AC fault characteristic analysis of AC-DC hybrid transmission grid

Taking the ±500 kV Bao-Dei and Gui-Guang II HVDC projects as model, and basing on a large number of RTDS experiments, we firstly analyzed the AC fault transient characteristic of the AC-DC hybrid transmission grid from the relay application viewpoint. The results show that the harmonic components of fault information are mainly expressed as low frequency and non-integer component, and its content basically does not decay with the passage of time. In addition, due to the nonlinear characteristics of converters, the magnitude and phase of the system positive sequence impedance are no longer equal to those of the negative sequence impedance. Those features have an unfavorable influence on many types of protection such as differential component, distance component, phase selection component, and directional component. The results can be adopted to establish a theoretical foundation for the development and application of relay protection applied to AC-DC hybrid grid.