Saturation of High-Frequency Current Transformers: Challenges and Solutions
Martin Fritsch, Martin Wolter
Abstract
When using high-frequency current transformers (HFCT) to measure partial discharges (PD) on power cables, core saturation caused by the 50 Hz operating current of the power cable is a major problem. Saturation leads to nonlinearity in the transfer function of the HFCT sensor and significantly reduces its sensitivity to the PD signals. To avoid magnetic saturation and ensure linear HFCT operation and thus maximum PD sensitivity, a split ferrite core can be used. However, finding the right air gap length between the two halves of the core is a complicated task, as the optimal length depends on several parameters and should be neither too small nor too large. This article deals with the air gap problem in detail and presents solutions for determining the optimal air gap length of HFCT cores. We present numerous measurements with exemplary split-core HFCTs from three different ferrite materials. Based on the obtained data, we then derive mathematical functions describing the optimal air gap length of the different ferrite cores. Based on these optimal air gap functions, we then derive and validate an HFCT split-core model. Using the model, the optimal air gap function for any ferrite material can be calculated with a few simple equations, significantly speeding up the air gap design process. In this way, the model improves the computer-aided design of split-core HFCTs. With optimal air gap length, the HFCT does not saturate and its sensitivity for PD measurements is maximized. Our results show that for online monitoring of power cables, the optimal air gap length depends mainly on the amplitude of the 50 Hz current and thus changes with the load on the power cable. Therefore, the conventional HFCT design with constant air gap length is not practical. Instead, we want to develop an improved HFCT with active air gap control in the future.