Schweitzer Engineering Laboratories SEL-421-4 - Figure 5.51 Unblocking of the MAB Signal by the 67 QUB Element; Figure 5.52 Directional Element Signals 67 QUBF and 67 QUBR

5.68
SEL-421 Relay Instruction Manual Date Code 20171021
Protection Functions
Out-of-Step Logic (Zero Settings)
To detect phase-to-phase faults, the relay uses a directional overcurrent element,
67QUBF, based on a negative-sequence directional element, 32QF, as shown in
Figure 5.52. 3IA2LFM is the negative-sequence current that the relay measures.
If 3IA2LFM exceeds a reference value (a2 • 3 • IA1LFM), Timer 1 starts. If
Timer 1 expires and the flow of negative-sequence current is in the forward direc-
tion (32QF asserted), then 67QUBF asserts.
Out-of-Step Tripping (OST)—Zero Settings Element
While the zero-setting out-of-step blocking function requires no settings, the out-
of-step tripping function requires eight blinder settings. These eight blinder set-
tings are the Zone 6 and Zone 7 blinder settings, and are common for the conven-
tional and the zero-setting OST function (see Figure 5.32).
Figure 5.53 shows the resistive and reactive blinders used in the OST scheme
logic. The OST logic uses the traditional OOS calculations for the left (R1R6),
right (R1R7), top (X1T6), and bottom (X1T7) blinders of Zone 6 and Zone 7.
Specify Settings X1B6 and X1B7 under the advanced settings option. For on-the-
way-out (TOWO) out-of-step tripping, these settings do not require any stability
studies. However, the out-of-step tripping on-the-way-in (TOWI) still requires
stability studies to determine the proper OST settings for right- and left-hand
blinders RR6 and RR7.
Figure 5.51 Unblocking of the MAB Signal by the 67QUB Element
Figure 5.52 Directional Element Signals 67QUBF and 67QUBR
MAB
Phases A-B
mho signal
OSB
67QUB
Phase A-to-B
distance logic
3IA2LFM
32QF
32QR
a2 • 3 • IA1LFM
67QUBF
67QUBR
0
CYC
1.5
Timer 1
IA2LFM
0.1 • I
NOM
0.5
CYC
0
Time 2