Analytical Investigation of Partial Discharge Characteristics in Cable Insulation Systems Under Nigerian Tropical Service Conditions

This study analytically and experimentally investigates partial discharge characteristics in high-voltage cable insulation systems subjected to the combined thermal, humidity, and contamination stresses representative of Nigerian tropical service conditions, addressing a gap in cable insulation diagnostics evidence for the Nigerian operating environment. Cross-linked polyethylene insulated cables operating in humid coastal environments such as Lagos and Port Harcourt experience accelerated insulation ageing mechanisms that differ from those in the dry and hot conditions of the Sahel zone where cables experience thermal cycling and UV exposure. Three cable insulation test specimens representing new, five-year, and ten-year service-aged cables of the same 11kV XLPE specification were sourced from EKEDC's decommissioned cable stock. Each specimen was subjected to accelerated ageing in an environmental chamber cycling between conditions simulating Lagos coastal humidity and Kano dry Harmattan exposure. Partial discharge measurements were conducted at test voltages from 1.0 to 2.0 per unit of nominal operating voltage using an IEC 60270 compliant partial discharge measurement system at three-month intervals throughout the 18-month experimental programme. Discharge pulse pattern analysis was performed using the PRPD method to identify defect type. Results confirm that thermal cycling under humid conditions produces significantly higher partial discharge activity than dry thermal cycling alone, suggesting moisture ingress is the dominant insulation degradation mechanism in coastal Nigerian cable environments. Moisture diffusion modelling using a Fickian diffusion approach accurately predicts the observed discharge inception voltage reductions. Keywords: partial discharge, cable insulation, XLPE, tropical conditions, Nigeria.