GE D90 Plus Instruction Manual page 211

CHAPTER 7: PROTECTION
Neutral instantaneous overcurrent
PLUS
D90 LINE DISTANCE PROTECTION SYSTEM – INSTRUCTION MANUAL
Reset
Range: Instantaneous, Timed
Default: Instantaneous
The "Instantaneous" reset method is intended for applications with other relays, such as
most static relays, which set the energy capacity directly to zero when the current falls
below the reset threshold. The "Timed" reset method can be used where the relay must
coordinate with electromechanical relays.
Block
Range: any FlexLogic™ operand or shared operand
Default: Off
Assertion of the operand assigned to this setting blocks operation of the neutral time
overcurrent element.
Events
Range: Enabled, Disabled
Plus
Default: varies with UR
This setting enables and disables the logging of neutral time overcurrent events in the
sequence of events recorder.
The logic for the neutral time overcurrent 1 element is shown below. The logic is identical
for all neutral time overcurrent elements.
Figure 158: Neutral time overcurrent 1 logic
The neutral instantaneous overcurrent element may be used as an instantaneous function
with no intentional delay or as a definite time function. The element essentially responds to
the magnitude of a neutral current fundamental frequency phasor calculated from the
phase currents. A positive-sequence restraint is applied for better performance. A small
portion (6.25%) of the positive-sequence current magnitude is subtracted from the zero-
sequence current magnitude when forming the operating quantity of the element as
follows.
The positive-sequence restraint allows for more sensitive settings by counterbalancing
spurious zero-sequence currents resulting from:
• System unbalances under heavy load conditions.
• Transformation errors of current transformers (CTs) during double-line and three-
phase faults.
• Switch-off transients during double-line and three-phase faults.
-series model; refer to the EnerVista UR
GROUPED PROTECTION ELEMENTS
Plus
Setup software
Eq. 20
203