Dry transformer shielding is often challenging due to the intense magnetic fields that they produce.
It is necessary to have a shielding system that features a high shielding factor which can only be achieved through the combined use of materials with magnetic and conductive behaviour. Based on the standard product, Sati Shielding developed an innovative localised transformer shield comprised of a free-standing wall to be installed near the transformer.
Description of the shielding materials and their shielding efficiency:
The magnetic induction mitigation system is achieved by installing magnetic shielding obtained by coupling two different materials:
• Material with high electric conductivity
• Material with high magnetic permeability
In the presence of a variable magnetic field (inducer field) the layer of material with high electrical conductivity becomes a place of circulation currents, which in turn generate a reaction magnetic field (induced field). The combined effect of the fields, induced and inducer, translates into an overall reduction of the total magnetic field.
The layer of material with high magnetic permeability reduces magnetic induction by absorbing the existing magnetic field. Its shielding behaviour, similar to an "umbrella" of protection against the magnetic field can be very intense next to the shield, but tends to decrease the further the distance from the shield.
The combination of the two materials, ferromagnetic and conductive, create a shield with excellent shielding capacities, both next to the shield, mainly thanks to the ferromagnetic shield, as well as far away from the shield, thanks to the conductive shield.
The shielding specifications are defined through the shielding factor measurement (SF: Shielding Factor) carried out at the Sati Shielding S.r.l. research and development laboratory in via Ferrero 10, Rivoli Cascine Vica (TO).
The tests were performed by measuring the magnetic induction values emitted by a resin transformer with a nominal power of 630 kVA before and after field mitigation activity.
The shielding factor suggested below is calculated as a ratio between the magnetic induction modules in the pre and post shielding situation, based on the distance from the shield.
The configuration of the tests and the obtained results are provided on the right:
Shield dimensions were adapted to the size and therefore the dimensions of the installed transformer. The dimensions of the wall that the experimental tests refer to are provided by way of example.
It represents a simple and immediate indication of how many times it is possible to reduce the magnetic induction value by applying the shielding solution.