4.42
SEL-351A Relay Instruction Manual Date Code 20080213
Loss-of-Potential, Load Encroachment, and Directional Element Logic
Directional Control Settings (Not in SEL-351A-1)
Z2F—Forward Directional Z2 Threshold
Z2R—Reverse Directional Z2 Threshold
Setting Range:
–128.00 to 128.00 Ω secondary
(5 A nominal phase current inputs, IA, IB, IC)
–640.00 to 640.00 Ω secondary
(1 A nominal phase current inputs, IA, IB, IC)
Z2F and Z2R are used to calculate the Forward and Reverse Thresholds,
respectively, for the negative-sequence voltage-polarized directional elements
(see Figure 4.9 and Figure 4.20).
If enable setting E32 = Y, settings Z2F and Z2R (negative-sequence
impedance values) are calculated by the user and entered by the user, but
setting Z2R must be greater in value than setting Z2F by 0.1 Ω secondary.
Z2F and Z2R Set Automatically
If enable setting E32 = AUTO, settings Z2F and Z2R (negative-sequence
impedance values) are calculated automatically, using the positive-sequence
line impedance magnitude setting Z1MAG as follows:
Z2F = Z1MAG/2 (Ω secondary)
Z2R = Z1MAG/2 + z (Ω secondary; “z” listed in table below)
Figure 4.25 and Figure 4.26 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
Setting Range:
0.25–5.00 A secondary (5 A nominal phase current inputs, IA, IB, IC)
0.05–1.00 A secondary (1 A nominal phase current inputs, IA, IB, IC)
The 50QFP setting (3I
2
current value) is the pickup for the forward fault
detector 50QF of the negative-sequence voltage-polarized directional
elements (see Figure 4.6). Ideally, the setting is above normal load unbalance
and below the lowest expected negative-sequence current magnitude for
unbalanced forward faults.
Relay Configuration z (Ω secondary)
5 A nominal current, 300 V voltage inputs 0.2
1 A nominal current, 300 V voltage inputs 1.0
NOTE: If the calculation of Z2F and
Z2R exceeds the setting range, the
quantity is set to the upper limit of the
setting range.
Courtesy of NationalSwitchgear.com
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