In power systems, high-impedance transformers are now widely used because they effectively limit short-circuit current. Among them, the built-in high-voltage-winding type—being less expensive—has the largest installation share, yet it produces the most severe inrush currents during no-load energization, with higher peaks and greater three-phase asymmetry. In recent years, mal-operation of the main protection of such transformers and even of back-up relays on adjacent equipment has been reported several times, seriously endangering secure system operation. This paper therefore reviews the types and construction of high-impedance transformers and explains why their inrush is particularly large. A simulation model of the built-in HV-winding transformer is developed, and its residual-flux evolution is compared with that of a conventional design. Dynamic analogue tests are finally carried out to explore suppression strategies suited to high-impedance units, providing a basis for refining future inrush-mitigation schemes and equipment.

High-impedance transformer,inrush-current suppression,no-load energization,residual flux,transformer tests