Effects of Moderate DC-magnetization on 3-D Loss Distributions of a 3-phase Model Transformer Core Edin MULASALIHOVIĆ, Helmut PFÜTZNER, Patrick ZANOLIN, Georgi SHILYASHKI, Viktor GALABOV Inst. of Electrodynamics, Microwave & Circuit Eng., Vienna Univ. of Technol., Austria (edin.mulasalihovic@tuwien.ac.at) Abstract – Unbalanced DC bias of 3-phase transformers may cause very strong increases of excitation currents and thus of stray fields. We investigated whether influx of the latter into the core yields planar eddy currents (PECs) as a source of planar eddy current losses. By use of the rise-of-temperature method, local distributions of total losses P were determined at 128 regions of the surface of a 3-phase/3-limb model core. Moderate bias in the middle limb yielded increases of P up to 100%, especially in limbs, while T-joint regions of rotational magnetization prove to be less affected. Local determinations of planar, classical eddy current losses PPEC by means of a field vector sensor revealed local increases up to a factor of 10. Finally, interior PECs were analyzed by means of thin silver layer sensors. As a result, the intensity of PPEC decreases exponentially towards the centre of the core. 1 Introduction As well known, unbalanced DC-bias of 3-phase transformers may cause very strong increases of excitation currents and thus of the corresponding stray field. According to experience, the latter may interact with magnetic components of the core like clamps and - in special - the tank. However, parts of the stray flux may also pass through the soft magnetic machine core. We investigated whether such influx may yield planar eddy currents (PECs). PECs may widely spread out since not being restricted by the thickness of lamination. Thus they represent a potential source of PEC-losses. So far, the literature has not reported the involved mechanisms. 2 Methodologies Experiments were performed on a 1m x 1m 3-phase/3-limb model transformer core exhibiting 56 layers of GO SiFe (300μm, M-5). The core was magnetized with 1.7 T by means of excitation coils of short length, thus offering free access to the ends of limbs, e.g. for stray flux investigation generated from windings and joint regions. Unbalanced DC-bias was simulated by DC-current impress into a secondary coil on the middle limb. Equivalent to a DC/AC excitation ratio r DC ≈ 1, the DC current intensity was close to the AC-current intensity for mere AC-magnetization (r DC=0). The following tests were made for the DC/AC-case in comparison to mere AC: (a) At the core surface, in a fully-automatic thermistor scanning chamber [1] applying the rise-of temperature method, the distribution of total losses P(x,y,z=0) was inves- tigated. (b) At the surface, by means of a multi-parametrical 4-needle-contact field sensor, the local distribution of classical eddy current losses PEC,C (x,y,z=0) was determined. (c) Interior analyses of PEC,C (x,y,z) were based on a thin-film technique [2]. Tests were based on approx. 15 μm thick silver paste strips, which were applied both on the top and bottom sides of a “sensor-laminate”. It detects eddy field components between lamination surface points of a "sensor" lamination which is arranged in different regions of the core package. Thus it allows an effective separation of planar eddy currents (PECs) and regular eddy currents (RECs). Measurements were taken for 23 layers L1...L23, i.e. considering almost one half of the stack.