Rocof Relay

The ROCOF (Rate of Change of Frequency) relay:

The place that is specified in the instruction manual that came with your unit.

"...Installation

For reliable operation the unit's signal connections should be connected as close to the generator as is practical. The unit uses the vector shift produced by the generators characteristic impedance when the generator load changes. Any additional impedance introduced by bus bars etc., which could introduce vector shifts during normal operation (not mains failure) should be avoided in the connection to the unit as it may be necessary to "de-sensitise" the vector shift trip point to avoids nuisance trips..."

From Googling on ROCOF relay: http://energy.te.com/china/crompton_doc/rocof.pdf page 23. If this is unclear hire a competent engineer to do this for you. Incorrect placement and/or settings will cause needless unit trips.

More information is required on the site installation and how they operate. Are you trying to disconnect one or all STGs upon loss of one STG prime mover when they are islanded?

If you want to disconnect only one generator or they are islanded from the mains supply, I would seriously think about using another form of protection to disconnect upon loss of prime mover. Say if generator#1 has a more sensitive set point than generators#2 and #3, then if either generator#2 or #3 looses the prime mover, then either generator#1 or more would trip.

ROCOF and / or Vector Shift protection should be used as one form of anti-islanding protection when connected to the mains supply, not local generator supply. As (in most cases) the mains supply frequency can be relied upon to be very stable, any rapid changes in frequency or vector shifts can be determined to be a loss of mains supply and the local generators should be disconnected from the mains grid ASA possible.

I have seen in the past where someone tried using ROCOF protection and it activated after the breaker closed when 2 GTGs were synchronised.

It would be best if a protection or power design engineer would be consulted / hired to assess the installation so the best solution can be provided.

Actually the three steam turbines also connected to utility grid, and we want to shed the load in case of generation failed. My thinking is, if we installed rocof, would the frequency of other generator, interfere the process of rocof, and how many rocof should be installed for such configuration. Thanks

Once again, not knowing the installation, if all local generators are connected to the utility, the utility (effectively an infinite supply capacity) would dictate the frequency for all generators. The ROCOF would be used to disconnect utility incomer circuit breaker when there is a loss of utility supply, not local generator supply. The settings for the ROCOF relay all depends on how stable the frequency is from the utility, so you can see there is no hard and fast rule for the settings, usually the protection engineer for the utility supplier should be able to advise any recommended settings and protections for anti-islanding.

If the local generators are operating islanded from the utility, then a reduced power relay could be used with a bit of logic to detect a minimum power level once the local generator is online. But I am sure there are better ways to detect a loss of prime mover for the local generator...