Most preferred type of PD sensor in EHV Cable

[ravindranathan]

Which is the most preferred type of PD sensor used for PD measurement in EHV cable? We have copper foil type PD sensor in our EHV cable network which are placed every 500 metre. What is the sensitivity of copper foil type sensor in terms of distance from occurrence of PD location? Understand that HFCT sensor can sense PD upto 1km on its either side and hence we may place HFCT sensors at a gap of every 2km. Please confirm if my understanding is correct.