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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]

This article brings forward a design method of virtual resistance for droop-controlled dc microgrids (DCMGs). Although droop control is widely employed in coordinated DCMGs to coordinate different energy sources, few works address thoroughly the principles for a proper virtual resistance design. In this article, dynamic stability and static voltage deviation constraints are taken into consideration as the main criteria to be complied with by the virtual resistance design. In critical cases, constant power loads (CPLs) may substantially decrease system damping and adversely affects the stability of the system. In this sense, the large-signal stability model is developed including CPLs by using the Lyapunov function, and it is subsequently analyzed to infer the stability criterion. Theoretically, bus voltage is the most important index when addressing the DCMG control; hence, the impact of virtual resistance on the voltage deviation is explored as well. The research presented in this article finds out that virtual resistance influence on system stability is opposite to that on voltage deviation; thus, a tradeoff method based on the containment principle is developed. Throughout the proposed compromised design, we can adjust the weight coefficient to satisfy different performance requirements of DCMGs. The effectiveness of the proposed scheme is validated through experimental results. [ABSTRACT FROM AUTHOR]

In the literature, many studies on stability analysis of dc microgrids have been conducted. However, most of them mainly focus on small-signal stability. On the other hand, few works analyze large-signal stability, but the major part of these works is based on a single unit or a simple cascaded system as a case study. Different from those, this article aims to address the large-signal stability analysis of a dc microgrid from a system-level perspective. First, the equivalent model of a droop-controlled dc microgrid is developed. Subsequently, the Lyapunov-based large-signal stability analysis and the stability criterion are derived, and mixed potential theory is used to make comparisons to verify the effectiveness of the derived criterion. The equilibrium point stability for different operation stages was obtained by means of theoretical calculation. Furthermore, the instabilities principle as well as their physical interpretation is revealed. In this article, the bus voltage is used as the only index to assess the microgrid power balance. Hence, the power load limit can be obtained by taking into consideration the stability and voltage deviation constraints. Finally, simulation and experimental results from a four-converter dc microgrid system verify the feasibility of the proposed theoretical analysis. [ABSTRACT FROM AUTHOR]

The modular multilevel converter (MMC) is one of the most attractive topologies for high-voltage and high-power applications. Reliability is an important challenge for MMCs due to large amounts of power switches in the phase leg of the MMC. To enhance the reliable performance of MMCs, an effective and accurate fault detection and localization strategy for MMCs with the nearest level modulation is proposed in this paper. According to the general characteristics of the MMC, the capacitor voltage of the faulty submodule (SM) is higher than the healthy ones’. Based on the sorting algorithm, the faulty SM will have the maximum voltage and it will be identified by the proposed fault detection method. The auxiliary monitoring is supplemented to the detection algorithm for the cases of faults occurring in multiple SMs. The faulty SM localization strategy is based on the differences of capacitor voltages. The faulty switches are located by the distinctions of capacitor voltage in different current intervals. The proposed strategy could be applied under the cases of single or multiple SMs failures and identifies three kinds of the power switches faults in the faulty SM. The validity of the proposed strategy is verified by the simulation results in MATLAB/Simulink. [ABSTRACT FROM AUTHOR]

Voltage source converter (VSC)‐based high‐voltage dc (HVDC) transmission is widely utilised nowadays. However, a VSC‐HVDC connected to a weak ac system still faces the problem of phase‐locked loop (PLL) synchronisation. To tackle this problem, this study proposes a dc voltage synchronisation control (DCSC) based on the power synchronisation control and the power balance equation of the virtual synchronous generator. To analyse the dynamic stability of DCSC, voltage sensitivity indicator (VSI) is used to reflect the voltage fluctuations. Considering the deviations of dc voltage and converter losses, a closed‐loop transfer function of dc voltage is designed. Three important parameters including VSI, virtual damping coefficient and inertia time constant are investigated. The dynamic stability analysis shows DCSC has strong controllability. Then, a three terminals VSC‐MTDC model is simulated in PSCAD/EMTDC. Herein, DCSC is used in one inverter and PLL is used in another inverter. The waveforms obtained by using DCSC are compared with that by using PLL. Simulation results clearly depict that the inverter with DCSC has a normal steady state and a better transient response than using PLL when connected to a strong ac system, also provides better power transfer capability when connected to a weak ac system. [ABSTRACT FROM AUTHOR]

This paper proposes a decentralized current-sharing control strategy to endow fast transient response to paralleled dc–dc converters systems, such as dc microgrids or distributed power systems. The proposed controller consist of two main control loops: an external voltage droop control for current-sharing proposes and an internal current loop. The external droop control loop is designed as a voltage loop with embedded virtual impedance, which avoids the use of a slow voltage loop and a separate extra virtual impedance loop that may limit the system bandwidth. The internal current loop, thanks to the external control loop simplification, plays a major role in the system bandwidth, so that an adaptive proportional-integral (PI) controller is proposed for this matter. In the paper, two different droop control methods have been modeling, designed, simulated, and tested: The conventional virtual-impedance-loop based V–I droop and the proposed embedded-virtual-impedance based I–V droop. In order to compare the dynamic response performances between two droop controllers, their state-space models have been developed and analyzed in this paper. The results show that the dynamic response of the I–V droop control is faster than that of the conventional V–I droop control. Furthermore, by analyzing the effects from I–V droop control parameters, the errors can be reduced faster by enlarging the proportional terms, but with no fluctuations, and then completely eliminated by restoring back to small proportional values. Meanwhile, there exists a tradeoff phenomenon between the fast dynamic response and good steady-state performance; thus, an adaptive PI controller is proposed to both improve dynamic response and guarantee good steady-state performance simultaneously. Experimental results are shown to verify the accuracy of the models and the effectiveness of the proposed control framework. [ABSTRACT FROM PUBLISHER]

