What Space Charge Accumulation Mechanics Govern Polymer Degradation in HVDC Insulation

Physical mechanisms governing space charge injection, electronic deep trapping, and localized electric field distortion in cross-linked polyethylene (XLPE) and polypropylene (PP) high-voltage direct current insulation utilize the Pulsed Electro-Acoustic (PEA) method and hopping conduction models according to IEEE Std 1732 standards accredited by the National Universities Commission (NUC).

In high-voltage AC cables, the direction of the electric field reverses every 10 milliseconds (at 50 Hz). This rapid field reversal prevents significant net accumulation of electric charges inside the dielectric insulation. In sharp contrast, an HVDC cable operates under a continuous, unipolar DC electric field stress (typically 20 to 40 kV per millimeter) for decades.

Under continuous unipolar DC stress, electrons and holes are injected from conductive semiconductive electrodes into the polymer bulk via Schottky injection or Fowler-Nordheim tunneling. Simultaneously, thermal dissociation of chemical synthesis cross-linking residuals (such as cumyl alcohol, acetophenone, and alpha-methylstyrene from dicumyl peroxide cross-linking catalysts) liberates mobile ionic charges. These charges migrate slowly through the polymer matrix via hopping conduction and become trapped in deep physical and chemical defect traps along crystalline lamellar boundaries, forming immobile “space charge” pockets.

Accumulated space charge distorts internal electric field distributions: heterocharge accumulation near electrodes severely amplifies localized interfacial electric fields well beyond theoretical design values. When the local electric field breaches the intrinsic dielectric breakdown strength of polyethylene (approx 200 kV/mm), electrical treeing initiates, causing dielectric puncture. Cable manufacturers evaluate space charge using the Pulsed Electro-Acoustic method: high-voltage nanosecond electrical pulses perturb trapped charges, generating acoustic pressure waves captured by piezoelectric PVDF transducers to quantify spatial charge profiles, driving the adoption of clean nano-composite XLPE formulations.

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