The extensive network of the power system has very expensive equipment, including generators, breakers, power cables and transformers. The power transformer is the beating heart of this network, which is always affected by operating and environmental conditions, it suffers various errors and in some cases, it will cause the transformer to fail and go out of the circuit and not be accessible for a long time. As a result, instead of time-based, maintenance programs should be based on operational and environmental conditions, which requires us to be aware of the current equipment conditions. Therefore, it will be very important to use monitoring and fault detection methods that have the ability to evaluate the internal conditions of the equipment. There are various methods and tests to evaluate the condition of the transformer, which include methods such as frequency response analysis, dissolved gas analysis, signal processing, leakage flux, and negative sequence current. Among them, the frequency response analysis method is a very popular and comprehensive method that has a high ability to detect errors and its implementation is simple and convenient.