Harmonic Analysis of Modular Multilevel Matrix Converter for Fractional Frequency Transmission System

Abstract

Fractional Frequency Transmission is a competitive technology for offshore wind power transmission. Modular Multilevel Matrix Converter (M 3 C) plays a key role in a Fractional Frequency Transmission System (FFTS) as the frequency changer. M 3 C is broadly considered as the AC-AC converter for the future with its attractive advantages in high voltage and high power applications. Due to the lack of a DC link, electrical quantities at different frequencies from two AC systems couple in M 3 C, resulting in a complex harmonic condition. Harmonics can lead to stability issues and its analysis is of great importance. This paper focuses on the harmonic analysis of M 3 C. The arm capacitor voltage ripples and the harmonic currents are analyzed at various frequencies. Major factors influencing the harmonics magnitude are discussed. Analysis is conducted on sub-module capacitance and arm inductance selection. A zero-sequence current mitigation controller for M 3 C is implemented and tested. It is found that for a FFTS, some current harmonics flow into AC systems even though the system is balanced, while the others circulate within the converter. The theoretical harmonic analysis is verified by simulations in Real Time Digital Simulator (RTDS) of a M 3 C system where each arm consists of forty sub-modules. Index Terms-Fractional frequency transmission system, harmonic analysis, modular multilevel matrix converter (M 3 C), offshore wind power, energy storage. I. INTRODUCTION U NDER the pressure of fossil energy depletion and environmental pollution, renewable energy development has drawn worldwide attention. Offshore wind power is favored with merits of not taking up land in cities, rich and stable resource and suitability for large-scale development. Three main offshore Manuscript

An Overview of Modelling Techniques and Control Strategies for Modular Multilevel Matrix Converters

“…All the previously discussed research areas are also complemented by the analysis of the M 3 C stability, which could be modified depending on the estimation methods, the control structure [62] or the interaction with the system connected with the M 3 C. However, only a few works have studied M 3 C stability. For example, the M 3 C stability is analysed in [90], considering the effects of the current harmonics in the CCVs. In this paper, a zero-sequence current mitigation controller is proposed and tested by the authors to enhance system stability.…”

“…In this paper, a zero-sequence current mitigation controller is proposed and tested by the authors to enhance system stability. Additionally, the tuning of this controller and its influence in the system performance is also addressed in [90]. The application of the M 3 C for wind energy application is considered in [91].…”

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“…AC/AC converter is undoubtedly the most important equipment in low‐frequency AC transmission. In order to realize the purpose of AC/AC conversion, 6,8‐11 two back‐to‐back MMC systems in Reference 8 can be used, but this method will increase the volume and cost of the equipment. To this end, Erickson R. and AI‐Naseem O. of the University of Colorado in the United States proposed a new cascaded H‐bridge AC/AC converter in 2001, namely, Modular Multilevel Matrix Converter 9 .…”

“…In order to realize the purpose of AC/AC conversion, 6,8‐11 two back‐to‐back MMC systems in Reference 8 can be used, but this method will increase the volume and cost of the equipment. To this end, Erickson R. and AI‐Naseem O. of the University of Colorado in the United States proposed a new cascaded H‐bridge AC/AC converter in 2001, namely, Modular Multilevel Matrix Converter 9 . M3C is a bidirectional switch composed of many H‐bridge units connected in series, which is not a semiconductor switching device in traditional matrix converter.…”

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“…Thus, the steady-state capacitor voltage ripples are ignored in its admittance modeling. However, for other cases with different parameters, the dynamics of capacitor voltage ripples and current harmonic components can have significant impact on the stability of an AC/AC MMC system [20]. Therefore, it is significant to develop an impedance model of AC/AC MMC considering its steady-state harmonic components to accurately assess the stability of the AC/AC MMC-based railway system.…”