Modular Multilevel Converter Technology Research Articles

Application of optimized photovoltaic grid-connected control system based on modular multilevel converters

Photovoltaic power generation is a promising method for generating electricity with a wide range of applications and development potential. It primarily utilizes solar energy and offers sustainable development, green environmental benefits, and abundant solar energy resources. However, there are many external factors that can affect the output characteristics of Photovoltaic cells and the effectiveness of the grid-connected control system. This study describes the introduction of Modular Multilevel Converter (MMC) technology into photovoltaic power generation systems to improve power generation efficiency. It proposes optimizing and improving the technology by adjusting the temperature and magnitude of lighting and combining traditional algorithms to propose a composite control algorithm. The photovoltaic power generation system employs the modular multi-level converter technology to enhance power generation efficiency alongside optimization and improvement. The temperature and size of light are regulated alongside the traditional algorithm to introduce the composite control algorithm. The improved composite algorithm surpasses the traditional one after experimental comparison of the results. The testing of a model photovoltaic power grid-connected system shows that the combination of modular multi-level converter technology and a photovoltaic grid-connected system, incorporating composite proportional integral control and quasi-proportional resonant control algorithms, yields improved results and feasibility. With rationality and effective control. The simulation results show that at 0.5 s, the light intensity suddenly increases from 750 to 1000 W/m2, and the direct-current voltage suddenly increases for a short time, but then decreases rapidly and finally returns to a stable level close to the rated voltage. From this, it can be seen that when the light intensity continues to change, the voltage value on the direct-current bus side of this MMC grid-tied photovoltaic system can still be maintained close to the rated value, ensuring the operational stability of the entire system. Sensibly and effectively controlled. The implementation of MMC technology in photovoltaic power generation systems enhances power generation efficiency, whilst simultaneously supporting the advancement of photovoltaic power generation and contributing towards environmental protection in the long term.

GriDConv – control, design, and experimental verification of a lab-scale high-voltage DC-DC converter

The advent of the Modular Multilevel Converter technology allows building multi-terminal high-voltage DC grids. In order to interconnect DC grids with different voltage levels, highly efficient and compact high-voltage DC-DC (HVDC) converters are required. This paper presents the control and design of a transformerless high-voltage DC-DC converter, which has been designed and built within the framework of the research project GriDConv at the Graz University of Technology. The converter basically consists of high-voltage half-bridges in combination with a series of submodules with full-bridge technology. A dedicated control strategy for the converter is developed, which allows advantageous soft switching of the high-voltage half-bridges, which considerably reduces switching losses of the half-bridges. In addition, a balancing strategy for the individual cell voltages as well as a control of total cell voltage are presented. The developed control strategies are discussed in detail and, in the end, verified through experiments on a constructed 50,text{k}text{W} lab-scale hardware demonstrator.

Systematic Analysis and Characterization of Extreme Failure for IGCT in MMC-HVdc System—Part II: Failure Mechanism and Short Circuit Characteristics

Short circuit failure mode (SCFM) of integrated gate commutated thyristor (IGCT) under extreme failure is of vital importance in high voltage direct current power transmission based on modular multilevel converter technology. Due to the sealing housing package of IGCT, the short circuit mechanism is hard to be clarified. Considering this limitation, the current density changes in IGCT with different failure modes are studied under different current levels in a built integrated test system with the placed magnetic sensor array and thermal couple array. Then, the short circuit mechanism of failed IGCT is proposed based on the experimental results. IGCT with turn-<small>off</small> failure performs self-triggering effect due to the focused short current and increased temperature in the initial destruction area, which is usually located in a single position of the outer cathode rings. While IGCT with surge current failure shares the short current among multiple initial destruction areas, which are usually distributed evenly in the wafer area. Scanning electron microscope picture and energy dispersive X-Ray spectroscopy (EDX) analysis show that the formed conducting alloy has spread into the molybdenum plate deeply and the phenomenon of atom diffusion (molybdenum, silicon and aluminum) near the interface is observed. Finally, long term short circuit tests of more than 12&#x00A0;h with both turn-<small>off</small> failure and surge current failure under 3000&#x00A0;A prove the stable SCFM of IGCT.

