The Rogowski coil is an old device for current measurement. It has been modified and improved over a century and is still being studied for new applications. The Rogowski coil has various advantages over conventional magnetic current transformers (CTs). Not only can it be used instead of CT, but also it has various utilizations in other fields. This paper provides a brief review on different aspects of the Rogowski coil and its advancement procedure, during last decades. In this paper, the history of the coil is brifely reviewed and its bases and applications are discussed. The Rogowski coil is analyzed from different points of view, including different integration techniques in the output stage, models for the Rogowski coil, experimental methods for parameter measurement in models, and method for determining the damping resistor. Finally, a brief review over different applications of the coil ends this paper.

With the increasing age of the primary equipment of the electrical grids there exists also an increasing need to know its internal condition. For this purpose, off- and online diagnostic methods and systems for power transformers have been developed in recent years. Online monitoring is used continuously during operation and offers possibilities to record the relevant stresses which can affect the lifetime. The evaluation of these data offers the possibility of detecting oncoming faults early. In comparison to this, offline methods require disconnecting the transformer from the electrical grid and are used during planned inspections or when the transformer is already failure suspicious. This contribution presents the status and current trends of different diagnostic techniques of power transformers. It provides significant tutorial elements, backed up by case studies, results and some analysis. The broadness and improvements of the presented diagnostic techniques show that the power transformer is not anymore a black box that does not allow a view into its internal condition. Reliable and accurate condition assessment is possible leading to more efficient maintenance strategies.

This study is an effort to evaluate the numerical indices for the assessment of frequency response analysis (FRA) to provide a deeper understanding of their characteristics. First, the study introduces the indices in the literature and categorises them into three groups. Then, the results of 11 case studies from an experimental setup, two transformer models, and a power transformer real case are employed to examine the indices from the monotonicity aspect. Moreover, a short review on the interpretation experiences with indices is reported for further understanding. This contribution continues with a novel uncertainty analysis on the measurement setup and, finally, evaluates the indices based on their vulnerability to these uncertainties. Although the results show the advantage of some indices over the others in the FRA interpretation, further studies, especially with large transformers in the field, are needed to reach a general conclusion.

Optical current transducers are developed vastly in recent decades. For an accurate transducer achievement, different factors should be considered. This paper reviews the most important sources of errors in these devices. First, the main types of transducers are introduced. Afterward, the effects of temperature and vibration on their operational performance are discussed. Then, the birefringence is addressed which is the main error source in the transducers. In each part, the proposed solutions in the papers are reviewed. Finally, the effect of nonideal optical parts on the measurement accuracy is discussed. This paper serves as a brief introduction to common errors in optical current transducers.