GE D90 Plus Instruction Manual page 215

CHAPTER 7: PROTECTION
NOTE:
PLUS
D90 LINE DISTANCE PROTECTION SYSTEM – INSTRUCTION MANUAL
Figure 161: 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 will be delayed by 1.5 of a power system cycle. The element is designed
to emulate an electromechanical directional device. Larger operating and polarizing
signals will result 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, should be used for the tripping direction. The reverse-
looking function is designed to be faster as compared to the forward-looking function and
should be used for the blocking direction. This allows for better protection coordination.
The above bias should be taken 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 item to open the neutral directional overcurrent configuration window.
GROUPED PROTECTION ELEMENTS
207