Analysis of a Novel Security Measure with Additional Synchronous Condenser for Multi-outfeed and Multi-infeed Ultra-HVDC of Similar Path

China power grid has the characteristics of long-distance transmission architecture and significant strong DC systems and weak AC systems pattern. This makes the converter commutation failure difficult to avoid, which may accelerate deterioration or even destroy the stability of the AC and DC systems. Recently, a novel security measure with additional large-capacity synchronous condenser (SC) has been proposed and installed in the AC support systems of multi-infeed Ultra-HVDC to ensure AC systems voltage stability and prevent commutation failures. This paper deduces the mechanism of SC to suppress commutation failure, compares and analyzes the advantages of SC compared with other reactive power compensation devices as well. According to the characteristics of multi-outfeed and multi-infeed Ultra-HVDC system, the electromagnetic model considering AC grid structure is constructed, and the role of SC in practical application system is simulated and tested. It is proved that this method can effectively reduce the number of consecutive commutation failures and shorten the commutation failure time, which can be widely applied to practical projects.