6.108
SEL-421 Relay Instruction Manual Date Code 20171021
Protection Applications Examples
EHV Parallel 230 kV Underground Cables Example
Directional Control
The SEL-421 uses an array of directional elements to supervise the ground-
distance elements and residual directional overcurrent elements during ground
fault conditions. Internal logic automatically selects the best choice for the
ground directional element (32G) from among the negative-sequence voltage-
polarized directional element (32QG), zero-sequence voltage-polarized direc-
tional element (32V), and the zero-sequence current-polarized directional ele-
ment (32I).
The relay setting ORDER determines the order in which the relay selects direc-
tional elements to provide the ground directional decisions. You can set ORDER
with any combination of Q, V, and I. The listed order of these directional ele-
ments determines the priority in which these elements operate to provide ground
directional decisions. Only one specific directional element operates at any one
time. Directional element classification is as follows:
➤ Q—Negative-sequence voltage-polarized directional element
➤ V—Zero-sequence voltage-polarized directional element
➤ I—Zero-sequence current-polarized directional element
Set ORDER to Q. This setting selects only the negative-sequence voltage-polar-
ized directional element. You rely on 32QG to provide high-speed reliable and
sensitive protection during unbalanced faults via the communications channel.
Cable zero-sequence impedance depends on the fault current return path; the neg-
ative-sequence impedance of the cable does not.
ORDER := Q. Ground Directional Element Priority (combine Q, V, I)
The relay hides the Z0F, Z0R, a0, and E32IV settings because ORDER does not
contain V or I.
Negative-Sequence Voltage-Polarized Directional Element Reverse
Decision (R32QG) Check
The setting Z2R is the reverse threshold for the negative-sequence voltage-polar-
ized directional element. If the apparent negative-sequence impedance (z2) that
the relay measures is greater than Z2R, the relay declares that an unbalanced fault
is reverse. For overhead transmission lines, ignore the shunt admittance that rep-
resents the charging capacitance. The shunt negative-sequence admittance of the
underground cable is significant and modifies the z2 measurement during reverse
faults. You must include this admittance in the cable model to verify proper oper-
ation of the default setting.
Figure 6.27 illustrates the effect of the shunt admittance at both ends of the cir-
cuit for a reverse unbalanced fault.
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