This paper simulates the voltage sags caused by HVDC failures in both the asynchronous interconnecting and the AC/DC parallel power systems and studies their relationships with the short circuit ratio (SCR) series indices. It's found that in the asynchronously interconnected power system the SCR indices can quantitatively indicate the voltage supporting ability of the receiving-end AC system under HVDC failures. However in the AC/DC parallel system the SCR indices are ineffective. An improved SCR index is then proposed to solve the problem. Simulations prove its feasibility in the AC/DC parallel power systems.
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