Two-level Voltage Source Converter Research Articles

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.

SiC-MOSFET or Si-IGBT: Comparison of Design and Key Characteristics of a 690 V Grid-Tied Industrial Two-Level Voltage Source Converter

In this paper, a design driven comparison between two 190 kVA industrial three-phase two-level voltage source converter (2L-VSC) designs based in silicon carbide (SiC) and silicon (Si) for 690 V grids is presented. These two designs were conceived to have the same nominal power, while switching at reasonable switching speeds and requiring the same case to ambient thermal impedance. Under these conditions, the designs were studied to detect the potential gains and limitations that a pragmatic converter design could feature when using these two technologies regarding cost, efficiency, size and weight. To achieve this, experimentally determined semiconductor characteristics were used to perform simulations, the results of which were then used to design the essential parts of the converter. These designed parts were then corroborated with manufacturers, from which physical characteristics of all designed components were obtained. The results show that the SiC based design presents substantial weight savings and an 11% system cost reduction, while preserving its traditional characteristics such as improved overall efficiency when compared to the silicon based design under the given design requirements and constraints.

Broadband Impedance Shaping Control Scheme of MMC-Based STATCOM for Improving the Stability of the Wind Farm

Sub and super-synchronous resonance problems caused by the increasing penetration of wind energy in the grid usually can be solved by reshaping the impedance of wind farms. Considering the feasibility and economics, the static synchronous compensator (STATCOM), which is already part of the wind farm in certain regions, becomes a preferred option for impedance shaping to improve the system stability. However, due to the difference between the application object and the application scenario, the control schemes for impedance shaping that widely used in the two-level voltage source converter are difficult to apply directly to the MMC-based STATCOM to solve the resonance problems in the practical wind farm. Therefore, in this article, a broadband impedance shaping control scheme for MMC-based STATCOM is proposed to generate an active parallel impedance, thus reshaping the impedance characteristics of the wind farm over the sub and super-synchronous frequencies for enhancing the system stability. Then, a passivity-based design method for this impedance is developed to ensure that the proposed control can provide sufficient damping at the desired frequency. With this broadband impedance shaping control scheme, the STATCOM can effectively suppress resonance in the wind farm under various operation conditions, which is confirmed by the hardware-in-the-loop experimental results.

Analysis on Response Characteristics of Two-Level VSC to Disturbances and Its Oscillation Suppression Strategy

To improve the stability of renewable energy power systems, the response characteristics of two-level voltage-source converter (VSC) to disturbances are analyzed, and some suppression strategies for oscillation problems are further proposed in this paper. Based on the balance principle of frequency components, the response characteristics of two-level VSC to its AC-side disturbances are investigated in detail, considering its complete control system. Specially, the coupling effect among disturbances with different frequencies are illustrated by the time-domain phasor and space vector analysis. Subsequently, the impedance model describing the port-characteristics of two-level VSC is established and verified through electro-magnetic transient simulation in PSCAD/EMTDC. On the basis, the conditions about oscillation problems caused by negative-resistance effect are concluded and verified in a two-level VSC test system. Finally, by analyzing the influence of controller parameters on impedance characteristics, the controller parameters adjustment based suppression strategy is proposed and verified. From case studies, it indicates that the oscillation instability is related to the resonance frequency and the negative-resistance effect of two-level VSC, and it could be improved by adjusting controller parameters.

High-Efficiency Silicon Carbide-Based Buck-Boost Converter in an Energy Storage System: Minimizing Complexity and Maximizing Efficiency

Energy storage (es) systems are key enablers for the high penetration of renewables. The buck-boost converter in a dc-coupled architecture for integrated photovoltaic (PV) and ES systems shows promising performance with a lower cost and higher efficiency. Silicon carbide (SiC) devices can benefit ES converters as well as the whole ES system. This article focuses on the development of a high-efficiency, SiC-based buck-boost converter in an ES system while emphasizing its unique design considerations. First, the topologies of the buck-boost converter, considering system requirements and device rating/availability, are discussed. A two-level voltage source converter (VSC) with paralleled discrete SiC devices in mature packages (e.g., TO-247) is selected because of its smaller converter complexity and better device availability. Then, the gate drive and layout design for current sharing between paralleled devices are presented. The impact of a device parameter mismatch (e.g., threshold voltage) on switching loss is also evaluated. Moreover, control and protection schemes for the buck-boost converter are discussed. Finally, a 60-kW ES converter with four 1,700-V SiC MOSFETs is prototyped and experimentally demonstrates up to 99.48% efficiency and satisfactory short circuit performance.

