Over- And Undervoltage Protection; Over- And Underfrequency Protection - Schweitzer Engineering Laboratories SEL-787-2 Manual

characteristic and a definite-time characteristic; or a
dual-level, definite-time element.
For any of these operating characteristics, the element
provides a linear reset characteristic with a settable reset
time. The torque control setting also supervises this
element. The tripping element has a percent-travel
operating characteristic similar to one used by an
induction-disk, time-overcurrent
characteristic emulates
overexcitation on transformer components.
Figure 7 SEL-5806 Volts/Hertz User Curve Design
Example

Over- and Undervoltage Protection

The SEL-787 with optional voltage inputs contains phase
over- and undervoltage and sequence overvoltage ele-
ments that help create protection and control schemes,
such as undervoltage load shedding or standby genera-
tion start/stop commands. All voltage elements provide
two pickup levels with definite-time delay settings. The
following over- and undervoltage elements are available:
➤
Phase undervoltage (27P) and overvoltage (59P) ele-
ments that operate on the measured phase-to-neutral
voltages.
➤
Phase-to-phase undervoltage (27PP) and overvoltage
(59PP) elements that operate on the measured
phase-to-phase voltages.
➤
Negative-sequence overvoltage (59Q) and residual-
ground overvoltage (59G) elements that operate on
the calculated negative-sequence and residual-ground
voltage, respectively.
Schweitzer Engineering Laboratories, Inc.
element. This
the heating effect of
➤
Phase undervoltage (27S) and phase overvoltage
(59S) elements that operate on VS channel voltage.
➤
Inverse-time overvoltage (59I) and inverse-time
undervoltage (27I) elements that operate on the mea-
sured phase-to-neutral voltages, phase-to-phase volt-
ages, positive-sequence voltage, or VS channel
voltage.
Loss-of-Potential Detection
The SEL-787 with optional voltage inputs contains LOP
detection logic on the three-phase voltage input to the
relay. The LOP logic detects open voltage transformer
fuses or other conditions that cause a loss of relay sec-
ondary voltage input. The SEL-787 with optional voltage
inputs includes LOP logic that detects one, two, or three
potentially open fuses. This patented LOP logic is
unique, because it does not require settings and is univer-
sally applicable. The LOP feature allows for the blocking
of protection elements to add security during voltage
transformer fuse failure.
Synchronism Check/Station DC
Battery Monitor
The SEL-787-3S allows you to program the VS/Vbat
voltage channel for use as either a synchronism check or
station dc battery monitor. When programmed as a
synchronism-check channel,
(phase-to-neutral or phase-to-phase) can be connected to
the voltage input for a synchronism check or hot/dead
line check across the circuit breaker to which the three-
phase voltages are assigned. When the channel is pro-
grammed for the battery monitor, the station dc battery
voltage can be monitored. The relay also allows you to
program over- and undervoltage elements on the voltage
channel.

Over- and Underfrequency Protection

The SEL-787 with optional voltage inputs contains four
frequency elements. Each element operates as either an
over- or underfrequency element with or without time
delay, depending on the element pickup setting.
If the element pickup setting is less than the nominal
system frequency setting, the element operates as an
underfrequency element, picking up if the measured
frequency is less than the set point. If the pickup setting
exceeds the nominal system frequency, the element
operates as an overfrequency element, picking up if the
measured frequency exceeds the set point.
The SEL-787 with optional voltage inputs uses the
positive-sequence voltage
frequency. All frequency elements are disabled if the
positive-sequence voltage is less than the minimum
voltage threshold.
11
single-phase voltage
to determine system
SEL-787-2, -3, -4 Data Sheet