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The capacitor condition monitoring is an important issue for the reliable operation of modular multilevel converters (MMCs) in high-voltage applications. The numerous capacitors make the monitoring for the capacitor condition complex and computation-consuming. This paper proposes a hierarchic capacitor condition monitoring strategy to assure the reliability of high-voltage MMCs. In the strategy, the switching times of the switching signals are counted to detect the condition of the capacitor in each submodule (SM). Then, the subsequent capacitance calculation method is only conducted to the abnormal capacitors indicated by the above condition detection algorithm. Meanwhile, the variations of the measured and reference capacitor voltages are employed to calculate the capacitance in the proposed method. The condition detection algorithm shrinks the scope where the abnormal capacitors locate, which avoids calculating the total capacitances in MMCs. In the proposed capacitance calculation method, the complex operations are further saved. Both the two measures reduce the computation burden effectively. The implementation of the whole monitoring strategy is simple and fit for high-voltage MMCs. Moreover, the strategy has no adverse influence on the normal operation of MMCs. The effectiveness of the proposed strategy is verified by the simulation studies with the professional tool Matlab/Simulink. [ABSTRACT FROM AUTHOR]

Modular multilevel converters (MMCs) in high- voltage applications contain large numbers of power switches, which increases the fault probability inside MMCs. This paper proposes a detection method for the faulty arm containing switch open-circuit failures in high-voltage MMCs to improve the reliable operations of MMCs. The proposed method uses the periodicity of the deviation between the maximum and minimum capacitor voltages to provide the detection reference. To yield a reliable reference, the repetitive signal generator is employed. Then, the fault is identified by the difference between the referenced and measured capacitor voltage deviations. An auxiliary criterion is also designated to avoid the false detection caused by the mutations in MMCs. The proposed method can reduce the difficulty in the selection of thresholds. Moreover, the proposed method is robust in high-voltage MMCs with inferior capacitor voltage balancing performance, which ensures detection rapidity and accuracy. Finally, the feasibility of the method is verified by simulation results in Matlab/Simulink. [ABSTRACT FROM AUTHOR]

• The faulty operation of MMC with hot-reserved SMs is analyzed. • NLM corrector based fault-tolerant strategy for high-voltage MMC is proposed. • The strategy features simplicity, low voltage stress and little influence scope. The reliable and consistent operations of modular multilevel converters (MMCs) under fault conditions are crucial for high-voltage and high-power applications. To strengthen the performances of MMCs under submodule (SM) faults, a fault- tolerant strategy based on the nearest level modulation (NLM) is proposed in this paper. According to the changed AC component in the faulty arm voltage, this method adds a corrector in NLM, which remedies the faulty arm voltage by adjusting the number of inserted SMs for the faulty arm. Then, the faulty operation can be recovered to the normal. The corrector is simple to realize and can effectively suppress the odd-order circulating currents. Under the proposed method, only the ripples of the capacitor voltages in the faulty arm rise. The average voltage stress on each capacitor is the same as that in normal operation since the rated capacitor voltage in the faulty arm has no rise. No measure and influence impose on healthy arms. With features of simplicity, low voltage stress and little influence scope, the proposed strategy is fit for high-voltage MMCs. The feasibility of the proposed strategy is verified by the simulation results with the professional tool Matlab/Simulink. [ABSTRACT FROM AUTHOR]

Voltage–current (V–I) droop control is becoming a promising alternative to achieve proper power sharing among the parallel-operated voltage-sourced inverters (VSIs). Compared with the conventional droop control, this method has a simpler structure and a better dynamic performance because it saves the power loop (including the low-pass filters). However, due to the absence of the power loop related to the system frequency, the synchronization of VSIs becomes an intractable issue. To tackle this, a communication-free algorithm to synchronize and control the VSIs is proposed. Initially, the principle of V–I droop control is presented straightforwardly based on the virtual impedance. Then, a synchronization algorithm is incorporated into the V–I droop controller for accurate power sharing. This algorithm adaptively adjusts the phase step of each VSI according to the phase differences between VSI and the ac bus. In this way, the phase differences of each VSI to the ac bus eventually converge to an identical value, thus realizing the synchronization of VSIs. A detailed parameter design method is also implemented considering both system stability and power quality. Finally, the proposed strategy is validated experimentally using a three-parallel-VSIs system. [ABSTRACT FROM AUTHOR]

DC grid has become an important application in power transmission. However, the exist works only have the method to obtain the droop coefficient in a radial topology. Also, this droop coefficient is for V-I droop control. There is no such like method to obtain the droop coefficient for the P-V droop control. This paper proposed a new method for a complex dc grid to obtain the droop coefficients. Firstly, based on the converter control strategies, the types of converter were classified. It will help to find out which kinds of converter can be participated into the dc-side voltage control. Secondly, a virtual node was defined, and the matrix of the dc grid was reshaped to fit the new added virtual node by using the proposed simplification method. An example was shown how this method worked. After that a new method of calculating the P-V droop coefficient was proposed. Finally, a simulation model was investigated to verify if the proposed method can operate in a real system. The results showed that the simplification method can ensure the mathematical relationships of the dc system. And simulations show good performances of the droop coefficient calculation method. [ABSTRACT FROM AUTHOR]

Modular Gravity Energy Storage (M-GES) systems are emerging as a pivotal solution for large-scale renewable energy storage, essential for advancing green energy initiatives. This study introduces innovative capacity configuration strategies for M-GES plants, namely Equal Capacity Configuration (EC) and Double-Rate Capacity Configuration (DR), tailored to optimize energy storage efficiency and stability. Through comprehensive MATLAB/Simulink simulations, we demonstrate each configuration's distinct power characteristics and operational advantages. Our findings reveal that while the EC configuration offers enhanced power accuracy and stability, it requires a larger number of units. In contrast, the DR configuration, characterized by fewer but larger units, shows a trade-off with reduced power stability. This paper contributes significantly to the field by presenting a first-of-its-kind comparative analysis of these configurations, offering insights into their practical applications and scalability. The proposed strategies and findings lay a foundation for future research and development in gravity energy storage systems, marking a step forward in pursuing sustainable and reliable energy solutions. [Display omitted] • Two typical unit capacity configuration strategies for M-GES power plants are proposed. • The unit scheduling method of the M-GES power plant is proposed. • The coordination control method is provided for each proposed configuration strategy. • Equal capacity configuration has better power accuracy but requires more units. • Double rate configuration needs fewer but larger units and less power stability. [ABSTRACT FROM AUTHOR]

This paper presents the first systematic study on power control strategies for Modular-Gravity Energy Storage (M-GES), a novel, high-performance, large-scale energy storage technology with significant research and application potential. Addressing the current research gap in M-GES power control technology, we propose two corresponding compensation modes and several power control strategies for Hybrid M-GES and M-GES. The effectiveness of these strategies is validated through simulations on the MATLAB/Simulink platform, including tests using sinusoidal power and feasibility verification based on a natural California load curve. Additionally, we analyze the normal and abnormal operating conditions of M-GES. In conclusion, the characteristics of the proposed control strategies are summarized, providing a guideline for further research. [Display omitted] • Modular-gravity energy storage (M-GES) power control system studied. • Two compensation modes and four control strategies are systematically studied and validated. • The center switching method with hysteresis control (CSM-H) excels in continuous compensation mode. • The center switching method with hysteresis control and dead zone (CSM-HD) excels in overlap compensation mode. • Based on the power control system proposed in this paper, the M-GES plant performs highly in natural power systems. [ABSTRACT FROM AUTHOR]