7-22 Maintenance and Testing Date Code 20011026
SEL-321/321-1 Instruction Manual
Consider the result of applying the following voltage and current signals to Equation 7.2:
ondarysec,amps902I
ondarysec,volts18010V
90ANG1Z
2
2
°−∠=
°∠=
°=∠
2
*
2
]
)
90-2 • 90(1•180Re[10
=c2Z
°∠°∠°∠
4
ohms 20-
=c2Z
ohms -5=c2Z
Coincidentally, these voltage and current signals could represent a forward single line-ground
fault on a system with a 90° impedance angle. Normally, a forward fault results in a negative
Z2c value.
Here is an example for a reverse single line-ground fault.
ondarysec,amps902I
ondarysec,volts18010V
90ANG1Z
2
2
°∠=
°∠=
°=∠
2
*
2
]
)
90+2• 90(1 • 180Re[10
=c2Z
°∠°∠°∠
4
ohms 20
=c2Z
ohms 5=c2Z
Notice that the result of Equation 7.2 is positive for a reverse fault and negative for a forward
fault. This result is consistent with actual behavior on the power system.
The relay determines fault direction by comparing the result of Equation 7.2 to forward and
reverse impedance thresholds that are dependent upon the Z2F and Z2R relay settings and the
magnitude of negative-sequence voltage divided by the magnitude of negative-sequence current.
When Z2c is less than the forward Z2 threshold, Z2FT, the fault is in the forward direction, so
the relay sets the 32QF element, if other supervisory conditions permit. When Z2c is greater
than the reverse Z2 threshold, Z2RT, the fault is in the reverse direction, so the relay sets the
32QR element, if other supervisory conditions permit.
The forward directional threshold is calculated using Equation 7.3 when Z2F is negative or
Equation 7.4 when Z2F is positive.
When Z2F ≤ 0:
m2 Z• 0.25 - F2 Z• 0.75=FT2Z Equation 7.3
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