A contactless and non-intrusive current measurement technique for low and medium voltage AC power systems

Abstract

The real-time state of current is essential to precisely monitor and control the AC power systems. It is also crucial for detecting various types of faults that may lead to long duration and wide area outages and affect the reliability and dependability. Traditional core wound window type current transformers (CTs) are widely used for current measurement at present. Increasing number of distribution energy resources integrated to the power systems network require a greater number of such instrument transformers for efficient monitoring and control of the grid. However, these CTs require complex and time-consuming operational procedure for installation and maintenance. In addition, they have a major drawback of saturation. To overcome this drawback, they need a higher accuracy leading to bigger size and higher costs and, therefore, beget the need for alternative current measurement techniques. They also pose a serious hazard of explosion if their secondary windings are left open circuited. In this thesis a technique of non-invasive contactless current measurement using Tunneling magnetoresistive (TMR) sensors is proposed and implemented for AC power systems. The proposed sensors overcome the aforementioned drawbacks of the CTs and provide more accurate outputs for asymmetrical currents during fault conditions. A thorough investigation is carried out to study the effect of distance, conductor insulation, and frequency of source current on their performance while applied for single-phase and three-phase current measurements. The sensors were calibrated to overcome the inequality in the sensed magnetic field due to the various aspects such as the distance from the source, minute structural variations, the magnitude of the source current, and harmonics. This thesis introduces a new technique to determine the phase angle error in absence of time-synchronized data. The weighted fusion technique is applied to six pair combinations from an array of four sensors in a three-phase triangular and horizontal structure for accuracy improvement. The measurement accuracy based on the sets of weighting factors corresponding to a minimum TVE showed promising and successful validation of the magnetic sensors for a possible replacement of CTs in the ac current measurement.