Abstract
Both LCC and MMC have AC/DC harmonic transformation affection in hybrid HVDC transmission system. their harmonic distribution characteristics are different and coupled though DC transmission lines, which leads to harmonic instability. In this paper, the harmonic transmission characteristics of DC transmission lines are studied. Based on the electromagnetic model of DC transmission lines, a DC transmission line model suitable for the analysis of harmonic current amplification characteristics is constructed. The analytic expression and calculation method of harmonic current amplification of DC transmission lines are given and proved by PSCAD. On this basis, this paper takes an actual HVDC project as an example to study the influencing factors of harmonic current of bipolar transmission lines. When 50 Hz symmetric harmonic voltage source is injected, if the total length of the line reaches a certain threshold, severe harmonic current amplification will occur. The maximum magnification will increase with the increase of overhead line height, the increase of pole distance and the decrease of wire equivalent radius, but it still appears at a fixed line length.
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