Abstract
With the increasing numbers of high-voltage direct current transmission (HVDC) projects based on the modular multilevel converter (MMC), the power quality of the grid-connected MMC has drawn attention. By using peripheral filtering equipment to mitigate the background harmonic effect on MMC leads to several numbers of problems, e.g., high costs, difficulty in parameter tuning, the resonance problem, etc. Therefore, this paper focuses on the mitigation of harmonic pollution for the MMC controller without filters. Based on the multi-layer coordinate transformation structures, a novel nested decoupled synchronous reference frame (NDSRF) transformation technique is presented to attain the harmonics compensation in the coordinate transformation process. Besides, analyses of the proposed NDSRF coordinate transformation technique, including harmonics transfer rules and harmonics rejection principles, are further given in the time domain. Furthermore, to reduce the harmonic content of MMC output voltages in case of a power grid with serious harmonic pollution, the harmonic resistant MMC controller based on the NDSRF technique is proposed. Finally, the simulation results are demonstrated to validate the background harmonic suppression function of the harmonic resistant MMC controller based on the NDSRF transformation technique.
Highlights
The modular multilevel converter (MMC) was first introduced in [1]
EXAMPLES AND ANALYSIS To verify the proposed nested decoupled synchronous reference frame (NDSRF) transformation technique and the MMC harmonic resistant controller based on the NDSRF, a detailed 21-level MMC-high-voltage direct current transmission (HVDC) model based on time-domain simulation software PSCAD/EMTDC is established
To further prove that the proposed novel MMC harmonic resistant controller is superior over those based on the conventional synchronous reference frame (SRF) and decoupled double synchronous reference frame transformation technique (DDSRF) techniques in terms of the harmonic distortion mitigation, the spectral analysis comparison of results is shown in Fig.18 as follows
Summary
The modular multilevel converter (MMC) was first introduced in [1]. The MMC stands for the significant technological innovation among the diverse voltage source converters (VSCs), and it has been avowedly identified as an appropriate solution for converters in high-voltage direct current transmission (HVDC) [2]. Under serious harmonic distortion, performances of SRF-PLL and DDSRF-PLL degrade, i.e., there are a large number of ripples in the synchronous frame (d-q) To deal with such issues, a novel nested decoupled synchronous reference frame transformation technique (NDSRF) is proposed in this paper, which can reduce the transfer of background harmonic from stationary frame (abc) to synchronous frame (d-q). Utilizing the multi-layer coordinate transformation structures instead of filters, the NDSRF technique extracts the fundamental frequency positive-sequence component from complex background harmonic voltages relatively accurately, which can mitigate the adverse harmonic effect on MMC controller. By using the transfer rule of the fundamental and harmonic components in different reference frames, the proposed NDSRF coordinate transformation technique mitigates the adverse effect of harmonic pollution while avoiding the disadvantages of filters, for example, higher complexity of parameter tuning, higher production and maintenance costs, the resonance problem, and so on.
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