GE D90 Plus Instruction Manual page 259

CHAPTER 7: PROTECTION
NOTE:
PLUS
D90 LINE DISTANCE PROTECTION SYSTEM – INSTRUCTION MANUAL
Figure 213: Neutral directional voltage polarized characteristics
The neutral directional overcurrent element incorporates a current reversal logic. If the
reverse direction is indicated for at least 1.25 of a power system cycle, the prospective
forward indication is delayed by 1.5 of a power system cycle. The element is designed to
emulate an electromechanical directional device. Larger operating and polarizing signals
results in faster directional discrimination bringing more security to the element operation.
The forward-looking function is designed to be more secure as compared to the reverse-
looking function, and therefore is to be used for the tripping direction. The reverse-looking
function is designed to be faster as compared to the forward-looking function and is to be
used for the blocking direction. This allows for better protection coordination.
Take this bias into account when using the neutral directional overcurrent element to
directionalize other protection elements.
Proper application of this element requires that the operating current and the polarizing
voltage (or current) be measured from the same side of the transformer.
Select the Settings > Protection > Elements > Group 1 > Current > Neutral Directional
OC menu to open the neutral directional overcurrent configuration window.
GROUPED PROTECTION ELEMENTS
249