The modular multilevel converter (MMC) has been a subject of increasing importance for medium/high-power energy conversion systems. Over the past few years, significant research has been done to address the technical challenges associated with the operation and control of the MMC. In this paper, a general overview of the basics of operation of the MMC along with its control challenges are discussed, and a review of state-of-the-art control strategies and trends is presented. Finally, the applications of the MMC and their challenges are highlighted.

Operation, Control, and Applications of the Modular Multilevel Converter: A Review

This paper describes a modified phase-shifted carrier-based pulsewidth-modulation (PSC-PWM) scheme for modular multilevel converters (MMC). In order to reduce the average device switching frequency, a reduced switching-frequency (RSF) voltage balancing algorithm is developed. This paper also proposes a circulating current suppressing controller (CCSC) to minimize the inner circulating current in an MMC. Based on the double line-frequency, negative-sequence rotational frame, the three-phase alternative circulating currents are decomposed into two dc components and are minimized by a pair of proportional integral controllers. Simulation results based on a detailed PSCAD/EMTDC model prove the effectiveness of the modified PSC-PWM method and the RSF voltage-balancing algorithm. The proposed CCSC not only eliminates the inner circulating current but also improves the quality of the converter ac output voltage. A simple loss evaluation demonstrates that the RSF voltage-balancing algorithm and the CCSC reduce the converter power losses.

Reduced Switching-Frequency Modulation and Circulating Current Suppression for Modular Multilevel Converters

Modular multilevel converters (MMC) are considered a top converter alternative for voltage-source converter (VSC) high-voltage, direct current (HVDC) applications. Main circuit design and converter performance evaluation are always important issues to consider before installing a VSC-HVDC system. Investigation into a steady-state analysis method for an MMC-based VSC-HVDC system is necessary. This paper finds a circular interaction among the electrical quantities in an MMC. Through this circular interaction, a key equation can be established to solve the unknown circulating current. A new steady-state model is developed to simply and accurately describe the explicit analytical expressions for various voltage and current quantities in an MMC. The accuracy of the expressions is improved by the consideration of the circulating current when deriving all the analytical expressions. The model's simplicity is demonstrated by having only one key equation to solve. Based on the analytical expressions for the arm voltages, the equivalent circuits for MMC are proposed to improve the current understanding of the operation of MMC. The feasibility and accuracy of the proposed method are verified by comparing its results with the simulation and experimental results.

A Steady-State Analysis Method for a Modular Multilevel Converter

Modular multilevel converter (MMC) is considered as a promising topology for voltage-source converter (VSC) high-voltage, direct current (HVDC) applications. This paper presents a new control strategy for MMC-HVDC under unbalanced grid conditions. First, a new inner loop current control strategy based on nonideal proportional resonant (PR) controllers in stationary αβ frame is designed, which is more concise compared to the existing dual sequence current control scheme. Second, an analytical expression for circulating current is obtained which shows that the circulating currents will be asymmetric under unbalanced grid conditions and can be decomposed into positive-, negative-, and zero-sequence component. In order to suppress all these components, a new circulating current suppressing strategy is analyzed and designed also based on nonideal PR controllers. Application of nonideal PR controllers makes the control system well adapt to the fluctuation of grid frequency. The effectiveness of the proposed control strategy is verified through a simulation case of a 251-level MMC-HVDC transmission system using real-time digital simulator.

Circulating Current Suppressing Strategy for MMC-HVDC Based on Nonideal Proportional Resonant Controllers Under Unbalanced Grid Conditions

An improved circulating current control method by applying a digital plug-in repetitive controller is discussed for harmonic elimination of a carrier-phase-shift pulse-width-modulation (CPS-PWM)-based modular multilevel converter (MMC) in this paper. The performance of the controller is analyzed in detail based on an improved circulating current control model with the consideration of the submodule voltage disturbance. It is shown that the proposed control method has the merits of simplicity, versatility, and better performance of circulating harmonic current elimination than the traditional proportional integral controller. The stability analysis of the proposed method are also discussed in the paper as well as the design principles. Finally, the experimental results including the steady-state performance and the transient response are given, which validates the feasibility and excellent performance of the proposed control scheme.

Circulating Harmonic Current Elimination of a CPS-PWM-Based Modular Multilevel Converter With a Plug-In Repetitive Controller

This paper describes two distinctive functions of the arm inductor in the modular multilevel converters (MMC): suppressing the circulating current and limiting the fault current rise rate. Based on the equality of the instantaneous power, the characteristics of the circulating current are analyzed and demonstrated that it flows through the three phase of the converter in negative sequence at double-fundamental frequency. The relationship between the amplitude of the circulating current and the value of the arm inductor is discussed, and the first principle to design the parameters is proposed. The second principle to design the parameters is based on the calculation of the fault current rise rates, with different values of the arm inductors. A detailed PSCAD/EMTDC model of MMC is developed. Steady state simulation proves the existence of the circulating current at double fundamental frequency. Simulation results show a close agreement with the calculation of the circulating current, with different arm inductor parameters. The fault between the DC terminals of the converter is investigated; the results verify the above parameter design principles of the arm inductor.

