High-voltage High-power Applications Research Articles

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.

FPGA Based Symmetrical Multi Level Inverter with Reduced Gate Driver Circuits

<span style="font-size: 9pt; font-family: 'Times New Roman', serif;">Multilevel converters tender advantages in terms of the output waveform quality due to the increased number of levels used in the output voltage modulation and have been widely accepted for high-power high-voltage applications. This paper introduces topology in multilevel dc link inverter (MLDCLI), which can significantly reduce the switch count and improve the performance.<strong> </strong>The preferred topology provides a dc voltage with the shape of a staircase approximating the rectified shape of a commanded sinusoidal wave, to the bridge inverter, which in turn gives the required alternating waveform<strong>.</strong> This topology requires fewer components compared to traditional Multi level Inverters (MLI).Therefore, the overall cost and complexity are significantly reduced particularly for higher output voltage levels. Finally, </span><span style="font-size: 9pt; font-family: 'Times New Roman', serif;" lang="EN-GB">Matlab/Simulink and XILINX are used as a simulation and compiler architecture of control circuit embedded in FPGA. Simulation and experimental results for fifteen-level inverter are presented for validation</span><span style="font-size: 9pt; font-family: 'Times New Roman', serif;">.</span>

Comparative analysis of modular multilevel converter with different modulation technique for control of induction motor drive

Modular multilevel converter (MMC) is an enhanced multilevel converter topology. It is widely used in research nowadays for its superior performance in high-voltage high-power applications. Modification is suggested in conventional MMC topology by connecting a single DC source to lower submodules This paper focuses on performance analysis of proposed configuration of MMC for control of induction motor (IM) drive with different modulation techniques. The modulation strategies used are sinusoidal pulse width modulation (SPWM) and selective harmonic pulse-width modulation (SHE-PWM). Simulation is carried out in MATLAB-SIMULINK with both techniques, to generate three level output by selecting suitable switching pattern. Initially, both the techniques are compared with same switching frequency of 500 Hz. Weighted total harmonic distortion (WTHD) is used as a performance indicator. Result shows that WTHD is less by employing SHE-PWM technique in comparison to SPWM technique. As WTHD is directly proportional to copper loss in IM, motor efficiency can be improved. Later, comparison is also carried out on basis of switching frequency required to eliminate same order of harmonics. Elimination up to 13th harmonic components requires frequency of 500 Hz in SHE-PWM and 1050 Hz in SPWM. As switching frequency required is almost double in SPWM, switching losses increases which is not desirable in high power application. Laboratory prototype was developed for the proposed configuration controlling IM drive. Experimental results were presented with both the techniques. SPWM pulses are generated using microcontroller DSPTMS320F2812. With advancement in microcontroller technology, SHE-PWM pulses are then generated with the help of microcontroller dsPIC33EP256MU810.

Evolution of Topologies, Modeling, Control Schemes, and Applications of Modular Multilevel Converters

Modular multilevel converter (MMC) is one of the most promising topologies for medium to high-voltage high-power applications. The main features of MMC are modularity, voltage and power scalability, fault tolerant and transformer-less operation, and high-quality output waveforms. Over the past few years, several research studies are conducted to address the technical challenges associated with the operation and control of the MMC. This paper presents the development of MMC circuit topologies and their mathematical models over the years. Also, the evolution and technical challenges of the classical and model predictive control methods are discussed. Finally, the MMC applications and their future trends are presented.

Dyna-C: A Minimal Topology for Bidirectional Solid-State Transformers

This paper presents a topology for bidirectional solid-state transformers with a minimal device count. The topology, referenced as dynamic-current or Dyna-C, has two current-source inverter stages with a high-frequency galvanic isolation, requiring 12 switches for four-quadrant three-phase ac/ac power conversion. The topology has voltage step-up/down capability, and the input and output can have arbitrary power factors and frequencies. Further, the Dyna-C can be configured as isolated power converters for single- or multiterminal dc, and single- or multiphase ac systems. The modular nature of the Dyna-C lends itself to be connected in series and/or parallel for high-voltage high-power applications. The proposed converter topology can find a broad range of applications such as isolated battery chargers, uninterruptible power supplies, renewable energy integration, smart grid, and power conversion for space-critical applications including aviation, locomotives, and ships. This paper outlines various configurations of the Dyna-C, as well as the relative operation and controls. The converter functionality is validated through simulations and experimental measurements of a 50-kVA prototype.

