Team,
We are monitoring CPU, memory and disk with RSP probe and have requirement If CPU utilization is >=90% for continuous 30 mins than only generate alarm.
You should be able to do this by configuring a Time Over Threshold dynamic alarm for the CPU metrics on the devices monitored by the rsp probe by configuring this through the probe's Configuration Page in Admin Console. For a discussion on Time Over Threshold alarms see The Time Over Threshold Event Rule WIki page.