Sel SEL-421 - Detailed Settings Description; 50 FP and 50 RP; Z2 F and Z2 R; A2 and K2

R.1.29
Date Code 20111215 Reference Manual SEL-421 Relay
Protection Functions
Ground Directional Element
Detailed Settings Description
If you set E32 to Y, you can change the settings listed in Tab le 1.28.
50FP and 50RP
Setting 50FP is the threshold for the current level detector that enables
forward decisions for both the negative- and zero-sequence voltage-polarized
directional elements. If the magnitude of 3I
2
or 3I
0
is greater than 50FP, the
corresponding directional element can process a forward decision.
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 impedance 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 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 1.28.
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 1.28.
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.