Some times adding of air-gap in transformers core may be a benefit design technique for transformers performance improvement and can be applicable in electrical systems. For example we can mention following items: – Shunt reactors – Constant-voltage transformer – Air-gapped current transformers – Linear current transformers – TPY and TPZ current transformers Shunt reactors Shunt reactors are available in two design configurations: coreless and iron core. Coreless oil-immersed shunt-reactor designs utilize a magnetic circuit or shield, which surrounds the coil to contain the flux within the reactor tank. The steel core that normally provides a magnetic flux path through the primary and secondary windings of a power transformer is replaced by insulating support structures, resulting in an inductor that is nearly linear with respect to applied voltage. Conversely, the magnetic circuit of an oil-immersed iron-core shunt reactor is constructed in a manner similar to that used for power transformers, with the exception that an air gap or distributed air gap is introduced to provide the desired reluctance. Because of the very high permeability of the core material, the reluctance of the magnetic circuit is dominated by the air gap, where magnetic energy is primarily stored. Inductance is less dependent on core permeability, and core saturation does not occur in the normal steady-state-current operating range, resulting in a linear inductance. A distributed air gap is employed to minimize fringing flux effects, to reduce winding eddy losses (adjacent to the gaps), and improve ampere-turns efficiency. Constant-voltage transformer A well-known solution for electrical