Schweitzer Engineering Laboratories SEL-421-4, Instruction Manual

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GL.8
SEL-400 Series Relays Instruction Manual Date Code 20171006
Glossary
DC Offset — Disconnect (Isolator)
DC Offset A dc component of fault current that results from the physical phenomenon
preventing an instantaneous change of current in an inductive circuit.
DCB (Directional
Comparison Blocking)
A communications-assisted protection scheme. A fault occurring behind a
sending relay causes the sending relay to transmit a blocking signal to a remote
relay; the blocking signal interrupts the tripping circuit of the remote relay and
prevents tripping of the protected line.
DCE Devices Data communications equipment devices (modems).
DCUB (Directional
Comparison Unblocking)
A communications-assisted tripping scheme with logic added to a POTT scheme
that allows high-speed tripping of overreaching elements for a brief time during a
loss of channel. The logic then blocks trip permission until the communications
channel guard returns for a set time.
Dead Band The range of variation an analog quantity can traverse before causing a response.
Deassert To deactivate. To remove the logic or electrical requirements needed to operate a
device. To clear a logic condition to its false state (logical 0). To open the circuit
or open the contacts across a relay input. To open a normally open output contact.
To close a normally closed output contact.
Debounce Time The time that masks the period when relay contacts continue to move after
closing; debounce time covers this indeterminate state.
Default Data Map The default map of objects and indices that the relay uses in DNP3 protocol.
Delta A phase-to-phase series connection of circuit elements, particularly voltage
transformers or loads.
Demand Meter A measuring function that calculates a rolling average or thermal average of
instantaneous measurements over time.
Differential Element The differential element calculates current differences across a zone of
protection.
Direct Tripping Local or remote protection elements provide tripping without any additional
supervision.
Directional Element The directional element determines the direction of power flow at a point in the
power system.
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.
Disconnect (Isolator) Mechanical switch that isolates primary equipment such as circuit breakers from
the electrical system.