An Alternative Method for Dealing with Saturation in High-Frequency Current Transformers

High-frequency current transformers (HFCT) are used to measure partial discharges (PD) or other transient current pulses on power cables. In addition to the PD signal, the power cable usually transmits a high current at a frequency of 50 Hz (or 60 Hz). Both signals are superimposed and inductively coupled by the HFCT sensor. In most cases, the magnetic field of the 50 Hz current is strong enough to drive the sensor's ferrite core into saturation. Saturation falsifies the measurement results, reduces the HFCT sensitivity and leads to unwanted voltage peaks at the sensor output. To avoid saturation, air gaps are usually inserted into the ferrite core, but this also has the disadvantage of reduced PD detection capability. Recent publications now claim that saturation could be reduced without this drawback by a frequency dependent impedance at the HFCT output. We investigate this alternative method in this paper and present our results. To this end, we test whether this method actually reduces the saturation of the HFCT core or only its negative effects. For this purpose, saturation and its effects are first explained in detail. Then, the influence of the frequency dependent impedance on both 50 Hz signals and high-frequency PD signals is investigated. The method is critically reflected and compared with the air gap method. We can conclude that the studied method is a real alternative to air gaps in the ferrite core.