Characterisation of Soft Magnetic Materials for Metering Current Transformers and Other Low Flux Density Applications

The current transformer is a critical component in power measurement on electrical power supply systems. Its accuracy depends on the use of a magnetic core whose magnetic characterisitics are accurately known and do not vary with operating conditions.The core is normally made from grain oriented electrical steel strip which operates at a far lower flux density (B), and at the same time over a much wider range of magnitude, than in any other large scale application. The material is assessed and graded according to its high B performance but this is not directly related to performance in the low B regime. Furthermore, it is now realised that the permeability, the most important magnetic parameter for CT accuracy, varies widely at low B even in material of the same nominal grade; this itself will give rise to inconsistent CT performance.In operation, a fault current passing through the primary winding of a CT may cause a temporary or long term change in performance which is not detectable. This uncertainty can no longer be tolerated as accountablity for power flow and losses in distribution systems becomes more critical with the advent of more expensive fuel and distributed generation in particular.A related area of growing concern is effective passive shielding of equipment such as high field medical magnetic scanners (MRIs) where electrical steel is widely used. The shielding process usually means the material is mainly magnetised to a low B level as in CT cores so unknown B-H characteristics make device modelling or comparison of material performance very questionable.The aims of the project are (i) to develop a means of testing materials at low B to an accuracy not reached previously but now believed to be essential for evaluation of CT core and shielding materials, (ii) to develop a means of predicting the low B performance from studies of Barkhausen noise, domain wall motion and measured B-H curves, (iii) to study the effect of simulated power system disturbances on CT performance, (iv) to relate the accuacy of fully assembled CTs with variablity of core material and degradation of properties caused by the core manufacturing process and (v) to develop electromagnetic models to predict B-H characteristics in the low B regime.Uk steel, core and CT manufacturers will collaborate in the work where an important aspect will be to track material through the various stages from steel production, through core winding to the final assembly and evaluation of around fifty CTs.The main outcome of the research will be a new understanding of low full density performance of engineering magnetic materials which provide manufacturers with a more reliable and meaningful foundation for their designs which will lead to improved metering CTs and greater confidence of users in the accuacy of large scale electrical power measurement.