High-voltage High-power Applications Research Articles

SVPWM Strategy With Minimum Common-Mode Voltage for Multilevel Converter Combining the Concept of the Nearest Level

Multilevel converters are widely used in medium- and high-voltage high-power industrial applications. Space vector pulse width modulation (SVPWM) is one of the very promising modulation strategies for multilevel converters. However, the SVPWM strategy is difficult to implement as the number of levels increases. The output common-mode voltage (CMV) of the converter is another problem in the implementation of SVPWM strategy. In this paper, a new SVPWM strategy with minimum CMV for multilevel converter is proposed by combining the concept of the nearest level. The proposed SVPWM strategy only utilizes simple arithmetic operations and logical judgments, and completely avoids trigonometric functions and irrational number operations. Compared with the existing SVPWM methods, the proposed SVPWM strategy can quickly calculate the switching state with the minimum CMV in real-time by the concept of the nearest level, and avoids solving the non-homogeneous linear equations in the process of calculating the switching state corresponding to the space vector. It can be extended to any level converter without additional computational burden. Finally, the proposed SVPWM strategy is verified with extensive simulation and experimental studies.

A Five Level Modified Cascaded H-Bridge Inverter STATCOM for Power Quality Improvement

Multilevel converters have received serious attention on account of their capability of high voltage operation, high efficiency, and low electromagnetic interference. It has many advantages compared to conventional two-level inverters such as high dc-link voltages, reduced harmonic distortion, fewer voltage stresses, and low electromagnetic interferences. The multilevel converters have been used for STATCOM widely as it can improve the power rating of the compensator to make it suitable for medium or high-voltage high power applications. While deploying multilevel STATCOMs, designer’s role is to reduce the number of switching devices since, the total switching losses are proportional to the number of switching devices. The reduction in the count of switching devices also reduces the size and cost. In this paper, a five-level modified cascaded H-bridge inverter STATCOM is proposed for mitigation of harmonics. Modified Five-level CHB configuration is the most suitable as with lesser number of switches, give better performance resulting in a compact system. The PQ theory-based controller is developed for control of STATCOM operation. MATLAB simulation results are presented to demonstrate mitigation of harmonics.

A Family of Interleaved High Step-Up DC–DC Converters by Integrating a Voltage Multiplier and an Active Clamp Circuits

A family of interleaved current-fed high step-up dc–dc converters are introduced and analyzed here by combining a voltage multiplier and an active clamp circuit for high-voltage high-power applications. Low input currents and output voltages ripples values and high voltage-gains characteristics of these converters make them suitable for lots of dc–dc applications. All power devices operate entirely under soft switching conditions, even when wide load and input voltage variations are applied. Thus, they can be designed at high-switching frequencies to reduce passive components sizes to achieve high-power density, one of the main targets of the power electronics researches. Also, their input and output ports common ground simplifies the gate-drives and control circuits. To verify the given analyses and simulations, a 120–320 V to 1 kV, 50–1300 W three-stage two-leg prototype converter has been implemented at 100 kHz. Based on the experimental results, maximum efficiency of 96.5% is achieved.

Hybrid Three-Phase Transformer-Based Multilevel Inverter With Reduced Component Count

This paper proposes a novel three-phase transformer-based multilevel inverter (MLI) topology to maximize the output voltage levels for high-power high-voltage applications while reducing component counts as compared to its transformer-based counterparts. The proposed hybrid topology is formed by connecting a three-level T-type module with full H-bridge cells through single-phase transformers. The T-type module is fixed while the full H-bridge cell can be repeated for enlarging the output voltage levels without increasing the voltage stress on switches. Key features of the proposed topology include low part count, capacitor-free, diode-free, voltage boosting, simple control, and modularity. Within the framework, a simple low-frequency pulse width modulation (LFPWM) switching scheme is used to control the output voltage, and the working principle is detailed for seven-, nine-, and N-level operation. The operability and performance of the proposed topology are numerically verified and experimentally validated at different loads. Moreover, its conversion efficiency is experimentally examined. Finally, a comparative study with existing transformer-based MLI circuits is conducted to prove its key merits.

