What is the principal of transformer and what is its frequency?

Electrical Power Transformer is a static device which transforms electrical energy from one circuit to another without any direct electrical connection and with the help of mutual induction between to windings. It transforms power from one circuit to another without changing its frequency but may be in different voltage levelA variable frequency transformer is used to transmit electricity between two (asynchronous or synchronous) alternating current domains. The VFT is a relatively recent development (first deployed in2004). Most asynchronous grid inter-ties use high-voltage direct current converters, while synchronous grid inter-ties are connected by lines & "ordinary" transformers, but without the ability to control power flow between the systems.