13-20
Testing and Troubleshooting
Date Code 20011205
SEL-311A Instruction Manual
Where: Re indicates the real part of the term in brackets, for instance Re[A + jB] = A
*
indicates the complex conjugate of the expression in parentheses, (A + jB)* =
(A
-
jB).
The result of Equation 13.1 is an impedance magnitude value that varies with the magnitude and
angle of applied current. An example illustrates this operation.
Consider the result of applying the following voltage and current signals to Equation 13.1:
ondary
sec
,
amps
90
2
I
ondary
sec
,
volts
180
10
V
90
ANG
1
Z
2
2
°
-
Ð
=
°
Ð
=
°
=
Ð
2
*
2
]
)
90
-
2
•
90
(1
•
180
Re[10
=
c
2
Z
°
Ð
°
Ð
°
Ð
4
ohms
20
-
=
c
2
Z
ohms
-5
=
c
2
Z
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.
ondary
sec
,
amps
90
2
I
ondary
sec
,
volts
180
10
V
90
ANG
1
Z
2
2
°
Ð
=
°
Ð
=
°
=
Ð
2
*
2
]
)
90
+
2
•
90
(1
•
180
Re[10
=
c
2
Z
°
Ð
°
Ð
°
Ð
4
ohms
20
=
c
2
Z
ohms
5
=
c
2
Z
Notice that the result of Equation 13.1 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 13.1 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 F32Q and F32QG elements, 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 R32Q and R32QG elements, if other supervisory conditions permit.
The forward directional threshold is calculated using Equation 13.2 when Z2F is negative or
Equation 13.3 when Z2F is positive.
Summary of Contents for SEL-311A
Page 6: ......
Page 8: ......
Page 10: ......
Page 24: ......
Page 40: ......
Page 66: ......
Page 96: ......
Page 98: ......
Page 112: ......
Page 114: ......
Page 122: ......
Page 170: ......
Page 202: ......
Page 204: ......
Page 246: ......
Page 248: ......
Page 286: ......
Page 290: ......
Page 304: ......
Page 306: ......
Page 334: ......
Page 384: ......
Page 386: ......
Page 394: ......
Page 398: ......
Page 402: ......
Page 404: ......
Page 410: ......
Page 412: ......
Page 426: ......
Page 444: ......
Page 460: ......
Page 474: ......