Study On Dynamic Equivalents Methods For Large-scale Power Systems

Power grids are developing quickly in China. Large-scale national power grid will be formed in the future with the progress of Ultra High Voltage (UHV) transmission project. However, some forthcoming technical problems, such as stability, dynamic security assessment and coordination control et al., will emerge. It is difficult to analyze such a large-scale power system thoroughly. So, this dissertation focuses on the dynamic equivalents methods for large-scale power systems using the power system simulation software PSS/E. The main results are organized as follow:(1) The theory of coherency equivalents is studied under power system simulation software PSS/E, in which the coherent generators are aggregated based on structure preservation of the coefficient matrices in time domain. The aggregation method obtains directly the parameters of the equivalent generator without iterations unlike traditional aggregation method which needs iterations to optimize the parameters by means of fitting the transfer function in frequency domain. The program in IPLAN language in PSS/E can be implemented directly in the system. The aggregation method was used to reduce East China power grid. The original system and the equivalent system are simulated respectively. The comparison shows that the equivalent system can preserve the main dynamic characteristics of the original system well.(2) A practical method for power system electromagnetic analysis is presented based on physical equivalence, which matches a short-circuit current after the reduction of the power system to that of the original system. It was applied to reduce Jiangsu power grid. Simulation on the original system and the reduced system were performed and compared. Results show that the reduced system can maintain the essential dynamic behavior of the original with good accuracy. The stability of Jiangsu power grid was studied in the reduced system when disturbance in AC system or DC system occurred. Results indicate that Jiangsu power grid is stable whether AC system or DC system is disturbed.(3) A new dynamic equivalent method based on PMU is proposed, which reduces the external system on the basis of the synchronous dynamic measurements obtained by PMU and parameter identification. There are the following advantages: (a) PMU can provide the high-precision phasor measurement synchronously insuring the accuracy and reliability of the measurements. (b) It does not need the detailed information about the configuration, parameters and operating state of the external system. Results of simulation and mode analysis on the original system and the reduced system show that the reduced system can resemble the main dynamic characteristic of the original system well.(4) An improved two PSO algorithm with mutation operator was presented to optimize and identify the parameter of the equivalent system. The algorithm constructs two populations separately, and makes them seek the best position with different steps in the problem space by setting different maximum velocities. In order to share optimal information between two populations, the global optimum of one population is used by the other in the evolution process. Furthermore, a mutation operator is introduced to assures the diversity of the individual in the evolution process. The algorithm can escape from the local optimum and enhance the search ability of the particles.