This paper presents the results of a field test conducted for a static var compensator installed at a 500 kV substation. The static var compensator with a damping control function improved power system damping performance significantly by increasing the synchronizing and damping torques. Frequency response analysis was employed for examining the synchronizing and damping torques. Simulations of the test cases were performed by time domain analysis and agreed well with the results of the field testing. Tokyo Electric Power Company (TEPCO) and Toshiba Corporation have jointly developed and installed a static var compensator (SVC) with multiple functions at the Shin-Shinano Substation of TEPCO [1][2]. The SVC entered operation in April, 1987. The SVC is connected to the tertiary winding of the main transformer (525/275/63 kV) at the Shin-Shinano Substation. It comprises a thyristor controlled reactor (TCR) and a fixed capacitor (FC). A 100 MVA TCR controls the reactive power from 80 Mvar capacitive to 20 MVar inductive with the 80 MVA FC. The TCR employs a directly light-triggered thyristor valves and a water cooling system for thyristor valve cooling. A triplicated direct digital control (DDC) system is applied to the control system of the SVC. The main functions of the SVC control system are voltage control, damping control and constant reactive power control, which are selective. As of August, 1988, the SVC has operated for approx. 4200 hours in the voltage control mode and approx. 6300 hours in the damping control mode with no system failures.