Linearity of shielded measuring transformer

Voltage and current measuring transformers must be accurate even in a case when measuring value is not strictly harmonic, but it contains some higher harmonics. For this reason, both measuring transformers must exhibit high linearity. This means that measuring transformers should not contain any ferromagnetic material, which is always nonlinear. Therefore, any measuring transformer must be air-cored. On the other hand, inside the unknown, undesirable external time varying magnetic field in the transformer´s windings the induced electromotive force (emf) appears, which has significant impact on measurement results. In order to keep necessary measurement accuracy, a shielding procedure must be applied. Against time varying magnetic field, shielding must be performed with a conducting ferromagnetic material, which also introduces a certain nonlinearity in the measuring transformer. Hence, the main goal of this investigation is to find the optimal shield, which will keep the transformer linearity as high as possible, while providing a good protection against external magnetic fields as well. For this reason a model of measuring transformer was constructed and harmonic input voltage or current, at different frequencies, was simulated. Electromagnetic field of this transformer was calculated for each input voltage or current harmonic separately, applying a FEM based program. Obtained results are then applied to determine the emf value induced in secondary transformer´s winding.