4.65
Date Code 20170927 Instruction Manual SEL-751 Relay
Protection and Logic Functions
Group Settings (SET Command)
Z0MTA—Zero-Sequence Maximum Torque Angle
The Z0MTA setting is at the heart of the zero-sequence voltage-polarized
directional element of Figure 4.27. Z0MTA is only available if both of the fol-
lowing conditions are true:
➤ enable setting EDIR := AUTO or Y
➤ setting ORDER contains the value “V” or “S”
Otherwise, Z0MTA is hidden and of no consequence. Z0MTA can be set one
of two ways:
➤ If enable setting EDIR := AUTO, then Z0MTA is automatically
set equal to the value of setting Z0ANG (the setting range of
Z0MTA encompasses that of setting Z0ANG).
As long as EDIR := AUTO, Z0MTA can be seen, but not
changed. This automatic setting mode is primarily for
traditional applications, where the angle of the zero-sequence
system impedance behind the relay is deemed to be essentially
the same as the angle of the zero-sequence line impedance in
front of it (see Figure 4.44 and Figure 4.45[a]).
➤ If enable setting EDIR := Y, then Z0MTA is set independently
within its setting range.
This option is primarily used for such applications as low-
impedance grounded systems, which are discussed in the
balance of this subsection.
The distribution system in Figure 4.46 is low-impedance grounded at the sub-
station by either of the following methods:
➤ a resistance in the transformer bank neutral
➤ a grounding bank with a resistance in its broken-delta
secondary (effectively making it a neutral resistance)
Figure 4.46 Low-Impedance Grounded Distribution System With a Ground
Fault on Feeder 1
NOTE: STILL MAKE SETTING
Z0ANG WHEN EDIR = Y
Even though setting Z0MTA is not
automatically set equal to the value of
setting Z0ANG when enable setting
E32 = Y, setting Z0ANG should still be
made for fault location purposes.
R
G
Ground
Fault
Feeder 1
Feeder 2
Feeder n
Relay 2
Relay 1
2
1
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