Protection Method Based on Wavelet Entropy for MMC-HVDC Overhead Transmission Lines

Recent technological developments in modular multilevel converter-based high-voltage direct current (MMC-HVDC) transmission systems have shown significant advantages over the traditional HVDC and two-level voltage source converter (VSC) transmission systems. However, there are a lack of studies on the protection methods for MMC-HVDC overhead lines where the protection method should be able to provide a fast and accurate response and be able to identify lightning strikes. In this paper, a wavelet entropy-based protection method is proposed. Due to the capability of revealing time–frequency distribution features, the proposed protection method combines wavelet and entropy to identify the time–frequency characteristics of different faults. Simulation results show that the proposed method can accurately and quickly determine the types of faults or disturbances with appropriate noise tolerance. In addition, the impact of the ground resistor and fault distance on the performance of the proposed method is studied.

Modified Nearest Level Modulation for Full-Bridge Based HVDC MMC in Real-Time Hardware-in-Loop Setup

Modular Multilevel Converter (MMC) is an emerging converter topology for medium and high voltage applications. Nearest level Modulation (NLM) is the conventional control topology used to control the MMC that produces the N+1 AC output waveform. In previous research work, the Modified NLM has been already proposed, producing a 2N+1 and 4N+1 output waveform while utilizing a half-bridge (HB) submodule (SM) topology. However, half-bridge-based MMC has a similar behavior as two-level Voltage Source Converter (VSC) and cannot block DC fault current in case of DC-side short circuit fault. So, in recent years, full-bridge-based MMC topology is preferably used by manufacturers as it has DC fault blocking capabilities. This paper presents the Modified NLM for Full bridge (FB) SM topology to take the critical benefits of FB SM topology and improve power quality. The proposed method is simpler to implement and produces a 4N+1 AC output waveform. The THD of the output voltage and current reduces to half compared to the conventional NLM method. The proposed method is verified using LabVIEW Multisim co-simulation and as well as real-time simulation.

Detailed Dynamic Modeling, Control, and Analysis of a Grid-Connected Variable Speed SCIG Wind Energy Conversion System

Wind energy as a renewable energy source continues to be a better alternative to fossil-fuel based generation due to its low cost and environmental benefits. While considerable research efforts have been focused on modeling and control of grid connected variable speed squirrel cage induction generator (SCIG) wind energy conversion systems (WECS), comprehensive models for grid integration studies have been almost non-existent. This paper presents the detail modeling, control and analysis of a grid-connected 2.25-MW variable speed SCIG based WECS that can be utilized for grid integration studies. The presented WECS model consists of a pitch regulated wind turbine connected to a SCIG through a gear box. Then, a full-capacity power electronic converter with maximum power point tracking (MPPT), dc bus voltage regulation and power factor correction, connects the SCIG to the grid. The power converter system comprises of back to back two-level voltage source converters linked through a dc-link capacitor. The generator and grid-side converters are controlled using indirect rotor field-oriented control (IR-FOC) and voltage-oriented control (VOC) respectively. The overall system is simulated in MATLAB/Simulink for a varying wind speed. Results show that the presented model is adequate and efficient in the representation of a variable speed SCIG WECS. In addition, the model meets all of the system performance objectives while simultaneously meeting all of the control objectives.

Two-Port Characterization and Transfer Immittances of AC-DC Converters—Part I: Modeling

This two-part paper presents a general method to model three-phase ac-dc converters in the frequency domain for power system stability studies. In the developed approach, an ac-dc converter is treated as a two-port network connecting an ac and a dc power system, and is characterized by a set of self and transfer immittances. The transfer immittances describe two types of small-signal coupling that are unique to three-phase ac-dc converters: The first type describes the coupling between the ac and dc port and involves a frequency shift by the fundamental; the second type models the coupling over frequency in the ac port current response and involves a frequency shift by twice the fundamental. Together with the self-immittances of each port, the developed models can be used as building blocks to support impedance-based stability analysis of various power systems that involve such converters, including ac, dc, and hybrid ac-dc power generation, transmission and distribution systems. Part I of the paper explains the two-port modeling approach and develops the associated models for a two-level voltage-source converter. Part II presents applications of the developed models for stability analysis of different systems.

Novel space vector-based PWM strategy with minimal circulating current and line-current ripple for two parallel interleaved inverters

Existing PWM methods suffer from a tradeoff between zero-sequence circulating current (ZSCC) and line-current ripple, which are two major concerns of two parallel interleaved two-level (2L) voltage-source converters (VSCs). Thus, this paper introduces a novel space vector-based PWM strategy to reduce both line-current ripple and ZSCC. Given the redundancy of the vector combination of two parallel interleaved converters, this paper proposes eight optimal vector sequences, which simultaneously optimizes the ZSCC and the line-current ripple. Then, a carrier-based scheme is applied to yield optimal vector sequences, which simplifies the implementation procedure. Furthermore, this paper quantitatively compares both the ZSCC and the line-current ripple of the proposed method to those of existing PWM schemes. This comparison confirms that the proposed method is superior to existing PWM schemes in terms of current ripple and ZSCC. Finally, experimental results further verify the theoretical analysis and the effectiveness of the proposed strategy.

