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The existing power decoupling control method of modular multilevel converter (MMC) was developed from that of conventional two-level voltage source converters. Therefore, it cannot fully realize power decoupling due to neglecting the coupling effects from submodule capacitor voltage, dc current, circulating current as well as control strategies of circulating current, which inevitably results in inaccurate regulation of active and reactive power, deteriorated system dynamic performance, possible overload operation and eventually threatens the safety and stability of MMC. This paper analyzes the intercoupling among all electrical quantities and corresponding control strategies in MMC. An accurate control model of active and reactive power/current with four coupling paths and influence factors in the rotating frame is also established. A full power decoupling control method has been proposed based on the developed model. The proposed control method can realize accurate and completely decoupled active and reactive power regulation, avoid overload operation, and significantly improve the dynamic performance of MMC. Moreover, it facilitates the parameters selection of MMC controller with no extra cost, which is of great significance for practical projects. The effectiveness and accuracy of the proposed analysis and control methods were verified by both simulation and experimental results. [ABSTRACT FROM AUTHOR]

Modular multilevel converters (MMCs) are multi-input multi-output (MIMO) nonlinear systems. The control systems for MMCs are required to simultaneously achieve multiple control objectives, e.g., output current regulation, submodule capacitor voltage control, and circulating ripple currents suppression. Existing cascaded control strategies for MMCs achieve those control objectives with relatively complex controllers, and the controller parameter design is normally difficult for such nonlinear systems with highly coupled states. In view of this, a feedback linearization-based current control strategy is proposed for an MMC system in this paper. The nonlinear state function model of the MMC is presented and transformed to a linearized and decoupled form with the help of the input–output feedback linearization technique. Based on the linearized system, simple linear controllers are employed to regulate the output and inner differential currents of the MMC, which significantly reduces the difficulty in controller design. The stability of the proposed control strategy is analyzed. The experimental verification results show that, compared to the conventional cascaded control strategies for MMCs, the proposed feedback linearization control strategy is able to achieve improved steady-state and dynamic performances. The robustness of the proposed control strategy against parametric uncertainties is experimentally investigated. [ABSTRACT FROM PUBLISHER]

Modularity and high reliability from redundancy are the two attractive advantages of modular multilevel converters (MMCs). This paper elaborates a switch open-circuit fault diagnosis and a fault-tolerant operation scheme for MMCs with distributed control. The proposed fault diagnosis and fault-tolerant control method can significantly improve the reliability of the MMC while maintaining the modularity of its software implementation. By distributing fault diagnosis into submodules, its local controller is capable of identifying the switching devices in open-circuit fault without extra hardware circuitry. Based on the real-time measurements of submodule terminal voltage and arm current, single, or multiple faulty switches can be identified within 3.5 ms without triggering faulty alarms. Furthermore, a new fault-tolerant operation is proposed to maintain the output current, internal dynamics, and switching harmonics unchanged after the faulty submodule is bypassed. This is achieved by resetting the period and phase registers in the local controller according to the information of bypassed submodules. The control loops of the MMC are not influenced by the proposed fault diagnosis and fault-tolerant operation, making the operation transition seamless and reliable. Experimental results show that fault identification and system reconfiguration can be completed within 5 ms, and the MMC can seamlessly and smooth ride through the switch open-circuit faults without severe malfunction and catastrophic damages. [ABSTRACT FROM AUTHOR]

Conventional centralized control strategies may reduce the flexibility and expandability of a modular multilevel converter (MMC) system. To tackle this issue, this paper proposes a distributed control architecture that is capable of assigning certain control tasks to distributed local controllers and improves the modularity of an MMC system. A central controller dealing with the output current regulation based on sensed arm currents is adopted. The control of MMC internal dynamics and the pulsewidth modulation (PWM) generation are distributed into local controllers. Unlike the conventional MMC control that needs all submodule capacitor voltages for capacitor voltage averaging, the proposed capacitor voltage control only relies on local submodule voltage measurement. Consequently, communication-intensive capacitor voltage transmission in each control cycle is not required and the communication burden of the control system can be significantly reduced. The control loops and possible control conflicts among submodules are presented and considered for system stability analysis. The effectiveness of the proposed distributed control architecture and capacitor voltage control for an MMC are confirmed by the start-up, steady state, and transient experimental results. [ABSTRACT FROM PUBLISHER]

Modular multilevel converters (MMCs) in high-voltage dc applications usually adopt a distributed control architecture to manage a large number of submodules (SMs) through a communication network. The communication congestion and network disconnection might lead to communication interruption (CI) and eventually cause the system to malfunction. In this article, a resilient operation strategy is proposed and studied to ride-through the CI fault, in order to prevent frequent fault SM bypassing, replacement, or even system shutdown. The analysis of the MMC distributed control system with the presence of CI indicates that the insertion index of the faulted SM might become constant, which distorts the output current and results in overvoltage of the communication interrupted SM (CI-SM). The CI-SM capacitor voltage prediction can be used to determine the MMC safe operation period after CI occurs. During the safe operation period, the CI-SM power balance is sustained by utilizing prestored phase signals to generate a sinusoidal insertion index according to its capacitor voltage tracking error. Two operation modes are proposed and analyzed to ensure the MMC stable operation under various conditions. The system protection is sensibly used only if the CI duration exceeds a safe operation period, which avoids frequent SM cut-off. Good agreement of the CI-SM capacitor voltage is achieved between the theoretical and simulation results. The effectiveness and robustness of the proposed MMC resilient operation are experimentally confirmed. [ABSTRACT FROM AUTHOR]

Indirect matrix converter (IMC) is an alternative for ac/ac energy conversion, usually operated with a voltage stepped-down gain of only 0.866. For applications like distribution generation where voltage-boost functionality is required, the traditional style of operating the IMC is therefore not appropriate. Like most power converters, the operation of the IMC can surely be reversed to produce a boosted gain, but so far its relevant control principles have not been discussed. These challenges are now addressed in this paper with distributed generation suggested as a potential application. Simulation and experimental results for validating various performance aspects of the proposed control schemes can be found in a later section of this paper. [ABSTRACT FROM AUTHOR]

This paper introduces a novel H-bridge structured ac–ac pulsewidth modulation (PWM) converter topology based on two-level non-differential ac chopper legs, which can work at the non-inverting and inverting modes for the utility voltage compensation. Compared to the traditional bipolar-type H-bridge ac–ac converter, no bidirectional switches are switched in a complementary manner to overcome the commutation problem due to the delayed response of electronic circuits and semiconductor switching devices. The detailed PWM control signals and operational principles are presented to regulate the output voltage in a bipolar manner. Especially the main advantage is that the proposed ac–ac converter has the same buck/boost operation process for non-inverting and inverting modes, which ensures the continuously average current supply to the low-voltage output side without a high-value capacitor to support power. Additionally, due to the common sharing ground of the input and output, the feature that output can reverse or maintain phase angle with input is supported well. Then, the detailed analysis, design conditions, and experimental verification based on a 1 kW experimental prototype are presented. [ABSTRACT FROM AUTHOR]