What may happen in transient when we supply high voltages to the transmission lines which are not energized ?

First clarify what u mean by transient?

transmission Lines can be , underground cable or OHL conductors, in Qatar Electricity Transmission HV/EHV is by AC transmission ,in several countries long distance transmission is achieved by HVDC lines also .Energization /Transmission project commissioning, of FEEDERS/CABLE/OHL or SUBSTATION is a long procedure consisting of number of safety precautions and test acceptance process.

finally there is an over all stability test from remote end(Live ) to the upcoming  commissioning end .

again final safety documents and various test results and healthiness reports are reviewed before energization /commissioning .