A Multi‐Objective Optimal Synchronous Modulation Strategy for Three‐Level Converter Through Switching Angle Regulation

This paper presents the FPGA based implementation of symmetrical switching strategy in SVPWM for three level NPC converter. A simplified switching strategy is proposed by exploiting the redundant states of switching vectors available in three level space vector resulting into symmetrical switching sequence. This switching strategy leads to generate switching sequence for all regions of every sector with each switch has to be turned “ON' once in time period resulting into lower device stress and reduced switching loss. The proposed switching strategy is developed using FPGA based Xilinx block sets and tested on hardware test bench consists of Induction motor integrated to the three level Neutral Point Clamped (NPC) converter controlled by FPGA based wavect controller. It is observed from the results, that the complexity of SVPWM for three level inverter is reduced to that of two level inverter and address the problem of narrow pulse width at the midpoint of NPC converter

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> High-power converters for regenerative drives and distributed generation need a network-friendly operation, low harmonic conductive electromagnetic emission for low electromagnetic interference and high electromagnetic compatibility reaching high reliability and performance. Nevertheless, the converters must be controlled with low switching frequency in order to reduce the commutation losses. These two requirements can be satisfied if an optimal modulation strategy is used. The selective harmonic elimination (SHE) is one of the low-switching-frequency strategies most used today. However, this strategy only eliminates a reduced set of harmonic components from the input current. This paper presents a novel optimal modulation strategy whose objective is to reduce the total harmonic distortion of the input current. Six- and twelve-pulse three-level neutral-pointclamped pulsewidth modulation rectifiers are used in order to implement both modulation techniques. The results confirm the advantages of the proposed strategy, namely, less input current distortion and remarkable reduction of higher order harmonics compared with the SHE method, while keeping a low-switching behavior. </para>

In this paper, in order to improve the control performance of a high-power electrically excited synchronous motor (EESM) under low switching frequency, and to improve the poor dynamic response of the traditional selective harmonic elimination pulse width modulation (SHEPWM) strategy, an optimized pulse width modulation (PWM) pulse mode with specific harmonic elimination and good dynamic performance is studied. Based on the fast response characteristics of predictive control, it combines the rolling time domain optimization theory with SHEPWM strategy. Firstly, the stator flux linkage of EESM is taken as the tracking target, and the real-time tracking error value of flux linkage is transformed into a voltage-second product; then, the predictive rolling time domain is established, the above error voltage-second product value is optimized in this time domain and the switching angle of SHEPWM is dynamically adjusted to minimize the stator tracking error so as to realize the dynamic adjustment of EESM at low switching frequency. Through MATLAB simulation and a 50 kW experimental platform, the optimized modulation strategy proposed in this paper has been verified to have good dynamic regulation performance at low switching frequency (≈500 Hz).

Low switching frequency is always used in an electric rail traction induction motor control system, in order to reduce switching losses and increase system reliability. But under low switching frequency, the conventional single pulse width modulation (PWM) technique for the whole speed region will produce a large number of harmonic components, which in turn cause motor heating, torque ripple, and other adverse effects. In this paper, a hybrid PWM technique formed by merging carrier-based asynchronous and synchronous modulation and optimal synchronous modulation is proposed to achieve low losses and total harmonics based on very low switching frequency. A simple low-cost and low-power digital signal processor (DSP) and field programmable gate array (FPGA) hybrid hardware structure is then adopted for implementing the technique, which fully utilizes advantages and resources of each processor. Total harmonic distortion of the system can hence be reduced by operating with the high-precision hybrid PWM, while not compromising the multirate and multitask processing requirements of traction control. Detailed implementations in both DSP and FPGA with high accuracy, low usage of resources, and computational burden are then described, focusing particularly at the smooth transitions among different PWM schemes. Simulations and experiments are eventually performed with results captured for validating the presented hybrid PWM technique.

