Currently, ultra-high-voltage (UHV) are under development in many countries around the world in order
to accommodate the increasing levels in demand for electricpower. In Korea, the commercialization of a 765kV ultra-high-voltage system has been ongoing since 2002. We have developed a number of protection relays suitable for the UHV
system to enable us to make a contribution to the 765kV UHV projects in Korea. During the development of the protection
relays for the 765kV UHV system we encountered several technical issues of particular interest in relation to the transformer
protection relay.In this paper, we introduce a number of technical issues, such as a development undertaken in response to the
need for an increase in transformer capacity to overcome consequent transmission restrictions, the implementation of a CT
saturation countermeasure and the requirement for high-speed operation of the current differential elements. In order to provide solutions to these issues, we have undertaken several developments, which we explain in this paper. Furthermore, with
the development of this UHV transformer protection technology, we have been able to complete the commercialization of the
transformer protection relay for the 765kV UHV system.