12
SEL Application Guide 2020-04
Date Code 20200326
Note 1.
When determining the CTD, you must evaluate the operating time of the tripping ele-
ments. You must consider both the source-to-impedance ratio (SIR) and the distance from
the relay. For the SEL-421-5, SEL-311C-1, and SEL-321-1, you can find this information in
their respective instruction manuals. Delay the CTD at the faster relay to coordinate with
the blocking elements of the slower relay. Add all of this to the communications channel
delay, plus additional margin. The CTD must be greater than the communications channel
delay.
In the SEL-421-5 and SEL-311C-1 relays, Z3XPU determines how long Zone 3 elements must
assert before the block is sealed in to provide security for current reversals. The Z3XPU timer is
hard coded in the SEL-321-1 at 2 cycles, which holds in the START bit. The default setting for
Z3XPU in the SEL-311L is 2 cycles, while in the SEL-421-5, the default is 1 cycle. As long as the
relay trip time, along with breaker open time, exceeds 2 cycles, there is no difference between the
two times regarding current reversal security for an external fault. It is not required to set Z3XPU
the same in the local and remote relays; they do not require coordination with each other.
Table 12
SEL-311C-1 DCB Settings
Setting
Setting Description
Setting Value
Description
ECOMM
Communication-Assisted Trip
Scheme
DCB
Enables the DCB logic in the SEL-311C-1.
Z3XPU
Zone 3 Reverse Pickup Time
Delay
1 (default)
Current reversal guard pickup timer.
Z3XD
Zone 3 Pickup Extension Time
Delay
6
Prevents a DCB scheme misoperation during current
reversal; this also matches the SEL-421-5 for better
coordination.
BTXD
Block Trip Receive Extension
Delay
1
Sets the reset time of a block trip received condition
after the reset of a block trip input.
21SD
Zone 2 Phase and Ground Coor-
dination Time Delay
See
Delays the M2P element output.
67SD
67G and 67Q Coordination
Time Delay
See
Delays the 67N2 element output.
Table 13
SEL-321-1 DCB Settings
Setting
Setting Description
Setting Value
Description
EDCB
Enable Directional Comparison
Blocking
Y
Enables the DCB logic in the SEL-321-1.
Z3XD
Zone 3 Pickup Extension Time
Delay
6
Prevents a DCB scheme misoperation during current
reversal; this also matches the SEL-421-5 for better
coordination.
BTXD
Block Trip Receive Extension
Delay
1 (default)
Sets the reset time of a block trip received condition
after the reset of a block trip input.
Z2PSD
Zone 2 Phase Coordination
Time Delay
See
Delays the M2P element output.
Z2GSD
Zone 2 Ground Coordination
Time Delay
See
Delays the Z2G element output.
67N2SD
67N Coordination Time Delay
See
Delays the 67N2 element output.
67Q2SD
67Q Coordination Time Delay
See
Delays the 67Q2 element output.
