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4.61
Date Code 20081022
Instruction Manual
SEL-787 Relay
Protection and Logic Functions
Voltage-Based Protection
power element(s) if the power element(s) are still receiving sufficient
operating quantities. Use the power element time delay setting to avoid such
race conditions.
Frequency Elements
NOTE:
The relay measures system
frequency for these elements with the
positive-sequence voltage if the
applied positive-sequence voltage is
greater than 0.75 volts for at least
four cycles. Otherwise, the relay
assumes the frequency same as
nominal frequency determined by
FNOM setting.
The SEL-787 provides four 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.
shows the logic diagram for the frequency elements.
Figure 4.39
Over- and Underfrequency Element Logic
Loss-of-Potential
(LOP) Protection
The SEL-787 sets Relay Word bit LOP (loss-of-potential) upon detecting a
loss of relay ac voltage input such as that caused by blown potential fuses or
by the operation of molded-case circuit breakers. Because accurate relaying
potentials are required by certain protection elements (undervoltage 27
elements, for example), you can use the LOP function to supervise these
protection elements.
The relay declares an LOP when there is more than a 20 percent drop in the
measured positive-sequence voltage (V1) with no corresponding magnitude
nor angle change (above a pre-determined threshold) in positive-sequence
(I1), negative-sequence (I2), or zero-sequence currents (I0).
Table 4.22
Frequency Settings
Setting Prompt
Setting Range
Setting Name :=
Factory Default
FREQ1 TRIP LEVEL
OFF, 20.0–70.0 Hz
81D1TP := OFF
FREQ1 TRIP DELAY
0.0–240.0 sec
81D1TD := 1.0
FREQ2 TRIP LEVEL
OFF, 20.0–70.0 Hz
81D2TP := OFF
FREQ2 TRIP DELAY
0.0–240.0 sec
81D2TD := 1.0
FREQ3 TRIP LEVEL
OFF, 20.0–70.0 Hz
81D3TP := OFF
FREQ3 TRIP DELAY
0.0–240.0 sec
81D3TD := 1.0
FREQ4 TRIP LEVEL
OFF, 20.0–70.0 Hz
81D4TP := OFF
FREQ4 TRIP DELAY
0.0–240.0 sec
81D4TD := 1.0
freq
81DnTP
81D1T
81D2T
81D3T
81D4T
81DnTP
<
FNOM
81DnTP
≥
FNOM
FREQTRK
81DnTD
0
Frequency Elements 1–4
Measured Frequency
Frequency Pickup Setting
Nominal Frequency Setting
Over- and Underfrequency Element Pickup Time Delay
Definite-Time Delayed Over- and Underfrequency Element Relay Word Bit
Relay Is Tracking Frequency
n =
freq =
81DnTP =
FNOM =
81DnTD =
81DnT =
FREQTRK =
Summary of Contents for SEL-787
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