This paper studies the impact of the inherent characteristics on the initial-stage short-circuit current of modular multilevel converter based multi-terminal direct current (MMC-based MTDC) ...transmission system. A short-circuit current calculation method based on input-output characteristics is proposed. The MMC's input-output characteristics are factorized into a series combination of a linear and a nonlinear part through the operator approach. The inherent characteristics of the system represent the inherent properties of the MMC-based MTDC system, which are unrelated to the short-circuit fault. Therefore, short-circuit currents are decoupled and quantitative indices are proposed to measure the impact of the system inherent characteristics on the initial-stage short-circuit current. The proposed decoupled short-circuit current calculation method and the quantitative indices are verified in a typical MMC-based MTDC transmission system under different system parameters and structures. The proposed quantitative indices can effectively evaluate the short-circuit current of the MMC-based MTDC system without calculating the short-circuit current, which indicates that they can provide guidance for the planning of DC system structure, determination of operational modes, and selection of system parameters from the perspective of short-circuit current.
Aluminum based metal matrix composites (AMMCs) have the potential for replacing conventional monolithic materials due to excellent fatigue, creep and wear resistance, high temperature retention, and ...high strength to weight ratio. Thus Aluminum based metal matrix composites have several applications in different industries including Aerospace and Automotive. This study revealed that Al 6063 based metal matrix composites reinforced with Al2O3, SiC, TiO2 have higher hardness, tensile strength, and yield strength as compared to the pure Al 6063. This study shows that hardness, tensile strength, and yield strength are increasing as increasing SiC particles content. Microstructure and surface morphological studies have been carried out to see their surface and particles distribution. Energy Dispersive X-ray Spectroscopy (EDS) and XRD analysis have been conducted to identify elements content. Fourier Transform Infrared Spectroscopy (FTIR) analysis is carried out to identify chemical functional group of the Al6063 metal matrix composites.
Power reversal control strategies for different types of hybrid line-commutated-converter (LCC)/modular multi-level converter (MMC) based high-voltage direct-current (HVDC) systems have been proposed ...with the consideration of system configurations and MMC's topologies. The studies show that the full-bridge (FB) MMC gives better performance than halfbridge (HB) MMCs in terms of power reversal in hybrid LCC/MMC systems. The modulation method employed in this paper can achieve a smooth online polarity reversal for hybrid LCC/FB-MMC HVDC systems. Additional DC switches and/or discharging resistors may be needed to reverse the DC polarity of LCC/HB-MMC HVDC systems. Based on the proposed strategies, the power reversal processes of the studied systems can be accomplished within several seconds. The speed can be changed according to system operation requirements. The effectiveness of the proposed control strategies has been verified through simulations conducted in PSCAD/EMTDC.
The modular multilevel converter (MMC) with half-bridge submodules (SMs) is the most promising technology for high-voltage direct current (HVDC) grids, but it lacks dc fault clearance capability. ...There are two main methods to handle the dc-side short-circuit fault. One is to employ the SMs that have dc fault clearance capability, but the power losses are high and the converter has to be blocked during the clearance. The other is to employ the hybrid HVDC breakers. The breaker is capable of interrupting fault current within 5 ms, but this technology is not cost effective, especially in meshed HVDC grids. In this paper, an assembly HVDC breaker and the corresponding control strategy are proposed to overcome these drawbacks. The assembly HVDC breaker consists of an active short-circuit breaker (ASCB), a main mechanical disconnector, a main breaker, and an accessory discharging switch (ADS). When a dc-side short-circuit fault occurs, the ASCB and the ADS close immediately to shunt the fault current. The main breaker opens after a short delay to isolate the faulted line from the system and then the mechanical disconnector opens. With the disconnector in open position, the ASCB opens and breaks the current. The proposed breaker can handle the dc-side fault with competitively low cost, and the operating speed is fast enough. A model of a four-terminal monopolar HVDC grid is developed in Power Systems Computer Aided Design / Electromagnetic Transients including DC, and the simulation result proves the validity and the feasibility of the proposed solution.
Here, the uncontrolled rectification pre-charging characteristics of the VSC-HVDC system with different modular multilevel converter (MMC) topologies such as full-bridge, half-bridge, and hybrid ...topologies are proposed based on theoretical analysis and PSCAD/EMTDC simulations. Different MMC topologies have a great impact on the selection of pre-insertion resistors. The surge energy of pre-insertion resistors is the lowest with the full-bridge sub-module (FBSM) MMC, while much higher with the half-bridge sub-module (HBSM) MMC. In order to prevent start-up failure caused by extremely non-uniform voltage between FBSM and HBSM of hybrid MMC, the FBSM should be turned to half-bridge state by valve control system as soon as possible so as to carry out the second charge, and then balance the sub-modules voltage by bypass certain high-voltage sub-modules. The half-controlled charging process has a great influence on the absorbed energy of the pre-insertion resistors of hybrid MMC. The research results can be used to guide the selection of VSC-HVDC pre-insertion resistors and start-up strategy making.
