Sel 411L - Figure 3.1 Sampling and Transmitting Instantaneous Local Currents in the 87 L Scheme (Breaker-And-A-Half Scheme); Figure 3.2 Consolidating Currents in the 87 L Scheme While Conserving the Channel Bandwidth

P.3.5
Date Code 20151029 Protection Manual SEL-411L Relay
Protection Functions
87L Theory of Operation
Figure 3.1 Sampling and Transmitting Instantaneous Local Currents in the
87L Scheme (Breaker-and-a-Half Scheme)
The sum of all such currents among all of the line terminals produces the true
differential current for the complete 87L zone (see Figure 3.2).
Equation 3.1
We can therefore refer to the transmitted instantaneous currents (i
LOC
) as
“partial differential” currents.
Figure 3.2 Consolidating Currents in the 87L Scheme While Conserving the
Channel Bandwidth
The relay transmits 87L data packets every 4 ms. Each packet contains four
consecutive 1 kHz samples of the local current. Transmitting more data in
each packet improves the payload-to-overhead ratio and optimizes further the
usage of the communications channel. Relays align all local and remote
currents they receive to compensate for independent, not simultaneous,
sampling of the relays in the 87L scheme as well as variable channel latencies.
At this stage, each relay uses the local frequency tracking algorithm to re-
sample the aligned 1 kHz samples to a variable rate of 16 samples per cycle.
IWIX
Time
ADC, 8 kHz
I
LOC
Downsampling, 1 kHz
Scaling
Transmit to
remote
terminals
Place in local
alignment table
Relay
Scaling
Σ
i
LOC
i
REM1
i
REM2
i
CT1
... i
CT6
i
DIF
=++=++
CT2
CT1
I
RST REM1
I
RST LOC
= I
CT1 MAG
+ I
CT2 MAG
I
RST REM2
i
REM1
i
LOC
= i
CT1
+ i
CT2
i
REM2
CT4
CT3
I
RST REM1
I
RST LOC
= I
CT3 MAG
+ I
CT4 MAG
I
RST REM2
i
REM1
i
LOC
= i
CT3
+ i
CT4
i
REM2
CT5 CT6
I
RST REM1
I
RST LOC
= I
CT5 MAG
+ I
CT6 MAG
I
RST REM2
i
REM1
i
LOC
= i
CT5
+ i
CT6
i
REM2
T1 T2
T3