Determination of the Optimal Air Gap of an HFCT

When using high-frequency current transformers (HFCT) to measure partial discharges (PD) on power cables, saturation of the sensor's ferromagnetic core is a known problem. Saturation is usually not caused by the PD signals, but by the 50 Hz (or 60 Hz) operating current of the power cable. Saturation falsifies the measurement results, reduces the HFCT sensitivity and should therefore be avoided as much as possible. A typical method to avoid saturation is to insert air gaps in the ferrite core, i.e., to use a split-core HFCT. However, finding the optimal air gap length is a complicated task as it depends on the core material and degree of saturation. If the air gap is too small, the saturation will not be reduced enough; too large an air gap reduces HFCT sensitivity. In order to find the optimal air gap of an exemplary split-core HFCT, we performed many measurements. This paper presents all the results and derives a function to calculate the optimal air gap for our HFCT. To achieve the highest possible sensitivity, the air gap must be adjusted depending on the 50 Hz operating current. Based on this result, we plan to develop a split-core HFCT with air gap control in the future.