Arm current reversal‐based modular multilevel DC‐DC converter

Nowadays, most of the converters used in high-power high-voltage (HV) applications are the conventional modular multilevel converters (MMC). However, in the case of DC-DC conversion, an imbalance of the capacitor voltages occurs and the conventional MMC fails to operate correctly. This paper introduces an arm current reversal-based modular multilevel DC-DC converter, which successfully provides balance among the capacitor voltages while operating in DC-DC conversion. The proposed configuration is used in medium voltage DC grids to feed DC loads or to interconnect between two DC grids of different voltage levels. The proposed converter is a two-stage DC-DC modular converter, which consists of a single-phase half-bridge MMC with half-bridge submodules (HBMMC) followed by a single-phase H-bridge MMC with half-bridge submodules (SMs). The operational concept of the proposed converter is based on reversing the arm current direction and reversing the output terminals with the help of the H-bridge MMC stage, which ensures the same direction of the voltage at the load terminals. The proposed converter provides a high conversion ratio, bidirectional power flow, simple architecture, and a simple control scheme. Detailed illustrations, analysis, and design of the proposed converter are presented. Besides, MATLAB-based and Opal RT-based simulation results and experimental results are presented to validate the proposed configuration claims.

Performance Analysis of Modular Multilevel Converter and Modular Multilevel Series Converter under Variable-Frequency Operation Regarding Submodule-Capacitor Voltage Ripple

The modular multilevel converter is capable to reach high-voltage levels with high flexibility, high reliability, and high power quality as it became the standard solution for high-power high-voltage applications that operate with fixed frequency. However, in machine-drive applications, the modular multilevel converter shows critical problems since an extremely high submodule-capacitor voltage ripple occurs in the machine start-up and at low-speed operation, which can damage the converter. Recently, a new converter solution named modular multilevel series converter was proposed as a promising alternative for high-power machine-drive applications since it presented many important structural and operational advantages in relation to the modular multilevel converter such as the reduced number of submodule capacitors and the low submodule-capacitor voltage ripple at low frequencies. Even though the modular multilevel series converter presented a reduced number of capacitors, the size of these capacitors was not analyzed. This paper presents a detailed comparison analysis of the performance of the modular multilevel converter and the modular multilevel series converter at variable-frequency operation, which is based on the proposed analytical description of the submodule-capacitor voltage ripple in such topologies. This analysis concludes that the new modular multilevel series converter can be designed with smaller capacitors in comparison to the modular multilevel converter if these converters are used to drive electrical machines that operate within a range of low-frequency values. In other words, the modular multilevel series converter experiences extremely low submodule-capacitor voltage ripple at very low frequencies, which means that this converter solution presents high performance in the electrical machine start-up and at low-speed operation.

Novel Converter Topology With Reduced Cost, Size and Weight for High-Power Medium-Voltage Machine Drives: 3x3 Modular Multilevel Series Converter

The modular multilevel converter is considered the state-of-the-art power electronic solution for high-power high-voltage applications that require a DC stage and that operate with fixed frequency at their AC terminals. However, in high-power AC-AC variable-frequency applications, it has been proved that this converter topology presents an unnecessarily large number of components and an intolerably high submodule-capacitor voltage ripple at low frequencies. The modular multilevel matrix converter was proposed as a promising alternative for high-power medium-voltage electrical-machine drives since it presented improved performance at low frequencies and a reduced number of components in comparison to the modular multilevel converter resulting in reduced costs and improved efficiency. Even though, the modular multilevel matrix converter is one of the converter solutions that stands out the most for high-power machine drives, it presents some operational drawbacks such the unstable behavior under unbalanced grid conditions and the extremely complex internal control and modulation. Recently, a new converter topology named modular multilevel series converter was proposed presenting attractive operational characteristics such as the high performance at low frequencies, high performance under unbalanced grid conditions, and straightforward control and modulation, which are important advantages compared to the modular multilevel converter and the modular multilevel matrix converter. Nonetheless, as demonstrated in this paper, the modular multilevel series converter presents large component count and poor efficiency. Thus, in this paper, a new converter solution is proposed, which is an enhanced version of the modular multilevel series converter that preserves the previously mentioned advantageous operational characteristics and that presents an improved efficiency and a considerably reduced number of components in comparison to the other three converter topologies, resulting in reduced cost, size and weight.

New AC–AC Modular Multilevel Converter Solution for Medium-Voltage Machine-Drive Applications: Modular Multilevel Series Converter

