Two-level Voltage Source Converter Research Articles

Analysis and Design of a Novel Hybrid Modular Multilevel Converter With Time-Sharing Alternative Arm Converter

This paper proposes a time-sharing principle-based modular multilevel converter (TS-MMC) for medium- to high-voltage power transmission applications, which is designed with a lightweight running feature. The TS-MMC consists of cascaded switch stacks (CSSs) and alternative arm converters (AACs), integrating the merits of both the MMC and the two-level voltage source converter (VSC). In TS-MMC, the AACs shape the required multilevel arm waveforms, and the CSSs arrange the AACs operating at different intervals for producing the desirable output waveforms. By applying a “time-sharing” principle for the AACs, a dramatic reduction of the sub-module (SM) number is achieved. Additionally, the TS-MMC does not require any arm inductors, whereas only three inductors are equipped in AC side. Moreover, featured by the specific structure, the DC fault blocking capability of the TS-MMC is also enabled. Furthermore, to explore the performance of the TS-MMC, mathematical model and control strategy of the TS-MMC are provided. Finally, the simulation and experimental results are carried out to verify the feasibility of the TS-MMC.

Part I—Advancements in Power Converter Technologies: A Focus on SiC-MOSFET-Based Voltage Source Converters

Power converter technologies have become vital in various applications due to their efficient management of electrical energy. With the growing prominence of renewable energy sources such as solar and wind, the high penetration of power electronic converters has been justified. However, ensuring power quality has emerged as a significant challenge for grid-connected power converters. The divergence from the ideal sinusoidal waveform in terms of magnitude and frequency impacts both grid-side currents and voltages. Several studies have proposed solutions to address power quality issues at the load side. The advancement of power converters has been fueled by the development of high-performance microprocessors and the emergence of high-speed switching devices, such as SiC-MOSFETs. This paper focuses on the design of voltage source converters, particularly those based on SiC-MOSFET semiconductor devices. The article presents the design of H-Bridge cells, discusses two-level voltage source converters based on cascade H-Bridge cells in a parallel configuration with experimental fault analysis, addresses the seven-level voltage source converter topology, and explores the design and experimental results of the matrix converter. The findings underscore the importance of considering the entire converter design for improved performance at high switching frequencies. The article concludes by summarizing the main outcomes and implications of this research.

Estimation of Stator Vibration in DFIM Under Various Types of Rotor Excitation

Variable speed power generation using Doubly Fed Induction Machine requires an adjustable frequency supply at the rotor terminals which can be achieved using power converters. Ideally, a variable frequency sinusoidal voltage should be applied, but it is not possible without the help of power converters. For a typical 250 MW DFIM with a speed variation of -10.73% to +8.33%, excitation power of 25 MW at 3.3kV and 11600 Amps is required. With this much high rotor current at low fundamental rotor frequency (3.75 Hz) electromagnetic forces in the air gap contain harmonic force components. Also, any time harmonic of radial force density in the rotor will be induced in the stator as a distinct time harmonic, modified by the influence of space harmonics in DFIM with regular three phase integral slot winding on both stator and rotor and vice versa. So, if the frequency and order of electromagnetic force components conflicts with mode shape and natural frequencies of the machine stator strong magnification in vibration will occur or more severely develops mechanical resonance. The stator vibration of electrical machine is generated by the electromagnetic forces acting on the surface of the stator tooths. This paper analyzes stator vibration for 250 MW DFIM in multiphysics simulation with its rotor winding excited through pure sinusoidal excitation at fundamental rotor frequency of 3.75 Hz and with two-Level voltage source converter at 3.75 Hz fundamental rotor frequency. Hardware results with supported multiphysics simulation on a laboratory prototype of 2.2 kW rating DFIM.

Improved Loss Minimization Control Based on Time-Harmonic Equivalent Circuit for Linear Induction Motors Adopted to Linear Metro

By the supply of two-level voltage source converter with low switching frequency, the linear induction motor (LIM) adopted to linear metro suffers from time harmonics greatly, which would aggravate the airgap flux density, cause extra harmonic loss, and reduce working efficiency for the LIM and drive. In order to quantitatively analyze and reduce the neglective influence of time harmonics, one loss minimization control (LMC) scheme based on improved loss model is proposed in this paper. First, the traditional loss model under sinusoidal excitation is introduced. Second, after analyzing the main harmonic components of converter, one improved loss model based on the equivalent circuit under harmonic excitation is innovatively proposed in this work. Third, one novel LMC scheme with loss minimization cost function is developed, which can find out the optimal flux linkage to get the minimal loss for the LIM. Finally, the proposed loss model considering harmonic and LMC scheme have been fully validated by comprehensive simulation and experiments in terms of one 3 kW prototype.