This paper compares two converter configurations for a multi-10 MVA gate-turn-off (GTO) reactive power compensator (STATCOM) from the viewpoints of converter connection and switching frequency. One is a single-bridge system consisting of a three-phase bridge converter unit and a transformer. Its pulsewidth modulation (PWM) frequency varies from 450 to 1800 Hz, and its line frequency is 50 Hz. The other is a multiconnected converter system consisting of plural, single-, or three-phase converter units and transformers. Its switching frequencies are chosen to be the lowest possible. The evaluated items are harmonic distortion, power loss, GTO utilization factor, and control response. Our simulation study shows that the multiconnected converter system with the lowest switching frequency is superior to the single-bridge system with the higher switching frequency in every case, even when there are severe line faults requiring very quick response. A new control strategy is adopted for obtaining quick response.

Since the introduction of modular multilevel converters (MMC), various additional features have been introduced for MMCs, among them parallel inter-module connectivity. However, such a parallel mode requires appropriate modulation and switching strategies to exploit its full potential. Low-frequency switching modulation is widely used with half-bridge modules and a large body of research focusses on the optimum control and scheduling of these methods. However, so far, a suitable adaptation for MMCs with parallel mode is missing, and existing methods perform suboptimum. This article proposes a generic scheduling algorithm for simple and low-cost integration with low-frequency switching modulation. The proposed method reduces control complexity, minimizes power loss, and can be combined with most modulation techniques, including nearest-level modulation (NLM) and selective harmonic elimination. The text in this article provides a general algorithm for integrating this method with other control levels and analyzes its effect on MMCs with parallel functionality. Moreover, it examines the performance of the system with NLM combined with the proposed scheduler and compares it to other alternatives. Results show up to a 50% reduction in power loss with a similar modulation technique as well as significantly lower THD and power loss compared to optimized phase-shifted carrier modulation as another sensorless alternative method.

As a result of increasing the demand of power converters in electric aircrafts and electric vehicles, the power converter must be designed to deliver better, faster and cheaper performance. Finding the appropriate LC filter is the first step of the power converter design. Considering the desired losses, weight and cost, the optimization method is key in LC filter design. The value of the filter components has been optimized to satisfy the design of specific objective function such as maximizing the quality factor of the filter or minimizing the attenuation of harmonics distortion. Controllers help reduce harmonics distortion in filters. The proportional-integral controller as the first control method, has been used to control the power converters (buck converter, single and three phase inverter). This type of the controller can maintain the bandwidth and the stability margin of the controlled system within desired range. The PI controllers in different systems have been tuned by root locos, bode plot and pole placement methods. Then the sliding mode controller, which does not need the average model of power converter and has a high level of stability in different performance condition (such as load variations), has been applied to the single phase inverter. As the last control step, the three phase inverter with low switching frequency has been controlled in stationary frame with multiloop controller which provides sufficient bandwidth to eliminate low level voltage harmonics even in low switching frequency. This type of the controller doesn’t need park transformation which reduces the calculation time. The optimized value of the controller gains has been investigated by minimizing the integral of time-weighted absolute error or reducing the multiplication of raise time, settling time and the overshoot of the output voltage. The Matlab simulation and optimization toolbox has been used to verify the performance of the proposed controllers and also find the optimized value of filters components and controllers gains.

In this article Fault analysis in 3 level and 5 level inverter to drive Induction motor is presented. Induction motors are widely used in industrial applications for their intrinsic ruggedness and reduced cost. Recently, the use of adjustable speed drives has spread in many applications. Most of the industrials motors are used today are in fact induction motors. Induction motors have been used in the past mainly in applications requiring a constant speed because conventional methods of their speed control have either been expensive or highly inefficient. Multilevel inverters have a high number of switching states so that the output voltage is stepped in smaller increments. This allows mitigation of the harmonics at low switching frequencies thereby reducing switching losses. Almost 38% of faults in industrial loads like variable speed ac drive. IGBT's are used in almost all converters as power handling capability is high. It is estimated that among all types of faults in variable speed ac drives in industry, 10 μs. The performance of the system is analyzed by adopting MATLAB/SIMULINK software.

