Recent developments of high power converters for industry andtraction applications

The introduction of new high power devices like integrated gate commutated thyristors (IGCTs) and high voltage insulated gate bipolar transistors (IGBTs) accelerates the broad use of pulse width modulation (PWM) voltage source converters in industrial and traction applications. This paper summarizes the state-of-the-art of power semiconductors. The characteristics of IGCTs and high voltage IGBTs are described in detail. Both the design and loss simulations of a two level 1.14 MVA voltage source inverter and a 6 MVA three-level neutral point clamped voltage source converter with active front end enable a detailed comparison of both power semiconductors for high power PWM converters. The design and the characteristics of a commercially available IGCT neutral point clamped PWM voltage source converter for medium voltage drives are discussed. Recent developments and trends of traction converters at DC mains and AC mains are summarized     

Distributed Control of Hybrid Motor Drives

The hybrid inverter fed motor drive with two cascaded multilevel inverters is an attractive option for high performance high power applications such as naval ship propulsion systems due to a number of unique features. There is a natural split between a higher-voltage lower-frequency “bulk” inverter and a lower-voltage higher-frequency “conditioning” inverter in the cascaded system which matches the availability of semiconductor devices. Furthermore, the bulk inverter may be a commercial-off-the-shelf (COTS) motor drive meaning that only the conditioning inverter needs to be custom made. However, a drive involving a COTS bulk inverter would require a distributed conditioning inverter control which works completely independent of the bulk inverter control. In this paper, a set of distributed control methods are developed for the hybrid inverter drive with cascaded bulk and conditioning inverters, requiring only single dc source. Moreover, a solution to the practical problem of instant synchronization between the two inverters is presented. Laboratory measurements on a 3.7-kW induction motor drive validate the proposed control. Various practical considerations (such as low$m$-index performance and capacitor precharging options) are discussed and their solutions provided.     

A generalized Undeland snubber for flying capacitor multilevel inverter and converter

This paper proposes a snubber circuit for a flying capacitor multilevel inverter and converter. It also explains the concept of constructing a snubber circuit for a multilevel inverter and converter. The proposed snubber circuit makes use of an Undeland snubber as a basic snubber unit, and thus can be regarded as a generalized Undeland snubber for a flying capacitor multilevel inverter and converter. It has such an advantage of Undeland snubber used in the two-level inverter. Compared with a conventional RLD/RCD snubber for multilevel inverter and converter, the proposed snubber keeps such good features as fewer number of components, reduction of voltage stress of main switching devices due to low overvoltage, and improved efficiency of system due to low snubber loss. In this paper, the proposed snubber is applied to a three-level flying capacitor inverter, and its features are in detail demonstrated by computer simulation and experimental result.     

Three-Dimensional Feedforward Space Vector Modulation Applied to Multilevel Diode-Clamped Converters

Simplified space vector modulation (SVM) techniques for multilevel converters are being developed to improve factors such as the computational cost, number of commutations, and voltage distortion. The feedforward SVM presented in this paper takes into account the actual DC capacitor voltage unbalance of the multilevel power converter. The resulting technique is a low-cost generalized feedforward 3-D SVM method and is particularized for three-phase multilevel diode-clamped converters. This new modulation technique can be applied to topologies where the gamma component may not be zero. The computational cost of the proposed method is similar to those of comparable methods, and it is independent of the number of levels of the power converter. Experimental results using a three-level diode-clamped converter are presented to validate the proposed modulation technique.     

Resonant link bidirectional power converter. II. Application tobidirectional AC motor drive without electrolytic capacitor

This paper proposes a new power converter which consists of two identical 3-phase PWM modulators and a novel resonant circuit. A new control strategy is integrated to realize the bidirectional power converter without an electrolytic link capacitor. The power flow between converters is bidirectional and the regenerative braking is inherent. The source side currents maintain sinusoidal waveforms with a unity power factor. It is observed in the experiment that by balancing the active power between the source and load side, the voltage across a small ceramic link capacitor can be maintained within a small deviation from the reference. Simulation results and experimental results are presented to verify the operational principles     

Control of cascaded multilevel inverters

A new type of multilevel inverter is introduced which is created by cascading two three-phase three-level inverters using the load connection, but requires only one dc voltage source. This new inverter can operate as a seven-level inverter and naturally splits the power conversion into a higher-voltage lower-frequency inverter and a lower-voltage higher-frequency inverter. This type of system presents particular advantages to Naval ship propulsion systems which rely on high power quality, survivable drives. New control methods are described involving both joint and separate control of the individual three-level inverters. Simulation results demonstrate the effectiveness of both controls. A laboratory set-up at the Naval Surface Warfare Center power electronics laboratory was used to validate the proposed joint-inverter control. Due to the effect of compounding levels in the cascaded inverter, a high number of levels are available resulting in a voltage THD of 9% (without filtering).     

