Ac Dc Parallel Transmission System Harmonic Analysis

With the rapid development of High Voltage Current Transmission, the interconnection of power networks has strengthened greatly, which is unavoidable to form a complex system called AC/DC hybrid transmission system. It is necessary, therefore, to comprehensively analyze the trait of such system.In pure HVDC system, harmonics generated by converters are relatively independent, so each converter can be studied as a single harmonic source. In AC/DC parallel transmission systems, however, harmonics generated by both converters can be transferred via the AC line, thus interacting with each other and generating new harmonics. At present, little research on the multi-harmonic sources system is known, so it is necessary to configurate a new harmonic analysis model and to explore its harmonic characteristics deeply, which will be more helpful to harmonic elimination.This thesis aims atmulti-harmonic sources in AC/DC parallel transmission systems. Based upon its inherent characteristics, the generation theory and relationship of harmonics generated by rectifier and converter are studied. In order to establish proper harmonic analysis model, the current harmonic analysis methods of HVDC are compared, and the advantages of Modulation Theory and Iterative Harmonic Analysis (IHA) for modeling multi-harmonic sources system is also proved. Modulation Theory regards the converters as modulation switches, simulating the no-nlinear properties of converters reasonably. Thus the interface problem of AC and DC systems can be solved successfully. IHA method can consider the interaction process of harmonic voltages and currents in both AC and DC networks, and then by iterative calculation, harmonic components in steady state can be obtained exactly.Based on the above analysis, a new method named Modulation and Iterative Harmonic Analysis(MIHA) method for harmonic analysis in the AC/DC parallel transmission system is proposed. By the detailed discussion of the equivalent circuits and harmonic parameters of each component, the algorithm models of AC and DC networks in steady state are established respectively. By using Modulation Theory, converter properties are described, and AC and DC network are integrated to form the whole system model. The interaction of multi-harmonic sources is described properly. This method can realize the separate calculation of AC and DC networks, and simplify the analysis process;the iterative method can ensure the accuracy of the calculation, and is helpful for analyzing the interaction process of harmonics in AC and DC networks and exploring the generation mechanism of harmonics.To demonstrate the usefulness of MIHA method, a typical AC/DC parallel transmission system with monopole and 12-pulse converters has been studied. By MINA method, the computer program for this system in steady state is developed. And the harmonic behaviors and characteristics of the AC and DC networks as well as converters have been studied deeply. This system is also simulated by simulation software, EMTDC/PSCAD. The simulated results accord with that of MIHA method, which proves that the proposed method and model in this thesis is valid and practical for the calculation and analysis of the multi-harmonic sources system.This thesis goes deep into the model and the harmonic characteristics of multi-harmonic source, which can enrich the contents of AC/DC hybrid transmission theory, and lay a good foundation for further research of harmonics in AC/DC hybrid transmission, and also provide instructive means for harmonic prevention, so it is meaningful in both theory and application.