A cascaded bridgeless totem-pole multilevel converter and its model predictive control (MPC) are proposed for the power interface of 400 V dc-powered data centers. Wide bandgap devices such as gallium nitride (GaN) or silicon carbide (SiC) MOSFETs with very low reverse recovery charge are utilized in the proposed totem-pole converter to reduce the switching loss. DC bus voltage error can be effectively mitigated by adopting the proposed MPC strategy when compared with a conventional proportional-integral (PI) control; the proposed MPC also exhibits smaller overshoot and much faster response when the load changes, and can operate at lower switching frequencies, thus further reducing switching losses. Furthermore, it is easier to balance the dc-bus voltages in each module using the proposed MPC control. The simulation and hardware-in-loop test results of conventional PI and proposed MPC controllers are presented to validate the effectiveness of the proposed converter and its control algorithm.

Over the past years, modular multilevel converters (MMC) have been widely researched as a preferred option among voltage source converters for HVDC transmission system. The converter presents modularity to generate high number of output voltage levels by simple series connection of modules, resulting in higher reliability and easy failure management/maintenance. Synthesis of near sinusoidal output voltage waveforms by multilevel operation requires pulse-width modulation at relatively lower switching frequencies, resulting in reduced switching losses. In this paper, asymmetrical submodules in the MMC based HVDC system have been investigated. Use of asymmetrical submodules with different capacitor voltages results in lesser number of semiconductor switches for the same number of output voltage levels, thus reducing the overall size of the converter system. The operating principle using different carrier based modulation schemes is discussed in detail. Modified voltage balancing algorithms based on well known sorting algorithm and PI controller have been investigated for their feasibility to asymmetrical-MMC operation. Simulations with preliminary experimental results are presented to validate the proposed converter operation.

In this paper, an efficiency comparison of neutral-point-clamped (NPC) inverters and bipolar switch NPC (T-Type) inverters is studied and the result shows that the T-Type inverter is more efficient at lower switching frequencies. Nevertheless, its efficiency suffers when the switching frequency increases due to high switching loss of the equipped high voltage power switches. In order to reduce switching loss and hereby enhance efficiency, a newly proposed circuit-level decoupling modulation (CLDM) scheme is applied for these two widely used three-phase three-level inverters, as well as their corresponding loss analyses are addressed. The switching loss reduction is evaluated comprehensively under variant modulation indices and load power factors. The analysis results reveal that the CLDM is an alternative discontinuous pulse-width modulation (DPWM) approach for inverters with high switching frequencies in order to achieve superior output voltage quality without lowering efficiency.

Reactive power handling by distributed power generation systems (DPGS) was prevented in the past in order to avoid additional power losses in the distribution feeders, however several new national grid codes request reactive power generation/absorption by the converters of active users. Actually almost no literature is focused on power losses analysis in case of reactive current circulation from the DPGS converter point of view. In the reactive power management, it has to be considered that the amount of the power losses varies on basis of the converter topology and the related modulation strategy. In this paper an accurate power losses evaluation is provided in case of two-levels and three-levels single-phase PWM inverters in grid-connected operation. The H-Bridge converter and the Neutral Point Clamped (NPC) half-bridge converter are compared. Reactive power handling feasibility is demonstrated up to low power factors (leading and lagging) involving a very limited increase of the power losses.

In recent years, manufactures of adjustable speed drive (ASD) make effort to reduce the acoustic noise emitted by electric motors. Traditionally, LC passive filters can be employed to mitigate the pulse-width modulation (PWM) generated current harmonics in the audible noise range. However, the bulky passive filters contribute to additional volume and cannot effectively eliminate harmonics at low switching frequency. In this paper, an active output filter (AOF) is proposed to replace passive filters to achieve higher power density and lower acoustic noise. The proposed AOF can be envisioned as a power semiconductor filter block and is implemented by the harmonic injection strategy, which can compensate voltage/current harmonics at the arbitrary frequency. Advanced wide-band gap devices can be employed in AOF to achieve low power loss and high switching frequency. Experimental results from a 2/3 horsepower ASD prototype verifies the advantages of the proposed AOF.

