Just guessing around...
If it's really the sed command failing (for whichever reason), try this instead of copying and then editing in place.
Code:
sed -e 's/600/900/' /etc/kdb.src/luigi/system/daemon/powerd/t1_timeout.bkp > /etc/kdb.src/luigi/system/daemon/powerd/t1_timeout
Also, you might want to try "sed -i -e 's/.../'".