A Novel State-of-Charge Balancing Method Using Improved Staircase Modulation of Multilevel Inverters

Multilevel converters have received more attention recently. Multilevel converters can be used not only in high-voltage high-power applications, but also in energy storage systems, thanks to reduced harmonics and electromagnetic interferences. State-of-charge (SoC) balancing of the storage system cells within cascaded multilevel inverters, e.g., batteries, can be one of the challenges in practical applications. Different methods have been studied and reported in technical literature. However, the SoC balancing is still a challenging issue. In this paper, a new method is proposed to solve this problem by applying several clever but simple changes to the staircase modulation switching scheme without changing the output waveform properties. Validations for this simple method were performed through simulation and experimental prototype. Despite the simplicity of the method, the simulation and experimental results show that SoC balancing of the cells has significantly improved.

High‐power bidirectional resonant DC–DC converter with equivalent switching frequency doubler

A high-power double frequency bidirectional symmetrical DC–DC resonant converter is proposed to reduce the sizes of the passive components and increase the cost-effectiveness. Inspired by the bipolar and unipolar modulations for the single-phase inverter, an asymmetrical duty cycle double frequency modulation is originated for high-power bidirectional resonant converters whose switching frequency and power density are significantly limited by the high-power devices. The equivalent operating frequency of the resonant tank becomes twice of the switching frequency, which effectively reduces the volumes and costs of the passive components, especially for high-power high-voltage applications. Moreover, the proposed converter has the same output characteristics for the two power flow directions, which not only symmetrises the bidirectional frequency control, but also simplifies the design of the resonant components. The converter operation principle is illustrated and the converter performances are analysed including the principle of the double frequency modulation, output characteristics and soft switching condition. Finally, a 5 kW prototype is built to verify the analysis, and the resonant components for the conventional bidirectional resonant converter are also designed and compared to illustrate the advantages of the proposed converter.

High Step-Up Interleaved Forward-Flyback Boost Converter With Three-Winding Coupled Inductors

A novel high step-up interleaved converter for high-power high-voltage applications is proposed in this paper. Through three-winding coupled inductors, a high step-up conversion with high efficiency is obtained. The proposed converter not only reduces the current stress, but also constrains the input current ripple, which decreases the conduction losses and lengthens the life time of input source. In addition, due to the lossless passive clamp performance, leakage energy is recycled to the output terminal. Hence, large voltage spikes across the main switches are alleviated and the efficiency is improved. Even, the low-voltage stresses on semiconductor components are substantially lower than the output voltage. Finally, the prototype circuit with input voltage 48 V, output voltage 380 V, and output power 2 kW is operated to verify its performance. The highest efficiency is 96.5%, and the full-load efficiency is 92.6%.

A New Active Fault-Tolerant SVPWM Strategy for Single-Phase Faults in Three-Phase Multilevel Converters

Multilevel converters have been well studied for high-voltage high-power renewable energy applications. Owing to the multibranch structure, these converters can be operated under fault conditions in case of a loss of a branch or voltage cell. In this regard, this paper proposes a new active fault-tolerant space vector pulsewidth modulation strategy for single-phase faults in three-phase multilevel converters. The proposed modulation strategy treats the multilevel converter as a two-level converter by introducing an offset vector. A generalized approach is proposed to search for the offset vector and to adjust the modulation of the converter online under different fault operation conditions. Several short- and open-circuit faults have been demonstrated on a seven-level hybrid input-switched converter to prove the effectiveness of the proposed fault-tolerant scheme.

Investigation of multimodule buck–boost inverter‐based HVDC transmission system

In high voltage direct current (HVDC) systems, the semiconductor devices have to be connected in series to obtain the required high-voltage ratings. This study proposes a new HVDC configuration, namely, multimodule buck–boost inverter for HVDC transmission applications which avoids series connection of large number of semiconductor switches. In addition, it provides a blocking capability against DC side faults. The proposed configuration consists of several simple buck–boost converters which are assembled together to meet the requirements of high-voltage high-power applications. This paper studies the dynamic performance of the proposed system under different operating conditions, and the results were satisfactory. The main advantages of the proposed configuration are: (i) pure sinusoidal output which minimises/eliminates the requirements for supplementary AC filters and offers an inherent suppression to the common mode voltages, (ii) very low dv/dt stresses and (iii) complete blocking capability of AC side contributions during DC side faults. This study discusses the system architecture, passive components selections, voltage and current ratings of its semiconductor devices and the required controllers. A comparison between the proposed configuration and other existing HVDC technologies is also presented in this study.

