CHAPTER 4: SETPOINTS PROTECTION
889 GENERATOR PROTECTION SYSTEM – INSTRUCTION MANUAL 4–143
Third Harmonic
Neutral Undervoltage
(27TN)
The Third Harmonic Neutral Undervoltage element detects a loss of third harmonic voltage
at the generator neutral. The percentage of stator winding covered by this function
depends on the pickup setting and the amount of third harmonic generated by the
machine at the time of the fault. A settable window of forward real power can supervise
this element for enhanced security. The element is also supervised by positive sequence
voltage measured at the generator output terminals. This element can be used with either
wye or delta connected VTs on the terminal side and requires that the machine neutral
voltage is connected via an auxiliary voltage channel of the relay Ax VT Bnk1-J2 bank
input.
NOTE:
The setpoint Aux VT Connection (under Setpoints > System > Voltage Sensing > Ph VT Bnk1-
J2) must be selected as “VN” for this element
Path: Setpoints > Protection > Group 1 > Generator > 3rd Harm Neutral UV
TRIP FUNCTION
Range: Disabled, Trip, Configurable
Default: Disabled
This setting enables the Third Harmonic Neutral Undervoltage trip functionality.
TRIP PICKUP
Range: 0.10 to 20.00 V in steps of 0.01 V
Default: 1.00 V
This setting specifies the trip pickup level in VT secondary for the magnitude of the 3rd
harmonic of the neutral voltage. The magnitude of the 3rd harmonic voltage at the
neutral point can be monitored under Metering > Generator > Stat Gnd 3rd Harm.
Measuring the actual value of the operating quantity for a specific machine under
variety of load conditions is helpful when selecting the pickup threshold for this feature.
TRIP PICKUP DELAY
Range: 0.00 to 600.00 s in steps of 0.01 s
Default: 1.00 s
This setting specifies a time delay for the trip function. Once the operating quantity 3rd
Harm VN is lower than the Trip Pickup level for the Trip Pickup Delay, a trip occurs
indicating a fault condition.
TRIP RELAY
Range: Do Not Operate, Operate
Default: Do Not Operate
ALARM FUNCTION
Range: Disabled, Alarm, Latched Alarm
Default: Disabled
This setting enables the Third Harmonic Neutral Undervoltage alarm functionality.
ALARM PICKUP
Range: 0.1 to 20.00 V in steps of 0.01 V
Default: 0.50 V
This setting specifies the alarm pickup level for the magnitude of the 3
rd
harmonic of the
neutral voltage.
NOTE:
Trip and alarm pickup levels are set in secondary voltage.
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