GE D90 Plus Instruction Manual page 266

GROUPED PROTECTION ELEMENTS
Negative-sequence instantaneous overcurrent
256
Figure 217: Negative-sequence time overcurrent scheme logic
The negative-sequence instantaneous overcurrent element may be used as an
instantaneous function with no intentional delay or as a definite time function. The
element responds to the negative-sequence current fundamental frequency phasor
magnitude (calculated from the phase currents) and applies a positive-sequence restraint
for better performance: a small portion (12.5%) of the positive-sequence current
magnitude is subtracted from the negative-sequence current magnitude when forming
the operating quantity.
The positive-sequence restraint allows for more sensitive settings by counterbalancing
spurious negative-sequence currents resulting from:
• System unbalances under heavy load conditions.
• Transformation errors of current transformers (CTs) during three-phase faults.
• Fault inception and switch-off transients during 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 how the test
currents are injected into the relay. The operating quantity for single-phase injection is:
The operating quantity for three-phase injection, opposite rotation, is:
Select the Settings > Protection > Elements > Group 1 > Current > Negative Sequence
IOC menu item to open the negative-sequence instantaneous overcurrent configuration
window.
Figure 218: Negative-sequence instantaneous overcurrent configuration settings
The following settings are available for each negative-sequence instantaneous
overcurrent element.
PLUS
D90 LINE DISTANCE PROTECTION SYSTEM – INSTRUCTION MANUAL
CHAPTER 7: PROTECTION
Eq. 32
Eq. 33
Eq. 34