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Mission profile emulation (MPE) for submodules (SMs) in the modular multilevel converter (MMC) is a crucial step before a full-scale system is put into field operation. Existing methods suffer from limitations of the testing accuracy due to the staircase-like arm voltages that result in distorted loading current and require large filter inductance. This is especially the case when a limited number of SMs are under test. In this article, an efficient MPE method is proposed to emulate the loading behaviors of multiple SMs in MMC. The testing circuit allows two arms of MMC with flexible SM number, operating in inverting and rectifying mode under nearest level modulation, to be tested simultaneously with reduced dc supply voltage. A common H-bridge circuit is introduced in the testing circuit, with each arm of the H-bridge circuit functioning as the voltage compensator by using feedforward control. In this way, the current distortion can be significantly suppressed, and testing accuracy can be guaranteed in a cost-efficient manner even though no large inductor is included in the testing circuit. The proposed method allows SMs under test to be loaded as in the actual back-to-back MMC system. Simulations and experimental measurements are given to validate the proposed method. [ABSTRACT FROM AUTHOR]

In this article, a modified LLC resonant converter with LC antiresonant circuit in parallel branch is proposed. The proposed converter is derived by placing LC antiresonant circuit on parallel branch. In comparison with LLC resonant converter, the equivalent inductance of parallel branch of the proposed converter decreases along with switching frequency within regulating frequency range. This advantageous property contributes for that the turn-off current and circulating conduction loss in the primary side can be reduced at unity voltage conversion ratio due to large impedance of LC antiresonant circuit. Moreover, by optimizing resonant parameters design of parallel branch based on time-domain analysis, the magnetizing current is in close proximity to a sine wave and the conduction loss in the secondary side will be reduced, especially in the light load. A higher voltage conversion ratio can be achieved at low switching frequency due to decreased impedance in parallel branch. In addition, a design flowchart and a design practical case are given to help to implement an efficiency optimization algorithm to decide five resonant design parameters. Finally, a 2 kW 250 V—400 V-input and 48 V-output prototype is built, and the main experimental results are provided to verify the feasibility of the proposed converter and compare with LLC resonant converter. Comparative state-plane trajectories are depicted to compare their characteristics and to verify the model-domain analysis. [ABSTRACT FROM AUTHOR]

Modular multilevel converter (MMC) is one of the most promising converter topologies for medium/high-voltage applications, where the nearest level modulation (NLM) is widely applied. However, the capacitor voltage balancing methods under this modulation mode exist some inefficient actions, which are useless for capacitor voltage balancing but lead to unnecessary switching of power semiconductor devices. In this article, the disturbance and unbalance mechanisms of capacitor voltages in MMC under NLM are analytically revealed and solved. The disturbing metrics for capacitor voltage are digitally extracted to identify the accurate time interval in which the switching actions have immediate effects on capacitor voltage balancing. Then, a demand-oriented scaling factor is introduced to adjust the balancing strength by changing the range of the time intervals which are highly efficient with respect to capacitor voltage balancing, thus flexible tradeoff between the switching frequency of power semiconductor devices and the balancing level of capacitor voltages can be achieved. Finally, a dedicated mission-profile-emulator for the testing of MMC is designed and built with comprehensive validations to verify the effectiveness of the proposed method. [ABSTRACT FROM AUTHOR]

• A decoupled mathematical model based on inner electromotive force (EMF) comprises comprehensive state variables of the MMC considering AC grid normal and asymmetric conditions are developed. • Considering that the established model can decouple the DC inner EMF from the SM-capacitor voltages, a novel DC-side voltage control technique is proposed to improve the voltage behavior. • The energy-based control method, which is only suitable for the PCMMC or VCMMC under short or medium DC transmission line conditions, is improved and extended to the entire HVDC system. Thus, both VCMMC and PCMMC can achieve full control of their inner dynamics regardless of DC line length. The interconnection of two non-synchronous AC grids is one of the promising applications in the modular multilevel converter (MMC) based high-voltage direct-current transmission system (MMC-HVDC). Due to the complicated internal dynamics of the MMC, energy-based control is becoming an emerging technique to guarantee the robust stability of the MMC. However, conventional energy-based control cannot be applied simultaneously to the power-controlled MMC (PCMMC) and voltage-controlled MMC (VCMMC) interconnected by short or medium DC transmission lines. To address this problem, this paper proposes an improved energy-based control for the MMC-HVDC system aiming to achieve full control of the inner dynamics of MMCs regardless of the DC line length. A completed decoupled mathematical modeling with comprehensive state variables of MMCs is established. Then, a unified control framework for the PCMMC and the VCMMC considering AC grid conditions is developed. A distinct advantage of the scheme is that direct and fast regulation of the DC-side voltage can be achieved to improve the voltage behavior. The other is that the dynamic performance of the submodule capacitor voltages of VCMMC and PCMMC can be conspicuously improved under normal, AC system fault and arm parameter asymmetry conditions. Comparative simulations and hardware-in-loop (HIL) experimental results validate the effectiveness of the proposed methodology. [ABSTRACT FROM AUTHOR]

