4.30
SEL-749M Relay Instruction Manual Date Code 20080918
Protection and Logic Functions
Frequency and Load Control Settings
Frequency and Load Control Settings
Frequency Tracking
The SEL-749M relay tracks frequency between 44–66 Hz. The tracking
algorithm uses V1 (Positive Sequence Voltage) if available, and greater than
0.75 V. For models without voltage inputs or when voltage is below 0.75 V the
relay uses I1 (Positive Sequence Current).
The relay word bit FREQ is asserted when the relay is tracking frequency. The
frequency is clamped at the maximum (66 Hz) or minimum (44 Hz) when it
crosses the bounds. When there is no frequency measurement due to low
voltage and current, the relay switches to nominal frequency.
Frequency Elements
The SEL-749M provides two warning and two trip over- or underfrequency
elements with independent level and time-delay settings. When an element
level setting is less than the nominal frequency setting, the element operates as
an underfrequency element. When the level setting is greater than the nominal
frequency setting, the element operates as an overfrequency element.
Figure 4.19 shows the logic diagram for the frequency elements.
Figure 4.19 Over- and Underfrequency Element Logic
Table 4.26 Frequency Settings
Setting Prompt Setting Range Setting Name := Factory Default
FREQ1 TRIP LEVEL Off, 55.0–65.0 Hz 81D1TP := OFF
FREQ1 TRIP DELAY 0.0–240.0 sec 81D1TD := 1.0
FREQ1 WARN LEVEL Off, 55.0–65.0 Hz 81D1AP := OFF
FREQ1 WARN DELAY 0.0–240.0 sec 81D1AD := 1.0
FREQ2 TRIP LEVEL Off, 55.0–65.0 Hz 81D2TP := OFF
FREQ2 TRIP DELAY 0.0–240.0 sec 81D2TD := 1.0
FREQ2 WARN LEVEL Off, 55.0–65.0 Hz 81D2AP := OFF
FREQ2 WARN DELAY 0.0–240.0 sec 81D2AD := 1.0
NOTE: The setting range for Trip and
Warn Levels shown in Tabl e 4. 26 are
for FNOM := 60 Hz. The setting ranges
will be “Off, 45.0–55.0 Hz” when
FNOM := 50 Hz.
NOTE: The relay measures system
frequency for these elements with the
positive-sequence voltage if the
voltage input option is present and
the applied positive-sequence voltage
is greater than 0.75 volts for at least
four cycles. Otherwise, the relay uses
positive-sequence current.
freq
81DnTP
81D1T
81D2T
81DnTP < FNOM
81DnTP FNOM
81DnTD
0
freq
81DnAP
81D1A
81D2A
81DnAP < FNOM
81DnAP FNOM
FREQ
Frequency Elements 1–2
Measured Frequency
Frequency Pickup Setting
Nominal Frequency Setting
Over- and Under Frequency Element Pickup Time Delay
Definite-Time Delayed Over- and Underfrequency Element Relay Word Bit
Relay Is Tracking Frequency
n =
freq =
81DnP =
FNOM =
81DnAD, 81DnTD =
81DnA, 81DnT =
FREQ =
81DnAD
0
Courtesy of NationalSwitchgear.com
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