5.36
SEL-421 Relay Instruction Manual Date Code 20171021
Protection Functions
Ground Directional Element
Setting 50RP is the threshold for the current level detector that enables reverse
decisions for both the negative- and zero-sequence voltage-polarized directional
elements. If the magnitude of 3I
2
or 3I
0
is greater than 50RP, the corresponding
directional element can process a reverse decision.
Z2F and Z2R
Setting Z2F is the forward threshold for the negative-sequence voltage-polarized
directional element. If the relay measures the apparent negative-sequence imped-
ance z2 less than Z2F, the relay declares the unbalanced fault to be forward.
Setting Z2R is the reverse threshold for the negative-sequence voltage-polarized
directional element. If the relay measures apparent negative-sequence impedance
z2 greater than Z2R, the relay declares the unbalanced fault to be reverse.
a2 and k2
Positive-sequence current restraint factor a2 compensates for highly unbalanced
systems. Unbalance is typical in systems that have many untransposed lines. This
factor also helps prevent misoperation during current transformer saturation. The
a2 factor is the ratio of the magnitude of negative-sequence current to the magni-
tude of positive-sequence current, |I
2
|/|I
1
|. If the measured ratio exceeds a2, the
negative-sequence voltage-polarized directional element is enabled. Typically,
you can apply the default calculations in Table 5.33.
Zero-sequence current restraint factor k2 also compensates for highly unbalanced
systems. This factor is the ratio of the magnitude of negative-sequence current to
the magnitude of zero-sequence current, |I
2
|/|I
0
|. If the measured ratio exceeds k2,
the negative-sequence voltage-polarized directional element is enabled. If the
measured ratio is less than k2, the zero-sequence voltage polarized directional
element is enabled. Typically, you can apply the default calculations that appear
in Table 5.33.
Z0F and Z0R
Setting Z0F is the forward threshold for the zero-sequence voltage-polarized
directional element. If the relay measures apparent zero-sequence impedance z0
less than Z0F, the relay declares the unbalanced fault to be forward.
Setting Z0R is the reverse threshold for the zero-sequence voltage-polarized
directional element. If the relay measures apparent zero-sequence impedance z0
greater than Z0R, then the relay declares the unbalanced fault to be reverse.
Typically, you can apply the default calculations that appear in Table 5 .3 3 for the
settings Z2F, Z2R, Z0F, and Z0R. For series-compensated lines, calculate each of
these settings separately. The forward threshold setting must be less than corre-
sponding reverse threshold setting to avoid the situation where the measured
apparent impedance satisfies both forward and reverse conditions.
a0
Positive-sequence current restraint factor a0 is the ratio of the magnitude of zero-
sequence current to the magnitude of positive-sequence current, |I
0
|/|I
1
|. If the
relay measures a ratio greater than a0, the zero-sequence voltage-polarized direc-
tional element is enabled. Typically you can apply the default calculations that
appear in Table 5.33.
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