Due to its scalability, reliability, high power quality and flexibility, the modular multilevel converter is the standard solution for high-power high-voltage applications in which an AC–DC–AC connection is required such as high-voltage direct-current transmission systems. However, this converter presents some undesired features from both structural and operational perspectives. For example, it presents a high number of components, which results in high costs, size, weight and conduction losses. Moreover, the modular multilevel converter presents problems dealing with DC-side faults, with unbalanced grid conditions, and many internal control loops are required for its proper operation. In variable-frequency operation, the modular multilevel converter presents some serious limitations. The most critical are the high-voltage ripples, in the submodule capacitors, at low frequencies. Thus, many different AC–AC converter solutions, with modular multilevel structure, have been proposed as alternatives for high-power machine-drive applications such as offshore wind turbines, pumped-hydro-storage systems and industrial motor drives. These converters present their own drawbacks mostly related to control complexity, operational limitations, size and weight. This paper introduces an entirely new medium-voltage AC–AC modular multilevel converter solution with many operational and structural advantages in comparison to the modular multilevel converter and other alternative topologies. The proposed converter presents high performance at low frequencies, regarding the amplitude of the voltage ripples in the submodule capacitors, which could make it very suitable for machine-drive applications. In this paper, an analytical description of the voltage ripples in the submodule capacitors is proposed, which proves the high performance of the converter under low-frequency operation. Moreover, the proposed converter presents high performance under unbalanced grid conditions. This important feature is demonstrated through simulation results. The converter solution introduced in this paper has a simple structure, with decoupled phases, which leads to the absence of undesired circulating currents and to a straightforward control, with very few internal control loops for its proper operation, and with simple modulation. Since the converter phases are decoupled, no arm inductors are required, which contributes to the weight and size reduction of the topology. In this paper, a detailed comparison analysis with the modular multilevel converter is presented based on number of components, conduction and switching losses. This analysis concludes that the proposed converter solution presents a reduction in costs and an expressive reduction in size and weight, in comparison to the modular multilevel converter. Thus, it should be a promising solution for high-power machine-drive applications that require compactness and lightness such as offshore wind turbines. In this paper, simulation results are presented explaining the behavior of the proposed converter, proving that it is capable of synthesizing a high-power-quality load voltage, with variable frequency, while exchanging power with the grid. Thus, this topology could be used to control the machine speed in a machine-drive application. Finally, experimental results are provided to validate the topology.

A General Fault-Tolerant Operation Strategy Under Switch Fault for Modular Series–Parallel DC–DC Converter

Modular series–parallel dc–dc converters (MSPDDCs) are good options in high-power high-voltage applications. A reliable operation should be a top concern in such critical situations. In this article, a general fault-tolerant method for switch fault is proposed for MSPDDC, including fast fault diagnosis (FD) and postfault reconfiguration. The FD is achieved using the relationship between output inductor voltage and gate driver signal in each module. The inductor voltage is acquired by adding a small auxiliary winding into the magnetic core, and no expensive sensors are introduced. A general and low-cost hardware circuit is designed to locate faulty module in a very short time. The reconfiguration process for four structures is then systematically presented. Through a careful timing sequence design, a fast and smooth hot swap is realized, and its impact on the output side and the other healthy modules is minimized. The principle of FD and reconfiguration, hardware implementation, and experimental verification are given in this article.

A Family of Ultrahigh Efficiency Fractional dc–dc Topologies for High Power Energy Storage Device

The application of a nonisolated bidirectional fractional dc–dc topology is proposed for high power energy storage device in this article. The proposed topology has the benefits of ultrahigh efficiency, simple structure, and low cost because it processes only a fractional of the total power. With the existence of an extra-low voltage (LV) power source, the converter voltage stress is only the difference between the dc bus and high voltage (HV) power source, therefore, low-cost LV transistors can be used in HV high power applications. Since a majority of the power is transferred directly between the energy storage device and dc bus, ultrahigh system efficiency can be achieved. Converter operating principle and design considerations such as battery capacity calculation methodologies and over voltage protection (OVP) are presented. A GaN transistor-based prototype is fabricated. Experimental results of a 1.2-kW battery energy storage prototype are presented to validate the analysis. The system efficiency is higher than 99% when a wide input–output voltage range is achieved. The fractional converter is convincing in achieving high efficiency and high conversion power with reduced cost for energy storage devices.

Modular multilevel DC–DC converter with arm interchange concept

Conventional modular multilevel converters (MMCs) are promising converters for high-voltage high-power applications. Unlike DC–AC conversion, DC–DC conversion of MMCs suffers from unbalanced capacitor voltages due to the energy drift problem. In this study, a new modular DC–DC converter is proposed, which consists of front-end half-bridge MMC with half-bridge submodules followed by full-bridge MMC with full-bridge submodules. The proposed modular multilevel DC–DC converter can be used effectively for connecting two different DC voltage levels in high-/medium-voltage DC grids. The proposed architecture provides successful DC–DC conversion with balanced capacitor voltages throughout the arm interchange concept. It also provides a high conversion ratio, bidirectional power flow, low losses, low current stresses, reduced number of submodules, and DC fault blocking capability. Detailed illustration, operational concept, and design of the proposed architecture with the arm interchange concept are presented. Simulation results for a high-voltage high-power DC–DC converter with the suggested approach are presented to demonstrate the system performance during different normal as well as abnormal operating conditions. Finally, a scaled-down prototype has been implemented for validation. The extracted simulation and experimental results elucidate the presented claims.

