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Inter-area oscillation has become a prominent stability problem in AC connected power systems, but wide area damping control (WADC) has great potential in solving this problem. The main difference between WADC and conventional local control is that in WADC signals are transmitted over long distances, and, thus, the time delay involved in the control loop cannot be ignored. An experimental system was built based on real-time digital simulators, a real WADC system and phasor measurement units, to investigate the impact of the time delay. In the experiments, the WADC was implemented on the Gao-Zhao HVDC supplementary control in the China southern power grid. The results revealed a phenomenon high-frequency constant amplitude oscillation caused by time delay. This phenomenon was successfully explained using the frequency-domain stability analysis method. Furthermore, a compensation block and a band-pass filter were used to prevent the negative impacts of time-delay on the stability margin and the damping performance. The method for designing their parameters is also described. Physical experimental results show the effectiveness of this method.