Underfrequency (81U) - GE 339 Instruction Manual

S3 PROTECTION

Underfrequency (81U)

NOTE:
NOTE
6–86
The 339 provides 2 Underfrequency Protection elements. Each element can be set to
either cause a trip or generate an alarm when the frequency of the input voltage (wye-
connected VT: VAN; delta-connected VT: VAB) drops below the pickup level for a specified
time delay. If it is desirable to have an alarm before a trip occurs, the user can set the
function of one Underfrequency element to ALARM, and the function of another element to
TRIP.
The Underfrequency element is blocked VAN is below the set MIN VOLTAGE. This setting
may be used to prevent nuisance alarms or trips when the bus is not energized. If a dead
source is desirable to be considered as a fault condition, set this setting to "0.00xVT".
This feature may be useful for load shedding applications on large motors. It could also be
used to load shed an entire feeder if the trip was assigned to an upstream breaker.
Underfrequency can also be used to detect loss of power to a synchronous motor. Due to
motor generation, sustained voltage may prevent quick detection of power loss. Therefore,
to quickly detect the loss of system power, the decaying frequency of the generated
voltage as the motor slows can be used.
The following path is available using the keypad. For instructions on how to use the
keypad, please refer to Chapter 3 - Working with the Keypad.
PATH: SETPOINTS > S3 PROTECTION > UNDER-FREQUENCY1(2)
When "VFD Not Bypassed" is asserted, frequency source is software calculated from the
phase A current. The underfrequency element is inhibited from operating unless this
current is above 10% of nominal.
UNDRFREQ 1(2) FUNC
Range: Disabled, Alarm, Latched Alarm, Trip
Default: Disabled
For details see Common
UNDERFREQ PKP
Range: 40.00 to 70.00 Hz in steps of 0.01 Hz
Default: 59.00 Hz
This setting defines the Underfrequency pickup level; it is usually set to a frequency level
considered dangerous for the stability of the system.
UNDRFREQ DELAY
Range: 0.1 to 600.0 sec in steps of 0.1 sec
Default: 2.0 s
This setting specifies the time delay before Underfrequency operation.
MIN VOLTAGE
Range: 0.00 to 1.25 x VT in steps of 0.01 x VT
Default: 0.70 x VT
The minimum operating voltage level is programmable to prevent undesired
Underfrequency operation before voltage VAN becomes available, such as on faults
cleared by downstream protection or fuses. This setting is not used in VFD mode.
OUTPUT RELAY X
For details see Common
BLOCK 1 to 3
For details see Common
setpoints.
setpoints.
setpoints.
339 MOTOR PROTECTION SYSTEM – INSTRUCTION MANUAL
CHAPTER 6: SETPOINTS