Harmonic Instability Caused by Interactions Between AC and DC Transmission Systems

For multi-infeed AC/DC schemes,the harmonic interaction not only takes place through the converters,but also through all AC network.Thus complexity of the situation is enhanced.In order to analyze the emerging risks with adverse interaction,all DC schemes that are electrically close must be considered.As an example,the electromagnetic transient simulation software PSCAD/EMTDC which includes detailed converter model is used to establish the complete electromagnetic transient simulation model for two HVDC transmission lines from Guizhou to Guangdong,and to analyze the harmonic instability problem at sending terminal of Guizhou power grid caused by the harmonic interaction.The simulation results show that harmonic instability will occur at the sending terminal caused by interactions among two HVDC transmission lines and the AC power system when the DC bipolar block fault occurs in the weak AC connection mode.But it is impossible to occur in other fault conditions.