4.59
Date Code 20170927 Instruction Manual SEL-751 Relay
Protection and Logic Functions
Group Settings (SET Command)
Z2F and Z2R Set Automatically. If enable setting EDIR := AUTO, set-
tings Z2F and Z2R (negative-sequence impedance values) are calculated auto-
matically, using the positive-sequence line impedance magnitude setting
Z1MAG as follows:
Z2F = Z1MAG/2 ( secondary)
Z2R = Z1MAG/2 + z ( secondary; “z” listed in table below)
Figure 4.44 and Figure 4.45 and supporting text concern the zero-sequence
impedance network, relay polarity, and the derivation of settings Z0F and
Z0R. The same general approach outlined for deriving settings Z0F and Z0R
can also be applied to deriving settings Z2F and Z2R in the negative-sequence
impedance network, though the preceding method of automatically making
settings Z2F and Z2R usually suffices.
50QFP—Forward Directional Negative-Sequence Current Pickup
50QRP—Reverse Directional Negative-Sequence Current Pickup
The 50QFP setting (3I2 current value) is the pickup for the forward fault
detector 50QF of the negative-sequence voltage-polarized directional ele-
ments (see Figure 4.23). Ideally, the setting is above normal load unbalance
and below the lowest expected negative-sequence current magnitude for
unbalanced forward faults.
The 50QRP setting (3I2 current value) is the pickup for the reverse fault detec-
tor 50QR of the negative-sequence voltage-polarized directional elements (see
Figure 4.23). Ideally, the setting is above normal load unbalance and below
the lowest expected negative-sequence current magnitude for unbalanced
reverse faults.
50QFP and 50QRP Set Automatically. If enable setting EDIR := AUTO,
settings 50QFP and 50QRP are set automatically at:
50QFP = 0.50 A secondary (5 A nominal phase current inputs, IA, IB, IC)
50QRP = 0.25 A secondary (5 A nominal phase current inputs, IA, IB, IC)
50QFP = 0.10 A secondary (1 A nominal phase current inputs, IA, IB, IC)
50QRP = 0.05 A secondary (1 A nominal phase current inputs, IA, IB, IC)
a2—Positive-Sequence Current Restraint Factor, I
2
/I
1
Refer to Figure 4.23.
The a2 factor increases the security of the negative-sequence voltage-polar-
ized directional elements. It keeps the elements from operating for negative-
sequence current (system unbalance), which circulates because of line asym-
metries, CT saturation during three-phase faults, etc.
a2 Set Automatically. If enable setting EDIR := AUTO, setting a2 is set
automatically at:
a2 = 0.1
Relay Configuration z ( secondary)
5 A nominal current 0.2
1 A nominal current 1.0
NOTE: If Z2F or Z2R exceeds the
setting range, the quantity is set to
the upper limit of the setting range.
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