Parameter design principle of the arm inductor in modular multilevel converter based HVDC

With the development of China's power system, more and more HVDC links are located in close proximity in a common AC system and thus multi-infeed HVDC systems are developed. Due to the interaction between AC system and DC systems, as well as the interaction amongst DC systems, the behavior of such multi-infeed HVDC system is much more complicated than a single-infeed HVDC system. This paper investigates into an example of such multi-infeed HVDC systems, i.e. Shanghai Power Grid. There are three HVDC links terminating in the load center of Shanghai Power Grid by 2010. The inverter stations of the three HVDC links are near to each other and thus the interaction of AC and DC systems is very strong and has a great impact on the secure and stable operation of the system. Transient stability under various faults, as well as the effect of load characteristics and spinning reserve, are studied in this paper. Conclusions are obtained according to the simulation results, which provide useful instruction for the operation of Shanghai Power Grid and other similar multi-infeed HVDC systems.

Transient stability analysis of Shanghai Power Grid with multiple HVDC links

The multi-infeed HVDC problems are the limiting factors for the application of the dc technology in the long-distance bulk-power transmission in China. Thus, this paper presents two new techniques to solve these problems: one is the grid dynamic segmentation technique and the other is the VSC-HVDC technique. Firstly, two aspects of the multi-infeed HVDC problem, i.e. concurrent commutation failure caused by ac short-circuit faults and power flow shift caused by bipolar faults of dc systems, are revealed based on the analysis of their physical mechanisms. Then, the large-impedance fault current limiter is used to fulfill the grid dynamic segmentation, which proves to be effective in solving the concurrent commutation failure problem. After that, the VSC-HVDC technique is used to eliminate both the problems with the multi-infeed HVDC system. Some important issues with the realization of the bulk-power VSC-HVDC overhead transmission, like the grounding mode selection, the bulk-power assembly converter, and the dc line fault self-clearing capability are also discussed and solved.

Two new techniques to solve the problems with multi-infeed HVDC systems

The Internet of Things (IoT) is an information network that connects gadgets and sensors to allow new autonomous tasks. The Industrial Internet of Things (IIoT) refers to the integration of IoT with industrial applications. Some vital infrastructures, such as water delivery networks, use IIoT. The scattered topology of IIoT and resource limits of edge computing provide new difficulties to traditional data storage, transport, and security protection with the rapid expansion of the IIoT. In this paper, a recovery mechanism to recover the edge network failure is proposed by considering repair cost and computational demands. The NP-hard problem was divided into interdependent major and minor problems that could be solved in polynomial time by using the Benders decomposition technique and cutting plane approximation. To ensure the nonincreasing character of the Benders upper limit, a local branching method was also added to improve the convergence. Simulation results indicated that the proposed method is superior to the existing method and has better overall performance.

An Optimized Approach for Industrial IoT Based on Edge Computing

It is of great significance to study the diffusion reaction and dynamic operation characteristics of solid oxide electrolytic cell (SOEC) by establishing physical model. Based on one dimensional mass transfer model in porous media and one-dimensional flow model, the dynamic SOEC diffusion-reaction model is established. According to the experimental results and parameters, numerical method is used to calculate the current density and water vapor concentration distribution. The influence of operating conditions, such as water vapor concentration and electrolysis voltage, are also explored. The results show that the current distribution is concentrated in the distance of 100 $\mu \mathrm{m}$ away from electrolyte layer, and the enhancement of oxygen ionic conductivity in cathode can benefit the current density and widen the current distribution. Strengthening the cathode porous structure can improve the current density of SOEC. The feed steam fluctuations lead to the performance fluctuation of SOEC, which can reach a new equilibrium within 0.02 s after changing the operating conditions. The SOEC exhibits fast responsiveness when considering the tolerance of the structure materials, and the mass diffusion cannot be ignored in the pores.

Hongyang Huang

Papers

Parameter design principle of the arm inductor in modular multilevel converter based HVDC

Transient stability analysis of Shanghai Power Grid with multiple HVDC links

Two new techniques to solve the problems with multi-infeed HVDC systems

An Optimized Approach for Industrial IoT Based on Edge Computing

Diffusion-reaction model and electrolysis dynamic characteristics of electrode surface and channel in solid oxide electrolytic cell