Quasi Two-Level Operation of Modular Multilevel Converter for Use in a High-Power DC Transformer With DC Fault Isolation Capability

DC fault protection is one challenge impeding the development of multiterminal dc grids. The absence of manufacturing and operational standards has led to many point-to-point HVDC links built at different voltage levels, which creates another challenge. Therefore, the issues of voltage matching and dc fault isolation are undergoing extensive research and are addressed in this paper. A quasi two-level operating mode of the modular multilevel converter is proposed, where the converter generates a square wave with controllable dv/dt by employing the cell voltages to create transient intermediate voltage levels. Cell capacitance requirements diminish and the footprint of the converter is reduced. The common-mode dc component in the arm currents is not present in the proposed operating mode. The converter is proposed as the core of a dc to dc transformer, where two converters operating in the proposed mode are coupled by an ac transformer for voltage matching and galvanic isolation. The proposed dc transformer is shown to be suitable for high-voltage high-power applications due to the low-switching frequency, high efficiency, modularity, and reliability. The dc transformer facilitates dc voltage regulation and near instant isolation of dc faults within its protection zone. Analysis and simulations confirm these capabilities in a system-oriented approach.

Predictive Sorting Algorithm for Modular Multilevel Converters Minimizing the Spread in the Submodule Capacitor Voltages

The modular multilevel converter is a suitable topology for bidirectional ac-dc conversion in high-voltage high-power applications. By connecting submodule circuits in series, a high-voltage waveform with excellent harmonic performance can be achieved with a very high efficiency and low switching frequency. The balancing of the capacitor voltages will, however, become increasingly difficult as the switching frequency is reduced. Although the capacitor voltages can be kept balanced over time even at the fundamental switching frequency, the spread and thus also the peak variation in the capacitor voltages will typically increase at lower switching frequencies. This paper presents a capacitor voltage balancing strategy which aims to combine a low switching frequency with a low capacitor voltage ripple. This is done by a predictive algorithm that calculates the amount of charge that must be stored in the submodule capacitors during the following fundamental frequency period. The converter is then controlled in such a way that the stored charge in the submodule capacitors is evenly distributed among all the submodules when the capacitor voltages reach their maximum values. In this way, it is possible to limit the peak voltage in the capacitor at switching frequencies as low as 2-3 times the fundamental frequency. The capacitor voltage balancing strategy is first validated by simulation results at 110 Hz switching frequency. It is observed that when the proposed method is used, the capacitor voltage ripple is 35% lower compared to the case when a conventional sorting algorithm is used. The capacitor voltage balancing strategy is also validated experimentally at 130 Hz switching frequency. The experimental results show that it is possible to combine the proposed method with previously presented circulating-current control methods.

Design and Implementation of a New Multilevel Inverter Topology

Multilevel inverters have been widely accepted for high-power high-voltage applications. Their performance is highly superior to that of conventional two-level inverters due to reduced harmonic distortion, lower electromagnetic interference, and higher dc link voltages. However, it has some disadvantages such as increased number of components, complex pulsewidth modulation control method, and voltage-balancing problem. In this paper, a new topology with a reversing-voltage component is proposed to improve the multilevel performance by compensating the disadvantages mentioned. This topology requires fewer components compared to existing inverters (particularly in higher levels) and requires fewer carrier signals and gate drives. Therefore, the overall cost and complexity are greatly reduced particularly for higher output voltage levels. Finally, a prototype of the seven-level proposed topology is built and tested to show the performance of the inverter by experimental results.

A Fault Detection and Protection Scheme for Three-Level DC–DC Converters Based on Monitoring Flying Capacitor Voltage

Fault detection and protection is an important design aspect for any power converter, especially in high-power high-voltage applications, where cost of failure can be high. The three-level dc-dc converter and its varied derivatives are attractive topologies in high-voltage high-power converter applications. The protection method can not only prevent the system failure against unbalanced voltage stresses on the switches, but also provide a remedy for the system as faults occur and save the remaining components. The three-level converter is subject to voltage unbalance in certain abnormal conditions, which can result in switch overvoltage and system failure. The reasons for the unbalanced voltage stresses are fully investigated and categorized. The solutions to each abnormal condition are introduced. In addition to the voltage unbalance, the three-level converters can be protected against multiple faults by the proposed protection method through monitoring the flying capacitor voltage. Phenomena associated with each fault are thoroughly analyzed and summarized. The protection circuit is simple and can be easily implemented, while it can effectively protect the three-level converters and its derivatives, which has been verified by the experiment with a three-level parallel resonant converter.

Buck-current-fed zero current switching converter for high voltage coupled cavity

This paper presents a design of buck-current-fed full-bridge zero current switching converters which are suitable for high-power high-voltage DC applications. This study shows the capability of incorporating the step-up transformer parasitic components to get the zero current turn-off characteristics for full-bridge switches. The buck stage of the converter uses pulse-width-modulation control to regulate the output voltage and the full-bridge stage operates at a constant on time mode to implement soft-switching commutations. In this study, the converter model is derived, and this model is implemented in an IsSpice circuit, which can reveal the transient characteristics of the converter. Steady state analysis of the converter is also presented. Finally, the simulation results of this converter for the application of coupled cavity traveling wave tube are given and major design issues are discussed.