Comparative Study of Small-Signal Stability Under Weak AC System Integration for Different VSCs

Voltage source converters (VSCs) with self-commutation ability are suitable for interconnecting weak ac systems. This article conducts a comparative study of the small-signal stability characteristics for three typical VSCs, namely, the two-level VSCs (TL-VSCs), the half-bridge modular multilevel converters (HB-MMCs), and the hybrid MMCs (HY-MMCs), under weak ac system integration with special consideration on both inverter and rectifier operation. The frequency responses based on impedance models are compared using the frequency-domain analysis. The oscillation frequencies and mainly participated state variables of the unstable modes are compared using root locus and participation factor analysis in the time domain. Proper parameter retuning approaches for stability enhancement are proposed. The above analysis is adequately validated by electromagnetic simulations.

Comparative Study of Dynamic Phasor and Harmonic State-Space Modeling for Small-Signal Stability Analysis

This paper conducts a comparison of two promising frequency-lifted representations used in the state-space modeling of power-electronic converters: dynamic phasors and harmonic state-space. These methods originate from similar hypotheses and aim to tackle the time-periodicity problem observed in systems with multiple harmonic components in steady state. The paper derives the theoretical foundations of the methods with an emphasis on their similarities and differences, and applies them to a two-level voltage-source converter. The application demonstrates the effects of different truncation orders of the models infinite formulations. Stability analysis tools such as eigenvalues and transfer functions are examined and benchmarked against a classical dq-frame small-signal model.

Determination of RMS Current Load on the DC-Link Capacitor of Voltage Source Converters Using Direct Current Control

RMS current load is one of the most important design criteria for the dc-link capacitors of voltage source converters. In the literature, determination of this rms current load is well documented for state-of-the-art modulation schemes, such as space vector modulation. But the methods presented cannot be applied for direct current control algorithms. Therefore, this article introduces a new approach especially for direct current controllers, which reflects and compares to the state-of-the-art calculation method for space vector modulation. As direct current control is a feedback control, no closed-form solution can be given. Instead, the dc-link capacitor rms current load is determined by an iterative calculation scheme as a function of the modulation level and the phase angle. Although the scheme presented is applied to a three-phase two-level voltage source converter, it can be adapted to any voltage source converter topology. An example is presented applying the calculation method to one specific direct current control algorithm. The calculated results are verified via a digital simulation model as well as test bench measurements showing good correlation. For correct comparison, different effects are considered and discussed between simulation, calculation, and measurement results. These contain transients, the commutation process between insulated gate bipolar transistor (IGBT) and diode, the current-dependent voltage drop of the semiconductors, and the stationary current error of the hysteresis control. The dc-link capacitor current load for space vector modulation is calculated analytically as a reference by using the state-of-the-art method. The comparison of results obtained in this article and the reference are very similar, which is discussed in the conclusion.

Comparison of common DC and AC bus architectures for EV fast charging stations and impact on power quality

Abstract This work covers the comparative analysis of common DC and AC bus architectures for grid-connected Electric Vehicle Fast Charging Stations (EVFCS) and addresses the relevant power quality issues. In this manuscript, Unipolar Common DC Bus and Common AC bus topologies are proposed for FCS. Two-level Voltage Source Converter (VSC) is used to connect the EVFCS to the grid through DC and AC buses in both the methods. Converters are connected to the grid through a Distribution Transformer (DT), and the output side is connected to the EV battery chargers. Grid-connected FCS are examined in terms of harmonic generation, and their impact on the host electric grid is evaluated. Voltage and current harmonics of both the topologies with different transformer configurations are compared, and an optimum transformer configuration is proposed. The Unit Template-based Control (UTC) algorithm is also proposed in this work, and the Constant Current-Constant Voltage (CC-CV) strategy is used for the charging control of the batteries. The work is focused on feasibility and comparison of common DC and AC bus FCS and impacts of harmonics generated due to EV charging on the grid power quality. The proposed architectures are then evaluated as per IEC and IEEE Power Quality standards. Power quality impacts are studied by varying the number of EV charging bays. The charging profile of both the architectures is examined by performing steady-state and dynamic analysis. Furthermore, a comparative analysis is also performed by considering different transformer configurations.

