Improved space vector modulation for matrix converter based isolated rectifiers

For most medium voltage high power applications, Neutral Point Clamped (NPC) three-level inverter is a preferred choice due to its advantages such as low cost, light weight and compact size. Space vector modulation is widely used in real-time digital control for power converters. It is especially suitable for use in NPC multilevel inverters due to its good harmonic profile, flexibility and easy digital implementation. This thesis focuses on the space vector modulation for high power three-level NPC inverter, where the switching frequency of the semiconductor devices is nominally below 1 OOOHz to reduce the power loss of the switching devices. The conventional space vector modulation (SVM) scheme for the three-level NPC inverter produces even order harmonics in the output voltages, which are not desirable for most industrial applications. In this thesis, the mechanism of even order harmonic generation is analysed. A new space vector modulation scheme, which can eliminate all the even order harmonics, is proposed. The performance of the new design is investigated and simulation results are provided for the verification purpose. The harmonic and THD profiles are compared with those of the conventional SVM scheme. The elimination of even order harmonics is achieved at the expense of a slight increase in the switching frequency. The proposed space vector modulation scheme can be applied to other types of converters for the even order harmonic elimination. An algorithm is developed to mitigate the neutral point potential deviation, which is a common problem in the NPC inverters. The simulation results show that this algorithm is suitable for both conventional and the proposed space vector modulation schemes.

A design methodology is presented for a full-bridge Power Factor Corrector (PFC) converter with two control schemes: the well-known Continuous Current Mode (C.C.M) and Triangular Current Mode (T.C.M). The second one is an extension of the Discontinuous Current Mode (D.C.M). The converter is composed of one high frequency leg and one leg switching at the grid frequency and includes the EMC (common and differential mode) filter. This work highlights the impact of the modulation scheme on the design of passive components and on the overall losses. A pareto front in the efficiency (η) vs power density (ρ) domain is derived for both modulation schemes and for natural convection cooling. We show that, when EMC issues are considered, the T.C.M poses more constraints for partial load operation, superseding the full-load design. Finally, it is shown that for non-interleaved converters C.C.M modulation is better than T.C.M modulation, resulting in higher density converters.

This paper presents a novel idea to integrate three of the Space Vector Modulation (SVM) schemes, including the alternating zero sequence, the symmetric sequence and the bus clamped, in only a single FPGA chip which provides many advantages. Flexibility, reliability and very compact system are obviously obtained from this design chip. Moreover, faster design and verification time, design change without penalty are also the benefits resulted in FPGA-Based IC design. The optimized SVM schemes frequencies and programmable deadtime that used to adjust output signals are obtained through either 4/spl times/4 keypad or microprocessor port.

This paper presents a new, simple and generalized approach to calculate flying capacitor currents for an n-level flying capacitor converter without the use of look-up tables. The performance of the proposed method has been studied for a three-, four- and five-level flying capacitor converter in MATLAB/Simulink environment.

Three-phase synchronous generator-rectifier systems (TPSGRSs) with LC filters would exist discontinuous current mode (DCM) at their DC sides in cases of light loading or large filter capacitors contained. It is difficult to establish mathematical model of these generator-rectifier systems operating in DCM. To solve this problem, a new modeling method is proposed by substituting controlled rectifier in series with Boost circuit for diode rectifier in a TPSGRS. Considering the influence of inner resistances of inductances, we establish a new large signal mathematical model of TPSGRSs using switching function and principle of harmonic balance. The new model is compared to detailed simulations as well as physical tests, and is validated to be accurate in capturing large disturbance behaviors of TPSGRS operating in both continuous current mode (CCM) and DCM.

Three-level inverter has been widely used in electric drives and renewable power generation system. However, the common mode voltage (CMV) and output current ripple reduction of three-level inverter should be carefully regulated for the appropriate operation. As these two targets are mutually interrelated, the conventional space vector modulation (SVM) scheme cannot cope with them decently. To overcome this drawback, this paper proposes a novel modified SVM scheme to realize CMV and current ripple reduction simultaneously. The generating mechanism of output current ripple is analyzed, and the relationship between current ripple and utilized vectors is quantitatively described. Based on the analyses, the proposed optimal current ripple and CMV reduction pulse width modulation (OCR-CMVRPWM) method adopts four voltage vectors (large, medium, small and zero vectors) to reduce root mean square (RMS) value of current ripple by allocating the sequence and dwell times of utilized vectors. Compared to the existing PWM methods, the proposed OCR-CMVRPWM scheme can reduce the peak of CMV to half of value using the conventional SVM, and obtain a significant reduction of output current total harmonic distortion (THD). The effectiveness of the proposed method is validated by both simulation and experimental results.

Switching power loss is a key issue in power electronic conversion systems. This paper presents a development of the modulation strategy for the reduced-switch topology of the nine-switch inverter, based on the generalized two-level space vector modulation (SVM) for the simplicities of mathematical algorithm and implementation. Unlike the conventional strategy, the proposed modulation strategy can further reduce the switching commutation of the nine-switch inverter with the elimination of a heavy switching in the middle switches for all the continuous SVM (CSVM) and discontinuous SVM (DSVM) schemes, and the development of the reduced switching utilization of all the DSVM schemes, as it has a significant improvement on the power conversion efficiency. To verify this idea, simulation and experimental analyses are presented.

