Voltage Source Converter Topology Research Articles

Experimental Investigation of Modified Level Shift PWM with Capacitor Voltage Balancing on Single Phase Modular Multilevel Converter

Modular Multilevel Converters (MMCs) are a prominent voltage source converter topology that is rapidly gaining popularity in medium/high power/voltage applications, including high-voltage DC transmission systems and electric vehicle systems. However, MMC has the critical issue of unbalanced submodule capacitor voltages and circulating current. The MMC distributes DC-link energy evenly among its submodule capacitors, rather than storing it in a large capacitor like conventional voltage source converters. In MMC, the submodule floating capacitors interface the DC input voltage and AC output voltage. Therefore, capacitor voltages must be balanced. The existing capacitor voltage control struggles to handle many gate pulses at medium and high voltages. An effective control scheme is needed to solve the issues mentioned earlier. This paper presents a performance analysis of a 1-Φ MMC using a modified level shift PWM technique based on Universal Control Modulation Scheme (UCMS) and a sorting-based capacitor voltage balancing algorithm. MATLAB/Simulink software implements the proposed control method for a 1-Φ , seven-level MMC. The outcomes of the simulation demonstration reveal that phase disposition PWM offers less harmonic distortion of output phase voltage than the other level shift PWM techniques, such as phase disposition PWM and alternate phase disposition PWM. The simulation results also demonstrate the effective use of the sorting-based capacitor balancing algorithm to regulate the voltages of the submodule capacitors. Finally, the real-time GUI tool validates the simulation results using an experimental prototype of 1-Φ MMC with the dSPACE MicroLabBox 1202.

Design of a high-efficient SiC-based interleaved voltage source converter

The paper deals with some peculiar aspects of the design of interleaved voltage source converter (IVSC) topology in high-power applications such as renewable energy systems and electric vehicles. The IVSC offers advantages like power sharing among multiple modules, improved power quality, redundancy, and fault tolerance. The design of an IVSC mainly involves the selection of the number of deployed converters and the choice of suitable semiconductor devices with the aim to reach a desired overall efficiency for an assigned operating condition. The paper develops an approach to characterize through a straightforward procedure the dependence of the IVSC' power losses on the number of the interlaced converters, on the employed semiconductor devices used and on a properly conceived set of key parameters which define the system operating conditions. The proposed methodology allows to highlight the system performance' dependence on the chosen power devices and it is showcased in this work by considering silicon carbide (SIC) devices with different current ratings. The main goal of the paper is to formulate a smart design procedure able to guide the configuration of an IVSC towards an optimal choice with respect to the power module rating, the number of interleaved levels, the system efficiency, and the output current waveforms. The proposed procedure has been experimentally validated.

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.

Modeling and Analysis on AC-DC Harmonic Coupling of the Three-Phase Voltage Source Converter under Asymmetric Condition

A three-phase voltage source converter (VSC) is the most common power electronic device in power systems, but its inherent nonlinearity leads to complex AC-DC harmonic coupling phenomena. Existing studies focus on the harmonic coupling characteristics of three-phase VSCs under three-phase symmetrical conditions, but the problem under asymmetrical conditions is rarely investigated. This paper proposes a practical modeling method for harmonic state space (HSS) modeling of the VSC topology, together with a classic T/4-delay-based decoupled double synchronous reference frame (DDSRF) control system. The steady-state phase shift characteristics of the T/4 delay link for each frequency component are first analyzed, and the expansion method of the DDSRF controller in the HSS frame is derived. Compared with the conventional method, the proposed technique can provide an accurate description of the steady-state effect of the delay link without introducing additional state variables and, therefore, can reduce model complexity and computation burden. Then, the small-signal close-loop HSS model of the VSC is established, and a harmonic transfer matrix between the AC positive sequence/negative sequence and DC current of different frequencies is developed, which reveals the global harmonic coupling relationship between AC and DC grids across the VSC. Analysis shows that the T/4-delay-link significantly weakens the AC-DC inter-harmonic coupling near the fundamental frequency. Finally, the electromagnetic transient simulations are carried out based on MATLAB/Simulink, and the validity of the proposed modeling method and the model and the correctness for the analysis of harmonic coupling characteristics are verified.