Modular multilevel converter (MMC) is a promising topology for medium- and high-power conversion systems. In recent years, it has been prominent over other power converters because of its exceptional ...features, including modularity and flexibility to adapt any voltage level, remarkable harmonic performance, no dc-link capacitors, transformerless operation, absence of ac-side filters, and capacitive nature of phase legs. However, there are some challenges, for instance, submodule (SM) capacitor voltage balancing, output current control, circulating current control, minimization of SM capacitor voltage ripple, dc-side faults, and efficient SM topologies that have been addressed by the research studies over time. In this article, we focus on the recent advancements in SM structures and their capability to overcome the faults, reduction in switching losses, decrease in voltage sensor count, and minimization of SM capacitor voltage ripple and provide the quick restoration of SM capacitor voltages after the removal of faults. Finally, the applications and development prospects of MMC are discussed in detail.
Based on a ±10 kV three-terminal MMC-MVDC (medium voltage DC distribution system based on modular multilevel converter) system, in this study, the authors mainly study on the influencing factors and ...protection schemes of the lightning intruding overvoltage (including direct lightning and back-flashover lightning). Firstly, a lightning intruding overvoltage simulation model of the MMC-MVDC distribution system using overhead lines is established based on the PSCAD/EMTDC program. The influence of steady-state operating voltage and the number of converter stations on the simulation results are simulated and analysed. Meanwhile, the proper simulation modelling method of the lightning intruding overvoltage for multi-terminal MMC-MVDC system is proposed. Secondly, the lightning intruding overvoltage is simulated when the line is struck by the lightning stroke current that occurs once in 20 years and the different lightning protection schemes are proposed. Finally, based on the lightning impulse withstand voltage, lightning trip-out rate and the cost of the protection schemes, the lightning protection schemes in different scenarios are proposed. The simulation results show that if the cost of the system is prioritised, the protection scheme of increasing the number of parallel stacks of the DL arresters can be selected, and if the power supply reliability of the system is prioritised, the protection scheme of installing the overhead ground wire as well as reducing grounding resistance can be selected.
The half-bridge modular multilevel converter (HB-MMC) is one of the most promising topologies for high-voltage direct current (HVDC) systems. Unlike full-bridge MMC (FB-MMC) and clamp double ...sub-module MMC (CDSM-MMC), HB-MMCs are defenceless against DC side faults. Different types of DC circuit breakers (DC CBs) assisted with arrester banks/damping resistors such as solid-state CB and hybrid DC CBs can be used to interrupt the DC fault current. Arrester banks are used to protect against overvoltage after interrupting the fault current and in turn to demagnetise the circuit inductors. The main disadvantage of the arresters is that they forcibly break apart when they are overloaded. In this study, an arrester-less fault current limiter is proposed for HB-MMC configuration, which provides an opposing voltage during the DC side fault. This is achieved by inserting external pre-charged capacitors in the DC current path during the fault. The selection of these external capacitors, their suitable initial voltages, and their charging stations are also presented in this study. A comparison between the proposed scheme and the CDSM-MMC has been held as well. The proposed protection scheme is assessed using a simulation study for a point-to-point nine-level HB-MMC-based HVDC system.
•In this paper, the loss and thermal stress distribution among SMs of MMC have been investigated which under different capacitance. The relationship between the degree of capacitance degradation and ...the junction temperature difference among SMs is revealed.•The switching principle of thermal balancing control is studied by analyzing the trend of junction temperature changes in power semiconductor devices.•A thermal balancing method is proposed, offering advantages such as reducing the demand for temperature sensors and eliminating the reliance on high-precision capacitance monitoring technology. The proposed method can effectively achieve balanced junction temperatures of power semiconductor devices among different SMs under capacitor deterioration.
Thermal control of semiconductor devices in the submodules (SMs) is a key factor for the safe and reliable operation of modular multilevel converters (MMCs). During normal operation, significant thermal differences can arise among SMs due to the manufacturing tolerance and aging effect of capacitors, which can affect the lifespan of semiconductor devices and even lead to equipment failure. To address the problem, this paper presents a method for achieving thermal balancing among SMs with different capacitances by adjusting their switching principle. Firstly, the relationship between the degree of capacitance degradation and the junction temperature difference among SMs is revealed. Then, by setting data windows to capture the dynamic losses of power devices, which can reflect the trend of junction temperature. The switching principle of thermal balance among SMs is studied based on the trend of junction temperature in power devices. The thermal balancing method has advantages including not requiring high-precision temperature sensors and eliminating the reliance on capacitance monitoring technology. Finally, the simulation and experimentation results demonstrate that the proposed method can effectively achieve balanced junction temperatures of power devices among different SMs.
Magnesium is one of the lightest structural metals currently available, and it can replace conventional alloys in mass-saving applications while still offering superior stiffness and strength. The ...presence of reinforcing components inside the metallic matrix has a substantial impact on stiffness, specific strength, wear behaviour, damping behaviour, and creep qualities when compared to conventional engineering materials. Due to their superior physical and mechanical characteristics, low density, and suitability for a variety of applications, magnesium metal matrix composites are ideal materials. This paper describes how to choose an acceptable technique and its process parameters for the synthesis of magnesium-based metal matrix composites (MMCs). Additionally, it gives a general summary of how different reinforcements affect magnesium and its alloys, highlighting both their benefits and drawbacks.
PDF