Modular multilevel converter (MMC)-based solid-state transformers (M-SSTs) show promise in hybrid ac/dc distribution grids where there is need for a medium-voltage dc (MVdc) interface. It can be used to interconnect several distribution networks with different voltage forms and levels and enable flexible power flow among them. In an M-SST, the MVdc short-circuit fault is a critical issue due to the rapidity of the fault current, which can cause device damage and power failure of the low-voltage (LV) grids. Aiming at addressing this issue, this article proposes a fault-mode control strategy applied to a hybrid M-SST topology for uninterrupted power supply of its LV ports. The topology consists of a hybrid MMC with both half- and full-bridge submodules and isolated bidirectional dc–dc converters. Through the proposed control, the fault current is eliminated when an MVdc short-circuit fault occurs. The hybrid M-SST can keep uninterrupted power interaction between the MVac port and the LVdc port during the fault, thereby improving power supply reliability. Moreover, an improved capacitor voltage balance control and an optimized modulation scheme are developed and included in the uninterrupted operation scheme to maintain the harmonic performance of the ac port. The feasibility and effectiveness of the proposed topology and control are verified by simulation and experimental results. [ABSTRACT FROM AUTHOR]

Recent studies have shown that the grid connection of offshore wind farms through modular multilevel converter–high voltage direct current (MMC-HVdc) transmission system has brought low inertia issue. It is an urgent requirement for offshore wind farms to compensate for the inertia. After analysis and evaluation in this article, it is found that integrating an ultracapacitor energy storage system (UESS) into an MMC has an advantage comprehensively considering cost, size, and dynamic. Based on the evaluation, this article proposes an approach to inertial compensation of HVdc offshore wind farms by MMC-UESS integration. An ultracapacitor is selected as an energy storage element (ESE) for its high power density since the inertial response is a short-term response and it is hoped that added ESE does not significantly increase the size of an MMC submodule. Batteries with high energy density but low power density are not suitable for such applications. The optimal operating voltage of a UESS and the control scheme between MMC loops and inertial emulation are proposed in this article. Experimental results from real-time simulation and a downscaled prototype are presented to verify the effectiveness of the proposed method. [ABSTRACT FROM AUTHOR]

Ring and meshed dc distribution grids have attracted increasing interest lately. Their multipoint and multiterminal structure facilitates flexible operational control and reliable power supply across the network. However, the dc grids implemented with existing power converters lack sufficient control freedoms for proper power flow adjustment; therefore, it is necessary to develop additional dc power flow controllers (DCPFCs) to be added to these grids for improved power flow control capability. Specific to multiterminal dc networks built with modular multilevel converters (MMCs), this article proposes an embedded DCPFC (e-DCPFC) directly integrated in the MMC topology. Unlike previously proposed solutions, the e-DCPFC configuration is derived from the conventional MMC and does not need any isolation transformers or external power supply. Its other main features include modular construction, convenience for multiport extension and bidirectional power flow adjustment capability. In this article, the topology, working principle, and the coordinated control strategy for e-DCPFC and its host MMC are elaborated. Experimental results from a lab prototype verify the proposed topology and its performance under typical working conditions. [ABSTRACT FROM AUTHOR]

The grid-connected converters are becoming a fundamental building block in the modern power grid, and the stability of grid-connected converters, especially when they are connected to weak grid, has drawn increasing attention. In order to test the interaction with various grid impedances and validate the stability behaviors, more accurate and advanced emulation of grid impedance is becoming an emerging need. However, the existing approaches are still difficult to flexibly shape the grid impedance under medium-to-high frequency range. In this article, a novel configuration for programable impedance emulation of power grid is proposed. The proposed grid emulator is composed of two power electronics converters with special control functions, i.e., the impedance forming converter (IFC) and power supporting converter (PSC). The IFC is designed with a higher switching frequency to virtually emulate different grid impedances under medium-to-high frequency band. On the other hand, the PSC is designed with a lower switching frequency to generate/absorb the power of the converter under test. Besides, a Norton-circuit-based virtual impedance control strategy is introduced to eliminate the derivation term in the control structure of virtual impedance. Finally, the performance of the proposed emulator is verified by various simulations and experimental prototype. [ABSTRACT FROM AUTHOR]