An Interactionless Duo Control Strategy for Bipolar Voltage-Source Converter in Renewables Integrated Multiterminal HVdc Grids

The dc voltage and frequency <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$ (PVF)$</tex-math></inline-formula> or modified dc voltage and frequency <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$ (PV^{2}F)$</tex-math></inline-formula> double droop control is commended for its ability to regulate both the dc voltage and the frequency in a decentralized approach. However, a convincing response is not achieved due to an interaction between the droop characteristics of dc voltage and frequency. This interaction affects the dc voltage and frequency support of the ac-system-surrounded multiterminal HVdc (AC-MTdc) grid. To overcome this effect, a Duo control strategy is proposed in this article, which takes advantage of a bipolar voltage-source converter topology in the MTdc grid. The virtue of the proposed control technique is emphasized by comparing it with the existing <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$ PV^{2}F$</tex-math></inline-formula> double droop control along with three case studies and two test systems. The validation of the interactionless Duo control strategy is carried out on a five-terminal CIGRE dc grid benchmark model integrated into a two-area power system (AC-MTdc grid-1) and a New England IEEE 39-bus system (AC-MTdc grid-2). These test systems are simulated in PSCAD/EMTdc software.

A Fusion Topology of Higher Efficiency and Lower Capacity Hybrid Parallel Multi-Converters for Power Quality Compensation

In high-power applications, different multilevel voltage source converter (VSC) topologies are usually applied. The hybrid parallel multiconverters, which is a combination of multiactive and hybrid VSCs connecting in parallel, can be a promising solution in a high-power grid. In this article, the fusion VSC topology concept is proposed in which fusion VSC takes the advantages of both active (inductive coupling) VSC and hybrid (capacitive coupling) VSC. It corresponds to overcoming the insufficient-capacity problems when active VSCs are employed alone and vice versa. This article proposes fusion active power filter topology by combing active and hybrid power VSC through their equivalent model for power quality compensation. With the proposed VSC&#x0027;s equivalent model, it can reduce the converter rating by 17&#x0025;&#x2013;80&#x0025; compared with the conventional approaches. Additionally, the corresponding control algorithm is proposed for the FAPF for power quality compensation. Finally, the simulation and real-time digital simulator hardware-in-the-loop experimental results of the proposed FAPF topology are given to demonstrate the validity and effectiveness for reducing its capacity and power loss compared with the conventional multi-VSC in power quality compensation.

A New Hybrid Voltage Source Converter With Reduced Active Switch Count for HVDC Applications

This paper presents a new hybrid voltage source converter topology with reduced active switch count for HVDC applications. It consists of a block of switches in the upper converter arms and cascaded switch-diode cells in the lower converter arms. The cascaded switch-diode cells in the lower converter arms produce a multilevel output voltage while the switch blocks in the upper converter arms perform rectification. The switches in the upper converter arms conduct for one-third of the fundamental line period as dictated by the relative magnitude of the input line-to-neutral voltages. Only one of the three phase blocks of switches conduct at a time. Compared to other voltage source converter topologies with similar structure and operation to this topology, the proposed topology has lower number of devices. Therefore, it has lower converter footprint and cost. More importantly, it reduces the semiconductor device power losses significantly due to the lower number of active switches (lower switching loss) and the lower number of devices in current conduction path (lower conduction loss). The basic operation principle of the proposed converter and its associated control strategy that regulates the power exchange between the converter, and the ac and dc networks are described in detail and verified using both simulation and experimental results.

A Modular multilevel converter with hybrid redundant strategy for HVDC system

As a typical voltage source converter topology, Modular multilevel converter (MMC) widespread used in high voltage direct current (MMC-HVDC) power systems due to its modularization, low harmonic, easy expansion, and high efficiency. The failure of SMs will cause inhibit circulating current growth and unbalanced system voltage. Therefore, the handling of faulty SM has an utmost significance for MMC-HVDC. This paper proposes a hybrid control method of redundant SMs with semi-hot and cold standby redundant SM control methods. In normal operation, some redundant SMs are pre-charged to become semi-hot standby SMs. When the SM fails, one semi-hot standby SM is charged to become a working SM, and one cold standby SM is charged to become a semi-hot standby SM. In this paper, the proposed method is verified by simulation. The simulation shows two working states of MMC-HVDC, one is the voltage balance control of semi-hot standby SM in the normal working condition, another is the SM failure and redundant SM insertion control. Compared to using only one SM strategy, the implementation of the hybrid redundant strategy is easier and the loss is also lower.

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.

Arm cortex M4 microprocessors based ± 100 kVAR energy quality regulator for reactive power/neutral current compensation, load balancing and harmonic mitigation

In this work, the design and implementation of a three-phase ± 100 kVAR energy quality regulator (EQR) which is capable of providing reactive power/neutral current compensation, load balancing as well as low order current harmonic mitigation is presented. The proposed EQR is based on three-leg four-wire voltage source converter topology with split capacitors in the DC link. At first, the power stage and the control system of the EQR are modelled and verified in PSCAD/EMTDC by extracting the compensation reference currents in synchronous reference frame. After that, the hardware of the EQR based on naturel air forced IGBT modules switching at 5 kHz is developed and practically implemented. The developed EQR control card is based on two Arm Cortex-M4 microprocessors © which handles all control and protection functions. Closed loop operation of the EQR is ensured with P, PI and PR type controllers due to their simplicity and reliability in practical applications. The field tests in a 50 Hz live distribution circuit are carried out by connecting the EQR to the low voltage side of a three-phase 34.5/0.4 kV 400 kVA distribution transformer. It is verified by real-time measurements that the EQR is able to perform the aforementioned compensation functions simultaneously.