Black Start Operation using a MMC based HVDC system

Blackouts could occur due to various faults and in order to keep the power stations running constantly to avoid consumers losing power a backup plan is necessary. Black start is the process of restoring power to the electric power stations. The Modular Multilevel Converter (MMC) topology has been propitious technology in High Voltage DC (HVDC) system. The research in MMC technology has significantly increased. The MMC based HVDC system has many advantages when compared to the other technology based HVDC system. In this paper, Black Start Operation using a MMC Based HVDC system is proposed to provide continuous power to the load for a certain period of time within which alternative methods of power generation can be made ready. The paper also provides a mathematical model of a continuous equivalent circuit of the MMC Phase Leg. The efficiency of the proposed idea has been verified by MATLAB/Simulink Software and discussed.

Conceptual design of the power supplies for DTT Neutral Beam Injector

The Divertor Tokamak Test facility (DTT) is the new tokamak whose construction is starting in Frascati (Italy), with the main aim of testing different divertor magnetic configurations and technologies in view of DEMO. In the present design, the DTT heating mix includes one Negative Neutral Beam Injector (NBI), with a power injected in the plasma of 10 MW and a beam energy of 500 keV.The NBI conceptual design is under development and includes solutions already adopted for ITER (RF-driven ion source, gas neutralizer, electrostatic Residual Ion Dump), but with some simplifications to reduce costs and complexity deemed feasible for DTT. In particular, the beam source is air-insulated and fed by the gas-insulated Transmission Line through gas-air bushings.This paper presents the status of the conceptual design of the main power supplies of the DTT NBI. The main ratings are collected from physics requirements and NBI design. On this basis, the reference technical solutions are selected, considering the experience gained at the ITER Neutral Beam Test Facility in Padova (Italy), the specific design of the DTT NBI and the layout constraints of the DTT Site. For the Acceleration Grid Power Supply, besides the ITER-like design, an innovative approach based on the Modular Multilevel Converter technology, never adopted in this application, is considered; pros and cons of the two solutions are evaluated.

A Review of Modular Multilevel Converters for Stationary Applications

A modular multilevel converter (MMC) is an advanced voltage source converter applicable to a wide range of medium and high-voltage applications. It has competitive advantages such as quality output performance, high modularity, simple scalability, and low voltage and current rating demand for the power switches. Remarkable studies have been carried out regarding its topology, control, and operation. The main purpose of this review is to present the current state of the art of the MMC technology and to offer a better understanding of its operation and control for stationary applications. In this study, the MMC configuration is presented regarding its conventional and advanced submodule (SM) and overall topologies. The mathematical modeling, output voltage, and current control under different grid conditions, submodule balancing control, circulating current control, and modulation methods are discussed to provide the state of the MMC technology. The challenges linked to the MMC are associated with submodule balancing control, circulating current control, control complexity, and transient performance. Advanced nonlinear and predictable control strategies are expected to improve the MMC control and performance in comparison with conventional control methods. Finally, the power losses associated with the advanced wide bandgap (WBG) power devices (such as SiC, GaN) are explored by using different modulation schemes and switching frequencies. The results indicate that although the phase-shifted carrier-based pulse width modulation (PSC-PWM) has higher power losses, it outputs a better quality voltage with lower total harmonic distortion (THD) in comparison with phase-disposition pulse width modulation (PD-PWM) and sampled average modulation pulse width modulation (SAM-PWM). In addition, WBG switches such as silicon carbide (SiC) and gallium nitride (GaN) devices have lower power losses and higher efficiency, especially at high switching frequency in the MMC applications.