Fast optimum-predictive control and capacitor voltage balancing strategy for bipolar back-to-back NPC converters in high-voltage direct current transmission systems

Multilevel power converters have been introduced as the solution for high-power high-voltage switching applications where they have well-known advantages. Recently, full back-to-back connected multilevel neutral point diode clamped converters (NPC converter) have been used in high-voltage direct current (HVDC) transmission systems. Bipolar-connected back-to-back NPC converters have advantages in long-distance HVDC transmission systems over the full back-to-back connection, but greater difficulty to balance the dc capacitor voltage divider on both sending and receiving end NPC converters. This study shows that power flow control and dc capacitor voltage balancing are feasible using fast optimum-predictive-based controllers in HVDC systems using bipolar back-to-back-connected five-level NPC multilevel converters. For both converter sides, the control strategy takes in account active and reactive power, which establishes ac grid currents in both ends, and guarantees the balancing of dc bus capacitor voltages in both NPC converters. Additionally, the semiconductor switching frequency is minimised to reduce switching losses. The performance and robustness of the new fast predictive control strategy, and its capability to solve the DC capacitor voltage balancing problem of bipolar-connected back-to-back NPC converters are evaluated.

New approach in back-to-back m-level diode-clamped multilevel converter modelling and direct current bus voltages balancing

Multilevel power converter structures have been introduced as the solution for high-power high-voltage applications and also for grid interface connection of renewable energy sources systems, where they have several advantages, namely low distortion voltages and currents and low switching losses resulting in higher efficiency. As a consequence of increasing interest on the multilevel converter applications, accurate models of these power converters are essential to be used in computer simulation studies. This study presents a new systematic modelling approach suitable to obtain global generalised models for back-to-back m-level diode-clamped multilevel converters connecting AC loads. Using the proposed converter modelling, an advantageous generalised method to balance the (m−1) DC bus capacitors, based on the DC bus average power flow is also presented. The generalised proposed model is particularised to the back-to-back multilevel structure for m=5. This back-to-back converter is studied working with bidirectional power flow, connecting an induction machine (in motor or self-excited generator mode) to the power grid as a flexible AC transmission system.

Cascode turn-off of field-controlled integrated thyristors

The residual voltage drop in the on state of an insulated-gate bipolar transistor, the basic device of today’s power electronics, is considerably higher than that of conventional thyristor-type devices, which is a serious disadvantage of the former. Field-controlled integrated thyristors offering a low residual voltage, as with conventional thyristors, and, at the same time, low turn-on and turn-off power losses in the control circuit, as is the case with insulated-gate bipolar transistors, seem to be promising for high-voltage high-power applications. Turn-off of these devices in the cascode mode with a series-connected low-voltage powerful FET is studied.

A Novel High-Power-Density Three-Level LCC Resonant Converter With Constant-Power-Factor-Control for Charging Applications

This paper proposes a variable-frequency zero-voltage-switching (ZVS) three-level LCC resonant converter that is able to utilize the parasitic components of the high turns-ratio transformer. By applying a three-level structure to the primary side, the voltage stress of the primary switches is half of the input voltage. Low-voltage MOSFETs with better performance can be used in this converter, and zero-current-switching (ZCS) is achieved for rectifier diodes. By applying a magnetic integration technique, only one magnetic component is required in this converter. The power factor concept of resonant converters is proposed and analyzed, and a novel constant power-factor control scheme is proposed. Based on this control strategy, the circulating energy of resonant converters is considerably reduced. High efficiency can be obtained for high-voltage high-power charging applications. The operation principle of the converter is analyzed and verified on a 700-kHz, 3.7-kW prototype, with which a power density of 72 W/inch3 is achieved.

Some Design Considerations For The 6.5 KV Igbt-based Half-bridge Dc/dc Converter

The 6.5 kV insulated gate bipolar transistor (IGBT) is a new promising power device for high-voltage high-power applications. Due to its increased voltage blocking capability, the new HV IGBT can serve as a good replacement for GTO and IGCT thyristors in medium or medium-to-high power applications. One of its general application fields is railway traction. The paper discusses design and development considerations of the 50-kW auxiliary power supply (APS) to be used in 3.0 kV DC commuter trains. Focus is on the implementation of 6.5 kV IGBTs in the primary inverter of APS to improve power density, integrity and reliability of the whole system. For instance, analysis and simulation of main functional blocks of a converter for different operation points are described. General design considerations mainly concerning the inverter stage, power transformer and output rectifier are specified in the paper. Some design optimization possibilities are proposed.