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CHAPTER 6: SETPOINTS
NOTE:
NOTE
339 MOTOR PROTECTION SYSTEM – INSTRUCTION MANUAL
All current elements will function properly with the exception of the neutral directional
element.
•
A filtering algorithm increases the trip and alarm times for the protection elements by
up to 270 ms when the level is close to the threshold. If the level exceeds the threshold
by a significant amount, trip and alarm times will decrease until they match the
programmed delay. The exceptions to this increased time are the short circuit ,
neutral, and ground fault, which continue to trip as per specification.
If FUNCTION is disabled, or is enabled but the "VFD Bypassed" operand is asserted (i.e.
BYPASS SWITCH is "Closed") then the frequency source is switched from current to voltage,
all protection elements and metering work as normal, and the VFD Not Bypassed operand
is de-asserted.
Voltage inputs to the 339 motor protection relay are normally measured at the busbar side
of the VFD where they are substantially sinusoidal. Since the output phase voltages from a
VFD are not sinusoidal and are distorted due to harmonics generated by the VFD, blocking
the voltage elements via the Block setting using the "VFD Not Bypassed" operand is
recommended. On the other hand, when voltages are measured at the motor side of the
VFD, voltages at the motor terminals and relay inputs are same, thus blocking the voltage
elements is not required. In addition, significant distortion in the voltage waveforms is not
always the case and really depends on the VFD type. If the input voltages are substantially
sinusoidal, which can be verified from 339 metering, oscillography, and the data logger,
then blocking the voltage elements is not required.
PATH:
SETPOINTS > S2 SYSTEM SETUP > VFD
FUNCTION
Range: Disable, Enabled
Default: Disabled
See
Common
setpointsfor details.
BYPASS SWITCH
Range: Off, Contact Input 1 to 10, Virtual Input 1 to 32, Remote Input 1 to 32, Logic
Element 1 to 16, CT Fail Alarm PKP, CT Fail Alarm OP
Default: Off
This setting defines the source used to determine if the motor is powered by the VFD or
directly from the AC source through the bypass switch. The "VFD Bypassed" operand will
be asserted when VFD is bypassed i.e. motor is directly powered by the AC system or the
utility. This operand can be used to generate a Logic Element for blocking voltage based
elements via the Block setting if this operand is not asserted i.e. VFD is not bypassed.
Blocking the voltage based elements via the Block setting of the desired elements is highly
recommended if "VFD Not Bypassed" is asserted
S2 SYSTEM SETUP
6–33
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