Take an example of Hi Pot test and Megger test used for insulation testing of the underground cable that is used to transmit power at 6.6KV.... What is the difference between the two except the voltage applied is high in Hi Pot test?...Please explain the two...

An insulation resistance (so called "megger" for meg-ohm) test is only checking the insulation system for continuity to ground and does not apply significant electric field stress so that is why it is considered a non-destructive test. A "megger" test is typically performed to verify no "grounds" are present or to quickly track down the location of a permanent ground fault failure in an insulation system.

A high potential (hi-pot) test applies significant electric field stress to the insulation to actively cause a failure at a weak point such as a poor cable termination and that is why it is considered a destructive test. Power cable sections complete with splices and terminations are typical applications of hi-pot testing although it can be applied to any generalized insulation system.

Megger is a maintenance tool and its use is to test the insulation system below the nominal maximum voltage stress level, rms x 1.414.

Hipot is a dielectric stress test typically done above nominal max voltage stress level and is a possible destructive test.

DC hipot is not recommended for cable testing as per IEEE 400.2. Today recommendation is AC VLF. There are both maintenance test level as in Tan Delta testing at .5, 1.0 and 1.5 Vo or acceptance testing and per IEEE 400.2 test table.