Reverse Power Protection

Time delay is absolutely required to prevent spurious tripping due to momentary load changes, during synchronisation, and to prevent turbine damage.

Multiple time delays are often used for backup relays to provide stepped redundancy, ie. one time delay value will be set for the primary relay and a slightly longer value for the secondary relay.

Some relays have two distinct adjustable time settings that can be critical for steam turbines. The shorter one of up to about 5 seconds is as above to prevent spurious tripping, the longer delay (often called turbine withstand) is provided to ensure the field remains offline long enough to prevent fatal overheating of the turbine due to steam loss.

Given the conditioning contacts from the MSV (Main Steam/Stop Valve) the designer of your protective relaying system is using the Reverse Power Relay for two separate purposes.

When the MSV is closed the turbine is not receiving any steam so the instantaneous element is being used as an "Inadvertent Energization" relay to remove any source of voltage from the generator terminals.

When the MSV is open, steam is being admitted to the turbine and the relay's time element is there to make sure that the generator doesn't act as a motor for very long.

The actual protective relaying action will depend upon the rest of the control logic, actual settings, zone control, reach, etc., etc., of which this is just one element.