look this example i have tested it is working
DECLARE @TODAY DATEtime
SET @TODAY = CONVERT(VARCHAR(10), GETDATE(), 108)
select CAST
(
(SUM (datepart(hh, convert (varchar, @TODAY, 108))) +
(sum(datepart(mi, convert (varchar, @TODAY, 108)))/60) ) AS VARCHAR(2)
)
+ ':' +
CAST
(
sum(datepart(mi, convert (varchar, @TODAY, 108))) - 60 * (sum(datepart(mi, convert (varchar, @TODAY, 108)))/60)
as VARCHAR(2))