This article proposed the concept of monopolar fault reconfiguration for bipolar dc distribution system realized with Bipolar Half Bridge (BiHB) converter. Monopolar fault reconfiguration oriented from bipolar high voltage direct current (HVdc) transmission grid. For dc distribution, the operation aim is to provide uninterrupted power supply to critical load under monopolar fault. It requires interfacing circuit with fault-tolerant capability as well as control design considering both bipolar and monopolar working mode. The proposed BiHB converter achieves mode switching by reconfiguring the converter switching pattern. Considering both monopolar and bipolar working mode, a bipolar modulation method is proposed for coordinate circuit design. Key influence factors relating to the transient voltage spikes have been identified, including the fault detection delay and the modulation method. Besides, to suppress the voltage spikes caused by monopolar fault, feed-forward compensation is adopted for smooth mode transition. The proposed BiHB converter and control strategy have been comparatively evaluated under fault protection and restoration. Uninterrupted power supply of load has been attained, in validation of the proposed monopolar fault reconfiguration concept. [ABSTRACT FROM AUTHOR]

A bleeding resistor is usually adopted to be parallel with the capacitor in the submodule (SM) to avoid static voltage imbalance caused by auxiliary power supply during start-up process of modular multilevel converters (MMCs). However, the bleeding resistor causes unnecessary power loss during normal operation. In this article, a resistor controllable SM based on a normally-on silicon carbide (SiC) junction field-effect transistor is proposed to reduce the power loss on bleeding resistor at normal operation of MMCs. The proposed control can not only ensure the static voltage balance during start-up of MMCs, but can also completely eliminate the power loss on bleeding resistor during normal operation of MMCs for high efficiency. In addition, the proposed control can shorten the SM capacitor discharging time at shut-down process. Experimental studies are conducted and the results confirm the effectiveness of the proposed control. [ABSTRACT FROM AUTHOR]

Modular multilevel converter (MMC) based solid state transformer (M-SST) shows promise in hybrid ac/dc distribution grids. To further meet low voltage dc (LVdc) users’ requirements, a bipolar-type M-SST (BM-SST) is developed in this article, which not only has medium voltage ac and dc ports but also directly provides a bipolar low voltage dc (BLVdc) port for various LVdc sources and loads such that an extra SST or voltage balancer for achieving similar outcomes could be saved. Featuring various ac and dc ports, easy voltage conversion, and flexible power control capability, BM-SST is most suitable for multiple ac and dc grids interconnection as an energy hub. The most crucial issue of BM-SST is how to deal with the unbalanced power caused by the bipolar LVdc port which leads to submodule capacitor voltage deviation and uncontrollable circulating current between upper and lower arms of MMC. An arm voltage regulation (AVR) combing with active circulating current injection (ACCI) method is proposed in this article to eliminate the unbalanced power and its negative effects. Based on detailed circuit analysis of the BM-SST, the coordinated control of the BM-SST with optimum AVR&ACCI is designed. Moreover, the effects of AVR&ACCI method on current stress, voltage stress, and power loss of BM-SST are evaluated. The feasibility of the proposed BM-SST has been verified by experimental results. [ABSTRACT FROM AUTHOR]

The mission profile emulator (MPE) proves to be an effective and cost-efficient approach to validate the design and reliability of the submodule (SM) in a modular multilevel converter (MMC). This article established the detailed models for the capacitor voltages of SM in the MMC system. The factors that can determine the dc component of capacitor voltages in SM of MMC are analytically solved. The difference between the capacitor voltage of SM in a complete MMC system and that of SM in MPE is discovered, explained, and verified. In order to improve the emulating accuracy for the capacitor voltage in SMs of the MPE, a novel voltage control method, which is more adaptive to the operating conditions of MMC, is proposed. The simulation models and experimental setup are built to verify the validity of the proposed methods. [ABSTRACT FROM AUTHOR]

Capacitance monitoring is one of the important issues for a modular multilevel converter (MMC) to obtain high reliability. Since pulsewidth modulation is usually implemented in the field-programmable gate array (FPGA), produced precise switching states in the FPGA cannot be directly accessed by the top digital signal processor (DSP) controller, which poses challenges to the existing capacitance monitoring methods based on precise switching states of the MMC. This article presents a submodule (SM) capacitance monitoring strategy for the MMC with a simple algorithm, where the fundamental frequency components of the SM capacitor voltage and current are extracted to estimate the SM capacitance based on reference, but not the precise switching states. The proposed scheme not only simplifies the implementation and calculation, but also avoids control limitations and heavy communication burden between the DSP and the FPGA. Besides, the proposed strategy effectively eliminates noise impact from sensors and increases accuracy. Simulation and experimental studies are implemented, and the results confirm the effectiveness of the proposed strategy. [ABSTRACT FROM AUTHOR]