Investigation and Reduction of Effects of Transient Signals for Switching Capacitor into a Power System by Using an Experimental Test Set

Abstract

This paper aims to investigate switching capacitor bank of the 115 kV Nong Chok substation under the Electricity Generating Authority of Thailand (EGAT). The substation comprises of 3 steps capacitor banks with reactive power of 48 Mvar in each step. In case study, the substation is downscaled to be an experimental unit with 415 V and 5 Mvar in each step in laboratory. Inrush currents, the behavior of transient signals, that occurs when capacitors are switched into the system are studied and analyzed. To reduce the effect of switching capacitors, current limiting reactors connected in series with the capacitors are proposed. In addition, a zero-crossing circuit is designed to control switching angle of the capacitors, since it has a significant effect on the inrush currents. The results of experiment are compared with two case studies: switching capacitors without integrated 7% of reactors and switching capacitors with integrated 7%. It can be summarized that the switching capacitor without integrated reactors has inrush currents change based on the switched angles of the capacitors. However, the switching capacitor with integrated reactors gives inrush current values are almost approximate in each angles of switching and they are lower the case of the switching capacitor without integrated reactors. Nevertheless, reactor integration into the system leads to high current values at the steady states.