Research On Harmonics Characteristics In AC And DC Hybrid System

With power electronics technology widely used in various aspects of power system, harmonics has become the most important issue of power quality. With the development of power grid, HVDC voltage level is becoming higher and higher. When UHVDC system is put into operation widely, due to their non-linearity and high power transfer capability, harmonics will lead to more serious distortion in power system.To study the characteristic of harmonics generated by HVDC system, based on Hami to Zhengzhou UHVDC project, this thesis does some research on the harmonics in Henan province power grid.Firstly, the simulation model has been established in PSCAD, based on the actual parameters of the AC power grid of Henan province and the UHVDC system from Hami to Zhengzhou. A criterion to determine whether the harmonic instability will occur has been applied to analyze the harmonic stability of Henan province power grid after UHVDC is connected. And the value of characteristic harmonics has been calculated.Secondly, this thesis extends the converter impedance-frequency characteristic of HVDC to UHVDC, and applies the theory to analyze the influence on Henan AC system impedance of Hami to Zhengzhou UHVDC system. In this algorithm, the UHVDC system can be seen as an impedance branch in AC system, by which the complex process of converter can be neglected when analyzing the background harmonic amplification characteristics in AC-DC hybrid system.Finally, the AC system harmonic model of Henan province has been established, and the harmonic admittance matrix can be formed. The matrix can be used to calculate the harmonic distortion generated by UHVDC, and the propagation coefficients of harmonic in different voltages have been calculated.