Negative sequence currents arise when the virtual representation of the phase rotation for an
unbalanced system appears negative.
Negative sequence currents can occur where there are single phase loads, unbalanced line
short circuits and open conductors, and/or unbalanced phase-phase or phase-neutral loads.
This protection is used to prevent the generator from overheating. Negative sequence currents
produce a magnetic field in the generator counter-rotating to the rotor. This field crosses the
rotor at twice the rotor velocity, inducing double-frequency currents in the field system and in
the rotor body.
The alarm response is based on the estimated phase-to-neutral current phasors, from the
source, as measured by the controller.
Controller types: This alarm is present in the GENSET and BUS TIE breaker controllers.
Configure the parameters under Configure > Parameters > [Source] > Current protections > Negative sequence current.
Table 3.41
Default parameters
Parameter Range Negative sequence current
Set point 1 to 100 % of nominal current 20 %
Time delay 0.1 s to 1 h 0.50 s
Enable Not enabled, Enabled Not enabled
Action Warning
3.4.13 Zero sequence current (ANSI 51Io)
Protection
IEC symbol (IEC60617) ANSI (IEEE C37.2) IEC 61850 Operate time
Zero sequence current 51Io PTOC < 200 ms*
*Note: This operate time includes the minimum user-defined delay of 100 ms.
Zero sequence currents arise when the phases rotation is positive, but the vector zero value
(star point) is displaced.
This protection is used for detecting earth faults.
The alarm response is based on the estimated phase-to-neutral current phasors from the
source, as measured by the controller.
Controller types: This alarm is present in the GENSET and BUS TIE breaker controllers.
Configure the parameters under Configure > Parameters > [Source] > Current protections > Zero sequence current.
Table 3.42
Default parameters
Parameter Range Zero sequence current
Set point 1 to 100 % of nominal current 20 %
Time delay 0.1 s to 1 h 0.50 s
DESIGNER'S HANDBOOK 4189340911K UK Page 73 of 521
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