GE D90 Plus Instruction Manual page 213

CHAPTER 7: PROTECTION
Neutral directional overcurrent
PLUS
D90 LINE DISTANCE PROTECTION SYSTEM – INSTRUCTION MANUAL
Block
Range: any FlexLogic™ operand or shared operand
Default: Off
Assertion of the operand assigned to this setting blocks operation of the neutral
instantaneous overcurrent element.
Events
Range: Enabled, Disabled
Plus
Default: varies with UR
This setting enables and disables the logging of neutral instantaneous overcurrent
events in the sequence of events recorder.
The logic for the neutral instantaneous overcurrent 1 element is shown below. The logic is
identical for all neutral instantaneous overcurrent elements.
Figure 160: Neutral instantaneous overcurrent 1 scheme logic
There are two neutral directional overcurrent protection elements available. The element
provides both forward and reverse fault direction indications the
operands, respectively. The output operand is asserted if the
NEUTRAL DIR OC1 REV
magnitude of the operating current is above a pickup level (overcurrent unit) and the fault
direction is seen as forward or reverse, respectively (directional unit).
The overcurrent unit responds to the magnitude of a fundamental frequency phasor of the
either the neutral current calculated from the phase currents or the ground current. There
are two separate pickup settings for both the forward-looking and reverse-looking
functions. If set to use the calculated 3I_0, the element applies a positive-sequence
restraint for better performance: a small user-programmable portion of the positive–
sequence current magnitude is subtracted from the zero-sequence current magnitude
when forming the operating quantity.
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.
The positive-sequence restraint must be considered when testing for pickup accuracy and
response time (multiple of pickup). The operating quantity depends on the way the test
currents are injected into the relay. For single-phase injection, the operating quantity is:
For three-phase pure zero-sequence injection, the operating quantity is:
-series model; refer to the EnerVista UR
GROUPED PROTECTION ELEMENTS
Plus
Setup software
NEUTRAL DIR OC1 FWD
and
Eq. 23
Eq. 24
Eq. 25
205