Traceable Calibration and Evaluation of Toroidal Current Transformers for High Impedance Fault Detection in Electricity Networks
Authors:   , Zhang Minyu 
Published: June 2024

<p><strong>This project delves into the realm of accurate sensors for current measurement in complex electrical systems, with a specific focus on their use for High Impedance Fault (HIF) detection. The journey spans calibration methodologies, sensor performance evaluations, and integration into experimental setups, resulting in a comprehensive exploration of the challenges and solutions in this critical domain.</strong></p><p>A meticulous sensor selection process was undertaken. Current Transformers (CT) were selected from two distinct categories of toroidal types: split core CTs (SCCT) and Rogowski coils (RCCT), which each has its own unique physical configurations and inherent attributes. The selection criteria used were multifaceted, extending beyond physical structure to incorporate aspects such as commercial market availability and other specific criteria. For the split core CTs, two sets of commercially available sensors were chosen for study and one set of Rogowski coils along with integrators. The rigorous selection process ensured not only technical alignment but also practical accessibility, enabling the seamless integration and effective utilization of the chosen sensors within the project's scope.</p><p>A pivotal aspect of the endeavour was the establishment of traceable calibration procedures. By tracing sensor measurements back to the International System of Units (SI) through rigorous calibration, their alignment was ensured with international measurement standards. Calibration efforts encompassed both 50 Hz frequency and an extensive 100 Hz to 2000 Hz broadband spectrum. Although the 50 Hz calibration demonstrated promising accuracy and consistency of both type of sensors, the RCCT performance was degraded at elevated frequencies. These assertions were then substantiated through empirical investigations conducted at the Robinson Fault Test Facility. Integration of the sensors into the Robinson Fault Test Facility and into the LabVIEW measurement data acquisition software marked a milestone. This integration enabled high-resolution data collection during High Impedance Fault (HIF) testing. In this test facility a preliminary set of experiments were performed, and the results examined; namely, Load Switching Events (LS), Low Impedance Fault Events (LIF), and HIF Events involving a tree branch. Comparative evaluations across the different sensor sets revealed diverse behaviours. One set of SCCTs evaluated exhibited exceptional accuracy and consistency, while the other set of SCCTs whilst displaying reasonable performance there was variation between the individual sensors. The Rogowski coil sensors encountered significant challenges at higher frequencies due to the resolution, noise, and bandwidth limitations of their integrators.</p><p>In synopsis, this endeavour constitutes an empirical exploration aimed at establishing a cohesive link between rudimentary metrological principles and their application within the expansive domain of industrial engineering, specifically focusing on power-related contexts. Furthermore, it augments contemporary current calibration methodologies. The work highlights the importance of verifiable calibration procedures, attention to detail when setting up the sensory instrumentation within the test architecture, and close scrutiny of operational procedures.</p>