Impedance Modeling and Stability Analysis of Energy Controlled Modular Multilevel Converter

In recent years, the modular multilevel converter (MMC) has been widely used in high-voltage dc (HVDC) transmission, which, however, brings about various stability issues, such as sub- -synchronous oscillation (SSO). To deal with these stability issues, many active and passive damping methods have been proposed. In fact, the MMC itself has more control degrees of freedom compared to the two-level voltage-source converter (VSC), such as internal energy, circulating current, etc., which can be exploited to improve the system stability. Based on the idea, the internal energy of MMC is utilized to suppress the SSOs in this paper. The frequency-coupling impedance model of MMC with energy controller is first developed based on the harmonic state-space (HSS) method by considering the internal dynamics and all control links, which is verified by frequency scanning result in simulation. The stability analysis of the energy controlled MMC-HVDC for wind farm integration is conducted and a modified energy control structure of MMC is also proposed to further improve the stability of the interconnected system.

A soft start-up method for DC micro-grid based on improved two-level VSC with DC fault ride-through capability

For the black start-up of DC micro-grids, three-phase charging resistors are required to limit the uncontrollable surge current. The main drawback of this start-up method is the difficulty in determining the appropriate resistance value to achieve a rapid start-up and limit the surge current with the change of grid parameters. To address this problem, this article proposes a soft start-up method for the DC micro-grid based on an improved two-level voltage source converter (VSC). Specifically, an silicon controlled rectifier and anti-parallel diode are added in each up-bridge-arm in the improved VSC. By conducting a dynamic control strategy of the firing angle on the SCRs, the start-up current can always be maintained near a given value to achieve rapid start-up. Moreover, the improved VSC has DC fault ride-through capability. The simulation results based on PSCAD/EMTDC are provided to validate the feasibility of the proposed start-up method.

Sliding-Mode Control for Interleaved Operation of Parallel-Connected Three-Phase Active Front-Ends

A converter system is considered that consists of parallel-connected two-level voltage source converters that share the same ac and dc terminals. A controller design is proposed that, in a first stage, controls the grid currents using hysteresis controllers. To this end, augmented grid currents are introduced, which feature decoupling between the phases. As a consequence, a switching action in one phase does not affect the derivatives of these currents in the other phases. In a second stage, the converter currents are controlled using hysteresis controllers such that a desired current distribution among the parallel converters may be ensured. This implicitly sets the circulating currents. The proposed approach is verified by measurements on an industrial megawatt-scale testbench with ten parallel converters. The results are compared with a benchmark approach, with the proposed approach showing lower semiconductor losses and grid current harmonic distortion.

Battery Storage System as Power Unbalance Redistributor in Distribution Grids Based on Three Legs Four Wire Voltage Source Converter

This article discusses the application of battery energy storage systems (BESSs) as power redistributors in three-phase distribution grids as an add-on functionality to typical BESS applications, such as congestion management and energy arbitrage. Combining those ancillary services into a single power unit is not yet performed in practice but may constitute an emerging business opportunity to increase the BESS revenues. The unbalanced operation of the BESS voltage source converter (VSC) leads to the circulation of low-frequency current harmonics in the dc-link through the capacitors and the battery cells. Therefore, it is particularly interesting whether relatively large 50- and 100-Hz currents can safely circulate within these components. Analytical modeling and design guidelines for the dc-link of a three-leg four-wire two-level VSC operating under unbalanced loads are detailed. Furthermore, a low-power VSC prototype is used to demonstrate the working principle of the BESS, providing power unbalance redistribution and symmetric power exchange. Additionally, the ICR18650-26F Lithium-ion cells are cycled to reach end-of-life with different current profiles and C-ratings. The analysis shows that charging with a 100 Hz ripple superimposed to the dc current leads to a 10% increment in degradation.

Accurate High-Frequency Modeling of the Input Admittance of PWM Grid-Connected VSCs

This article addresses high-frequency admittance modeling of current-controlled voltage source converters (VSCs). Recent studies have shown that harmonic instability may also occur at frequencies above the Nyquist frequency. To form an accurate multiple-frequency model in this frequency range, sidebands that originate from modulation and sampling must be examined. In this article, an accurate small-signal model is developed taking into account an adequate digital pulsewidth modulator (DPWM) representation, which allows to predict dependence of the frequency response on the steady-state dc (SS-dc) operating point. It is shown that when center-pulse sampling is implemented, pulsewidth modulation sidebands do not create additional loops leaving only sampling sidebands to be considered. Using the same approach as for SS-dc operation, a model that accurately represents admittance measurements during sinusoidal ac (S-ac) operation is developed. Its basis is a novel DPWM model suitable for S-ac regime, which allows to predict dependence of the VSC’s input admittance on the grid voltage magnitude. Experimental and simulated admittance measurements, performed on a single-phase two-level VSC during various SS-dc and S-ac regimes, match with the proposed models up to twice the sampling frequency.