Multilevel inverter units for stand-alone solar photovoltaic systems are gaining popularity in the present industrial environment. The number of switching devices, total harmonic distortion, number of output levels and number of sources are the governing factors in a multilevel inverter topology. The classical multilevel inverter topologies produce a considerable amount of switching losses due to handful number of switching devices as the number of output levels is increased. Therefore to overcome these drawbacks a multilevel inverter topology to maximize the number of output levels with reduced number of switching devices is opted. The staircase output voltages representing the total number of levels are obtained by the additive and subtractive combination of asymmetrically arranged voltage sources thereby reducing the total number of DC sources required. The reduction of THD and switching losses is done by selecting the suitable PWM technique with low switching frequency. Unlike in the central MPPT architecture, the distributed MPPT architecture is employed for the extraction of maximum power. A multilevel inverter for stand-alone solar PV system incorporating the features like reduced number of switches, reduced number of sources and maximizing the number of output levels with distributed MPPT architecture is designed. By this a better performance compared to the conventional methodology can be attain. The multilevel voltage source inverter with/without distributed MPPT and comparative analysis of the three types of Multicarrier Sinusoidal PWM techniques at low/high carrier wave frequency is modeled and simulation results have been taken. The simulation of the Multilevel VSI for stand-alone solar PV system is done using MATLAB R2014a software.

THIS dissertation addresses the technical challenges associated with the operation and control of high-power modular multilevel converters. To improve the performance of modular multilevel converter (MMC), a generalized three-phase mathematical model with common-mode voltage (CMV) is proposed. By using the proposed mathematical model, the magnitude of circulating currents, capacitors voltage ripple, and the ripple in DC-link current during balanced and unbalanced operating conditions can be minimized. The modulation scheme and switching frequency are directly affected the output power quality and the performance of the converter and control method. In this dissertation, a novel sampled average and space vector modulation scheme is proposed. These modulation schemes are suitable to control the MMC with any number of submodules (without modifications), operates at low switching frequency, minimizes the ripple in output current and voltage harmonic distortion, and reduces the output filter size. For reliable operation of MMC, the voltage balancing among submodules is mandatory. This dissertation proposes a generalized single-stage balancing approach with reduced current sensors to control the MMC. The proposed balancing approach is suitable to implement with both phase-shifted and level-shifted pulse width modulation schemes. With the proposed approach, it is also possible to control the MMC with half-bridge and three level flying capacitor submodules. Also, an improved balancing approach often referred as the dual-stage balancing approach is proposed to minimize the voltage harmonic distortion and device power losses. This dissertation also proposes a direct model predictive control (D-MPC) approach to minimize the ripple in submodule capacitors voltage. To implement D-MPC approach, a discrete-time model of MMC with CMV is proposed. With the use of proposed model, the D-MPC approach does not require a cost function to minimize the circulating currents. The computational complexity is one of the major issues in the implementation of D-MPC approach for MMC. In this dissertation, a novel reduced computational MPC approaches named as dual-stage D-MPC and indirect model predictive control (I-MPC) approach are proposed. These approaches significantly minimize the computational complexity and, improve the voltage and current waveform quality while operating at the low switching frequency. Finally, the simulation and experimental studies are presented to validate the dynamic and steady-state performance of proposed methodologies. Index Terms • Modular Multilevel Converters. • Capacitors Voltage Balancing. • Pulse Width Modulation Schemes. • Circulating Currents. • Capacitors Voltage Ripple • Direct Model Predictive Control. • Dual-Stage Direct Model Predictive Control. • Indirect Model Predictive Control. • Total Harmonic Distortion.