Full-bridge Modular Multilevel Converter Research Articles

Arm Current Deviation Minimizing Method for Co-Phase Traction Power Supply System Using Direct AC/AC Full-Bridge Modular Multilevel Converter

Arm Current Deviation Minimizing Method for Co-Phase Traction Power Supply System Using Direct AC/AC Full-Bridge Modular Multilevel Converter

An accelerated modelling method of full-bridge modular multilevel converter considering dead-time

To avoid short-circuits of the same side switching of power converters, the dead time is of significance in the pulse trigger gating signal for full bridge modular multilevel converter (FB-MMC). However, the previous equivalent models use the binary resistance switching model to equivalent the switching state which is impossible to simulate the switching with dead time, and the detailed model (DM) with higher levels is time-consuming and inefficient in electromagnetic transient (EMT) simulation. To address this problem, based on the state-space averaging method, a speed-up average value model (AVM) of FB-MMC considering dead time is proposed. Firstly, the working state of FB-MMC is obtained through analysing the switching states with dead time and is classified into three cases according to the capacitor. Secondly, derived from state-space equation, the three cases are integrated into a unified model by introducing the duty cycle, and then the equivalent controlled source is used as the interface circuit between the external grid and capacitor. Finally, a 49-level two-terminal MMC-HVDC system consisting of FB-MMC is constructed in PSCAD / EMTDC simulations, showing that the proposed AVM compared with DM has high accuracy and operation efficiency.

An Embedded DC Power Flow Controller Based on Full-Bridge Modular Multilevel Converter

Meshed dc grids help improve the current network's configuration, flexible power management, and reliable power supply. However, the existing power converters implemented in dc grids do not have enough controllability in adjusting each feeder's power, leading to transmission congestion, overloads, and high-power loss issues. The DC power flow controller (DCPFC) is prominent in addressing problems with better system performance. This paper proposes an embedded DCPFC (eDCPFC) with full-bridge modular multilevel converter (FB-MMC) topology. The eDCPFC is directly integrated into a host MMC. The device's energy is balanced through the coordinated control of the eDCPFC and the host MMC without using isolation transformers or external power supplies. The working principle and the corresponding control strategies of the FB-MMC type eDCPFC and its host MMC are elaborated. Experimental results conducted in a scaled-down prototype validate the feasibility and effectiveness of the proposed topology and control schemes under typical working conditions.

Current Integral Comparison-Based Power Loss Balancing Control for Full-Bridge Modular Multilevel Converters

Current Integral Comparison-Based Power Loss Balancing Control for Full-Bridge Modular Multilevel Converters

An Adaptable Fault Current Derivative Calculation Method for Derivative Relays in HVDC Grids

This paper presents an adaptable method for fault current derivative calculation in high voltage direct current (HVDC) grids composed of modular multilevel converters (MMCs). The proposed method can be used for current derivative calculation under different fault scenarios including pole-to-pole, pole-to-ground, and pole-to-metallic. The proposed method is adaptable as it can provide accurate fault current derivatives in various grid topologies such as symmetric monopole, asymmetric monopole with ground or metallic return, and bipole with ground or metallic return, as well as grids with different types of converters with and without fault-blocking capability including full-bridge MMCs (FB-MMCs), half-bridge MMCs (HB-MMCs), or a mix of HB- and FB-MMCs. The paper also demonstrates how the proposed current derivative calculation method can be used to form a derivative relay, which is fast, selective, computationally efficient, and insensitive to fault resistance. Furthermore, using the proposed current derivative calculation method, all relay settings are analytically calculated instead of being obtained through time-consuming simulation studies. Simulation results for various fault scenarios, grid topologies, and converter configurations show that the calculation method is accurate and the presented relaying algorithm can detect various faults within 10 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\mu$</tex-math></inline-formula> s, even when the fault resistance is as high as 500 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$Upomega$</tex-math></inline-formula> .

