Abstract: This paper investigates retrofit technology without outage to address the problem that the 10 kV busbar cannot be powered off during main transformer protection retrofit in substations. By adopting techniques such as conductor selection and potential measurement, installation of trip output links, addition of intermediate relays, use of maintenance links, and digital simulation testers, the correctness of the low voltage side backup protection of the main transformer and the tripping and blocking circuits of the bus-tie breaker is verified without physically operating the bus-tie breaker. Meanwhile, for the difficulty of accessing the TV secondary voltage circuit, the advantages, disadvantages, and risks of two schemes, namely TV outage transfer and voltage acquisition from the fault recorder panel, are compared. Through scheme comparison and optimization, a safe and efficient typical work procedure and corresponding risk management system for retrofit without outage are finally developed.