4–146 889 GENERATOR PROTECTION SYSTEM – INSTRUCTION MANUALPROTECTION CHAPTER 4: SETPOINTSThird HarmonicVoltage DifferenceThis element responds to third harmonic voltage measured at the generator neutral andoutput terminals. When used in conjunction with the neutral overvoltage (fundamentalfrequency) element, it provides 100% ground fault protection of the stator windings. Sincethe amount of third harmonic voltage that appears in the neutral is both load and machinedependent, the protection method of choice is an adaptive method. The following formulais used to create an adaptive operating quantity based on the amount of third harmonicthat appears at the generator terminals.Where:VN(3rd) is a magnitude of the the 3rd harmonic in the voltage measured at the machineneutral point measured via an auxiliary channel of the VT bank, and V0(3rd) is a magnitudeof the 3rd harmonic in the zero-sequence voltage measured at the machine terminals.NOTENOTE:The setpoint Aux VT Connection (under Setpoints > System > Voltage Sensing > Ph VT Bnk1-J2) must be selected as “VN” for this elementNOTENOTE:This element requires wye-connected VTs for measurement of the third harmonic in thezero-sequence voltage at the generator output terminals.Path: Setpoints > Protection > Group 1 > Generator > 3rd Harm Voltage DifferenceTRIP FUNCTIONRange: Disabled, Trip, ConfigurableDefault: DisabledThis setting enables the Third Harmonic Voltage Difference trip functionality.TRIP PICKUPRange: 0.005 to 0.900 in steps of 0.001Default: 0.100This setting specifies a pickup level for the operating quantity. It may be beneficial tomeasure the operating quantity under various load conditions for a particular machinein order to optimize this setting. This can be achieved using the actual values menu ofthe 889.TRIP PICKUP DELAYRange: 0.00 to 600.00 s in steps of 0.01 sDefault: 0.50 sThis setting specifies a time delay for the trip function. A trip will occur once theoperating conditions remains valid for the Trip Pickup Delay time.TRIP SUPVRange: 0.5 to 30.0 V in steps of 0.1 VDefault: 5.0 VThis setting specifies a signal level supervision for the vectorial sum of the thirdharmonic at the machine neutral and the zero-sequence terminal voltage. The safevalue of this setting can be established after putting a given machine into service andreading the quantity from the relay under actual values menu (Metering > Generator >Stat Gnd 3rd Harm).TRIP RELAYRange: Do Not Operate, OperateDefault: Do Not Operate
