4.124
SEL-751 Relay Instruction Manual Date Code 20170927
Protection and Logic Functions
Group Settings (SET Command)
Figure 4.75 Over- and Underfrequency Element Logic
Rate-of-Change-of-
Frequency (81R)
Protection
Frequency changes occur in power systems when there is an unbalance
between load and active power generated. Typically, generator control action
adjusts the generated active power and restores the frequency to nominal
value. Failure of such control action may lead to system instability unless
remedial action, such as load shedding, is taken. You can use the rate-of-
change-of-frequency element to detect and initiate a remedial action. The
SEL-751 provides four rate-of-change-of-frequency elements. Table 4 .49
shows the settings available for the elements.
freq
81DnTP
81DnT
81DnTP < FNOM
FREQTRK
81DnTC
81DnTD
0
n = Frequency Elements 1–6
freq = Measured Frequency
81DnTP = Frequency Pickup Setting
FNOM = Nominal Frequency Setting
81DnTD = Over- and Underfrequency Element Pickup Time Delay
81DnT = Definite-Time Delayed Over- and Underfrequency Element Relay Word Bit
FREQTRK = Relay Is Tracking Frequency
81DnTC = SEL
OGIC Setting for Torque Control of Frequency Element
81DnTP ≥ FNOM
Table 4.49 Rate-of-Change-of-Frequency Settings (Sheet 1 of 2)
Setting Prompt Setting Range
Setting Name :=
Fac to ry Defa ult
ENABLE 81R N, 1–4 E81R := N
81R VOLTAGE SUP OFF, 12.5–300.0 V 81RVSUP := 12.5
81R CURRENT SUP OFF, 0.1–2.0 * I
NOM
a
81RISUP := OFF
81R1 TRIP LEVEL OFF, 0.10–15.00 Hz/sec 81R1TP := OFF
81R1 TREND INC, DEC, ABS 81R1TRND := ABS
81R1 TRIP DELAY 0.10–60.00 sec 81R1TD := 1.00
81R1 DO DELAY 0.00–60.00 sec 81R1DO := 0.00
81R1 TRQCTRL SEL
OGIC 81R1TC := 1
81R2 TRIP LEVEL OFF, 0.10–15.00 Hz/sec 81R2TP := OFF
81R2 TREND INC, DEC, ABS 81R2TRND := ABS
81R2 TRIP DELAY 0.10–60.00 sec 81R2TD := 1.00
81R2 DO DELAY 0.00–60.00 sec 81R2DO := 0.00
81R2 TRQCTRL SEL
OGIC 81R2TC := 1
81R3 TRIP LEVEL OFF, 0.10–15.00 Hz/sec 81R3TP := OFF
81R3 TREND INC, DEC, ABS 81R3TRND := ABS
81R3 TRIP DELAY 0.10–60.00 sec 81R3TD := 1.00
81R3 DO DELAY 0.00–60.00 sec 81R3DO := 0.00
81R3 TRQCTRL SEL
OGIC 81R3TC := 1
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