Rate Of Change Of Frequency (ROCOF) Protection df/dt working Principle:
Rate Of Change Of Protection is used for load shedding in situations where sudden loss of generating capacity on a system will be accompanied by a decrease in system frequency. In such a situation of load Generation mismatch, the system frequency tends to fall. The df/dt relay can control the circuit breakers and allow feeders to be disconnected from the network, one by one. As a defense mechanism, df/dt relays are particularly effective in arresting the frequency collapse of a grid in the event of sudden loss of major generation. This is because by measuring the frequency decay rate, the corrective action can be initiated much ahead of the time when frequency of the synchronous interconnection would have actually dipped to a point at which generator under-frequency relays or unit auxiliaries would trip / operate leading to a complete system shutdown.
Principle of rate of change of frequency relay:
Whenever the load increases in the generator the frequency falls down. Consider two parallel generators (example 1MW, 1.5MW), both is running with the grid and the auxiliary load of the two generator is 1.1 MW. Here, if the grid also failed and 1.5 MW generator also tripped. Therefore, all the auxiliary loads will fall on the single generator and the generator frequency drops as the load increases. Here, The auxiliary loads should to be properly shedded to avoid the tripping of the remaining generator. This action will be done by df/dt relays.
The rate of change of frequency measurement is based on two successive frequency measurements and the time difference between the frequency measurements. The measured frequency value for df/dt calculation is averaged over three cycles. The accuracy of df/dt measurement depends on the accuracy of frequency measurements. This frequency measurement is carried out through time measurement of a cycle (time between two zero crossing).
Rate Of Change Of Frequency (ROCOF) Protection ANSI code: 81R
- ROCOF trips the field breaker for load shedding
- Trips the 110kV Field breaker for single generator
- In Annunciation, df/dt Opted