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Wide area damping control (WADC) is a novel technique used to solve the inter-area oscillation problem. Unlike the local control, time delay is involved in the transmission of wide area signals, which can not be ignored. In this paper, time delay effects on WADC are analyzed from the view of frequency domain. A new stability criterion is proposed to calculate the time tolerance based on Pade approximation. The stability criterion is verified by simulations of a two areas system with HVDC. The results show that high frequency oscillations can be caused by a certain value of time delay.