A Hierarchical Small-Signal Controller Stability Analysis Method for the MMCs

Differing from a two-level voltage source converter (TL-VSC), the internal dynamics of a modular multilevel converter (MMC) are much more complicated. Since the fluctuation of submodule capacitor voltage is unavoidable when transmitting power, the MMCs output voltages cannot track the references perfectly. The interaction between the time-varying capacitor dynamics and controllers may lead to self-instability of the MMCs. In order to address this self-instability issue, this paper first establishes the small-signal closed loop transfer function matrices (TFMs) of the external and internal control loop by the harmonic linearization method. Further, a novel design principle of MMC controllers is proposed. Then, the critical impact factors on the stability of circulating current controller and AC voltage controller are investigated based on the generalized Nyquist criterion(GNC). The proposed models and analysis methods are validated by time-domain simulations in PSCAD/EMTDC.

Hybrid Si/SiC Switch Modulation With Minimum SiC MOSFET Conduction in Grid Connected Voltage Source Converters

In this article, a hybrid Si/Si carbide (SiC) switch (HyS) modulation with minimum SiC MOSFET conduction (mcHyS) is experimentally characterized, so as to derive its conduction and switching performance. These are later used to derive a silicon (Si) area analytical model for the HyS configuration. The chip area model is used to benchmark the mcHyS modulation concepts against single-technology switches and typical HyS modulation when considering the implementation of a 100-kW two-level voltage-source converter (VSC) deployed for three industrial applications: photovoltaic inverter, electric vehicle fast-charging station, and battery storage systems for grid ancillary service. The two additional switching events of the SiC MOSFET, which differentiate the mcHyS modulation from the typical HyS one, are proven to happen in soft switching; therefore, the mcHyS switching performances are not penalized. Furthermore, the analysis presented shows how the studied mcHyS modulation performs against the single semiconductor technology and the typical HyS solution in terms of cost and power conversion efficiency. More specifically, it is shown that the HyS solutions are particularly competitive versus the full Si-based VSCs when the application at hand often operates at low partial loads. Finally, a 10-kW two-level VSC assembled with mcHyS is tested, so as to compare its efficiency versus single-technology switches.

Multi-Sampled Grid-Connected VSCs: A Path Toward Inherent Admittance Passivity

This article investigates the capability of multi-sampled pulsewidth modulator (MS-PWM) control to improve the input admittance properties of voltage source converters (VSCs). Due to delays found in digital control systems, the VSC admittance features a negative real part above a certain frequency, which may result in lowly damped or even unstable grid dynamics. To prevent the occurrence of harmonic instability, recent standards require the admittance of every grid-connected VSC to behave as a passive network. In this article, it is shown that the MS-PWM control significantly improves passivity measures by reducing the overall system delay. The passive behavior is achieved effortlessly, using a single-stage control loop. This avoids the need for passive or active damping techniques, which are associated with increased losses and number of sensors, sensitivity to noisy measurements, ambiguity of damping filter design, and overall system complexity. The experimental verification, performed on a two-level single-phase VSC, shows a very good match between admittance measurements and analytic modeling even above the switching frequency. Grid-connected operation is tested to demonstrate the improvement of resonance damping obtained with MS-PWM control.

Optimal Periodic Variable Switching PWM for Harmonic Performance Enhancement in Grid-Connected Voltage Source Converters

Variable switching frequency PWM (VSFPWM) modulation can be advantageously implemented in industrial applications, such as renewable energy, motor drives, and uninterrupted power supply (UPS) systems, to reduce the injected current harmonic amplitudes, to suppress audible noise, and to improve semiconductor power efficiency. In this article, the usage of periodic VSFPWM methods in a voltage source converter (VSC) is proposed, analyzed, and benchmarked in terms of harmonic spectrum spreading, following the IEEE-519 current harmonic standard for the connection to the distribution grid. Particular attention is paid to the influence of VSFPWM on the ac filter design. First, the analytical model of the voltage harmonic spectrum generated by a three-phase three-wire two-level VSC implementing several periodic VSFPWM methods is derived. Subsequently, a design guideline for the commonly used <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">LCL</i> filter in the grid-tied VSC application is proposed, which minimizes the size requirement of the necessary components. The voltage spectrum models of the proposed VSFPWM method and the optimal switching profiles are verified by MATLAB/Simulink simulations and a 5-kW three-phase two-level VSC hardware demonstrator. The study shows that the ac filter power density for the studied VSFPWM methods can be greatly increased when compared with the conventional and widely employed constant switching frequency continuous PWM strategies.