Comparison of voltage source converters and Z-source converters for electric vehicles

Voltage source and current source converters are used in industrial applications as either boost or buck converters for a defined power flow direction. These operational modes limit these converters to be utilized on their own in hybrid and electric vehicle (EV) applications. Z-source (impedance source) converters, on the other hand, have the capability to be operated as buck and boost converters to have a wide range of output levels. The boost ratio of the direct current link voltage of a Z-source converter is set by short-circuiting the phase legs of the converter and defining the overlap duration at each switching period. In order to have the optimum efficiency with the lowest possible total harmonic distortion, a modified space vector pulse width modulation method with an impedance network could be selected for closed-loop control of the quasi-resonant Z-source converter to drive permanent magnet synchronous motors (PMSMs). In this study, closed-loop vector-controlled drive systems built with two-level and three-level boosted voltage source converter topologies and two- and three-level quasi Z-source converters for field-oriented control of a PMSM in EV motor drive applications were compared under constant motor speed–constant load, constant speed–variable load and variable speed–constant load operating conditions.

Interoperability of modular VSC topologies in Multi-converter multiterminal DC systems

The ongoing transformation of power systems driven by the need for more interconnected networks will eventually lead multiterminal dc (MTDC) grids to become more commonplace in future electricity networks. Addressing interoperability challenges associated with multiple converter topologies for future MTDC implementations is a paramount requirement. This paper introduces a multi-converter MTDC (MC-MTDC) system combining both state-of-the-art and emerging modular voltage source converter (VSC) topologies. The developed MC-MTDC system, (i) demonstrates operation of the modular multilevel converter (MMC) and the alternate arm converter (AAC) in different MTDC configurations, and (ii) sets the basis for further development of MTDC configurations with diverse modular VSC topologies. The developed MC-MTDC system will also facilitate accurate testing and validation of multi-converter interoperability using real-time electromagnetic transient (EMT) models. The design parameters of the AAC-based terminals are determined ensuring compliance with existing MMC-based dc power systems. Detailed analysis and a complete set of results based on real-time implementation for different operating scenarios including steady-state, transient, and contingencies, verify multi-converter interoperability and associated control functions in different MTDC configurations.

FFS modulated VSC with different phase‐shifted 12‐pulse configured transformers for grid integrated large‐scale solar PV plant

The requirement for high capacity power conditioning unit (PCU) has increased for application in large-scale solar photovoltaic (PV) plant for optimisation of the balance of system cost. For high power applications, one of the most important criteria for designing of PCU is to have lower switching losses to minimise the heat generated in the power switching devices. Conventionally, PCU utilises traditional two-level or three-level voltage source converter (VSC) topology with high-frequency switching techniques and due to this, they have lower conversion efficiency and higher device switching losses. In this study, fundamental frequency switching (FFS) modulated three-level neutral point clamp (3L-NPC) VSC is used for MW scale PCU, as they have lower switching losses, higher conversion efficiency and higher AC/DC voltage ratio. The 3L-NPC VSC generates lower order current harmonics, which are mitigated by using different phase-shifted 12-pulse PCU transformers located at different pooling location inside solar PV plant. The PV plant configuration for a 40 MW (AC) plant capacity is developed in the Matlab/Simulink environment and implemented in real-time simulator OPAL-RT to validate the proposed concept. Harmonics, steady-state and dynamic performances are demonstrated at constant and changing solar irradiance levels.

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.

A New Seven-Level Topology for High-Power Medium-Voltage Application

In this article, a new seven-level voltage source converter topology is proposed for high-power medium-voltage applications. The proposed converter has less number of components in the circuit compared to the existing advanced seven-level topologies. All the switches have the same voltage rating. Space vector modulation technique is developed for the proposed topology to balance the flying capacitors voltage. The performance of the proposed converter is evaluated at both steady-state and transient conditions in MATLAB/Simulink. A scaled-down prototype is built to verify the feasibility and performance of the proposed converter at different operating conditions.

