New adaptive coordination approach between generator-transformer unit overall differential protection and generator capability curves

This paper presents a new method to increase the reliability of the generator-transformer unit overall differential protection with use of the capability charts. The main objective of proposed method is preventing the false tripping under both heavy external solid-faults associated at Generator Step-Up Transformer (GSUT) HV Side and Unit Auxiliary Transformer (UAT) LV Side, load rejection either house load rejection or zero load rejection, and synchronization. Also, the proposed method simultaneously detects both faults at low voltage side of UAT where the fault current is very small and near to normal load current comparing to generator capacity and single-line-to-ground faults at GSUT Low voltage side (generator terminal) due to the transformer delta connection that isolates the zero-sequence component from the network side. The presented method is based on using adaptive setting (three selected settings) for the generator-transformer unit overall differential protection to raise the characteristic setting under the external fault, load rejection and synchronization and decrease the characteristic setting under internal high-impedance fault. The suitable setting is selected according to the location of the generator operating point inside the generator capability curves. The generator capability curves are divided into four operating regions, where each one has a different differential characteristic setting having dual-rate of change of the differential current. The four operating regions are normal load operation and high impedance internal fault; heavy external and internal faults; load rejection and synchronization; and at under excitation operation without faults, respectively. The real dynamic simulation of the power station has been conducted by ATP/EMTP software for the large steam turbine driven synchronous generator. Extensive simulation case studies for internal faults and system disturbances are investigated as well as practical recorded signals.