Dynamic average-value modeling of a four-level drive system

Multilevel power converters have gained much attention in recent years due to their high power quality, low switching losses, and high-voltage capability. These advantages make the multilevel converter a candidate for the next generation of naval ship prolusion systems. Evaluation of these systems is typically assisted with a dynamic average-value models in order to rapidly predict system performance under several operating scenarios. In this paper, an average-value model is developed for the four-level diode-clamped converter which takes into account the active capacitor voltage balancing control. This model performance prediction is compared to a detailed model and laboratory measurements on an 18 kW rectifier/inverter test system.     

PWM AC／DC功率双向变流器建模方法的研究（一）

基于脉宽调制（PWM）的AC/DC功率双向变流器作为主要功率变换电路，在许多实际应用的电力电子系统中得到广泛应用，如统一电能质量调节器中的有源电力滤波器系统，交-直-交变频调速系统中的整流或逆变部分，电源系统的逆变器，再生能源并网发电系统中的逆变器以及灵活交流输电系统中的统一潮流控制器等等。在各种应用系统中，人们建立其数学模型进行系统分析和控制设计以满足系统目标。PWM整流器数学模型的研究是PWM整流器及其控制技术研究的基础，本文研究介绍了目前较为流行的四种建模方法，并分别分析其特点及应用。     

Single-switch parallel power factor correction AC-DC converters with inherent load current feedback

Single-stage power factor correction (PFC) ac-dc converters usually suffer from high bulk capacitor voltage stress and extra switch current stress. Bulk capacitor voltage feedback with a coupled-winding structure can dramatically alleviate the stresses. However, this type of feedback is indirect because the feedback only occurs after the bulk capacitor voltage increases. This paper presents a family of single-switch single-stage parallel PFC ac-dc converters with inherent load current feedback. Unlike the bulk capacitor voltage feedback, which utilizes the decreased duty ratio and the increased bulk capacitor voltage to reduce the input power at light load, the load current feedback can reduce the input power automatically at light load while maintaining an unchanged duty ratio. The proposed converters combine the advantages of simple topology, low bulk capacitor voltage, and no extra current stress across the switch. The concept is verified using an ac-dc converter with universal-line input and 5-V, 60-W output power. The input current harmonics meet IEC1000-3-2 Class D requirements.     

Development of a new adaptive sigma-delta modulator scheme applied to multilevel inverter control

Interest in multilevel inverters is increasing in recent years, especially in high voltage applications. Traditional pulse width modulation (PWM) techniques can be applied to control this type of switched converter. However, it is necessary to adapt these techniques to the particularities of multilevel operation. As an alternative, other modulation techniques, such as sigma-delta modulation (SDM) can be used. In this case, it is possible to use advanced modulation techniques, such as adaptive modulation, to adapt the SDM to the multilevel operation. This article presents a new adaptive sigma-delta modulator that controls a five levels multilevel inverter. The algorithm that generates the sigma-delta signal is implemented in a field-programmable gate array (FPGA). This allows a very fast operation independent of the overall control circuit operation response. In addition, the FPGA can generate the control signals necessary to drive the power semiconductors in the power stage. The resulting modulator is simpler than the PWM version.     

