Protective Elements
percentage restrained characteristic that allows to compensate both the static error and the dynamic error. The
static error accounts for transformer static magnetizing current and current measurement circuit calibration errors.
The dynamic error may be caused by Tap Changing (OLTC) and by CT saturation caused by heavy fault currents.
In addition, the static tripping characteristic can be modified temporarily at the User’s choice to prevent some
nuisance tripping from the harmonic inrush during energization, over-excitation, or deep CT saturation. The
harmonic inrush is evaluated through 2
nd
, 4
th
harmonics and 5
th
harmonics transient is monitored through the CT
saturation detector.
Phase Differential Protection Applications
The phase differential protection can be used for two application scenarios:
(1) Generator Phase Differential Protection - 87 GP
For this application, the phase differential protection will detect phase faults in the generator stator windings. The
differential zone is between the current transformers (CT) installed at the generator neutral and mains side.
For detailed application configuration, please see also the application table on the next page.
(2) Generator Phase Differential Protection with Unit Step-Up Transformer included in differential zone - 87 UP
For this application, the phase differential protection will detect phase faults both in generator and step-up
transformer. The differential zone in this case is between the current transformers installed at the generator neutral
and transformer main side (high voltage side). That means that all of the eletrical equipment, including generator,
transformer and the cable connections between them, are inside the phase differential zone (Overall Differential).
For detailed application configuration, please see also the application table on the next page.
It is to note that for both above mentioned applications, the phase differential protection base current Ib, to which
both the differential and restraining currents are scaled, will be calculated from the set generator ratings as follows:
To Next Page
