Schweitzer Engineering Laboratories SEL-749M, Instruction Manual

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4.27
Date Code 20080918 Instruction Manual SEL-749M Relay
Protection and Logic Functions
Voltage-Based Protection
If the real three-phase power falls below the warning or trip level for longer
than the time delay setting, the relay can issue a warning or trip signal. The
underpower elements are disabled when the motor is stopped or starting.
Figure 4.16 shows the logic diagram for the underpower elements. These
elements operate in addition to the load loss function, and you can use them to
detect motor load loss and other underpower conditions.
You can disable the elements by setting the underpower warning level
(37PAP) and underpower trip level (37PTP) settings to OFF.
Figure 4.16 Underpower Element Logic
Power Factor
Elements
If the measured power factor falls below the leading or lagging level for longer
than the time delay setting, the relay can issue a warning or trip signal. The
power factor elements are disabled when the motor is stopped or starting.
Figure 4.17 shows the logic diagram for the power factor elements. These
elements can be used to detect synchronous motor out-of-step or loss-of-field
conditions.
Refer to Figure 5.3 for the relay power measurement convention.
For application on an induction motor, disable the elements by setting all four
power factor level settings to OFF.
NOTE:
Underpower Trip and Warning
Level settings are in primary kW.
37PAD
0
37PA
Switches close when RUNNING = 1
RUNNING
Measured
Real Power
37PAP
37PTP
37PTD
0
37PT
Relay
Word
Bits
Relay
Word
Bit
Settings
Table 4.25 Power Factor Settings
Setting Prompt Setting Range Setting Name := Factory Default
PF LAG TRIP LEVL Off, 0.05–0.99 55LGTP := OFF
PF LD TRIP LEVEL Off, 0.05–0.99 55LDTP := OFF
PF TRIP DELAY 0–240 sec 55TD := 1
PF LAG WARN LEVL Off, 0.05–0.99 55LGAP := OFF
PF LD WARN LEVEL Off, 0.05–0.99 55LDAP := OFF
PF WARN DELAY 0–240 sec 55AD := 1
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