A New Hybrid Modular Multilevel Rectifier as MVac–LVdc Active Front-End Converter for Fast Charging Stations and Data Centers

As the power demands from DC energy-storage and loads continue to grow in electric vehicle (EV) fast charging stations and data centers, the power delivery infrastructure faces challenges with the installation of bulky, heavy, and slow responsive line-frequency power transformer (LFT). These transformers are required to step-down the feeder voltage from medium-voltage (MV) ac grid-service to low-voltage (LV) ac, followed by LVac-LVdc rectifiers. This approach results in a large equipment footprint, heavy conductor copper usage, and lower efficiency. Consequently, there is increasing interest in exploring direct interface from MVac to LVdc without the need for LFT. This paper proposed a new solution called MVac-LVdc hybrid modular multilevel rectifier (HMMR). The HMMR serves as a centralized step-down active front-end converter, enabling power delivery to LVdc with a reduced number of dc/dc back-end isolated converters. Compared to the modular multilevel converter (MMC) used as the MVac interface solution, the proposed HMMR could save the submodule number by 40%, reduce losses by 22%, and significantly reduce the footprint area by 37%, effectively increasing the power density and reducing the construction cost. Moreover, the proposed HMMR has the potential to operate in both unity and non-unity power factor modes, allowing it to provide the grid-support functionality. The performance of HMMR is evaluated and compared with full-bridge MMC in the case of 13.8 kV ac to 6 kV dc. The feasibility of the proposed converter is verified by the simulation results with the same specifications. Finally, a scale-down 1.4 kV HMMR prototype is developed to validate the effectiveness of the proposed converter.

An active pole-to-ground fault location scheme for FB-MMC-MTDC system using the short-time blocking of sub-modules

The pole-to-ground (PTG) fault current is small in the multi-terminal direct current (MTDC) system, which cannot be used to locate the fault. This paper proposes a PTG fault location scheme for MTDC based on full bridge modular multilevel converter (FB-MMC), by actively blocking the sub-modules (SMs). The SMs blocking generates a voltage pulse wave which travels between the MMC and the fault point. By using the wave polarity analysis method, the reflected wave from fault point can be identified. The improved complete ensemble empirical mode decomposition with adaptive noise (ICEEMDAN) is used to detect the reflected wave arrival time. The time difference between pulse start time and reflection arrival time is processed for fault location. A four-terminal MMC-MTDC simulation model established in PSCAD/EMTDC validates the proposed method have not affected the power transmission. Afterwards, the fault location accuracy under the influences of fault type, fault location and fault impedance issues are also confirmed.

Fault-tolerant strategies in MMC-based high power magnet supply for particle accelerator

Many particle accelerators require to supply chains of magnets with high quality, high magnitude, cycling currents. To do this, the power converters need to provide high output voltages, reaching in some cases tens of kilovolts. Additionally, converters are required to store the magnet energy during de-magnetization cycles. For such application, Full-bridge Modular Multilevel Converters (FB-MMC) could be used given their capacity to store energy and their inherent reliability. In this sense, one of the most interesting features of the proposed topology is the possibility of bypassing one or several submodules in the event of a fault or malfunction. By doing this, it is possible to ride-through the failure of a component and avoid the interruption of the accelerator operation.However, when the number of submodules is small, this operation could lead to an excessive charge of the healthy cells, increasing the risk of secondary failures. Besides, undesired harmonic content could appear on the output current, degrading the operation of the accelerator. It is then necessary to implement strategies that allow to remove a faulty cell without significantly impacting the operation of the remaining ones and of the converter itself.Accordingly, the purpose of this article is to investigate several of these strategies and assess them. By means of detailed computer simulations, the behaviour of the converter during normal and submodule fault conditions is analysed. Then, several fault-tolerant strategies are described, verified and compared with the aid of simulation tools. The results show the effectiveness of the analysed strategies in avoiding the overvoltage on the healthy submodules after a cell bypass and the little impact of this operation on the quality of the converter output current.

