Novel principle for flux sensing in the application of a DC + AC current sensor

Abstract

Magnetostriction describes the geometrical change in length of a ferromagnetic material in dependence of its internal magnetic flux density value. By detecting the vibrations caused by these dimensional changes with a piezo-electric transducer, the instantaneous value of the magnetic flux inside a magnetic core can be sensed from DC up to a few kilohertz. This principle, together with a high bandwidth current transformer, was utilized in order to construct a current sensor capable of measuring currents ranging from DC to several MHz. As will be shown in this paper, an additional sinusoidal AC-excitation of the core material provides higher sensitivity of the length measurement and overcomes the high-pass characteristic of the piezo sensor. In order to prove the principle and to demonstrate the capabilities of this new sensor, a series of experimental measurements and implementation results are presented. I. INTRODU CTIONPrecise current measurement is a mandatory requirement of modern power electronic systems as it enables the im plementation of high performance current control loops, monitoring and safe shut-down in case the maximum allowed current value has been exceeded, among others.Depending on the specific application and the required performance of the measurement system, the existing current measurement concepts can be classified according to their key operating principles. Fig. 1 gives an overview of the most common current measurement methods that are applicable in power electronics whereby their key features are presented in the following. A. Isolated Current Measurement ConceptsIf a galvanic isolation between the current to be mea sured and the sensor is required, the measurement principle is typically based on Ampere's Law where effects caused by the magnetic field of the current are exploited. There, Rogowski coils or AC current transformers are commonly used when only AC currents need to be measured, e.g. in power transmission systems [1] [2]. On the other hand, if AC as well as DC current compo nents need to be captured, several techniques are applicable. Magneto-resistive sensors make use of the fact that some ma terials change their resistance in the presence of a magnetic field [3]. Magneto-optical current sensors exploit the Faraday Effect and are usually applied in high-current applications [4]. There are many methods involving a saturable magnetic material in order to measure current. Sensors of this kind can be operated in open or closed loop systems and usually require more than one magnetic core [5]. Current can also be measured by introducing a semiconductor Hall-effect sensor in the magnetic path in order to directly measure the magnetic field caused by the current. These current sensors can be operated in open or closed loop configurations as well and they can be combined with a current transformer to achieve a higher bandwidth [6]. B. Non-isolated Current Measurement ConceptsIf no galvanic isolation is required, the current can be measured by using Ohm's Law and a shunt resistor [6].In...

Optimization Possibilities for DC Current Transformer

“…Magnetic sensor can be Hall generator, flux-gate sensor, GMR sensor, etc Concentrated magnetic field can be detected using different types of sensing element, placed in the gap of the ferromagnetic core. Apart to Hall generator as a magnetic field sensing element, fluxgate sensor, or GMR [12] sensor can be used, as well as some more magnetic sensor types.…”

DC-Compensated Current Transformer

“…The DC flux component in the CT core can be detected by an additional sensor inserted into the core airgap, e.g., a Hall plate or a magnetostrictive element [ 24 ]. The basic disadvantage of the airgap is that it distorts the symmetry of the magnetic circuit, which results in dependence on the position of the current conductor in the sensor ring and increased leakage of the external magnetic fields into the sensing core, which cause sensitivity to external electric currents [ 25 ].…”

Low‐power design for DC current transformer using class‐D compensating amplifier

“…The used magnetic core can be completely closed or with a gap. Types of DC-CT, which use a closed magnetic core, operate on the fluxgate principle [3,9,10] or magneto-striction effect [11]. In a gapped case, the core acts as a flux concentrator for a field probe that is placed in the air gap of the core [1][2][3]5].…”