Schweitzer Engineering Laboratories SEL-321 - Page 312

7-42 Maintenance and Testing Date Code 20011026
SEL-321/321-1 Instruction Manual
()
()
TEST2
CBTESTA
CB
2
A2
CB
2
A
3
1
2
II3
:so,0IIandII
IaIaII3
IaIaII
=⋅
===
⋅+⋅+=⋅
⋅+⋅+⋅=
With a 50QF setting of 0.5 amps, 50QF picks up when I
TEST
is greater than 0.5 amps.
Select the magnitude of I
A
greater than the 50L2, 50G2, and 50QF settings, but less
than the maximum current output capability of the current test source.
In this example, select │I
TEST
│ = 2.5 amps. This selection fulfills the supervisory
overcurrent conditions described above.
The reach of the distance element under test in secondary ohms is defined by the
element setting. In this case Z2MG = 9.36 Ω. The impedance measured by the relay
for a ground fault is determined by the faulted phase voltage, faulted phase current,
and the residual current multiplied by the zero-sequence current compensation
factor, k0. The SEL-321 Relay uses k01M and k01A settings to define the zero-
sequence current compensation factor for Zone 1 ground distance elements. The
k0M and k0A settings to define k0 for the remaining zones. The impedance
measured by the relay ground mho distance element for a Zone 2 fault is defined by
the following equation:
RA
A
AG
I0kI
V
Z
⋅+
=
Equation 7.12
Where: k0 = k0M ∠k0A°
For a fault on a radial system and when testing a ground distance element using a
single current source, I
A
= I
R
. In this case, Equation 7.12 can be simplified:
() 0k1I
V
Z
A
A
AG
+⋅
=
Equation 7.13
If we multiply both sides of Equation 7.13 by the quantity (1 + k0), the result is the
impedance applied by the A-phase test signals, V
A
and I
A
.
()
A
A
AG
I
V
0k1Z =+⋅
Equation 7.14
Since we are testing the Zone 2 mho ground distance element, the intended value of
Z
AG
= Z2MG = 9.36 Ω. The angle of Z2MG is equal to the angle of positive-
sequence impedance, Z1ANG.
Because the element under test is not a Zone 1 element, the k0 value is defined by
the relay settings k0M and k0A. When you test the relay Zone 1 ground elements,
use k01M and k01A to define the magnitude and angle of k0.
Calculate the value of Z
AG
⋅(1 + k0) based upon the information above.