P.3.65
Date Code 20151029 Protection Manual SEL-411L Relay
Protection Functions
87L Differential Elements
Extended Security Setting Switchover Logic
The 87L function of the relay supports two sets of Alpha Plane settings,
normal and extended security. The phase, negative-sequence, and ground 87L
elements each have two values of pickup, blocking radius, and blocking angle
settings in each of the relay setting groups (see 87L Elements for a detailed
description of the Alpha Plane settings). The relay provides extended security
settings to ensure secure operation of the 87L elements under rare and unusual
circumstances without penalizing sensitivity under normal and prevailing
conditions. These settings apply to the differential elements only and are
under automatic control of the 87L logic, so the switchover logic includes
alternate extended security settings separate from the setting groups.
Furthermore, the switchover logic is independent for the phase, negative-
sequence, and ground differential elements.
The extended security settings are in use when the Relay Word bits 87LPSEC,
87LQSEC, and 87LGSEC assert for the 87LP, 87LQ, and 87LG elements,
respectively.
The logic controls these Relay Word bits as shown in Figure 3.38. The logic
engages the extended security settings for the following.
The line charging current compensation is enabled, but the compensation
is unavailable because the 87L scheme has no voltage suitable for
compensation. This may be because the voltage inputs suffer a loss of
potential condition, the logic used bus-side voltages for charging current
compensation and the line breaker is not closed in all three poles, or the
relay switched to the ALINEV voltage input in response to the ALTV
SEL
OGIC control equation. The 87CCU Relay Word bit signals this
unavailability of line charging current calculation and calls for more
secure settings to accommodate higher values of the standing or transient
charging current, for which the relay provides no compensation at the
time (see Charging Current Compensation Logic for more details). If the
charging current compensation is disabled, the 87CCU Relay Word bit
remains deasserted, and the extended security switchover logic will not
engage in response to the LOP condition or breaker status.
The quality of data alignment and synchronization is less than normal,
potentially leading to a standing differential current because of slight
misalignment in local and remote current samples. The 87SYNL Relay Word
bit signals the decreased quality of synchronization. This bit asserts if the
logic detects in channels necessary for the 87L scheme any indication of
errors in synchronizing data from relays at both ends of the channel. See
Configuration, Monitoring, Alarming, and Logic for 87L Channels for more
details on data synchronization and on the 87SYNL Relay Word bit in particular.
➤ The EFD logic detects an external fault (see External Fault
Detection for more details).
➤ The 87L function is in the test mode (87TEST Relay Word bit
asserted), and the secure Alpha Plane settings are under test
(test variable 87TESTS = S). In the test mode, you can choose
to force the extended security settings switchover logic to
apply normal security settings or extended security settings.
The 87TEST Relay Word bit and the 87TESTS test variable
force the logic to apply normal or extended security settings as
necessary. See 87L Multiterminal Test on page P.11.21 and 87L
Single-Terminal Test on page P.11.25 for more details on
testing the 87L elements.
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