National Electric Power Dispatching and Control Center，Beijing100031，China

With the rapid development of the ultra high-voltage direct current（UHVDC）power transmission system，the correlation between AC and DC systems or between DC systems is becoming increasingly close.A single AC fault at the receiving end power grid may trigger simultaneous commutation failure in multi-infeed HVDC transmission systems，which will significantly impact both the sending and receiving end power grid.The paper presented a detailed analysis of an actual commutation failure in multi-infeed HVDC transmission systems caused by AC fault，the impact on the sending and receiving end power grid based on electromechanical transient simulation，the correlation effect of DC systems and the range of the AC fault that can trigger the commutation failure by means of electromagnetic transient and electromechanical transient hybrid simulation.Finally，relevant suggestions are made.