GL.7
Date Code 20020501
SEL-421/SEL-421-1 Relay
Glossary
Demand Meter
—
Echo Duration Time Delay
Demand Meter
A measuring function that calculates a rolling average or thermal average of
instantaneous measurements over time.
Direct Tripping
Local or remote protection elements provide tripping without any additional
supervision.
Directional Start
A blocking signal provided by reverse reaching elements to a remote terminal
used in DCB communications-assisted tripping schemes. If the fault is
internal (on the protected line), the directional start elements do not see the
fault and do not send a blocking signal. If the fault is external (not on the
protected line), the directional start elements start sending the block signal.
Directional Supervision
The relay uses directional elements to determine whether protective elements
operate based on the direction of a fault relative to the relay.
Disabling Time Delay
A DCUB scheme timer (UBDURD) that prevents high-speed tripping
following a loss-of-channel condition.
Distance
Calculation Smoothness
A relay algorithm that determines whether the distance-to-fault calculation
varies significantly or is constant.
Distance Protection Zone
The area of a power system where a fault or other application-specific
abnormal condition should cause operation of a protective relay.
DMTC Period
The time of the demand meter time constant in demand metering.
DNP (Distributed
Network Protocol)
Manufacturer-developed, hardware-independent communications protocol.
Dropout Time
The time measured from the removal of an input signal until the output signal
deasserts. You can set the time, in the case of a logic variable timer, or the
dropout time can be a result of the characteristics of an element algorithm, as
in the case of an overcurrent element dropout time.
DTE Devices
Data terminal equipment (computers, terminals, printers, relays, etc.).
DTT (Direct Transfer
Trip)
A communications-assisted tripping scheme. A relay at one end of a line
sends a tripping signal to the relay at the opposite end of the line.
Dumb Terminal
See ASCII terminal.
DUTT
(Direct Underreaching
Transfer Trip)
A communications-assisted tripping scheme. Detection of a Zone 1 fault at
either end of a line causes tripping of the local circuit breaker as well as
simultaneous transmission of a tripping signal to the relay at the opposite end
of the line. The scheme is said to be underreaching because the Zone 1 relays
at both ends of the line reach only 80 percent (typically) of the entire line
length.
Echo
The action of a local relay returning (echoing) the remote terminal permissive
signal to the remote terminal when the local breaker is open or a weak infeed
condition exists.
Echo Block Time Delay
A time delay that blocks the echo logic after dropout of local permissive
elements.
Echo Duration Time Delay
A time delay that limits the duration of the echoed permissive signal.
Summary of Contents for SEL-421
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