A Three-Phase Active-Front-End Converter System Enabled by 10-kV SiC MOSFETs Aimed at a Solid-State Transformer Application

The use of high-voltage silicon carbide (SiC) devices can eliminate multilevel and cascaded converters and their complicated control strategies, making converter systems simple and reliable. A three-phase two-level voltage-source converter system serves as a simple converter system for interfacing any dc source to a three-phase grid. However, when the high-voltage devices are used in two-level converters, they are exposed to a high-voltage peak stress and a high <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$dv/dt$</tex-math></inline-formula> (up to 100 kV/ <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). Operating these semiconductor devices at these stress levels requires careful design not only of the semiconductor die and the module, but also of the gate drivers, busbars, and passive filters. This article demonstrates the operation of 10-kV SiC <sc xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">mosfet</small> s and discusses the design considerations, advantages, and challenges associated with the operation of the three-phase two-level medium-voltage converter system used as the active-front-end converter system. Reliable operation of the medium-voltage converter system requires the development of reliable high-voltage modules and auxiliary parts, such as gate drivers, busbars, inductors, voltage and current sensors, and proper design of the controller system. Successful tests demonstrating continuous field operation of the medium-voltage active-front-end converter at a nominal rating of 7.2-kV dc-link voltage is demonstrated for the first time in the literature. The results indicate that these devices can accelerate the growth and deployment of medium-voltage SiC devices for field operation, as demonstrated by the operation inside the mobile container.

Dc Link Voltage Balance Of Grid Integrated Pv Module Using Pi-Awt

The purpose of this paper is to investigate the performance of a PV-based grid-connected system. In the suggested arrangement, there are two phases in the conversion process. The conversion of alternating current voltage to direct current voltage is the first stage. It is decided to use the usual two-level converter at this point. The second stage consists of converting the direct current voltage to alternating current voltage using a typical two-level voltage source converter (VSC). A control technique based on the Anti-windup tuned PI (PI+AWT) is being developed to govern the dc link voltage at VSC of the second stage conversion. The proportional resonant controller is used to control the current signals and also to regulate the reference signals in the pulse width modulation (PWM) scheme, which is described in detail below. The sinusoidal pulse width modulation (PWM) technique is used to regulate VSC switches. The suggested controller’s performance is evaluated under three different conditions: continuous irradiation, change of irradiation, and three-phase fault. MATLAB/SIMULINK is used to calculate the parameters of the results.

Improving the Stability of Standalone MMCs by Shaping the AC Side Impedance Using Insertion Index Compensation

Due to the significant coupling of the internal dynamics on the ac-side in a modular multilevel converter (MMC), its terminal characteristics are quite different from a conventional two-level voltage source converter (TL-VSC). Thus, the stability analysis and controller parameters design of the MMC without ignoring internal dynamics must adopt complicated modeling approaches, like the harmonic state-space (HSS) method, which doesn’t have an intuitive insight. Therefore, to simplify the modeling process and give a clear physical meaning, this paper proposes a decoupling method with ac-side impedance shaping by feedforwarding the capacitor voltages to compensate the arm insertion indices, which can also improve the stability of the MMC. Consequently, the ac-side impedance characteristics of the MMC are similar to that of the TL-VSCs, and the stability and controller parameters can be design-oriented by using a simple averaged state-space model. Finally, the effectiveness of the theoretical analysis and correctness of the proposed approach are verified by time-domain simulations in PSCAD/EMTDC.

Stability Analysis of Chaotic Grey-Wolf Optimized Grid-Tied PV-Hybrid Storage System during Dynamic Conditions

