P.3.29
Date Code 20151029 Protection Manual SEL-411L Relay
Protection Functions
87L Differential Elements
The settings described in this section apply only to the 87L function (phase-
sequence, negative-sequence, and zero-sequence differential elements).
Detailed Description of Settings
87CTWL and 87CTXL
These SELOGIC control equations control whether a given relay current input
terminal, W and X respectively, serves as the current input to the 87L function.
➤ If you program the SELOGIC control equation to a value other
than 0, the corresponding current terminal of the relay is a part
of the 87L function in terms of being a part of tap calculations,
scaling, and consequently impacting the per-unit base quantity
of the 87L differential zone.
➤ If the SELOGIC control equation asserts at a given time, the
relay includes the corresponding current as an input to the 87L
function.
➤ If the SELOGIC control deasserts at a given time, the
corresponding current is not an input to the 87L function; the
relay substitutes zeroes for this current input to the 87L
function.
Through the use of the 87CTWL and 87CTXL SEL
OGIC control equations,
you can switch local currents dynamically as inputs to the 87L function in
such advanced applications as breaker substitution (see 87L Theory of
Operation for more details).
Exercise care in designing an SEL
OGIC control equation for dynamic
switching of the current inputs to the 87L function. Spurious assertion or
deassertion of the equation while the associated current is other than zero will
upset the differential balance and may cause misoperation. The 87L element
incorporates local and remote disturbance detection supervision. The remote
supervision allows the function to ride through potential race conditions when
switching the currents. It is generally safe to switch the currents by asserting
or deasserting the 87CTWL and/or 87CTXL SEL
OGIC control equations when
the switched current is zero (the circuit breaker is opened). Otherwise, you
may consider programming a supervisory condition (phase undervoltage, or
negative-sequence overvoltage, for example) to accommodate potential race
conditions between the current value and the SEL
OGIC control equation and
any temporary unbalance in the differential current that a race condition might
cause.
Also be careful when using auxiliary contacts of disconnect switches or
breakers in the 87CTWL and 87CTXL SEL
OGIC control equations. An
accidental assertion or deassertion will cause current unbalance and likely
result in misoperation of the 87L function, depending on settings and load at
the time. If you use the 89 auxiliary contacts, you should apply proper
debounce settings for the relay inputs to deal with noise and battery ground
faults and/or use both the 89a and 89b contacts to deal with performance
issues of the 89 auxiliary contacts.
These considerations are similar to those in the art of low-impedance bus
protection for reconfigurable buses where bus protection zones are
dynamically switched to follow actual station configuration.
Most relay applications use static assignment to the relay current terminals to
the 87L function, resulting in 87CTWL and 87CTXL SEL
OGIC control
equations becoming constants, either 0 (the current is permanently excluded)
or 1 (the current is permanently included).
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