A hierarchical distributed control method for I–V droop‐controlled‐paralleled DC–DC converters in DC microgrid is proposed. The control structure includes primary, secondary, and tertiary levels. The secondary control level is used to remove the DC voltage deviation and improve the current sharing accuracy. An improved dynamic consensus algorithm is used in the secondary control to calculate the average values of bus voltage and voltage restoration in distributed control. In the tertiary control level, as the main contribution in this study, the system conversion efficiency is enhanced by using the average restoration value obtained in the secondary control level, instead of using the total load current which needs more communication traffic. When the converters are connected to batteries, the method for the state of charge (SoC) management is proposed so that the SoC balance can be guaranteed. The effectiveness of the proposed method is verified by detailed experimental tests based on four 0.7 kW DC–DC converters. [ABSTRACT FROM AUTHOR]

Modular multilevel converter (MMC)-based multiterminal HVdc (MTdc) transmission is a favorable option for asynchronously interconnecting several weak ac systems. This paper investigates the potential resonance of such an MMC-MTdc system excited by background harmonics in any of the MMC's ac systems and the undesirable transfer of these harmonics to the remote ac systems. First, the harmonic transfer rule across one MMC is established using phase leg voltage analysis and it is then used to develop an equivalent circuit model of the MMC-MTdc system. This model is used to investigate the mechanism of resonance excited by ac-side background harmonics. Finally, a control strategy is proposed to mitigate this resonance. Analytically calculated results from the derived model and the proposed control strategy are validated via electromagnetic transients simulation of a five-terminal MMC-MTdc network. [ABSTRACT FROM PUBLISHER]

Abstract: This paper presents an extensive analysis of the influence of the DC voltage on the compensation performance of Static Var Generator (SVG). A method to design the DC voltage rated value is proposed according to the compensation performance. First, a required lowest value of the DC voltage for full compensation (a unity total power factor) is obtained by using the analysis of the equivalent circuit. For the situation where the DC voltage is smaller than the above-obtained lowest value, two basic options which are the sinusoidal modulation and the over modulation for the PWM control of SVG are considered. Then, the quantitative relationship between the DC voltage rated value and the compensation performance is detailed. In addition, the lowest value of the DC voltage in the SVG with a battery energy storage device is also obtained for the reactive and active output current. Based on the above analysis, the design of the DC voltage rated value for SVG is quite straight forward. Both simulation results and the hardware experimental results are finally presented to verify the validity of the analysis results and the proposed design method. [Copyright &y& Elsevier]

• The study of the combined operation of RSFCLs and DCCBs has shown effective protecting results. • Detailed models of the RSFCL and DCCB are built in this paper. • The influence of four kinds of different parameters on the fault clearing process are investigated. • Converters in the system remain unblocked, making the fast recovery of the system possible through DCCBs' reclosing. The high rate of rise and large amplitude of a dc fault current in MMC-MTDC system make its interruption difficult for DC Circuit Breakers (DCCBs). In order to tackle this issue, this paper proposes a combined operation of a Resistive-type Superconducting Fault Current Limiter (RSFCL) and a DCCB. By analyzing the fault current in a symmetrical monopole MMC-MTDC system, the effectiveness and necessity of adding a resistive type fault current limiting device into the system after fault is established. With detailed modeling of the RSFCL and DCCB including the quench and recovery processes of the RSFCL, their combined operation is investigated. The influence of different parameters, such as the maximum quench resistance of the RSFCL, the insertion time of the energy absorption branch in the DCCB, and the quench and recovery process of RSFCL, on the fault clearing process are investigated in detail. The effectiveness of the combined operation is verified by extensive simulations in PSCAD/EMTDC. The results indicate that the combination of RSFCLs and DCCBs can obtain a suppression rate of at least 25% on the peak value of the fault current. Besides, the energy that DCCBs need to absorb in their arresters has been reduced by more than 60%, and the total fault clearing time is also reduced. Furthermore, all the converters remain unblocked, making the fast recovery of MMC-MTDC system possible through DCCBs reclosing. It is concluded that the combination of RSFCLs and DCCBs provides an effective solution to handle short circuit faults in MMC-MTDC systems. [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]

This paper investigates interconverter interactions in multi-infeed systems that include both rectifiers and inverters. Performance indices used earlier for multi-inverter systems are extended to include the rectifier-inverter mix. These indices can be used to predict the maximum available power (MAP) and power stability limits of the converters as well as transient phenomena, such as transient overvoltages (TOV) and inverter commutation failures (CF ).The results show that the presence of a rectifier in the neighborhood of inverters is effectively similar to an increase in the inverters' short-circuit ratio in that the inverters achieve a higher MAP, are less susceptible to commutation failure, and have smaller TOV. [ABSTRACT FROM AUTHOR]

The limitation of existing point to point high-voltage DC connections for offshore wind farm transmission is that it does not support power exchange or trading between two or multiple DC systems. However, proposed by many researchers, interconnected voltage-source converter based multi-terminal high-voltage DC systems provide system redundancy, higher flexibility and capability of exchanging power between multiple areas. This paper presents a detailed review of several key technical and operational aspects of multi-terminal high-voltage DC systems for large scale offshore wind farm transmission. This review not only summarizes the development of high-voltage DC system technology in the past decades such as line-commutated converter based and traditional 2-level voltage-source converter based high-voltage DC technology, but also provides an overview of contemporary challenges and opportunities for multi-terminal high-voltage DC systems including modular multi-level converter technology, DC breakers and network topologies. A case study of a six-terminal MTDC system was carried out in order to better compare these different topologies in terms of transmission line capacities, total length of transmission line, capital cost, and running costs. [ABSTRACT FROM AUTHOR]