Flexible current transformers for instrumentation and energy harvesting applications

Among the various sensing techniques to measure current, current transformers (CTs) are still widely used in most of the applications. The output current of a CT is inversely proportional to the turn-ratio of secondary and primary windings. The maximum induced output voltage across the burden resistor in the secondary circuit is directly proportional to operating frequency, the maximum flux density of the magnetic core, cross-section area of the magnetic core and the number of turns in the secondary winding. Besides transforming primary current to secondary current, the CT itself consumes a portion of the primary current for magnetizing current, eddy current loss and hysteresis loss. These cause magnitude and phase errors in the measurement. Magnetic cores with the larger cross-sectional area are preferred to prevent saturation of the core and obtaining higher secondary induced voltage. Commercially available CTs are usually bulky, rigid and heavy. The motivation of this thesis is to explore the use of thin and highly flexible magnetic core for CT design. A comprehensive literature review on currents sensing techniques, magnetic materials and their properties is presented. The major contribution of this thesis describes a comprehensive design guide, from analysis to fabrication of an ultra-thin and highly flexible CT. The proposed CT is flexible enough to wrap around an electrical cable for current sensing without taking much space. A measurement setup is developed to extract BH characteristics of flexible magnetic materials. Based on an improved Jiles-Atherton (JA) hysteresis technique, a circuit model of a CT that has the ability to handle the nonlinear behaviour of the flexible magnetic core is validated experimentally for two types of magnetic materials, Supermendur® and MuMETAL®. With the circuit model, a flexible CT for current sensing applications has been designed and fabricated using MuMETAL® magnetic core. The magnetic core is sandwiched by copper tracks on two flexible PCBs with vias, which form the secondary winding of the CT. Two compensation methods have also been explored to restore the distorted waveforms due to the non-linearity of the core so that the CT can be used for high current measurement. The waveform restoration using the two methods are verified experimentally. Also, wireless sensor network (WSN) forms the backbone to realize Internet of Things (IoT). Autonomous devices with energy harvester are in demands. Among the available types of energy, the magnetic field around a power cable can be tapped for such purposes. Flexible CT can also be developed as an electromagnetic energy harvester to energize wireless transceivers or any other low power sensor-actuator device. Since priority is given on harvesting maximum possible power, a flexible magnetic material with higher saturation flux density is required to optimize design analysis. A flexible CT with Supermendur® as a core material is developed and together with buck and boost converters, it can be used for energy harvesting application. The energy harvester drives a Bluetooth low energy (BLE) transceiver directly, and the excess energy is stored in a coin-cell Li-ion battery when the energy source is unavailable. This thesis presents an overall development of autonomous online-condition monitoring systems to monitor current flow in power cables for energy management, safety and protection by integrating flexible CTs for sensing and energy harvesting.Doctor of Philosophy (EEE