Research On Small Signal Stability Of Hybrid Dual-infeed HVDC System

With the widely application of LCC-HVDC and the development of VSC-HVDC,there have been many hybrid multi-infeed HVDC systems formed by LCC-HVDC and VSC-HVDC lines fed into the same load center at home and abroad.The coupling mechanism between different systems is more complex and abstract,and the stability of small signal is one of the important problems encountered in the system operation when the parameters of the control system and the strength of the AC system change.To certificate the damping character and high responding speed of the system,this paper takes the basic hybrid double-infeed HVDC system as an example,studys the influence of control system parameters and the strength of the AC system change on small signal stability of hybrid double-infeed HVDC system,reveales the system oscillation mechanism,and proposes a new type of a Supplementary Virtual-Resistor Damping Control to improve the damping characteristics of the system.The research results of this paper have important theoretical and engineering significance.Firstly,the LCC-HVDC and VSC-HVDC state space models are established,and the two subsystem are connected by the interface model.Based on the state space models,a complete mathematical model of small signal of hybrid double-infeed HVDC system is established.The time domain simulation model of the system is established in PSCAD/EMTDC,and the nonlinear mathematical model of the system is built in Matlab.Secondly,the control system parameters that have a great impact on the small signal stability of the hybrid double-infeed HVDC system are obtained through the participation factor and sensitivity analysis method.Namely,PLL parameters of LCC-HVDC subsystem and VSC-HVDC subsystem,and constant AC bus voltage controller parameters of VSC-HVDC subsystem.And then this paper uses the eigenvalues analysis method respectively study the influence of the three parameters on the system small signal stability,and analyze the coupling relationship between different control systems parameters.Finally,the impact of the strength of AC system on the small signal stability of hybrid dual-infeed HVDC system is investigated by eigenvalue analysis.As for the instability issue of hybrid dual-infeed HVDC system caused by the decreased strength of AC system,this paper proposes a Supplementary Virtual-Resistor Damping Control.Based on the current flowing through the tie-line and introduced virtual resistor,the SVRDC approach introduces an additional component to the input of AC voltage controller of VSC subsystem,improves the system damping characteristics by enhancing the virtual damping and electrical coupling,enhances the stability of the system without increasing the power loss of the overall system.