Sel 411L - Page 179

P.3.71
Date Code 20151029 Protection Manual SEL-411L Relay
Protection Functions
87L Differential Elements
Charging Current
Compensation Logic
The relay provides line charging current compensation by compensating
instantaneous phase currents (samples) in the time domain based on measured
line voltages and line parameters you provide as settings. As 87L Theory of
Operation explains, this approach has many advantages.
This section describes the line charging compensation logic and settings in detail.
As Figure 3.40 shows, the relay determines whether to perform or inhibit
compensation at any given terminal of the 87L scheme by asserting the
87CCC Relay Word bit. When the 87CCC bit is asserted, the local relay uses
its local voltage to calculate the total line charging current and subtracts a
portion (which it determines from the number of relays compensating for line
charging current) of this current from the measured local current prior to using
it in the 87L function and transmitting it to the remote relays.
The 87CCC Relay Word bit asserts when you enable the feature by setting
E87LCC to Y. The 87CCC Relay Word bit deasserts dynamically if the
following are true.
➤ The local terminal is in the stub bus configuration (ESTUB
Relay Word bit asserted). This prevents compensation for line
charging current in the local current in use for the local 87
function between the local breaker (or breakers) and the
opened line disconnect switch.
➤ The voltage source the logic uses to calculate line charging
current at the local terminal suffers a loss of potential condition
(LOP Relay Word bit asserted) and becomes unreliable.
➤ The local relay is in the outstation mode (the 87SLV Relay
Word bit asserted), so it does not receive data from at least one
remote relay in the 87L scheme. This relay can no longer
determine how many other relays compensate for the line
charging current. It cannot assess what portion of its own total
charging current to use for compensation. Therefore, the
outstation relays provide no compensation but allow the master
relay to compensate for the line charging current.
➤ The voltage source the logic uses to calculate the line charging
current is located on the bus side of the line breaker (or
breakers) as the 87CCLPT setting specifies, and the breaker is
not closed in all three phases (Relay Word bits SPO or 3PO). In
an open-pole condition, the bus voltage is not necessarily an
accurate reflection of the line voltage, so line charging current
compensation should not use this value.
87OCTB Open-circuited CT detected, B-phase
87OCTC Open-circuited CT detected, C-phase
87ROCT Open-circuited CT not suspected in any phase
87ROCTU Open-circuited CT not suspected, system appears balanced
87ROCTA Open-circuited CT not suspected (reset condition), A-phase
87ROCTB Open-circuited CT not suspected (reset condition), B-phase
87ROCTC Open-circuited CT not suspected (reset condition), C-phase
RSTOCT User-programmable condition to reset the open CT logic asserted
Table 3.27 Open CT Logic Relay Word Bits (Sheet 2 of 2)
Name Description