Martin Fritsch

High-frequency current transformers (HFCTs) are widely used for online partial discharge (PD) monitoring in rotating machines due to their non-intrusive nature and high sensitivity to fast current transients. Unfortunately, the scientific literature often lacks detailed descriptions of the physical principles governing HFCT operation and the influence of sensor design parameters on PD detection are considered insufficiently. This paper presents a focused review of HFCT operating principles and their use in PD monitoring of rotating machines. The electromagnetic behavior of the sensor is revisited, highlighting the role of core geometry, material properties, air-gap configuration, and transfer impedance on the frequency response. In addition, the propagation of PD pulses along power cables is discussed to clarify the combined effects of signal attenuation and sensor bandwidth. The analysis contributes to a clearer understanding of HFCT behavior and supports more informed application of this technique in online PD diagnostics in rotating machines.

This article reviews various designs of electrical peak detect and hold circuits and tests their applicability for the detection of partial discharge pulses. Since partial discharges are very short pulses in the nanosecond range, the peak detector circuit must be very fast and thus work precisely over a wide bandwidth up to the GHz range. The article begins with a comprehensive literature review and gives an overview of the state of the art of peak detector circuits. As a result of the literature research, four different peak detector designs are identified that appear suitable for measuring partial discharges. These four circuits are then simulatively tested for their performance in measuring the pulse height of various partial discharge pulses. It turns out that two of the identified circuits are able to measure nanosecond pulses of small amplitude. These two circuits must be further optimized for partial discharge detection in the future. In addition, the circuit should be tested prototypically with real partial discharge pulses.

This article deals with partial discharge (PD) measurement on power cables while they are in operation (online monitoring). In terms of sensitivity, high-frequency current transformers (HFCTs) are the most suitable sensors for this task, but they are prone to magnetic saturation when monitoring power cables online. To counteract saturation, the magnetic ring core of the HFCT is usually split into two halves to create air gaps. We show that a variable air gap length is required to maximize the HFCT sensitivity, and that the optimal air gap length depends on the actual operating point of the power cable. Accordingly, we present a concept for the construction of an HFCT capable of self-adjusting its air gap length during operation. The air gap control strategy is explained in detail and tested with a prototype. In addition, we propose a method for PD detection based on a combination of an analog peak detector circuit followed by a software algorithm. The developed PD sensor is immune to magnetic saturation, always operates at its optimum operating point and can therefore detect PD pulses with much higher sensitivity than comparable sensors with a fixed air gap length. Our tests prove that the sensor system works as intended and will be further improved in the future.

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.

High-frequency current transformers (HFCTs) are well suited as sensors for measuring transient current signals on power cables, such as partial discharges (PD). If the HFCT is well designed and its measurement bandwidth matches the bandwidth of the signal to be measured, high sensitivities can be achieved. However, optimizing the HFCT design for a specific measurement bandwidth is a difficult task because various design parameters affect its transfer function. In addition, there is little helpful literature on the relationships between HFCT design and resulting sensitivity/bandwidth. To give guidance and fill this research gap, this article aims to provide all the necessary information on HFCT development. For this purpose, an analytical HFCT model is derived and validated with measurements from various self-made HFCT sensors. The method for measuring their transfer function (or transfer impedance) is presented and the measurement data of all our manufactured prototypes are given. Based on these data, valuable relationships between various design parameters and the HFCT transfer function are analyzed. Based on our experience, detailed information about the sensor manufacturing process is provided. The developed HFCT model is an effective tool to simulate and optimize the sensor transfer function before it is built. In this way, HFCT sensors can be designed quickly and cost-effectively specifically as required.

Medium voltage cables are poor transmitters of signals with high frequency spectrum such as partial discharges. The propagating signals are strongly attenuated and distorted depending on the transmitted distance. In order to understand this process, this article provides a model to simulate the transmission of such signals on medium voltage cables. Compared to earlier approaches, this model does not neglect the wave character of the high frequency signals. Therefore, to describe the signal transmission, a comprehensible solution of the telegrapher's equations is provided. To work with this solution, the propagation constant of the medium voltage cable used must also be known. This propagation constant is calculated on the basis of the individual cable layers, considering all ohmic and dielectric losses. In contrast to previous methods, all primary line constants are modeled frequency dependent. The final transmission model is able to predict the spectrum of a transmitted signal at any distance from its origin. Validation measurements show that prediction and measurement agree with good accuracy. A possible application of the developed model is the investigation of the transmission of partial discharges on medium voltage cables. This model is probably also suitable for high voltage cables.