10.1.3.5 Voltage restrained overcurrent protection for generator and step-uptransformerExample will be given how to use one CVGAPC function to provide voltage restrainedovercurrent protection for a generator. Let us assume that the time coordination studygives the following required settings:• Inverse Time Over Current TOC/IDMT curve: ANSI very inverse• Pickup current of 185% of generator rated current at rated generator voltage• Pickup current 25% of the original pickup current value for generator voltages below25% of rated voltageThis functionality can be achieved by using one CVGAPC function. The following shallbe done in order to insure proper operation of the function:1. Connect three-phase generator currents and voltages to one CVGAPC instance (forexample, GF05)2. Set CurrentInput to value MaxPh3. Set VoltageInput to value MinPh-Ph (it is assumed that minimum phase-to-phasevoltage shall be used for restraining. Alternatively, positive sequence voltage can beused for restraining by selecting PosSeq for this setting parameter)4. Set base current value to the rated generator current primary amperes5. Set base voltage value to the rated generator phase-to-phase voltage in kV6. Enable one overcurrent step (for example, OC1)7. Select CurveType_OC1 to value ANSI Very inv8. If required set minimum operating time for this curve by using parameter tMin_OC1(default value 0.05s)9. Set PickupCurr_OC1 to value 185%10. Set VCntrlMode_OC1 to On11. Set VDepMode_OC1 to Slope12. Set VDepFact_OC1 to value 0.2513. Set VHighLimit_OC1 to value 100%14. Set VLowLimit_OC1 to value 25%Proper operation of the CVGAPC function made in this way can easily be verified bysecondary injection. All other settings can be left at the default values. Furthermore theother built-in protection elements can be used for other protection and alarming purposes.10.1.3.6 Loss of excitation protection for a generatorExample will be given how by using positive sequence directional overcurrent protectionelement within a CVGAPC function, loss of excitation protection for a generator can beachieved. Let us assume that from rated generator data the following values are calculated:1MRK 505 337-UUS - Section 10Multipurpose protection269Application manual