This paper presents the stability improvement of the three-phase four-wire (3P-4W) grid-tied PV-hybrid energy storage system (HESS) using chaotic grew wolf optimization (CGWO) for DC bus voltage (Vdc) and AC bus voltage (Vpcc) control. The CGWO tuned fractional order proportional–integral (FOPI) controllers reduce the Vdc and Vpcc variations during diverse, dynamic conditions, i.e., sudden irradiation variations, deep voltage sag/swell, etc. The DC bus is responsible for the current injection/extraction control, maximum PV power extraction, bi-directional power flow, dc second-harmonics component elimination, and active power balance. At the point of common coupling (PCC), the AC bus is accountable for bi-directional power flow and active and reactive power control. The two-level voltage source converter (VSC) is controlled by a novel variable step-size incremental least mean square (VSS-ILMS) in zero voltage regulation (ZVR) mode. Due to its varying step size, VSC control is less prone to noise signals offers better stability, improved convergence rate, dc offset rejection, and tracking speed during dynamics, i.e., large oscillations. A battery and ultracapacitor are coupled to the DC link by buck-boost converters in the HESS. To regulate power transit between the DC bus and the grid, the HESS current control technique is designed to shift frequently from charging to discharging stage and vice versa. The novelty of the PV-HESS system lies in CGWO tuned VSS-ILMS control of VSC, which effectively and efficiently filter out the active fundamental constituents of load current and eliminate dc offset from VSC output. The HESS control maintains the DC bus voltage profile by absorbing and delivering energy (during dynamic conditions) rather than curtailing it. The presented system is simulated in a MATLAB/SIMULINK environment. The simulation results in graphical and numerical forms verify the stable and satisfactory operation of the proposed system as per IEEE519 standard.

Low Switching Frequency Operation Control of Line Voltage Cascade Triple Converter

With the increasing power level of wind power generation system, the traditional topology of power converters can no longer meets the demand of high-power wind power generation systems due to the limitation of device performance. The line voltage cascade type multiple PWM converter (LVC-VSC) is a kind of converter that uses the traditional two-level and six-switch voltage source converter as the basic component unit, and each unit is combined with the line voltage cascade method. This type of converter is suitable for medium-voltage and high-power applications such as wind power generation and metallurgical drives because of its easy modularization, strong scalability and low number of isolated power supplies required. However, for medium-voltage and high-power applications, the switching frequency of power devices in the converter is low, usually limited to a few hundred hertz. The traditional modulation method of line voltage cascade converter has a large number of redundant states, and simply reducing the carrier ratio will cause serious degradation of control performance and system instability. To address this problem, this paper proposes a modulation strategy and a corresponding control method for low switching frequency. The modulation strategy is based on the vector relationship of finite switching states, and the optimal switching sequence is selected according to the modulation system by removing redundant states, thus ensuring the application of different modulation sequences under different modulation depths and ensuring the current quality on the basis of the minimum switching frequency, which effectively solves the control problems at low switching frequency. The experimental results show the correctness and effectiveness of the proposed modulation strategy and control method.

Modular Multilevel Converter for Low-Voltage Ride-Through Support in AC Networks

New grid-connected systems have imposed additional requirements regarding reliability, power quality, high levels of power processing capacity, and fault support, where power converters have a crucial role in fulfilling these requirements. Overcoming one of these challenges, this paper proposes a new alternative application to improve the low-voltage ride-through (LVRT) support based on the arm impedance employment of the modular multilevel converter (MMC) by attenuating the fault impacts, avoiding overcurrents and overvoltages. This proposal does not require additional hardware or control loops for LVRT support, only using PI controllers. This paper evaluates symmetrical and asymmetrical grid fault impacts on the converter DC side of four converter topologies: two-level voltage source converter topology (2L-VSC), neutral point clamped (NPC), MMC, and 2L-VSC equipped with a DC-chopper, employing the same control structure for the four topologies, highlighting that the MMC contributed better to LVRT improvement under severe grid conditions.

Control and Validation of a Reinforced Power Conversion System for Upcoming Bioelectrochemical Power to Gas Stations

This paper presents a proposal for potential bioelectrochemical power to gas stations. It consists of a two-level voltage source converter interfacing the electrical grid on the AC side and an electromethanogenesis based bioelectrochemical system (EMG-BES) working as a stacked module on the DC side. The proposed system converts CO2 and electrical energy into methane, using wastewater as the additional chemical energy input. This energy storage system can contribute to dampening the variability of renewables in the electrical network, provide even flexibility and grid services by controlling the active and reactive power exchanged and is an interesting alternative technology in the market of energy storage for big energy applications. The big challenge for controlling this system lays in the fact that the DC bus voltage of the converter has to be changed in order to regulate the exchanged active power with the grid. This paper presents a cascade approach to control such a system by means of combining external control loops with fast inner loops. The outer power loop, with a proportional-integral (PI) controller with special limitation values and anti-windup capability, is used to generate DC bus voltage reference. An intermediate loop is used for DC bus voltage regulation and current reference generation. A new proportional resonant controller is used to track the current reference. The proposed scheme has been validated through real-time simulation in OPAL OP4510.