According to its importance to save life and money, electrical power system networks must be protected to prevent any abnormal condition and to ensure the continuation of electrical power without any interruption to the healthy parts. Transformers are one of the most important parts of the power system and the researchers deal with developing the methods of protecting it to ensure secure operation. Protective relays are the most important part of the protection system. The relay does not prevent the incidence of faults. It can take action only after the fault has occurred. There are many types of relays depending on the strategy of detecting the fault and the part to be protected. In this research, a literature review for power transformer digital protection has been listed to present the development of the protection strategy for this part of the power system.