Frequency-Dependent Space Charge Dynamics of LDPE in Fractional Frequency Transmission Systems

Fractional frequency transmission (FFT) technique has emerged as a prospective solution for achieving carbon neutrality and enabling long-distance transmission of offshore wind power. During the long-term operation of FFT lines, main cable insulations inevitably face challenges of charge accumulation and electrical aging under complex conditions involving electric fields, temperature variations, and frequency drops. Herein, we have developed a modified space charge measuring system, with rapid detection capabilities, to evaluate space charge characteristics of low-density polyethylene (LDPE) used in FFT lines. Pulse electroacoustic (PEA) testing results indicate that charge accumulation intensifies at low-voltage frequencies, while elevated temperatures significantly increase carrier mobility, leading to hetero charge accumulation and deteriorated electric field distortion. Comprehensive theoretical analysis through bipolar charge transport (BCT) modeling and numerical simulations further support experimental findings, revealing that as the frequency increases, energy release concentrates at sample edges and causes material degradation. This study offers a rational pathway for understanding charge behaviors and is beneficial for the optimal design of cable insulation in FFT systems (FFTSs).