Third harmonic injection control for co‐phase traction power supply system using direct AC/AC full‐bridge modular multilevel converter

AbstractAs the full‐bridge modular multilevel converter (FB‐MMC) enables direct three‐phase AC voltage to single‐phase AC voltage conversion with good power quality and high voltage level, it is suitable for the co‐phase traction power supply system. However, voltage deviation occurs between arms when the direct AC/AC FB‐MMC operates in equal or similar frequency conditions. To solve this problem, a voltage balancing control based on third harmonic injection of co‐phase power supply system using direct AC/AC FB‐MMC is proposed. With the third harmonic injection, voltage balancing between arms is ensured and the capacity of FB‐MMC is enhanced. The third harmonic voltage is directly obtained from existing signals and the third harmonic current reference is produced by simple proportional‐integral (PI) controller. The effect of the third harmonic currents under different voltage ratios and phase difference is quantitatively analyzed, optimized range of voltage ratio and phase difference are obtained to minimize arm currents increment and reduce overrating of converter currents. In addition, a comparison between the proposed and the conventional control scheme is studied, showing the superior of proposed method in cost and performance. Finally, the effectiveness of the proposed scheme is verified by simulations and experiments.

Hybrid Modular Multilevel Converters for High-AC/Low-DC Medium-Voltage Applications

With ever-increasing power-density requirements, technologies such as energy storage systems and electric-vehicles can benefit greatly from interfacing medium-voltage (MV)-AC grid like 13.8kV or 30kV using high-AC/low-DC voltage converter. Using modular high-AC/low-DC voltage converter can help increase power-density and efficiency, while reducing total conversion steps and providing flexibility. Full-bridge modular multilevel converters (FB-MMC) and solid-state transformers are existing solutions for such operations, but suffer from limitations of high semiconductor requirements, large submodule capacitors and/or many high-frequency transformers. Three new hybrid-MMC (HMMC) topologies are proposed in this paper as alternative solutions for such high-AC/low-DC voltage operations. Each of the three developed HMMCs utilizes a unique combination of low-frequency high-voltage switches and fast-switching low-voltage switch based submodules to generate multilevel-AC voltage. HMMCs are compared extensively to state-of-the-art FB-MMC and are shown to have semiconductor savings of over 27%, 38% lower submodule capacitor size, and 53% lower losses for 13.8-kV-AC/6-kV-DC operation. Due to these benefits like higher efficiency, significantly smaller submodule capacitance requirements, and fewer semiconductors, HMMCs can be an excellent option for high-AC/low-DC applications. Practical considerations like snubber and DC split-capacitor requirement are also elaborated for developing and commercializing HMMCs. Comparison results are verified using a 17.5 kW three-phase MV laboratory prototype.

Control strategy for DC series-connected wind farm without energy curtailment

With lower construction costs and power losses, the DC wind farm (DCWF) based on series-connected DC wind turbines (DCWT) is a potential collection scheme for large-scale offshore wind power. However, energy curtailment caused by unequal wind speed exists in DCWTs and results in energy waste, weakening the advantages of DCWTs. The energy curtailment mechanism is analyzed for DCWF, where single active bridge (SAB) is used as DC/DC converter in DCWTS and a full-bridge modular multilevel converter (FB-MMC) as the receiving end converter. The linear relationship between input and output of single active bridge (SAB) is interpreted, and a novel coordinate control strategy for DCWF is proposed to solve the wind energy curtailment caused by unequal wind speed. In addition, an auxiliary control strategy for receiving end converter is proposed to reduce the loss on HVDC link. A complete DCWF is built in PSCAD/EMTDC software package, and the results show that the proposed control strategy makes DCWF can operate well without energy curtailment under unequal wind speeds.

