| High power rectifiers will produce a large number of harmonics pollution to grid-side. These problems could be well resolved by multi-pulse-rectifier system, but low harmonics would not be totally eliminated by this system. In order to achieve better harmonic suppression effects, a system which needs to be isolated between DC and AC side for harmonic suppression on grid-side with single phase inverter based on the main circuit topology of traditional 18-pulse-rectifier system has been researched in this thesis.Main circuit topology of traditional 12-pulse-rectifier is studied in this thesis. The Fourier expression of grid-side current and DC voltage has been calculated based on the conversion relationship between DC side variables and AC side variables. On the basis of the theory of harmonic suppression on DC side, this thesis introduces unified modulation on DC side and regulated output voltage on DC side based on traditional 12-pulse rectifier system. MATLAB has been used to simulate the effect of the two method of harmonic suppression.A system with single phase inverter on DC side for harmonic suppression, which is based on traditional 18-pulse-rectifier system, has been studied in this thesis. The auxiliary square voltage wave coupled to the DC side through high-frequency transformer, is produced by the single phase inverter, which is used to improve the waveforms of DC side current to suppresses harmonics of grid-side current. At last, the input current and the output voltage of the traditional 18-pulse-rectifier system has been compared with the variation of the system with the auxiliary circuit.The main reason causing unbalance output voltage of multi-pulse-rectifier system is unbalance input voltage and asymmetry configuration of transformer. Firstly, via analyzing the effect of unequal turns on multi-pulse-rectifier system output voltage, the relation between turns and output voltage unbalance factor of transformer is obtained. MATLAB is used to simulate the variation rule between turns and voltage unbalance factor. Secondly, the relation between unbalance input voltage and output voltage unbalance factor of multi-pulse-rectifier system is analyzed under symmetry configuration of rectifier transformer. Lastly, the change rule of grid current total harmonic distortion under every unbalanced condition is analyzed, which is used to verify the correctness by comparing the results by simulating.Finally, the 18-pulse rectifier system experimental platform is built, the detailed selection process of main circuit device types and software realization flowchart of the control system are presented. Moreover, the 18-pulse rectifier system control system has been debugged and experiments are carried out to verify the validness of the theory researched above. |
PDF Full Text Request