Optimal LCL-filter design method for grid-connected renewable energy sources

This paper presents a design method based on the extended harmonic domain modeling for L and LCL passive filters of active-front-end PWM two-level voltage source converters for grid-connected renewable energy sources. The proposed design method is tested under several operating scenarios, such as high and low power levels and switching frequencies. Also, a dedicated graphical user interface (GUI) with five other design methods is provided to ease the evaluation of the proposed design method. The GUI also computes the power quality indexes and the steady-state waveforms for each design methodology. In addition to this, an in-depth overview of filtering technologies and applications is presented to identify the primary practical use of passive filtering and to justify the use of LCL filter in grid-connected systems. The obtained results show that the proposed design method leads to the smallest passive filter parameters, and it is the only method that fulfills all the imposed power quality requirements as well as power specifications.

Quantitative Analysis of AC/DC Side Harmonic Transfer Characteristics for Modular Multilevel Converter and Suppression Measures

MMC (Modular Multilevel Converter) is the preferred topology for renewable energy delivery, but renewable energy resources, such as wind farms, contains much harmonic components. Compared with two-level VSC (Voltage Source Converter), the internal dynamic characteristics of MMC have great impact on the harmonic transmission. This paper quantitatively analyzes the harmonic transferring mechanism from one side to the other side of MMC, and deduces the time-domain analytical expression of the harmonic response on the MMC’s other side. Firstly, the influence of background harmonic on MMC’s control system and electrical system are quantitatively analyzed. On this basis, the equivalent circuit of MMC’s electrical system considering its internal dynamics is established to accurately calculate the effect of background harmonic on the MMC’s other side. In addition, the harmonic transferring impedance in frequency-domain is also defined in this paper, based on that the influence of background harmonic on the MMC’s other side can be calculated quickly. According to the harmonic transferring characteristics analyzed above, a zero-sequence virtual damping strategy is proposed for the first time to suppress the background harmonic transferring. Finally, a 232-level 500kV MMC simulated model is built in MATLAB/Simulink to verify the accuracy of the harmonic transferring calculation method and the effectiveness of the virtual damping suppression strategy.

Performance Assessment of the RTDS/RSCAD VSC Model

This document presents a performance assessment of the three-phase two-level voltage source converter (VSC) model of the real-time digital simulator RTDS/RSCAD. This evaluation is conducted through two case studies. One case is a simple two-bus power electronic-based system, and the second case is a three-bus system with a frequency-dependent transmission line model and a closed-loop controlled VSC with communication delay. The results obtained reveal that the RTDS/RSCAD VSC model presents some issues on stability, harmonic spectrum, ripple, and power quality in general, as compared with the PSCAD VSC solutions. These results also suggest the need for better power electronic components models.

DC Fault Analysis Models of Three Converter Topologies Considering Control Effects

Existing converter models are normally reduced as simplified electrical components, whereas it overlooks the impacts of fast dynamics of converter current control loops. Therefore, the dc fault current calculation with the traditional simplified converter models becomes invalid when the simulation time is longer. To overcome this problem, this article incorporates the control effects of converters during the transient process and proposes three dc fault analysis models of classical three converter topologies for the high-voltage dc (HVdc) application including line communicated converter (LCC), two-level voltage-source converter (VSC), and modular multilevel converter (MMC). For an LCC-based rectifier, the nonlinear part of original model is first linearized with the least square method due to the large dc-link voltage deviation during dc fault, and the proportional integral (PI) current regulator with the reduced linearized model can be equivalent as an RC circuit for dc fault analysis. However, for two-level VSC or MMC, the dc-link voltage deviation does not drop that much with a large dc-link capacitor discharging process during transient dynamics. As a result, the original model is linearized relying on the small-signal method based on prefault operation points, and the dynamics of fast inner current loop with the linearized model can be represented as an RL circuit for dc fault current calculation. for dc fault analysis The proposed dc fault analysis models of three converters as equivalent RLC circuits are well testified under different fault resistances and dc reactors, control parameters, and operating conditions. Moreover, dc fault simulations of a hybrid HVdc system have verified the effectiveness of the proposed converter models.

Equivalent Model of Modular Multilevel Converter Considering Capacitor Voltage Ripples

Modular multilevel converter (MMC) is regarded as a topology with high potential for high-voltage direct current transmission system. However, the distributed capacitors lead to significant submodule capacitor voltage ripples, which have great influence on the terminal characteristics of MMC. In this paper, an equivalent model considering submodule capacitor voltage ripples and circulating current suppression control is established to explore the terminal characteristics of MMC. In the model, MMC is equivalent to the cascade of buck/boost converter and two-level voltage source converter (VSC) from a terminal perspective. The influence of capacitor voltage ripples is represented by the equivalent series impedances, whose characteristics are investigated in detail with harmonic linearization method. Then, the simplified equivalent series impedances are obtained. Based on the equivalent model, the study of terminal characteristics is greatly simplified by directly applying conclusions of two-level VSC and buck/boost converter to MMC. Furthermore, the control strategy design and impedance modeling of MMC have been carried out to show how the equivalent model can be used in practical applications. Finally, the effectiveness of the proposed model is verified by detailed simulation results.