This paper analyses the problem that the converter protections act uncertainly under different kinds of AC fault. Various internal faults and protections of converter zone are summarized, and the types of protection which can response to AC faults and the response mechanism are analyzed based on the RTDS experimental results of an actual UHVDC transmission system. This paper also analyses the uncertain electrical characteristics when faults occur as well as the timing principle of converter protections in detail, and points out that these are the two causes of the uncertain action of a converter protection. Further more, the logics of the protections whose trip order are uncertain under different faults are compared and the characteristics of their trip timing orders under different faults are analyzed based on the RTDS experimental results above. It reveals that the setting trip times of these protections are about the same, and one can probably be blocked by the others so that the trip timing orders under different faults determine the different action results.