Improved two-level voltage source converter for high-voltage direct current transmission systems

Adam, Grain Philip and Abdelsalam, Ibrahim and Fletcher, John Edward and Xu, Lie and Burt, Graeme M. and Holliday, Derrick and Finney, Stephen (2017) Improved two-level voltage source converter for high-voltage direct current transmission systems. IEEE Journal of Emerging and Selected Topics in Power Electronics. ISSN 2168-6777

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    Abstract

    this paper presents an improved two-level voltage source converter for dc transmission systems with relatively low rated power and dc operating voltage. Unlike conventional two-level converter, the presented converter employs two distributed cell capacitors per three-phase; thus, do not contribute any current when converter is blocked during dc short circuit fault as in modular multilevel converter case. The use of three-phase cells is proven to be beneficial because the arm currents do not contain 2nd order harmonic currents, and cell capacitors tend to be small as they only experience high-order harmonic current associated with the switching frequency. For the same rated dc link voltage and switching devices, the rated power of the improved two-level converter will be twice that of the conventional two-level converter. Average, switching function and electromagnetic transient simulation models of the improved two-level converter are discussed and validated against detailed switch model. The viability of the improved two-level converter for HVDC applications is examined, considering dc and ac short circuit faults. Besides, reduced complexity of the control and power circuit of the improved two-level converter, it has been found that its transient responses to ac and dc faults are similar to that of the modular multilevel converter.