Systematic Synthesis and Derivation of Multilevel Converters Using Common Topological Structures With Unified Matrix Models

Multilevel converters (MLCs) have been increasingly adopted in both low-power low-voltage systems and high-power high-voltage applications. In recent years, many new topologies have been proposed, and a few of them have been successfully implemented in industry. While searching for new topologies, synthesis and derivation principles of multilevel topologies are critical for converter design. In this article, a generalized synthesizing approach of multilevel topologies is proposed and analyzed with the considerations of the voltage source, current source, and matrix-type MLCs. Based on the proposed method, the topological relationship among these three types MLCs is revealed through the stage-based common circuit structure. In addition to the graphic-based approach, a mathematical approach is proposed for representing the MLC topologies in this work. The matrix-based model is utilized to unify and verify the derivation and simplification process in a systematic way through many MLC examples in this article. Finally, demonstration examples are presented to show how the proposed principles can be used to derive new topologies covering five-level to nine-level converters. With the ever-increasing research efforts on MLCs, it is hoped that this article can provide a new approach that inspires the development of more interesting and practical MLC topologies for various applications.

Control and Experimental Validation of the Series Bridge Modular Multilevel Converter for HVDC Applications

The series bridge converter (SBC) is a modular multilevel converter (MMC) recently developed to enhance power density in high-voltage high-power applications. The MMC is a well-established solution, widely researched, and exploited in practical HVdc connections, thanks to its high power quality and high efficiency. However, the main limitation of the MMC is the relatively large energy storage, also due to the fact that power ripples in the submodule capacitors include a component at the fundamental ac frequency. As a result, volume becomes critical in applications such as offshore or city center in-feeds, where space is restricted and expensive. The SBC offers a more compact footprint by exploiting a series connection on the dc side and by operating the submodules with rectified waveforms, thus moving the minimum component of the instantaneous power to twice the ac fundamental and reducing capacitors size. The drawback of the converter is a more complex energy control compared to the MMC. This paper proposes the first experimental validation of the SBC, using a 2-kW laboratory-scale prototype. Since the basic converter design has been discussed in previous papers, the focus of this paper is on converter control design and experimental validation.

Switch Fault Diagnosis Method for Series–Parallel Forward DC–DC Converter System

Series–parallel forward dc–dc converter system is a good option in high-voltage high-power applications. A switch fault, in this converter, should be detected as soon as possible, to avoid further damage and save maintenance cost. In this paper, a new switch fault diagnosis method, which treats output inductor voltages as diagnostic criteria, is proposed for this system. With the proposed method, expensive sensors are needless, since diagnostic signal can be easily acquired by adding a small auxiliary winding into inductor magnetic core. Based on the feature of diagnostic signals in such a system, a low-cost diagnostic circuit is designed. The failed module caused by open-circuit fault or short-circuit fault could be detected within one switching period, allowing for an immediate fault-tolerant action. This method also shows generality for different connections and buck-type isolated topologies. Besides, it works reliably under input-voltage variation and load step, and no misdiagnosis will occur during normal dynamic process. Finally, experimental result verifies the effectiveness of this method. The proposed scheme has a good potential to be applied in real applications for its simplicity and robustness.

Modular Multilevel Converters

The papers in this special section focus on the latest achievements on MMCs regarding the development of new circuit topologies, modeling, operation, and fault analysis. Modular multilevel converters (MMCs) have emerged as an attractive solution for high-voltage high-power applications, such as high-voltage direct current (HVdc) transmission systems, due to their attractive features such as modularity, reliability, high voltage capability, and small footprint. However, new power converter topologies and recent advances in terms of control have expanded the application of the MMC to power conditioning and machine drives, among others.

Asymmetric Pulse Frequency Modulation With Constant On-Time for Series Resonant Converter in High-Voltage High-Power Applications

The series resonant converter (SRC), controlled by the traditional pulse frequency modulation (PFM) with constant on-time, can operate in discontinuous conduction mode (DCM) and is applicable for high-voltage high-power applications with the requirement of a wide output voltage range. However, in the traditional PFM with constant on-time, the resonant capacitor voltage will be higher than the input voltage during the zero current stage, leading to a higher maximum magnetic flux density (MMFD) case. To avoid this, a novel asymmetric pulse frequency modulation (APFM) with constant on-time is proposed for SRC operating in DCM, where the MMFD of transformer core varies linearly with the operating frequency and output voltage among the whole output voltage range. The high-power transformer can be designed according to highest operating frequency and the transformer turns ratio can be designed to be small. Furthermore, the proposed APFM leads to smaller peak current for all switches and fully zero-current-switching can be achieved. The output power and voltage can be still regulated, meeting the high-voltage high-power applications. For the proposed APFM, there are four different driver combinations with exact the same effects and advantages. The theoretical analysis has been validated by the established simulation model and experimental platform.

