Issue in subtracting 2 times , i need to get result in hh:mm:ss format

Hi guys,
 this is my query

  select CONVERT(Varchar(12), DATEADD(MS, DATEDIFF(MS,'24:00' , '27:1' ), 0), 114)

 I am facing this error : Conversion failed when converting date and/or time from character string.
 i need to get result in hh:mm:ss format
 Please help me out on this issue.

 Thank you,
Arjun.