Voltage Control Schemes to improve Short-term and Long-term Voltage Stability in KEPCO system

Recent technology advancement related to computer & communication and measuring devices allows system operators to adopt more intelligent monitoring and control systems to their power systems in order to prevent massive system blackout. Among them, wide-area monitoring and control (WAMAC) system based on synchrophasor and advanced voltage control technologies have been widely applied to power systems according to their own purposes. In this paper, the study on the development of under voltage load shedding (UVLS) scheme to mitigate short-term and long-term voltage instability in KEPCO system and recent implementation results of a Voltage Management System(VMS) in KEPCO system are introduced as the voltage control schemes to improve voltage stability in the system. And methodology to develop multi-step load shedding schemes based on EMS and synchrophasor data is presented in the paper. The scheme is to improve one of the existing special protection schemes (SPS) which have been applied to the low probability and high impact contingencies in KEPCO system since 2004. Among them, the SPS for voltage instability due to the tripping of 765kV lines in the Seoul metropolitan area is considered in the paper, which has a form of event based UVLS with a single-step scheme. By applying response-based multi-step UVLS schemes based on EMS and Synchrophasor data to severe contingencies in the system; it is shown that more effective results than those of the existing single-step SPS can be obtained in the paper. A centralized UVLS scheme with a hierarchical structure is dealt with in the study. The FV (interfacing power flows vs. bus voltage) curve like PV in voltage stability study with real-time indication of current operating voltage on the critical bus, which can be made from Synchrophasor data, is used as an index for voltage instability in the paper. The scheme has three steps to shed the loads in the system. The first step is to stabilize the short-term voltage instability, which is an event-based operation like the existing SPS in case of tripping 765kV lines. It should be automatically done because there is no time to delay. Then, if the system voltage is not recovered to the appropriate point on the FV curve within the threshold defined in advance, the second load shedding is occurred to stabilize the system. The operator can do its operations manually or automatically with monitoring the real-time index for system voltage stability. It is the same things for the next steps. The remaining steps are prepared to block the long-term voltage instability of the system. Simulation results at the test-bed based on RTDS show that the proposed scheme is more reliable and reasonable way to prevent system blackout due to the voltage instability in KEPCO system compared to existing SPS.