6.4.2 FunctionalityShort circuit between the phases of the stator windings causes normally very largefault currents. The short circuit gives risk of damages on insulation, windings andstator iron core. The large short circuit currents cause large forces, which can causedamage even to other components in the power plant, such as turbine and generator-turbine shaft.The task of Generator differential protection GENPDIF is to determine whether a faultis within the protected zone, or outside the protected zone. If the fault is internal, thefaulty generator must be quickly tripped, that is, disconnected from the network, thefield breaker tripped and the power to the prime mover interrupted.To limit the damage due to stator winding short circuits, the fault clearance must be asfast as possible (instantaneous). If the generator block is connected to the powersystem close to other generating blocks, the fast fault clearance is essential to maintainthe transient stability of the non-faulted generators.Normally, the short circuit fault current is very large, that is, significantly larger thanthe generator rated current. There is a risk that a short circuit can occur between phasesclose to the neutral point of the generator, thus causing a relatively small fault current.The fault current can also be limited due to low excitation of the generator. Therefore,it is desired that the detection of generator phase-to-phase short circuits shall berelatively sensitive, detecting small fault currents.It is also of great importance that the generator differential protection does not trip forexternal faults with large fault currents flowing from the generator. To combine fastfault clearance, as well as sensitivity and selectivity, the generator differentialprotection is normally the best choice of protection for phase-to-phase generator shortcircuits. A negative-sequence-current-based internal-external fault discriminator canalso be used to determine whether a fault is internal or external. The internal-externalfault discriminator not only positively discriminates between internal and externalfaults, but can independently detect minor faults which may not be detected (until theydevelop into more serious faults) by the "usual" differential protection based onoperate-restrain characteristic.Adaptive frequency tracking is included to ensure proper operation of the generatordifferential protection during varying frequency conditions.An open CT circuit condition creates unexpected operations for Generator differentialprotection under the normal load conditions. It is also possible to damage secondaryequipment due to high voltage produced from open CT circuit outputs. Therefore, itmay be a requirement from security and reliability points of view to have open CTdetection function to block Generator differential protection function in case of openCT conditions and at the same time produce the alarm signal to the operationalpersonal to make quick remedy actions to correct the open CT condition.Generator differential protection GENPDIF is also well suited to generate fast,sensitive and selective fault clearance, if used to protect shunt reactors or smallbusduct.Section 6 1MRK 502 048-UEN ADifferential protection122Technical manual
