Abstract: When converters are connected to grids,the negative-sequence control （NSC） may deteriorate the system’s stability since it changes the typical PLL-based vector control,especially in weak grids. This paper analyzes this issue.Firstly,the proportional-integrational quasi-resonant （PIR） control,as one of the widely-used NSC methods,is introduced.Then,we derive the amplitude-phase impedance model and primal-dual complex circuit of the converter-connected grid for the case that the grid voltage is balanced,in which the converter applies the PIR control. Then,we analyze the impacts of the change caused by the PIR control on the admittance elements of the converter’s amplitude-phase impedance. Besides,the generalized impedance criterion is applied to analyze the impact of NSC on the system ’s stability. The analysis results show that the NSC may cause the sub-synchronous oscillation issue and nearly 100 Hz oscillation issue. The simulation results verify the analysis results.