-It is well known that line switching operations like reclosing are able to cause transient power oscillations which can stress or damage turbine generators. This paper presents a reclosing scheme to reduce the shaft torsional torques of turbine generators by inserting an additional reactor. A novel method to determine optimal reactor capacity to minimize the torsional torque generated in a turbine generator is also proposed. In this paper, the turbine generator shaft is represented by a multimass model to measure torsional torques generated in the shaft between the turbine and the generator. Transmission systems based on actual data from Korea are modeled to verify the proposed scheme using ElectroMagnetic Transient Program (EMTP) software. The simulation results clearly show the effectiveness of the proposed scheme and torsional torque can be minimized by applying the proposed scheme.

-Most of faults occurring in transmission system are transient. To cope with such faults, automatic reclosing is applied and it is economic and effective method to improve the reliability and transient stability of power system. Many countries applying 3-phase and leader-follower reclosing scheme use a synchronism-check relay to limit impacts associated with automatic reclosing under Live-Bus/Live-Line (LBLL) conditions. Most utilities generally use a standardized conservative setting that is in the range of 20 o to 30 o . However, it might be sometimes not a good idea because each transmission line in system has different system conditions such as system structure, line length and load flow that might result in undesirable system impacts. Therefore, it is necessary to derive the Optimal Synchronism-Check (OSC) phase angle which is appropriate for each transmission system. In this paper, a novel method composed of three steps is proposed to derive the OSC phase angle. The proposed method is applied to transmission system based on actual data from Korea. Computer simulations are performed using ElectroMagnetic Transient Program (EMTP) and simulation results show that proposed method is valid and effective.

-It is important to clear the fault and prevent resulting in damage to power system. Although the frequency of generator internal fault is relatively low, it can lead to incalculable damage to power system as well as generator. Especially, loss of field on generator can cause the generator to lose synchronism for a short time if it is not removed promptly. Therefore, it is needed to conduct research on loss of field relay for detecting or clearing the loss of field. However, the setting of the relay may vary in generator operator or engineer, and the relay is not coordinated well with other elements associated with loss of field. In this paper, we address specifically the coordination of positive offset mho loss of field relay which is one of the protection schemes for loss of field. Computer simulations are performed by using ElectroMagnetic Transients Program-Restructured Version (EMTP-RV) based on actual data.

-Jeju power system is connected to the mainland power system through HVDC, and receives about half of required electricity through the HVDC lines. Jeju power system already experienced black out when a generator tripped at the moment of the HVDC line faults. But, UFR operated as was expected when HVDC line fault occurred at that time. As the penetration level of wind turbines increases, it is required to set UFR again considering intermittent wind turbin outputs. In this paper, we address a new way of UFR setting through computer simulation.