Sel 411L - Table 3.26 Open CT Logic Settings

P.3.69
Date Code 20151029 Protection Manual SEL-411L Relay
Protection Functions
87L Differential Elements
If the system meets these conditions for 1/8 of a power cycle, the logic
declares an open CT by setting the latch and driving the 87OCT Relay Word
bit. The logic ORs the 87OCTA, 87OCTB, and 87OCTC Relay Word bits for
your convenience into the 87OCT Relay Word bit.
The open CT logic does not self-reset. You can program the reset condition
via the RSTOCT SEL
OGIC control equation, a common control variable for all
three phases of the open CT logic.
In one protection philosophy, the reset can occur only manually after
inspection of the measured currents and attending to actual CT problems as
necessary. Following a different philosophy, you can allow automatic reset of
the open CT alert if the current measurements return to normal for any reason.
To aid this application, the scheme asserts a second set of output Relay Word
bits to indicate absence of the conditions that caused the CT trouble. Note that
use of these Relay Word bits to reset the logic does not occur by default; you
can use these bits in the RSTOCT SEL
OGIC control equation as necessary.
The 87ROCTU Relay Word bit asserts if the averaged restraining current is
significant (in excess of the 87
THR
threshold, defined previously as the lesser
of the 87LP pickup setting and 0.25 pu of the 87L base), and the magnitude of
the full-cycle filtered negative-sequence differential current is less than 1/8 of
that phase averaged restraint. The 87ROCTU Relay Word bit asserts if these
conditions are true for all three phases, signaling that the differential system
appears balanced.
The 87ROCT Relay Word bit asserts in a similar manner but requires the
filtered phase differential current magnitude to be low compared with the
averaged phase restraint current.
The 87ROCT Relay Word bit asserts if the phase differential is low in all three
phases (87ROCTA, 87ROCTB, and 87ROCTC asserted) or if the differential
current appears balanced (87ROCTU asserted).
You can use 87ROCT as the RSTOCT SEL
OGIC control equation. Note that
the open CT condition can still exist, but the affected CT carries no significant
current, resulting in removal of open CT condition symptoms.
Table 3.26 Open CT Logic Settings
Setting Description Range Default
E87OCTL Enable open CT logic Y, N N
RSTOCT Reset condition for open CT logic SEL
OGIC control
equation
NA