Concept of a Split-Core HFCT with Air Gap Control for Partial Discharge Measurements

Intelligent asset management requires condition-based maintenance. To monitor the condition of power cables online, partial discharge (PD) measurements are the most common method. For such measurements, high-frequency current transformers (HFCT) are usually used as sensors. However, a major problem when using HFCTs to measure PDs online is magnetic saturation of the sensor core due to the high 50Hz operating current of the power cable. To prevent saturation, air gaps are inserted into the ferrite core, i.e., a split-core is used. These air gaps should be large enough to avoid saturation, but no larger, or the sensitivity of the HFCT will be unnecessarily reduced. In addition, the optimum air gap length varies with the amplitude of the 50Hz operating current of the power cable. Therefore, based on our previous work, in this paper we present a concept for an improved split-core HFCT capable of self-adjusting its air gap length. The air gap is controlled in real-time based on the saturation level of the HFCT core. For this purpose, a microcontroller will be used to control a servo motor that optimally adjusts the air gap length of the HFCT. A prototype of the described HFCT is currently being produced and tested.