Advertisement
GEK- 264 23
The system constant s in the above equation s should be evaluate d for a single phase to ground fault in
the non-trip ping direction at the relay terminal s.
This fault location is designat ed as Fl in Figure 5.
T
=
Minimum permissi ble tap setting in percent.
C
=
Positive sequence distribut ion constant
I'/I.
C 0
=
Zero sequence distribut ion constant I0'/IO•
=
System positive sequence impedance as viewed from the fault.
Z 0
=
System zero sequence impedance as viewed from the fault.
9
=
The angle of the system positive sequence impedance Z
1
.
K 5
=
Constant depending on the ratio of Z
0
/ Z
1
.
See curves on Figure 14.
A
=
Angle depending on the ratio of ZQ/Z
1
.
See curves on Figure 14.
The system constant s in these equation s should be evaluate d for a single phase to ground fault in the
non-trip ping direction at the relay terminal s.
This fault location is designat ed as Fl in Figure 5.
Under some system conditio ns it is possible during double phase to ground faults in the non-trip ping
direction that the unit on the unfaulte d phase will pick up.
Since this can result in a false trip, it is
necessar y to limit the reach setting of the units to prevent them from picking up on reverse double phase
to ground faults.
Equation JIc gives this limit.
K (C
-
C)
T
=
3Z
0
Cos (9
-
60)
lic
where:
K
=
Design Constant
100 for 1.0 basic minimum tap
200 for 2.0 basic minimum tap
300 for 3.0 basic minimum tap
9
=
The angle of the system zero sequence impedance Z
0
.
All other terms are defined above.
Note that C 0 and C in equation tIc have the same values as they have in equation s ha and lIb.
After the values of T have been calculate d for equation s ha, hib and TIc above, the largest of the
three values should be selected and then some margin, such as 10: (not 10 percenta ge points), should be
added to this setting.
This value of tap setting is then the minimum permissi ble tap setting for the re
lay at the terminal under consider ation.
If any (or all) of the values of T calculate d from the three
equation s is negative , that signifies that the particula r equation (or equation s) offers no limitatio n
on the minimum permissi ble tap setting.
Aside from all of the above, the relay should never be set on a tap that is lower than 10 percent.
All voltages , currents and impedances in the above equation s are in terms of secondary quantitie s
based on the CT and PT ratios of the protected line.
The effects of arc resistanc e have not been included in these calculati ons.
19
Advertisement
