Study On Dynamic Analysis Methods For AC/DC Power Systems

China's power grids are developing quickly. With the application of nation-wide power grid interconnection and China's power transmission from the West to the East, a national hybrid AC/DC power grid will be formed in the future. However, that will bring some technical problems, such as low frequency oscillations, security problem in the system with several HVDC converters terminating in it. By using the power system simulation software PSS/E and combining the strategic planning of China's national power grids, this dissertation focuses on the dynamic analysis of AC/DC power systems. The main works are organized as follows:The HVDC models for electromechanical transient simulations of AC/DC power systems are studied. According to whether considering the inductive transient processes in DC transmission line and the dynamic characteristics of the high frequency DC controller or not, the HVDC models can be classified into two kinds: HVDC response model and HVDC detail model. And the validity of these HVDC models is discussed. Through the analysis and comparison of the HVDC models in PSS/E, it is shown that the HVDC response model CDC4 is simple and suitable for the transient stability analysis of real large-scale AD/DC power systems.A practical analysis method of low frequency oscillations for large power systems is presented. This method combines the advantages of the QR eigenvalue algorithm and the Prony method. And it is easy to be realized in the existing power system software. The computation results of multi-machine power systems show that the frequency, mode shape and participation factors of each oscillation modes calculated with the classical generator models are basically identical to those with detailed generator models and exciter models. Thus, this proposed method has two steps: first, the basic inherent characteristics of the system oscillation modes are calculated using the QR eigenvalue algorithm with the classical generator models; second, the damping can be calculated using the Prony analysis method with detail generator models and exciter models.A novel HVDC supplementary controller based on Phasor Measurement Unit (PMU) is proposed. This controller's input signal is the voltage phase angle difference between two areas obtained by PMU, which can effectively reflect the inter-area low frequency oscillation characteristics. The analysis results show that the proposed controller is capable of enhancing thesystem damping and the AC system's transfer capability over a wide range of operation conditions. This controller can efficiently damp the system's low frequency oscillation and works well under the condition of large disturbances. In addition, it is also robust.An observational linearization and optimal HVDC supplementary controller based on Phasor Measurement Unit (PMU) is presented. This controller utilizes the real-time state variables obtained by PMU to linearize the state equations of the system, and then the linear optimal control strategy is used to design HVDC supplementary controller. Compared with the controllers based on the models linearized in one operation point, the non-linearity of the system is considered in the presented controller, and its control rule varies with the real-time status of the system.Several important computations for the strategic planning of nation-wide power grid interconnection are accomplished, including the analysis of the low frequency oscillations and transient stability in the interconnection schemes of national power grids in the year of 2015, and the transient stability analysis in the pure DC interconnection scheme in the year of 2030. And the AC/DC interaction characteristics within the multi-infeed HVDC systems are also investigated.