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Power Automation and Protection Division
5.5
Zero sequence impedance to Positive sequence impedance ratio (ZR)
This setting is used for all ground fault measurements. It reflects the magnitude ratio of the as-
sumed zero sequence impedance to the positive sequence impedance of the line.
For Example:
If Zl0 = 65 ohms at 60°, and Zl1 = 19 ohms at 75°, then set ZR = 65/19 = 3.42.
6.
BACK - UP SYSTEM SETTINGS
6.1
Loss of Potential Block enable (LOPB)
This setting enables the loss of potential logic (Vo and not Io) to block all Zone 2 and Zone 3
impedance units if the logic is satisfied.
For Example:
Set LOPB =
6.2
Forward Directional Phase Unit (FDOP)
If the system has voltage inputs, then the high set overcurrent units (IPH) can be made direc-
tional if FDOP is in. Set FDOP = IN to make the IPH units directional.
6.3
Forward directional ground unit (FDOG)
If the system has voltage inputs, then the high set overcurrent unit (IGH) and the time delayed
ground back-up unit (TOG) can be made directional if FDOG is in. Set FDOG = N to the make
the IGH and TOG units directional.
6.4
Ground directional unit polarization options (DIRU)
The ground directional unit can be zero sequence or negative sequence polarized. When zero
sequence polarized it uses all zero sequence quantities to determine the power flow direction
and it is very sensitive to zero sequence mutuals between parallel lines. When negative se-
quence polarized it uses all negative sequence quantities to determine the power flow direction
and its operation is negligibly affected by the presence of mutual effects. Set DIR = ZSEQ if mu-
tuals are not a consideration and DIR = NSEQ if strong zero sequence mutuals are present in
the neighborhood of the transmission line.
6.5
Medium set zero sequence overcurrent unit (IOM)
This overcurrent unit supervises the ground trips of the ground units in the impedance back-up
system. If TOG is being used it is also used for tripping after a time delay, TOG. It is measuring
the ground return current or 3Io. It should be set above the maximum expected unbalance in the
normal load current flow in the line.
The recommended setting is IOM = 0.5 amps.
REL 352 Version 1.00
YES to enable the logic.
I.L. 40-201.9
5
5-7
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