P.3.72
SEL-411L Relay Protection Manual Date Code 20151029
Protection Functions
87L Differential Elements
➤ The relay switches to the alternate voltage source the ALINEV
setting specifies in response to the ALTV SEL
OGIC control
equation. The 87L line charging current compensation keeps
this auxiliary feature simple by not following the voltage
redundancy principle. Otherwise, additional settings would be
required to specify the location of the alternate voltage source
(bus-side, line-side), and there may be different secondary
values for line susceptances because of a different ratio.
For security, the line charging current control logic keeps the 87CCC Relay
Word bit deasserted for two power system cycles after the clearing of any
condition(s) that caused the 87CCC Relay Word bit to deassert.
The local relay transmits the 87CCC bit together with current values to all
other relays in the 87L scheme. Likewise, the local relay receives the 87CCC
bits from all of its remote peers.
By counting asserted 87CCC bits (both local and remote), the local relay
determines how many relays perform the line charging current compensation
at any given time. Note that the number of compensating relays can change
dynamically per Figure 3.40 in response to the LOP condition, stub bus
configuration, or even breaker position.
Based on the number of relays in the 87L scheme that compensate at a given
time, the local relay calculates a multiplier, 87MCC, that it then uses to
determine the portion of the total line charging current that the local terminal
will subtract. For example, with two relays compensating, each will subtract
50 percent of the total line charging current (87MCC = 1/2); with three relays
compensating, each relay will subtract 33 percent of the total charging current
(87MCC = 1/3). If the local relay is the only relay that compensates, it will
subtract 100 percent of the total charging current (87MCC = 1). If the local
relay (and all other relays) provide no compensation, the multiplier is 0
(87MCC = 0). The control logic of Figure 3.40 drives the 87MCC multiplier.
Figure 3.41 explains use of the multiplier.
By applying the dynamic multiplier as explained previously, relay logic allows
line charging current compensation to ride through problems with voltage sources.
The control logic of Figure 3.40 drives three Relay Word bits that signal the
quality of the line charging current compensation at any given time. For this
purpose, the logic provides an 87CCN setting to indicate the number of line
terminals that normally perform the compensation. Typically, each line
terminal will be set to perform the compensation. In some cases, however, a
voltage source may be unavailable at a given terminal, preventing the scheme
from using this voltage for compensation. In such a case, the 87CCN number
is less than the number of line terminals. For example, a three-terminal line
application may have voltage transformers (VTs) installed at two terminals
only. If both terminals are configured to perform line charging current
compensation, the 87CCN setting should be 2.
The control logic of Figure 3.40 asserts as follows after comparing the number
of relays that compensate at any given time with the expected number of
compensating terminals, 87CCN.
➤ Best possible line charging current compensation (87CCB
Relay Word bit) when all expected terminals compensate.
➤ Degraded line charging current compensation (87CCD Relay
Word bit) when some but not all expected terminals compensate.
➤ Line charging current compensation unavailable (87CCU
Relay Word bit) when no terminals compensate.
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