P.3.28
SEL-411L Relay Protection Manual Date Code 20151029
Protection Functions
87L Differential Elements
tests will assert in the remote relay to remove from service the
relay, its remote 87L function, and all other instances of the
87L function via the provisioned blocking bit in the 87L data
packet. The disturbance detection logic therefore provides
extra time to the self-test logic and, in combination with this
logic, dramatically improves security of the 87L scheme.
➤ Problem with data alignment. Assume a hypothetical channel
event or a single upset event in an internal component of the
relay that leads transiently to a misalignment of the local and
remote data. Under this scenario, the 87L function may
spuriously pick up as a result of comparing incorrect local and
remote current data. However, the portion of the disturbance
detection that responds to local currents and voltages will not
assert because it bypasses the alignment table, preventing
misoperation of the 87L scheme.
Because of the exceptional sensitivity, minimum potential impact on
dependability, and extra benefits that disturbance detection provides, the relay
applies disturbance detection trip supervision permanently without an option
to disable it.
Cross-connection of relays is another communications event to consider. In
such a scenario, a given relay is inadvertently connected to the incorrect
remote relay, or the relay is looped back into itself inadvertently. To guard
against this threat, the relay uses transmit addresses and checks them upon
reception against the expected receive address setting. When the received and
expected addresses do not match, the relay does not use the data. In such a
situation, the relay removes the 87L function from service and sets a user
alarm.
87L Differential Elements
87L Elements
This section describes and explains logic diagrams, settings, analog quantities,
and Relay Word bits associated with configuration of current inputs for the
87L function; 87L phase, negative-sequence, and zero-sequence differential
elements; time-overcurrent differential protection for tapped loads; stub bus
condition; and direct transfer tripping embedded in the 87L function.
Configuration of the 87L Current Inputs
The relay offers flexibility in the current inputs you can select for its 87L
function. As 87L Theory of Operation explains, you can facilitate advanced
applications by dynamically including or excluding relay W and X current
terminals in the 87L function. In addition, you can invert the polarity of a
given current prior to using it in the 87L function. The relay allows different
CT ratios and ratings among various terminals of the 87L scheme.
This section describes the basic settings related to the current inputs of the
87L function in typical applications without in-line transformers
(87XFMR = N). Refer to 87L Differential Applications With In-Line
Transformers if you apply the relay to a line including a power transformer
connected in series.
Be sure to properly configure the relay current terminals via the CTRW and
CTRX current transformer ratio settings prior to selection of settings
pertaining to the 87L function.
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