The on-load tap changer（OLTC）

the only part which can perform mechanical actions in power transformer

is an important part for stabilizing the voltage level of the power grid

and plays an important role in ensuring safe and reliable operation of the system and the flexibility of power grid dispatching. In view of main OLTC technology used in the present transformers and the new types of OLTC which have been developed in recent years

the voltage regulation process and operating principles of mechanical

power electronic and hybrid OLTC are summarized

compared and analyzed successively and respectively. The structure types and the existed technical difficulties of various types of OLTCs are introduced in particular

and the advantages as well as disadvantages of them are pointed out. As for the mechanical and electrical defects in the operation of the on-load tap-changer of the transformer

the development status of its fault diagnosis technology is analyzed

and the main developments in the research of vibration method

driving motor current method and dissolved gas in oil method are introduced. Finally

the further development of transformer OLTC technology and fault diagnosis methods are prospected

which can provide a definite guidance and reference for future research work.