4.3 APPLICATION NOTES4.3.1 SETTING GUIDELINESFollowing changes on the network caused by faults or other operational requirements, it is possible that varioussubsystems will be formed within the power network. It is likely that these subsystems will suffer from ageneration/load imbalance. The "islands" where generation exceeds the existing load will be subject tooverfrequency conditions. Severe over frequency conditions may be unacceptable to many industrial loads, sincerunning speeds of motors will be affected. The overfrequency element can be suitably set to sense thiscontingency.An example of two-stage overfrequency protection is shown below using stages 5 and 6 of the f+t elements.However, settings for a real system will depend on the maximum frequency that equipment can tolerate for agiven period of time.Stage Element Frequency Setting (Hz) Time Setting (Sec.)1 Stage 5(f+t) 50.5 302 Stage 6(f+t) 51.0 20The relatively long time delays are intended to provide time for the system controls to respond and will work well ina situation where the increase of system frequency is slow.For situations where rapid increase of frequency is expected, the protection scheme above could be supplementedby rate of change of frequency protection elements.In the system shown below, the generation in the MV bus is sized according to the loads on that bus, whereas thegenerators linked to the HV bus produce energy for export to utility. If the links to the grid are lost, the generationwill cause the system frequency to rise. This rate of rise could be used to isolate the MV bus from the HV system.E00857LoadLoadTo utilityLocal generationIPP generationHV busMV busFigure 128: Power system segregation based upon frequency measurementsNote:This section refers to advanced frequency protection. The basic frequency protection works in a similar manner, but thesetting names and DDB signal names are different.P14x Chapter 11 - Frequency Protection FunctionsP14xEd1-TM-EN-1 241
