When a core loosing fault of power transformer occurs

the fundamental frequency component（100 Hz） in its vibration signal will change

but the fundamental frequency component cannot be used for accurate diagnosis of the fault because many factors influence the component. It is known that the new characteristic frequencies of the 50 Hz frequency component and some of its harmonic components

as well as the harmonic components of the fundamental frequency

can also reflect the fault of transformer core loosing.In this study

a fault diagnosis method of transformer core loosing was proposed based on the relationship among the characteristic frequencies. This method can diagnose whether a fault exists in transformer core

and determine the fault type.In order to verify the proposed method

a deformation fault was set to the actual transformer core. The vibration signals were sampled from the monitoring points

and de-noising and conversion processing was performed to diagnose the fault with the proposed method. The diagnosis result is the same with the actual fault

indicating that the proposed diagnostic method is accurate and feasible. Fault diagnosis system of power transformer based on vibration has been developed by using the proposed diagnosis method and applied to the actual substation.