4.56
SEL-351A Relay Instruction Manual Date Code 20080213
Loss-of-Potential, Load Encroachment, and Directional Element Logic
Directional Control Settings (Not in SEL-351A-1)
Ungrounded/High-Impedance Grounded System Considerations for
Setting E32IV
On ungrounded/high-impedance grounded systems (when setting
ORDER = U), phase-to-phase or unbalanced three-phase faults can cause the
ungrounded/high-impedance grounded element to operate on false quantities.
To prevent this situation, SEL
OGIC setting E32IV may be used as follows:
E32IV = V1GOOD * !32QE
The V1GOOD Relay Word bit (see Figure 4.1) deasserts during a three-phase
fault, and the 32QE Relay Word bit (see Figure 4.6) asserts during a phase-to-
phase fault. If either one of these occur, the E32IV setting evaluates to
logical 0, and the ungrounded/high-impedance grounded directional element
is blocked (see Figure 4.8).
When a switch or breaker closes, the poles can close sequentially (not at the
same time), creating a momentary current unbalance condition. To avoid any
possible operation of the ungrounded/ high-impedance grounded element for
this momentary current unbalance condition, set 3PO (three-pole open
condition; see Figure 5.3) in SEL
OGIC setting E32IV as follows:
E32IV = … + !3PO (= … + NOT[3PO])
The 3PO dropout time (setting 3POD) provides the extended blocking (3PO =
logical 1; !3PO = logical 0) for this momentary current unbalance condition.
Courtesy of NationalSwitchgear.com
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