Modeling Strategy for Back-to-Back Three-Level Converters Applied to High-Power Wind Turbines

Three-level converters are becoming a realistic alternative to the conventional converters in high-power wind-energy applications. In this paper, a complete analytical strategy to model a back-to-back three-level converter is described. This tool permits us to adapt the control strategy to the specific application. Moreover, the model of different loads can be incorporated to the overall model. Both control strategy and load models are included in the complete system model. The proposed model pays special attention to the unbalance in the capacitors' voltage of three-level converters, including the dynamics of the capacitors' voltage. In order to validate the model and the control strategy proposed in this paper, a 3-MW three-level back-to-back power converter used as a power conditioning system of a variable speed wind turbine has been simulated. Finally, the described strategy has been implemented in a 50-kVA scalable prototype as well, providing a satisfactory performance     

Voltage and current stress reduction in single-stage power factorcorrection AC/DC converters with bulk capacitor voltage feedback

Single-stage power factor correction (PFC) AC/DC converters integrate a boost-derived input current shaper (ICS) with a flyback or forward DC/DC converter in one single stage. The ICS can be operated in either discontinuous current mode (DCM) or continuous current mode (CCM), while the flyback or forward DC/DC converter is operated in CCM. Almost all single-stage PFC AC/DC converters suffer from high bulk capacitor voltage stress and extra switch current stress. The bulk capacitor voltage feedback with a coupled winding structure is widely used to reduce both the voltage and current stresses in practical single-stage PFC AC/DC converters. This paper presents a detailed analysis of the bulk capacitor voltage feedback, including the relationship between bulk capacitor voltage, input current harmonics, voltage feedback ratio, and load condition. The maximum bulk capacitor voltage appears when the DC/DC converter operates at the boundary between CCM and DCM. This paper also reveals that only the voltage feedback ratio determines the input current harmonics under DCM ICS and CCM DC/DC operation. The theoretical prediction of the bulk capacitor voltage as well as the predicted input harmonic contents is verified experimentally on a 60 W AC/DC converter with universal-line input     

DC-side shunt-active power filter for phase-controlled magnet-loadpower supplies

Phase-controlled thyristor rectifiers are still the preferred choice in high-power AC/DC converters. This paper shows that their steady-state and dynamic performance can be greatly enhanced for applications requiring high-precision fast-response performance by means of a hybrid structure using a shunt pulse-width modulation (PWM) active filter. In this hybrid structure, the rectifier is designed to handle the bulk of the output power, whereas the PWM converter is only used for harmonic cancellation and current-error compensation under transient conditions. This results in a small power rating for the shunt-active filter. A suitable control scheme is proposed and implemented in this paper for the rectifier and PWM converter. Experimental results are provided to validate the proposed concept     

A novel control scheme for the multilevel rectifier/inverter

A novel three-level pulsewidth modulation (PWM) rectifier/inverter is proposed: this single-phase three-level rectifier with power factor correction and current harmonic reduction is proposed to improve power quality. A three-phase three-level neutral point clamped (NPC) inverter is adopted to reduce the harmonic content of the inverter output voltages and currents. In the adopted rectifier, a switching mode rectifier with two AC power switches is adopted to draw a sinusoidal line current in phase with mains voltage. The switching functions of the power switches are based on a look-up table. To achieve a balanced DC-link capacitor voltage, a capacitor voltage compensator is employed. In the NPC inverter, the three-level PWM techniques based on the sine-triangle PWM and space vector modulation are used to reduce the voltage harmonics and to drive an induction motor. The advantages of the adopted th-ree-level rectifier/inverter are (1) the blocking voltage of power devices (T1, T2, S_a1-S_c4) is clamped to half of the DC-link voltage, (2) low conduction loss with low conduction resistance due to low voltage stress, (3) low electromagnetic interference, and (4) low voltage harmonics in the inverter output. Based on the proposed control strategy, the rectifier can draw a high power factor line current and achieve two balance capacitor voltages. The current harmonics generated from the adopted rectifier can meet the international requirements. Finally, the proposed control algorithm is illustrated through experimental results based on the laboratory prototype.     

Trinary Hybrid 81-Level Multilevel Inverter for Motor Drive With Zero Common-Mode Voltage

This paper proposes a trinary hybrid 81-level multilevel inverter for motor drive. Benefiting from the trinary hybrid topology of the inverter, 81-level voltages per phase can be synthesized with the fewest components. Bidirectional DC-DC converters are used not only to inject power to the DC links of the inverter but also to absorb power from some DC links in cases with a lower modulation index. The higher bandwidth of DC-DC converters alleviates the ripples of DC-link voltages caused by the load current. The space vector modulation used here, which selects voltage vectors that generate a zero common-mode voltage in the load, works at a low switching frequency. With up to 81-level voltages per phase, the total harmonic distortion is small, and the relationship between the fundamental load voltage and the modulation index is precisely linear. A vector controller is used to control an induction motor, which results in a high dynamic response for speeds or torques. The performance of the proposed inverter for the motor drive is confirmed by simulation and experiment.     

