Yiqiao Liang

Analysis and Control of Modular Multilevel Converters Under Unbalanced Conditions

This paper investigates the contents of submodule voltage ripples, circulating currents, and internal converter voltage in modular multilevel converters (MMCs) theoretically. The operation of MMCs is studied under asymmetry of the upper and lower arm, as well as the unbalanced ac system. In three-phase MMCs, the analysis shows that the second harmonic circulating currents will be asymmetric under an unbalanced ac system and can be decomposed into positive-, negative- and zero-sequence parts. The positive- and negative-sequence components affect neither the ac side nor the dc side of MMCs while the zero-sequence components will flow into the dc side, aggravating the power fluctuations of the dc side. In order to solve this problem, a new controller based on the instantaneous power theory and proportional-resonant scheme is designed. Simulations with a detailed switching model on the PSCAD/EMTDC platform verify the theoretical analysis, and demonstrate that the proposed controller eliminates the active power fluctuations and suppresses the harmonic circulating currents as well.

Energy-balancing Control Strategy for Modular Multilevel Converters Under Submodule Fault Conditions

The modular multilevel converter (MMC) is a newly introduced switch-mode converter topology with the potential for high-voltage and high-power applications. This paper focuses on the design and control methods for fault-tolerant operation using redundant submodules (SMs), which is one of the most important features in the MMC topology. By comparing three design schemes of redundant SMs, the most economic and reliable scheme is identified. In addition, a mathematical model of the MMC with arms containing different numbers of SMs is developed. Based on the identified scheme and mathematical model, an energy-balancing control strategy is proposed to keep the MMC operating normally under SM fault conditions. Finally, time-domain simulations of a 61-level MMC system are performed, using the PSCAD/EMTDC software, while experiments are carried out on a 41-level MMC prototype. The simulation and experimental results validate the design scheme, mathematical model, and proposed energy-balancing control strategy.

A Prototype of Modular Multilevel Converters

This paper introduces a modular multilevel converter (MMC) based prototype for investigating the control of large-scale MMCs. The prototype contains 264 submodules (SMs) totally, which are designed in such a way that they can be rearranged between the half-bridge model and full-bridge model conveniently. Therefore, the characteristics of various typologies based on voltage-sourced converters can be studied. Meanwhile, a novel control system for large-scale MMCs is proposed, which is characterized for modular design and excellent expansibility. The designed control system is composed of one central control unit and several arm control units, while each arm control unit is composed of several valve-group control units. Besides, a new voltage-balancing method composed of internal voltage balance and external voltage balance is proposed to balance the SM voltages. Finally, experiments on a three-phase dc/ac MMC are carried out to verify the prototype and the control system, as well as the new voltage-balancing method. The results show that the proposed control system is feasible and the new voltage-balancing method is valid.

Zero tracking error nearest level modulation for modular multilevel converters

The nearest level modulation is the simplest and most practical method for a modular multilevel converter due to its large number of sub-modules. Traditional nearest level modulation ignores voltage fluctuation of a capacitor, when it calculates the number of inserted sub-modules. This will cause errors between the actual voltage generated by sub-modules and its reference. These errors will lead to the fluctuation of circulating currents and additional losses. This paper proposes a zero tracking error nearest level modulation, which can eliminate the errors. This method is validated by 71 voltage-level modular multilevel converter simulation results.

Analysis of Hybrid Cascaded Multilevel Converter based STATCOM

A new Static Synchronous Compensator (STATCOM) based on the Hybrid Cascaded Multilevel Converter (HCMC-STATCOM) is presented in this paper for the sake of decreasing the number, size and stored energy of the capacitors used in the STATCOM. The HCMC-STATCOM is comprised of one conventional three-phase two-level converter and one Wave-shaping Circuit (WSC) consisting of cascaded H-bridge Sub-modules (SMs). An in-depth analysis and design of the capacitor parameters is done to reduce the cost and footprint of the HCMC-STATCOM. Moreover, a detailed comparison between the HCMC-STATCOM and the conventional Cascaded H-bridge STATCOM (CHB-STATCOM) is made, which proves that the number and stored energy of the capacitors in the HCMC-STATCOM are obviously smaller than that of the CHB-STATCOM. Finally, a 35kV/±100Mvar HCMC-STATCOM mode based on PSCAD/EMTDC is established and then the technical feasibility of the proposed topology and the accuracy of the theoretical analysis are verified.

A control system for large-scale modular multilevel converters

This paper proposes a new control system for the large-scale modular multilevel converters (MMCs). The designed control system, characterized for modular design and excellent expansibility, is composed of one central control unit and several arm control units while each arm control unit can be divided into a few valve-group control units. As traditional voltage balancing method is incompetent for the proposed control system, a new voltage balancing method composed of internal voltage balance and external voltage balance is proposed. Finally, experiments on a three-phase MMC based inverter with 40 sub-modules per arm demonstrate that the proposed control system is feasible and the new voltage balancing method is valid.

Power characteristics analysis of bidirectional full-bridge DC-DC converter with triple-phase-shift control

This paper analyzes the control performances of the triple-phase-shift (TPS) control and the operating mode of bidirectional full-bridge DC-DC converter under this control. Then the corresponding mathematical model of the converter is established and the related power characteristics are also analyzed. Moreover, the related formulas of the converter under TPS are derived for the convenience of comparative analysis between TPS control mode and dual-phase-shift (DPS) control mode. Then the paper points out that triple-phase-shift control not only retains the advantages of dual-phase-shift control, but also shows some other obvious advantages such as a small peak current and RMS current, low reactive power, and high efficiency, etc. At last, simulation base on matlab/simulink verify the excellent performance of the triple-phase-shift control.