The use of power frequency current transformers as partial discharge sensors for underground cables

Abstract

Partial discharge (PD) diagnostics can be used to diagnose insulation defects in medium voltage cables and thereby inform required maintenance planning to extend the lifespan of ageing assets. High frequency current transformers (HFCT) installed on the earth strap at the cable termination are widely used to detect the fast varying pulses resulting from PD. Due to constructional reasons earth strap accessibility in underground cables is often limited. Furthermore in three core cables, phase angle dependency of pulses resulting from PD sometimes fails to induce pulses in the earth screen. This paper investigates an alternative method of PD detection using conventional power frequency current transformers (PFCT) principally used for protection and measurement purposes. This investigation assesses the feasibility of this approach by conducting a magnetic study of the transformer core, frequency response analysis (FRA), and finally capturing and analyzing onsite PD measurements using conventional PFCTs and the developed PD data acquisition/interpretation system. Based on the laboratory and on-site measurement results, PFCTs can be considered for detecting PD. This approach does not require retrofitting additional PD sensors and can also be applied to older switchgear design without any outage. Hence this approach can be considered as a low cost and noninvasive method of PD detection in cables.