will concentrate electric stress once energized. Poor workmanship during cable joint and terminal installation also creates risks. Dust, moisture, gaps and improper crimping at the interface easily induce surface discharge.
Operating environment also contributes a lot. Long-term moisture ingress, high temperature cycling, mechanical squeezing and ground settlement damage the insulation structure. Water tree defects we mentioned before will further evolve into partial discharge under continuous working voltage. Overvoltage from switching surges or lightning strikes can raise local field strength and activate latent discharge defects.
Common Types of Partial Discharge in Cables
Internal discharge happens within air cavities embedded in solid insulation. It is the most frequent type for XLPE power cables. Discharge repeats inside voids every power cycle, slowly etching surrounding dielectric material.
Surface discharge appears along insulation surfaces, mostly at cable terminals and joint gaps. Dust pollution and moisture reduce surface creepage resistance, forming creeping discharge traces.
Corona discharge usually occurs at sharp edges of metal shielding layers or exposed conductors. It often comes with audible buzzing noise and ozone, common in poorly sealed outdoor cable accessories.
Testing Methods for Partial Discharge
Offline partial discharge test is widely accepted for factory delivery and periodic maintenance. It applies specified test voltage following IEC standards and captures discharge signals via high-frequency sensors. VLF testing is often combined with partial discharge measurement for medium voltage underground cables.
Online partial discharge monitoring runs while the cable remains energized. Sensors are installed on joints or along cable routes to track discharge trends continuously. This method fits critical circuits for data centers, solar farms and industrial plants, helping engineers catch developing defects without power cut.
Acoustic and UHF detection are two popular field technologies. UHF sensors pick up high-frequency electromagnetic waves generated by discharge, which has strong anti-interference ability on site. Acoustic sensors identify discharge through ultrasonic noise, suitable for preliminary screening of cable accessories. Field diagnostic strategies are well summarised in CIGRE technical guidance for MV cable asset management.
Risks Ignoring Partial Discharge Problems
Early partial discharge shows almost no obvious symptoms. Once the damage accumulates, insulation performance drops rapidly. Unexpected cable faults bring unplanned downtime, production loss and high emergency repair cost. For renewable energy and grid transmission projects, outages may cause cascading influence on connected equipment.
Safety risk cannot be overlooked. Severe discharge can generate heat and ozone, increasing fire hazard at cable trenches and joint wells. In some cases, breakdown accidents hurt field operation staff. Regular partial discharge inspection helps shift maintenance from breakdown repair to predictive maintenance.
Practical Ways to Suppress Partial Discharge
Strict quality control at production stage is the foundation. Reduce voids and impurities during insulation extrusion and shielding layer co-extrusion. Select high-quality cable accessories matching the cable voltage class.
Standardized installation is critical. Keep the working environment clean and dry when making joints and terminals. Control cutting precision, remove burrs on metal parts and ensure tight interface fitting to avoid air gaps.
Reasonable operation management helps reduce triggering conditions. Avoid long-term overloading and frequent switching overvoltage. Arrange periodic diagnostic testing for aged underground cables and cables running in wet, corrosive locations.
FAQ
1.What is the difference between partial discharge test and insulation resistance test?
Insulation resistance test checks overall insulation integrity with DC voltage. Partial discharge test captures tiny local defects that insulation resistance test may miss. They complement each other for cable condition assessment.
2.Can partial discharge be repaired?
If discharge comes from minor joint assembly defects, rework on accessories may solve the problem. Once internal insulation has formed obvious electrical trees, cable replacement is usually required.
3.How often should we carry out partial discharge testing?
New cables usually take the test before handover. Critical MV/HV cables are recommended for testing every 1 to 3 years, while cables with known water tree history need more frequent monitoring.
We supply medium and high voltage XLPE power cables and matched cable accessories with factory partial discharge testing. Share your project voltage level, laying condition and maintenance plan, our technical team can support you with reliable cable selection and installation suggestions.