Control of Single-Phase-to-Three-Phase AC/DC/AC PWM Converters for Induction Motor Drives

This paper proposes a novel control scheme of single-phase-to-three-phase pulsewidth-modulation (PWM) converters for low-power three-phase induction motor drives, where a single-phase half-bridge PWM rectifier and a two-leg inverter are used. With this converter topology, the number of switching devices is reduced to six from ten in the case of full-bridge rectifier and three-leg inverter systems. In addition, the source voltage sensor is eliminated with a state observer, which controls the deviation between the model current and the system current to be zero. A simple scalar voltage modulation method is used for a two-leg inverter, and a new technique to eliminate the effect of the dc-link voltage ripple on the inverter output current is proposed. Although the converter topology itself is of lower cost than the conventional one, it retains the same functions such as sinusoidal input current, unity power factor, dc-link voltage control, bidirectional power flow, and variable-voltage and variable-frequency output voltage. The experimental results for the V/f control of 3-hp induction motor drives controlled by a digital signal processor TMS320C31 chip have verified the effectiveness of the proposed scheme     

Implementation of a three-level rectifier for power factorcorrection

In this paper, a new single-phase switching mode rectifier (SMR) for three-level pulse width modulation (PWM) is proposed to achieve high input power factor, low current harmonics, low total harmonic distortion (THD) and simple control scheme. The mains circuit of the proposed SMR consists of six power switches, one boost inductor, and two DC capacitors. The control algorithm is based on a look-up table. There are five control signals in the input of the look-up table. These control signals are used to control the power flow of the adopted rectifier, compensate the capacitor voltages for the balance problem, draw a sinusoidal line current with nearly unity power factor, and generate a three-level PWM pattern on the AC side of adopted rectifier. The advantages of using three-level PWM scheme compared with two-level PWM scheme are using low voltage stress of power switches, decreasing input current harmonics, and reducing the conduction losses. The performances of the proposed multilevel SMR are measured and shown in this paper. The high power factor and low harmonic currents at the input of the rectifier are verified by software simulations and experimental results from a laboratory prototype     

Multilevel voltage-source duty-cycle modulation: analysis and implementation

Multilevel converters have become increasingly popular due to high power quality, high-voltage capability, low switching losses, and low electromagnetic compatibility concerns. Considering these advantages, the multilevel converter is a suitable candidate for implementation of future naval ship propulsion systems. This paper focuses on modulation techniques for the multilevel converter. In particular, a novel voltage-source method of multilevel modulation is introduced and compared to existing methods. The proposed method is discrete in nature and can therefore be readily implemented on a digital signal processor. The method is also readily extendable to any number of voltage levels. Results of experimental implementation are demonstrated using a four-level rectifier/inverter system, which incorporates diode-clamped multilevel converters and an 11-level cascaded multilevel H-bridge inverter.     

一种改进的二极管箝位型多电平变换器拓扑

多电平电路在高压大功率领域的拓展受到其复杂电路拓扑的制约,因此近年来不断有新型多电平电路结构被提出。本文在传统多电平逆变器拓扑结构的基础上,提出了一种新型单相七电平电压源逆变器拓扑。新型电路拓扑是在传统的单相全桥五电平箝位二极管电路基础上,增加了两个开关器件,利用10个开关器件以及4个箝位二极管产生了7种不同的电平输出。详细分析了该逆变器的拓扑结构,给出了PWM控制策略。最后通过仿真实验验证了这种拓扑的可行性。该逆变器对传统箝位二极管逆变器在结构上做出了优化。     

Series equivalence/operation of current-controlled boost-typesingle-phase voltage source converters for bidirectional power flow

Single-phase voltage source power converters (VSCs) under consideration are AC-DC current-controlled boost-type power converters with bidirectional power-handling capability. Equivalence between two series-connected two-level power converters and a single three-level power converter is considered here. Further considered is the series operation of three-level power converters. Simulation results and experimental verification for both are provided. Economical configurations of three-level power converters leading to multilevel waveforms are presented thereafter