CHAPTER 4: SETPOINTS PROTECTION
889 GENERATOR PROTECTION SYSTEM – INSTRUCTION MANUAL 4–147
ALARM FUNCTION
Range: Disabled, Alarm, Latched Alarm
Default: Disabled
This setting enables the Third Harmonic Voltage Difference alarm functionality.
ALARM PICKUP
Range: 0.005 to 0.900 V in steps of 0.001
Default: 0.150
ALARM 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 alarm function. An alarm will be generated
once the operating conditions remain valid for the Alarm Pickup Delay time.
ALARM SUPV
Range: 0.5 to 30.0 V in steps of 0.1
Default: 5.0 V
This setting specifies a signal level supervision for the vectorial sum of the third
harmonic at the machine neutral and the zero-sequence terminal voltage. The safe
value of this setting can be established after putting a given machine into service and
reading the quantity from the relay under actual values menu (Metering > Generator >
Stat Gnd 3rd Harm).
ALARM RELAY
Range: Do Not Operate, Operate
Default: Operate
BLOCK
Range: FlexLogic operand
Default: Off
EVENTS
Range: Enabled, Disabled
Default: Enabled
TARGETS
Range: Self-reset, Latched, Disabled
Default: Latched
Example 1: Operating quantities under normal conditions
Consider the next figure shown. In the case of a high impedance grounded machine, the
neutral resistor is typically chosen such that power dissipated in the resistor during a
single line to ground fault is approximately equal to the reactive power produced by the
zero sequence capacitance of the stator winding and the generator step-up low voltage
winding (X
oc
). At power system frequencies (F
n
), the neutral resistance is therefore equal to
X
oc
/ 3, and at 3 x F
n
, the neutral resistance is X
oc
.For analysis, assume that E
3
= 10 V, R =
5, and Xc = 5.
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