Schweitzer Engineering Laboratories SEL-321 - Page 318

7-48 Maintenance and Testing Date Code 20011026
SEL-321/321-1 Instruction Manual
The 50G overcurrent elements operate based upon the magnitude of the residual
current. Using the current connections shown in Figure 7.6, the magnitude of I
R
is
equal to the magnitude of the applied test current.
With a 50G2 setting of 0.60 amps, 50G2 picks up when I
TEST
is greater than 0.60
amps.
The 50QF negative-sequence overcurrent element operates based upon the
magnitude of 3I
2
applied. Using the current connections shown in Figure 7.6,
calculate the magnitude of 3I
2
applied based upon the magnitude of I
TEST
.
()
()
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.
We calculated │I
A
│ = │I
TEST
│ = 2.49 amps which satisfies the supervisory
overcurrent conditions described above.
Select .volts12067Vandvolts12067V
CB
°∠=°−∠=
The ground distance elements are supervised by the negative-sequence directional
element. Check the negative-sequence quantities applied and verify that the 32QF
element should assert. Calculate the magnitude and angle of negative-sequence
voltage and current applied for the test quantities listed above. Then calculate the
negative-sequence impedance, Z2c, applied using Equation 7.2.
()
()
CB
2
A
3
1
2
CB
2
A
3
1
2
IaIaII
VaVaVV
⋅+⋅+⋅=
⋅+⋅+⋅=
() []
2
2
22
I
*IANG1Z1VRe
2Z
⋅°∠⋅
=
Equation 7.2
2
2
I
V
m2Z
=
m2Z25.0F2Z25.1FT2Z ⋅−⋅= Equation 7.4
Taking the test signals from Table 7.5.
°∠=
°−∠=
°∠=
120V0.67V
120V0.67V
0V4.40V
C
B
A
()
volts1800.9V
volts1200.6712011200.67240104.40V
2
3
1
2
°∠=
°∠⋅°∠+°−∠⋅°∠+°∠⋅=