Sel 411L - Page 252

P.3.144
SEL-411L Relay Protection Manual Date Code 20151029
Protection Functions
Ground Directional Element
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 magnitude 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 3.75.
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 3.75.
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 Tabl e 3.75 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 the corresponding 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 directional element is enabled. Typically you can apply the default
calculations that appear in Table 3.75.
ORDER
The relay uses Best Choice Ground Directional Element® logic to determine
the order in which the relay selects 32QG, 32V, or 32I to provide directional
decisions for the ground-distance elements and the residual ground directional
overcurrent elements. Directional element classification is as follows:
➤ Q—Negative-sequence voltage-polarized directional element
(32QG)
➤ V—Zero-sequence voltage-polarized directional element (32V)
➤ I—Zero-sequence current-polarized directional element (32I)