A Line-Frequency Commutated HV VSC Embedded Modular Multilevel Converter With DC Fault Blocking Capability

Modular multilevel converter (MMC) has become one of the most favored topologies in medium-voltage and high-voltage (HV) applications. However, it still suffers from some drawbacks, including a lot of sizeable capacitors, high conduction losses, and no dc-fault ride-through capability. To circumvent these issues, this article presents a modified MMC, namely hybrid-leg MMC (HL-MMC). HL-MMC integrates a pair of three-phase line-frequency commutated HV voltage source converters (VSC) to replace up to 40% MMC submodules (SMs), leading to lower cost and conduction losses. Due to the ripple energy cancellation in the three-phase HV VSCs, the total energy storage requirement in HL-MMC could be also reduced, leading to up to 30% energy storage capacitance reduction compared with MMC. In addition, by combing the dc hybrid circuit breaker, the HV VSCs in the proposed HL-MMC also help provides the dc fault blocking capability, enabling a dc-fault tolerant HL-MMC without using the full-bridge (FB) MMC SMs. Therefore, a significant device and loss saving can be attained compared with the traditional MMC with FB SMs. The detailed converter operational analysis, control method, high-voltage direct current simulation model, and scale-down prototype are provided to validate the effectiveness of proposed topology.

Analytical Modelling and Optimization of Output Voltage Harmonic Spectra of Full-Bridge Modular Multilevel Converters in Boost Mode

This article develops a new closed-form analytical solution to the harmonic spectrum of the full-bridge (FB) modular multilevel converter (MMC) with phase shifted carrier pulse width modulation. It considers FB-MMC operation in boost mode where the negative-voltage states of the FB submodules (SMs) are utilized to extend the ac operating voltage range and/or reduce the dc link voltage which is known to reduce the energy storage requirement. The solution is based on double Fourier series expansion in two variables. The harmonic distribution of the output voltage is analyzed and the effects of the modulation index and the number of SMs on the output voltage harmonics are revealed. The influence of the upper/lower arm carrier phase shift angle is also investigated. Using the analysis, selection of the number of SMs, the modulation index and the upper/lower arm carrier phase shift angle are specified for output voltage harmonic minimization. The findings are verified by simulation and experimental results.

Fast recovery strategy for MMC-HVDC based on coordination of fault current suppression and adaptive restart

Abstract The rapid suppression of fault current flowing through overhead transmission lines and safe restart of the system are critical problems to be solved urgently to improve the stability of the DC grid. A novel adaptive restart strategy combined with fault current suppression is proposed, which suitable for full-bridge modular multilevel converter high voltage direct current system. The resistance-capacitance energy transfer branch is used to achieve a large reduction and rapid attenuation of the fault current amplitude at restart time. The transient fault and permanent fault can be discriminated in a short interval after restarting based on the frequency domain amplitude characteristics. The proposed scheme has both restart-current protection and fault property identification functions, overcoming the high cost and low component reusability of traditional single-function fault current limiting schemes. The simulation results show that the scheme has a significant fault current suppression effect and accurately discriminates the fault property to realize rapid recovery.

Research on Capacitor-Switching Semi-Full-Bridge Submodule of Modular Multilevel Converter Using Si-IGBT and SiC-MOSFET

This article presents a capacitor-switching semi-full-bridge (CS-SFB) submodule (SM) using Si-IGBT and SiC-MOSFET for a modular multilevel converter (MMC). Compared with the conventional hybrid half-/full-bridge MMC, the CS-SFB MMC has the same dc fault ride-through capability and lower power losses. The conduction loss reduces due to the current sharing phenomenon between Si-IGBTs and SiC-MOSFETs. The switching loss reduces in virtue of the fast recovery characteristic of SiC-diode. The derivation process and working principle of the proposed SM are first introduced. Then, the power loss models are established in the switch, SM, and MMC system levels, respectively. Afterward, the piecewise calculation and simulation results reveal the low-power-loss characteristics of the proposed SM and MMC. Finally, these features are verified by experimental results acquired from a downscaled MMC prototype with Si-IGBTs and SiC-MOSFETs.

Protection of Single-Phase Fault at the Transformer Valve Side of FB-MMC-Based Bipolar HVdc Systems

Although the probability of occurrence of ac grounding faults at the valve side of the interface transformer of a high-voltage dc (HVdc) link is low, they may cause high risks to the converter when compared to grid-side ac faults. This article analyzes the characteristics of valve-side ac single-phase-to-ground faults in full-bridge modular multilevel converters (FB-MMCs)-based bipolar HVdc systems. Overcurrents in the converter arms are analyzed and it is shown that overvoltages in FB submodules occur without an appropriate protection in place. Two strategies are investigated to protect the FB-MMC during the fault and corresponding controllers are designed. The effectiveness of the presented strategies for the prevention of overcurrents and overvoltages, upon nonpermanent and permanent faults, and system postfault restoration is investigated. For completeness, the strategies are also verified by conducting simulations in PSCAD/EMTDC.

