Study On Core Saturation Harmonic Instability Of Converter Transformer In HVDC Transmission System

With the development of HVDC transmission system and tight connection in power grid,the harmonic problem becomes more significant than before.It is necessary to study the harmonic in DC system systematically in order to reduce it's influences on the system and devices,so that the stability of the power grid can also be improved.The dissertation mainly focuses on the research and analysis of harmonic instability in DC system.The main concerns include the iron model of 500 kV AC transformer and converter transformer,analysis of harmonic characteristics of converter transformer under dc bias,studies the cause and effect of the 2nd harmonic in AC/DC hybrid system.The simulation model of transformer is established by PSCAD-EMTDC,the saturation characteristics of different transformer structures are analyzed according to the model.Based on Nyquist stability criterion,a system harmonic instability criterion caused by core saturation of converter transformer is proposed.This dissertation introduces the instability which caused by 2nd harmonics,and the transmission of it in HVDC.The 2nd harmonic current caused by the direct current in the transformer winding is simulated.Conclusion is drawn that there is linear relationship between the 2 harmonic current amplitude values and the DC current,The harmonic instability criterion caused by core saturation of converter is related to the transformer ratio N and X,correlation coefficient is(1/N-X),The correlation coefficient is negative correlation between X and transformer rated excitation current Im.According to equivalence principle,the author establishes a model transformer in order to investigate the 2nd harmonic source characteristics of the transformer.The experimental results show that DC current injects into the converter transformer winding causes distortion of excitation current,asymmetry of positive and negative half-wave waveform,and harmonic distortion rate increase.The injection of DC current makes the 2nd harmonics in the excitation current increase greatly,The second harmonic current generated by transformer shows a linear relationship with the DC current.With the frequency of harmonics increases,the linearity between harmonic and DC current becomes worse,which no longer satisfies the linear relationship.The ratio of the 2nd harmonic to the DC current varies with the excitation state of the transformer,which is the highest in overexcitation and the lowest in underexcitation.