On control and balancing of rigid bipolar MMC-HVdc links enabling subsystem-independent power transfer

High voltage direct current transmission schemes equipped with modular multilevel converter technology are considered in several realized and planned bulk power transmission projects. Especially in the case of dc cable links with high system lengths, the rigid bipolar configuration presents a suitable compromise between power rating, cost-efficiency and converter redundancy. Caused by the absence of a metallic return path, system degrees of freedom are reduced in contrast to the bipolar configuration with dedicated metallic return. Therefore, the dc currents in positive and negative bipolar subsystem are not independent. This leads to new challenges and motivates the development of advanced control and balancing concepts for rigid bipolar schemes. This paper focuses on balanced and unbalanced ac grid operating conditions with subsystem-independent active power set points in an ac split-busbar scenario, which has not yet been covered in literature. In order to validate the presented methodology, case studies are performed in electromagnetic transient software.

Investigating Developments in Modular Multilevel Converters [From the Editor

The modular multilevel converter (MMC) has come a long way since first proposed for high-voltage dc (HVDC) applications nearly 15 years ago. Ongoing improvements and advances in MMC technology have extended the reach of the MMC to many more applications in the last 10–15 years, which include flexible ac transmission systems and high-power motor drives. To better understand the MMC, our theme for this issue is the modular multilevel converter.

Power flow and transmission loss analysis of modular multi‐level converter based multi‐terminal high‐voltage DC systems

Interconnected voltage-source converter based multi-terminal high-voltage DC (HVDC) systems provide better system redundancy, higher flexibility and capability of exchanging power between multiple areas. Recent developments in modular multi-level converter technology makes multi-terminal HVDC (MTDC) transmission more promising than before. This study provides an in-depth analysis of the power flow and transmission loss for MTDC systems with different grid topologies. A voltage–power droop control based algorithm is applied to solve the power flow problems. The main contribution of this study is the novel way of determining and calculating the transmission line losses for different MTDC network topology configurations. Three MTDC system topologies are investigated. Simulated case studies are used to observe the system power flow and transmission losses for all three topologies.

Research on Overvoltage for XLPE Cable in a Modular Multilevel Converter HVDC Transmission System

Due to the rapid development of the voltage-source converter (VSC), the demands for HVDC cables have increased significantly. Although more than ten VSC-HVDC projects are under construction and many more are in operations globally, the testing criteria for HVDC XLPE cables are mainly referenced to test guideline TB 496 recommended by CIGRE. Due to the large stray capacitance of the VSC-HVDC cables, the switching impulse overvoltage stress of the cables is the main electrical parameter concerned, a unified overvoltage factor of 2.1 p.u. being generally accepted. However, the switching impulse overvoltage stress of a VSC-HVDC cable system depends on not only the VSC topology but also the cable parameters and the volt-ampere characteristics of the dc cable surge arresters used. This paper studies the switching impulse overvoltage stresses for the dc cable system of a symmetrical monopole VSC-HVDC transmission system using the modular multilevel converter technology. The studies were performed according to the key technical parameters for a typical 1000-MW/320-kV VSC-HVDC transmission scheme, the XLPE dc cable links being assumed. The results show that a switching impulse overvoltage level of 2.3 p.u. should be applied during the type test of the overvoltage capability of the HVDC cables.

Soft-Switching Operation of Isolated Modular DC/DC Converters for Application in HVDC Grids

High-voltage dc/dc converters play an important role in HVDC grids. Isolated modular dc/dc converters (IMDCCs) based on modular multilevel converter technology provide a good solution to high-voltage applications. In order to reduce the size of the system, the IMDCC is required to be operated with a high ac-link frequency, but this will lead to increased switching loss and thus degraded efficiency. This paper proposes a soft-switching operation scheme for such an IMDCC. In this scheme, a quasi-square-wave (QSW) modulation method is employed, where the chain-links generate quasi-square terminal voltages with reduced dv / dt . With such chain-link terminal voltages, the arm currents which provide good condition for the soft-switching operation of the QSW-IMDCC can be obtained. Since soft switching can be achieved for the power switches, the proposed scheme will suffer less switching loss, thus improving the efficiency of the converter. Moreover, a capacitor voltage-balancing control strategy is proposed. This strategy does not need any arm current sensors, thus reducing the cost. The proposed soft-switching operation scheme and capacitor voltage-balancing control strategy are verified by the simulation results.