Research On Electromagnetic Transient Simulation And Stability Control For Large-scale AC/DC Power Systems

The reverse distribution between the national energy resources and power demands determines our country has to implement the national power development strategy such as power transmission from the western to the eastern region, power exchange between the southern and northern region, and nation wide interconnection. The application of regional power grid interconnection and high-voltage direct-current(HVDC) transmission technology in power system makes the national power system develop large-scale, cross-region, and AC/DC interconnected features. The fact that the dynamic behavior of large-scale AC/DC power systems being more complex because of AC/DC interconnection makes the traditional electromechanical transient simulation technology can not completely meet the research requirements; in addition, the security and stable operation of large-scale AC/DC power systems have to face more challenges under these new features, this in turn demands for higher performance of power grid security and stability control technology. Therefore, the author conducted intensive studies on the electromagnetic transient simulation and stability control technology for large-scale AC/DC power systems.The main works are organized as follows:(1) A novel dynamic reduction approach suitable for electromagnetic transient simulation of large-scale AC/DC power systems is proposed. The backbone(i.e., buses and their connecting lines at the500kV voltage level), important HVDC links and their AC connection with backbone are retained, meanwhile, the reduced network are calculated in such a way that the support strength of the equivalent generator to each500kV bus on backbone is equal to that of the original lower voltage network in the original system. Also, it has to be mentioned that it is not necessary for this approach to trip the electromagnetic loop between500kV and220kV voltage levels, and the reduced power system can be used for detail simulation of large-scale AC/DC power systems in the electromagnetic transient simulation platform. The proposed dynamic reduction approach has been applied to the China southern power grid (CSG). Simulation results show that the equivalent system of the CSG can represent the dynamic characteristics of the original system very well.(2) A electromagnetic transient simulation modeling approach of large-scale AC/DC power systems based on the platform of PSCAD/EMTDC is proposed. First, the corresponding relationship between the component models of electromechnial transient simulation program PSS/E and those of electromagnetic transient simulation program PSCAD/EMTDC are elaborated on, then the method of custom components in PSCAD/EMTDC is also addressed. The proposed approach has been applied to construct CSG in PSCAD/EMTDC, and the validity of the proposed modeling approach is verified by comparison of system responses in PSS/E and PSCAD/EMTDC under several fault scenarios. Finally, the effect of asymmetrical AC faults on the HVDC system and the AC system is further analyzed, which indicates the limit of the electromechanical transient program and the necessity of the electromagnetic transient program in studying system responses of large-scale AC/DC power systems under asymmetrical faults.(3) A wide-area measurement based robust controller design method for HVDC links to enhance the transient stability of AC/DC power systems is proposed. The inaccuracies of the reduced HVDC model, signals’time delays and uncertainty of outside disturbance are considered in the control design process. Firstly, the sliding mode manifold is designed according to the dynamics of the center of inertia(COI) and the sliding mode control design method; and then the Lyapunov stability criterion is adopted to guarantee that the AC/DC power system will be asymptotically stable in the presence of the uncertainties; finally, in order to extend the wide-area measurement based robust controller to large-scale AC/DC power systems, the control sensitivity index of HVDC link versvus generators is addressed. The superiority of the WMRC is verified in a simple2-area4-machine AC/DC power system, a multi-infeed AC/DC power system and Zhejiang AC/DC power grid.(4) A robust damping control design approach for HVDC supplementary controller based on multi-operating synthetic residues and HVH, hybrid control is proposed. This approach proposed a multi-operating synthetic residue index to determine the optimum scheme by comprehensively considering the joint controllability and observability, the coupling effect among the dominant mode and others, and the robustness to multi-operating conditions from the damping controller installation site to the feedback signal, and the validity of the synthetic residues under multi-operating conditions is verified by comparing the H2/Hx performance of the designed damping controllers. Finally, the multi-operating synthetic residues and H2/H?. based robust damping control design approach is verified in a large-scale AC/DC hybrid power system subject to multi-operating conditions and multiple faults.(5) The impact of the voltage fluctuation of HVDC converter AC bus on the HVDC transmission power is analyzed in the case of emergency power support for large-scale AC/DC power systems. Firstly, the operating modes of the HVDC system varying with HVDC converter AC bus voltages at both ends are obtained according to the HVDC Ud-Id characteristic curve, and then the actual transmission power of the HVDC system for each possible operating mode is calculated. The research results show that in emergency, only when the HVDC converter AC bus voltages at both ends maintain at a certain voltage level, can the actual transmission power of the HVDC system follow the order of HVDC emergency power support reference.