Phase-Shifted Carrier Pulse-Width Modulation Algorithm With Improved Dynamic Performance for Modular Multilevel Converters

High modularity, easy scalability, and superior harmonic performance, comprising the topology with the most potential for medium- to high-voltage high-power applications, are representative features of a modular multilevel converter (MMC). Each application of a MMC requires a proper scheme such that it conforms to control objectives such as correct submodule capacitor voltage balancing control and the suppression of circulating current. Over the past few years, various studies have been presented that meet the MMC requirements through both classical control-based pulse-width modulation and model predictive control, although some drawbacks exist for both control concepts. The dynamic performance of the classical control methods with proportional-integral or proportional-resonant controllers is unsatisfactory, and the requirement of proportional-integral parameters tuning procedure makes converter operation performance depends on proportional-integral parameters adjustment. Meanwhile, model predictive control performance depends significantly on the mathematical model of a system, and weighting factor selection is tedious. In this paper, we propose an improved phase-shifted carrier pulse-width modulation method and capacitor voltage balancing control that inherits the merits from both classical control and model predictive control. Meanwhile, the proposed control method eliminates the requirement of the tedious proportional-integral parameters tuning procedure and improves the dynamic performance compared with the conventional phase-shifted carrier pulse-width modulation-based proportional-integral controller method. Simulations and experiments were conducted to demonstrate the proposed method's properness.

An Optimal Full Frequency Control Strategy for the Modular Multilevel Matrix Converter Based on Predictive Control

The modular multilevel matrix converter (M3C) is a promising topology for high-voltage high-power applications. Recent researches have proved its significant advantages for adjustable-speed motor drives compared with the back-to-back modular multilevel converter. However, the branch energy balancing in the M3C presents great challenge especially at critical-frequency points where the output frequency is close to zero or grid-side frequency. Generally, this balancing control depends on the appropriate injection of inner circulating currents and the common-mode voltage (CMV), whereas their values are hard to determine and optimize. In this paper, an optimization-based predictive control method is proposed to calculate the required circulating currents and the CMV. The proposed method features a broad frequency range balancing of capacitor voltages and no reactive power in the grid side. For operation at critical-frequency points, there is no increase on branch voltage stresses and limited increase on branch current stresses. A downscaled M3C system with 27 cells is designed and experiment results with the R–L load and induction motor load are presented to verify the proposed control method.

Analysis of Strategy for Achieving Zero-Current Switching in Full-Bridge Converters

Conventional zero-voltage and zero-current-switching (ZVZCS) full-bridge converters are widely used because they can realize zero-voltage switching (ZVS) for the leading leg and zero-current switching (ZCS) for the lagging leg. However, the large turn-off loss of the leading leg is still unacceptable in high-voltage high-power applications where insulated-gate bipolar transistors (IGBTs) with current tailing are employed. Hence, it is preferable to achieve ZCS for all IGBTs of full-bridge converters. This paper analyzes the ZCS conditions for pulsewidth modulated (PWM) full-bridge converters and proposes a general strategy for achieving ZCS for main switches, including the control strategy and design method of the topology. Based on this theory, a novel ZCS PWM full-bridge converter with simple auxiliary circuit is derived and tested. It can achieve ZCS not only for the main switches of full-bridge converter, but for the auxiliary switch as well, and obtain high efficiency. The operation principle of the proposed ZCS PWM full-bridge converter and the design procedure are presented. The experimental results of a 600 V/2.5 kW prototype are also included to verify the theoretical analysis.

Optimized Branch Current Control of Modular Multilevel Matrix Converters Under Branch Fault Conditions

The modular multilevel matrix converter (M3C) is a promising topology for high-voltage high-power applications. It features easy scalability, high-quality input and output waveforms, superior availability, etc. This letter aims to further increase the availability of the M3C when one or more of the nine branches are lost. A general nonlinear multivariable optimization model is set up to optimize the branch current configuration after the branch lost. The proposed method reduces system power capability loss after the branch failure. It achieves a smooth transition from a full M3C to a reduced topology without shutdown of the system. A M3C prototype with 27 cells is designed, and experiment results are presented to confirm the validity of this method.