Suppression of Reactive Power in Isolated Modular Multilevel DC–DC Converter Under Quasi Square-Wave Modulation

Abstract

An isolated modular multilevel dc-dc converter (IMMDCC) consisting of two modular multilevel converters (MMCs) coupled via a medium-frequency transformer is competitive for medium-and high-voltage applications because of its high modularity and high-voltage handling capability. Employing the quasi square-wave modulation (QSM) scheme for MMCs of the converter can increase the dc-link voltage utilization ratio and realize soft-switching operation, compared with sinusoidal wave modulation methods. However, the accurate reactive power control at the ac side of the IMMDCC can hardly be achieved under the QSM mode. The ac-side reactive power and current stress increase considerably when the conversion ratio deviates from the transformer turn ratio. In order to address this issue, this paper presents an arm voltage decoupling control scheme for the IMMDCC by introducing a virtual buck/boost conversion stage between the dc-side voltage and the transformer voltage. The transformer primary and secondary voltages are kept matched to suppress the reactive power when the dc-side voltage varies. The theoretical analysis shows that the proposed control enhances the power transfer capability and reduces the ac-side current stress of the IMMDCC. In addition, submodule voltages are maintained even when the dc voltages exceed the designed value under the conventional control scheme, which implies that the voltage range of the converter is expanded. The simulation and experimental results confirm the effectiveness of the proposed modulation and control strategy. INDEX TERMS DC-DC power conversion, modular multilevel converter, reactive power suppression.

Transient Oscillation Suppression Method of Modular Multilevel DC Transformer

“…where Ts is the switching period and <uca_sum>Ts and <icma>Ts represent the averaged value of uca_sum(t) and icma(t) in each switching period in the phase leg a, respectively. Based on the traditional 50% square wave modulation [18], the relationship between the system-level and the SM-level voltages and currents are able to be established, as depicted in Fig. 3.…”

“…Their corresponding waveforms are presented with the solid lines in comparison with the waveforms using the conventional 50% square wave modulation with dotted lines. Note the small inter-SM phase shift angles [18] for reducing dv/dt stress across the transformer are neglected here for brevity.…”

“…In order to further improve the MMDCT system damping and accelerate the oscillation attenuation rate, a closed loop control method for suppressing oscillation is required. Since the mathematical model is nonlinear as per (18) when the OSA δd is seen as an input variable, a small signal model that describes the linear relationship between the input signal δd and the state variable (seen as the output variable) needs to This work is licensed under a Creative Commons Attribution 4.0 License. For more information, see https://creativecommons.org/licenses/by/4.0/ This article has been accepted for publication in a future issue of this journal, but has not been fully edited.…”

“…These strategies are inapplicable for the MMDCT scenario because the DC transformer typically adopts the square wave modulation for achieving higher DC voltage utilization and smaller lower frequency energy ripples [10] [11]. Therefore, because they do not need to buffer low-frequency power ripples, the specific characteristics of the RLC series configuration in MMDCT are more worthy of attention since the submodule capacitors in MMDCT are usually designed smaller than those in traditional MMCs [18] [19]. The smaller submodule capacitance value increases the possibility of oscillation, which seriously threatens the stability of the system.…”

Adaptive Modulation Strategy for Modular Multilevel High-Frequency DC Transformer in DC Distribution Networks

“…However, it may suffer the dv/dt stress problem since it produces a destructive dv/dt stress upon HFL transformer where DC voltage instantly varies, especially in HVDC grids [28]. To solve the dv/dt stress problem and maintain high DC voltage utilization, the quasi-square-wave modulation generating the quasi-square HFL voltages with staircase rising and falling transitions of controllable time is proposed, and it becomes an effective modulation scheme for MDCT in the MVDC and HVDC occasions [29], [30]. Although the terms of staircase rising and falling transition in quasi-square-wave method is small compared with the phase-shift angle between primary and secondary HFL voltages, it dose reduce the DC voltage utilization and degrade power transmission capability of DC transformers [31], [32].…”