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Date Code 20011112
Line Current Differential, Distance, Out-of-Step, Overcurrent,
3-21
Voltage, Synchronism Check, and Frequency Elements
SEL-311L Instruction Manual
Three-Terminal Protection During 87L Channel Loss or With Setting E87L = 3R
Figure 3.15 shows a typical three terminal communications channel arrangement. Notice that if
any of the three communications channels is lost, the relay connected to two healthy
communications channels still has all of the information necessary to perform three-terminal
protection. In that case, the other two relays automatically disable 87L protection and enable
direct-transfer tripping,
even if setting EHSDTT = N
.
SEL-311L
SEL-311L
M311L096
SEL-311L
Internal fault causes
communications channel failure
DTT
DTT
Figure 3.15: Three-Terminal Line With Internal Fault and Channel Failure
Alternatively, install only two communications channels as shown in Figure 3.16. Set E87L = 3R
in the relays connected to a single communications channel. Set E87L = 3 in the relay connected
to two communications channels. The relays with setting E87L = 3R enable direct-transfer
tripping, and setting EHSDTT is hidden.
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