Impact of proportional resonant controller parameters of VSC connected to AC grids with variable X/R characteristic on the small signal stability

The purpose of this paper is to assess the impact of proportional resonant controller (PR) parameters of a Voltage Source Converter (VSC) and AC grid topology on the small signal stability of a VSC-AC system. Based on this, a small signal model is constructed, step by step, and validated against EMT simulations. Following this validation, several scenarios containing different PR controller gains are proposed and compared by means of trajectory of eigenvalues. A discussion of the best adequacy of PR controller parameters and AC grid strength in combination with inductive-resistive characteristic (X/R) is developed to ensure the stability of the system. The previous eigenvalue analysis is verified in time domain by means of EMT simulations. The analysis performed concludes that, while the AC grid strength in combination with PR controller parameters plays an important role in determination of stability, variations in the inductive-resistive AC grid characteristic need to be included in the analysis, since further instabilities can appear even when the PR controller is well designed for a specific AC grid configuration.

Sensorless SynRG Based Variable Speed Wind Generator and Single-Stage Solar PV Array Integrated Grid System With Maximum Power Extraction Capability

This article presents a grid-integrated hybrid renewable energy sources based system comprising a solar photovoltaic (PV) array and a wind energy conversion system (WECS). The WECS uses a position-sensorless synchronous reluctance generator (SynRG) for the electric power generation from the wind turbine (WT), wherein, a sensorless field-oriented control (FOC) is made use of for the maximum power extraction (MPE). A second-order flux estimation (SFE) method along with frequency-locked loop (FLL) is utilized for the accurate flux estimation from the SynRG stator voltages and currents. A set of back-to-back connected three-phase two leg voltage source converter (VSC) topology is selected for the grid integration of WECS. This system has a common dc link where the solar PV array and the machine-side VSC (MSC) of the wind generator, are directly connected. The power output from the solar PV array and WECS, is shared between the grid and the local loads. The maximum power generation from SynRG in the WECS, is achieved by operating the SynRG at the speed estimated by the MPE algorithm. The maximum power is drawn from the solar PV array by adjusting the dc-link voltage, which is decided by the algorithm. For the proper power control and power quality improvement, the grid-side converter (GSC) is adequately controlled by implementing an observer-based control technique. The real-time validation of the system, is carried out using a developed laboratory prototype.

Alternate Arm Converter Energy Balancing Under Parameter Variation

Energy balancing of the alternate arm converter (AAC) is a major challenge and overlap period-based circulating current control methods are commonly used. Component parameter variation occurs due to tolerances, component aging, and internal faults leading to stored energy imbalances of modular voltage source converter (VSC) topologies. If not controlled, such variations can severely impact the operation of the AAC. Hence, component parameter variation is a key consideration for energy balancing of all sorts of modular VSC topologies, but has not been addressed in the existing literature for the AAC. This paper investigates the energy balancing capability of the AAC in high-voltage direct current (HVDC) applications under component parameter variations and proposes a compensation method to further improve its energy balancing performance. The effectiveness of the proposed method is verified in a real-time model of an AAC-HVDC transmission system from widely accepted existing CIGRE benchmark models.

Comprehensive understanding of DC pole‐to‐pole fault and its protection for modular multilevel converters

A modular multilevel converter (MMC) is one of the promising voltage source converter topologies in the field of high-voltage direct current (HVDC) transmission. MMCs are vulnerable to the direct current (DC) short-circuit faults due to the high discharge current stress. This study first introduced the simplified equivalent circuits of MMCs under both normal and DC fault operation conditions. Then the MMC fault spread loops and fault energy distribution principles are presented. Moreover, a detailed comparative study of the existing DC fault handling schemes, which are classified as the rigid blocking and flexible blocking methods are carried out. It provides a technical reference for further study on the MMC DC fault protection schemes for the HVDC transmission system.

Modular Embedded Multilevel Converter for MV/HVDC Applications

Many of the renewable energy resources, like offshore wind, utility PV farms, are located far away from the load centers, which requires dc power transmission system to efficiently deliver power. Voltage-source converter solution is required in many applications due to the stability and compensation requirement. This paper presents a new voltage source converter topology for medium to high voltage dc application, named modular embedded multilevel converter (MEMC). MEMC is based on a three-level topology structure and consists of series-connected insulated-gate bipolar transistor (IGBT) bridge stacks, like the popular modular multilevel converter (MMC) and series-connected thyristor or press-pack high voltage integrated gate-commutated thyristor or IGBT stacks. The basic operation principle, control solutions, and the methods to commutate the thyristors are explained in detail. Due to the three-level structure, the total number of IGBT stacks are reduced by half compared with MMC, leading to much lower energy storage, weight, volume, and system complexity. Also by replacing part of the IGBT stack in MMC with thyristor stack, both of the system conduction loss and switching loss can be further reduced. There are other topology variations of MEMC with a similar operational principle but different characteristics.