Abstract
MMC (Modular Multilevel Converter) is the preferred topology for renewable energy delivery, but renewable energy resources, such as wind farms, contains much harmonic components. Compared with two-level VSC (Voltage Source Converter), the internal dynamic characteristics of MMC have great impact on the harmonic transmission. This paper quantitatively analyzes the harmonic transferring mechanism from one side to the other side of MMC, and deduces the time-domain analytical expression of the harmonic response on the MMC’s other side. Firstly, the influence of background harmonic on MMC’s control system and electrical system are quantitatively analyzed. On this basis, the equivalent circuit of MMC’s electrical system considering its internal dynamics is established to accurately calculate the effect of background harmonic on the MMC’s other side. In addition, the harmonic transferring impedance in frequency-domain is also defined in this paper, based on that the influence of background harmonic on the MMC’s other side can be calculated quickly. According to the harmonic transferring characteristics analyzed above, a zero-sequence virtual damping strategy is proposed for the first time to suppress the background harmonic transferring. Finally, a 232-level 500kV MMC simulated model is built in MATLAB/Simulink to verify the accuracy of the harmonic transferring calculation method and the effectiveness of the virtual damping suppression strategy.
Highlights
Modular multilevel converter (MMC) has the advantages of easy expansion, which can effectively implement the electrical energy transformation under high voltage level [1], [2]
Compared with traditional VSC topologies, in MMC, its arm voltage will play a key role in the harmonic transferring process, but it has not been studied in detail
The derivation process is complicated and the coupling relationship between the background harmonic and the MMC’s steady-state electrical quantities is not considered, it cannot be used for quantitative analysis of harmonic transferring characteristics
Summary
Modular multilevel converter (MMC) has the advantages of easy expansion, which can effectively implement the electrical energy transformation under high voltage level [1], [2]. Compared with traditional VSC topologies, in MMC, its arm voltage will play a key role in the harmonic transferring process, but it has not been studied in detail. The derivation process is complicated and the coupling relationship between the background harmonic and the MMC’s steady-state electrical quantities is not considered, it cannot be used for quantitative analysis of harmonic transferring characteristics. The influence of background harmonic on MMC’s control system is quantitatively analyzed in time domain, the coupling relationship between its control system and the electrical system is deduced. The influence of background harmonic on MMC’s other side can be quickly calculated by transfer impedance compared with the time-domain method.
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