A Unidirectional Hybrid HVDC Transmission System Based on Diode Rectifier and Full-Bridge MMC

To reduce the cost of bulk power transmission using voltage source converter high-voltage DC (HVDC) technology, a unidirectional hybrid converter is proposed, where a diode rectifier (DR) and a modular multilevel converter (MMC) based on full-bridge (FB) submodules are connected in series on DC side. The FB-MMC controls its DC voltage to regulate the transmitted power. The majority of the power transmission is via the DR considering its cost and efficiency superiority and only low power rating FB-MMC is required. A thyristor valve is equipped at the DC side of the FB-MMC to prevent potential overcharge of the FB submodules during DC faults. Compared to conventional MMCs, losses can potentially be reduced by around 20%. An active power controller is proposed to regulate the DC voltage of the FB-MMC so as to control the transmitted power. With the inverter station controlling its DC terminal voltage constant, the FB-MMC increases the output DC voltage to increase the transmitted power and vice versa. To alleviate overvoltage of the HVDC link during AC grid faults of the inverter station, a dynamic DC voltage limiter is designed to actively reduce the DC output voltage of the FB-MMC. Simulation results confirm the proposed converter operation and control.

Studies of commutation failures in hybrid LCC/MMC HVDC systems

A hybrid of line commutated converters(LCCs) and modular multi-level converters(MMCs) can provide the advantages of both the technologies. However, the commutation failure still exists if the LCC operates as an inverter in a hybrid LCC/MMC system. In this paper, the system behavior during a commutation failure is investigated. Both halfbridge and full-bridge MMCs are considered. Control strategies are examined through simulations conducted in PSCAD/EMTDC.

Studies of commutation failures in hybrid LCC/MMC HVDC systems

A hybrid of line commutated converters (LCCs) and modular multi-level converters (MMCs) can provide the advantages of both the technologies. However, the commutation failure still exists if the LCC operates as an inverter in a hybrid LCC/MMC system. In this paper, the system behavior during a commutation failure is investigated. Both half- bridge and full-bridge MMCs are considered. Control strategies are examined through simulations conducted in PSCAD/ EMTDC. Additionally, commutation failure protection strategies for multi-terminal hybrid LCC/MMC systems with AC and DC circuit breakers are studied. This paper can contribute to the protection design of future hybrid LCC/MMC systems against commutation failures.

A Hybrid Three-Level and Modular Multilevel Converter With DC Fault Blocking Capability and Reduced Semiconductor Losses

This article proposes a hybrid three-level converter (H3LC) with AC-side cascaded full-bridge submodules (FBSM) for high voltage direct current (HVDC) transmission. The three-level converter operates at fundamental frequency switching to modulate square-wave output voltages. The AC outputs of the three-level converter are connected to cascaded FBSMs, which shape the square-wave voltages into multilevel sinusoidal voltages at the point of common coupling. Using third-order harmonic voltage injection, the AC-side chainlink voltage can be limited to within a quarter of the DC-side voltage which reduces the number of FBSMs and the associated semiconductor losses. The H3LC is capable of blocking DC pole-to-pole fault using the AC-side FBSMs and the proposed protection branches, each of which consists of a low-loss load-current carrying branch and a main breaker. Simulation studies demonstrate the effectiveness of the proposed converter for independent real and reactive power control as well as DC pole-to-pole fault blocking. Compared to other state-of-the-art HVDC converter topologies with DC fault blocking capability, i.e., the mixed half- and full-bridge modular multilevel converter and the hybrid two-level converter with AC-side cascaded FBSMs, the proposed H3LC provides reduced semiconductor losses due to fundamental frequency switching of the three-level converter and the reduced number of submodules.