Harmonic Analysis In High Voltage Direct Current Transmission System

High-voltage direct current transmission (HVDC) technology which started from the industrialization period in1954has experienced nearly6decades’ development. Much technology is already mature. The expansion of power grid scale and introduction of new computer and electronics technology also bring itself more complex problems, and harmonic problem is one of important aspects. Depend on our country’s situation, the development tend of HVDC system in China is to built a massive hybrid power system, whose cooperations and harmonic interactions between AC and DC systems will become very complicated, so, the analysis of harmonic problem in HVDC system is very important. This paper based on the theory of modulation, issues of harmonic and interharmonic were discussed, including the following aspects.Firstly, generation mechanism and the reasons for harmonic in the HVDC system were analyzed, transmision process of AC and DC side harmonic was described using modulation theory, and summarized its law by simulating the three-phase six-pulse rectifier to get the change law of non-characteristic harmonic in DC side with the change of commutation angle when harmonic was superposed on AC source. The derivation of mathematical algorithms that calculate the voltage and current harmonics in HVDC system was described to calculate the AC and DC side current harmonics of the two ends and12-pulse transmission system and the results were compared by using MATLAB/SIMULINK simulation software. This method can be used as the direct algorithm of the harmonic estimate, or as one step in the iterative process.Secondly, the generation mechanism and composition of interharmonics of HVDC system was analyzed. DC side harmonic component, the inter-modulation product was taken as the first analysis points, then, to draw the harmonic component of AC side current. The method can resolve the difficulty in analysing interharmonic when it contains multi-harmonic source, and the explaination for the mechanism is reasonable, simple and effective. This can be proved by corresponding simulations and the calculation and simulations conform with the basic theory. The simulation included the generation mechanism and compositions of inter-harmonic of HVDC system when the system is imbalanced or with background harmonic. Finally, the phenomenon of harmonic instability was described to compare the two analysis methods. The equivalent impedance of AC and DC systems whose parameters were provided by CIGRE was formulated by switch function method, where the frequency characteristic curve of DC equivalent impedance of inverter in HVDC system was represent. The method could determine the converter’s impedance-fequency characteristic when the AC and DC circuits were converted in the same fame, thus harmonic instability problem could be studied easily. By reproudcing this phenomenon of harmonic instability, this paper got the further description about the impact of DC impedance on the resonant impedance of AC system. Summed up the strategy of suppression of the emergence of the harmonic instability in last.The way of Harmonic analysis of the HVDC system in this paper can be extended to analyse multi-source harmonic. Looking for a simple, effective method to analyse harmonic, to research the harmonic generation mechanism and the interaction between harmonics from AC to DC system has an important significance.