In the Hainan–Guangdong interconnection project

oil leakage in oil-filled cables required replacing the faulty core with a single core from a three-core cross-linked polyethylene (XLPE) cable

leading to mixed operation of XLPE and oil-filled cables. However

systematic quantitative analysis on the three-phase unbalance caused by differences in material parameters between the two cable types is lacking. This study focuses on the mixed operation system

using a real-time digital simulation platform to build power flow and current distribution models. Finite element thermal analysis and accelerated thermal ageing experiments were conducted to reveal the evolution of XLPE cable insulation dielectric parameters during ageing. Two evaluation indices

the impedance mismatch coefficient and the length mismatch coefficient

were proposed to quantify how material degradation worsens system unbalance. Results show that after 1440 h of ageing

the system's negative-sequence and zero-sequence unbalance increased by 24.4% and 28.7%

respectively. A critical impedance mismatch threshold is identified; once exceeded

unbalance escalates and the system's stability margin diminishes. This research provides theoretical support for recognising ageing-induced sequence impedance mismatch and related unbalance effects in hybrid submarine cable systems and lays a foundation for optimising operational stability and designing compensation strategies.