In this article, a band-limited three-level modulation scheme is proposed for a three-phase neutral-point-clamped (NPC) converter with an aim to balance the average capacitor voltages under all operating conditions. When conventional carrier-based pulsewidth modulation is applied, the amplitude of neutral point (NP)-voltage ripple tends to increase under linear imbalances, while the NP voltage can shift, leading to instability in the presence of nonlinear loads. A previously proposed three-level modulation scheme was able to eliminate all low-frequency capacitor voltage ripple by switching a phase between three levels in each switching period. This approach significantly increased the switching loss. The proposed modulation scheme limits the increase in switching loss by employing two operational bands, one fixed band applied to the capacitor voltages and another variable band applied to the phase currents. If the instantaneous capacitor voltages exceed the voltage band, then an appropriate phase-leg is switched in three levels to guarantee that the capacitor voltages cannot diverge further. If the instantaneous capacitor voltages reside within the voltage band, a variable current band is used to regulate switching in three voltage levels in order to obtain lower switching losses as compared to the conventional three-level modulation scheme. Therefore, the proposed modulation scheme presents a tradeoff between switching losses and capacitor voltage ripple. The experimental results are obtained under different loading conditions for validating the performance of the proposed modulation scheme.

Electric vehicles provide portable energy storage that can be used in Vehicle-to-Grid (V2G) applications. In order to source power to the grid, the power electronics on-board these vehicles must be bidirectional. A single stage bidirectional isolated AC-DC converter is discussed. A modulation scheme for this topology is proposed to provide control of real and reactive power. A control scheme is coupled with the modulation and allows the topology to act as an inductive or capacitive or load while sinking or supplying power. The proposed modulation scheme can charge the electric vehicle battery as well as provide ancillary services to the grid. The modulation and control scheme are analyzed and discussed. Simulation and hardware results are presented.

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.

The three-phase buck-type rectifier has advantages as front-end converter for high efficiency power supplies in telecommunication and data centers. In this paper, the different commutation types of a three-phase buck rectifier with a freewheeling diode are analyzed through experiments using different semiconductor devices. Further, the switching loss of the converter is modeled and calculated for four space vector modulation schemes. It is shown that when the switches include minority carrier devices, such as Si PiN diode, IGBT and Reverse Blocking IGBT (RB-IGBT), more switching loss will occur in the commutation between two switches than between a switch and the freewheeling diode. This difference can be reduced if majority carrier devices, such as SiC Schottky diodes, are used in series with the switches. The modulator can be arranged to eliminate the specific transition which has the most switching loss. According to the analysis, each modulation scheme has its own field for high efficiency application. The advantageous modulation scheme is given for different device combinations in this paper.

This paper proposes to use variable-saturation regulators for induction motor drives operating in field-weakening region, and presents a comparative dynamic-performance analysis between the traditional indirect rotor field oriented (IRFO) control scheme and a new stator flux oriented - sliding mode (SFO-SM) control scheme. The traditional IRFO control scheme has the d-axis current component, torque, rotor flux, and speed loops with four PI-type controllers. The SFO-SM control system is a direct torque control - space vector modulation (DTC-SVM) scheme with closed loops of torque and stator flux without current PI-type controllers. Both control schemes use regulators with variable-saturation in such a way that maximum dc-bus voltage utilization is reached and overcurrent problems are prevented. The experiments are carried out using dSpace digital controllers and comparative results show that the SFO-SM controlled induction motor (IM) drive and IRFO one are characterized by similar dynamic responses in spite of structural simplicity of the SFO-SM control scheme

Series-end winding voltage-source inverters (SEW-VSIs) can effectively improve the dc-link voltage utilization and provide the capabilities to handle the zero-sequence loop. However, the existing modulation schemes for SEW-VSIs cannot fully utilize the output capability of SEW-VSIs while guaranteeing simple implementation. To this end, a three-dimensional space vector modulation (3D-SVM) scheme is proposed for driving three-phase SEW-VSIs in this article. The modulation is conducted in the 3D-space made up of the <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">αβ</i> plane and the zero-sequence-axis, and the reference voltage vector in the 3D-space is synthesized by three adjacent basic voltage vectors and the null voltage vector, which can fully utilize the output capability of three-phase SEW-VSI. Then, a convenient implementation method based on a look-up table is developed to find three basic voltage vectors adjacent to the reference voltage vector. Moreover, in the 3D-space, the feasible region and the linear modulation region of the proposed 3D-SVM are investigated. Accordingly, intuitive overmodulation between different subspaces and the switching sequence generation are developed. A series of simulation and experimental results are presented to validate the effectiveness of the proposed 3D-SVM for three-phase SEW-VSI drives.

This paper proposes and studies the phase-shifted chaotic space vector modulation (SVM) scheme for harmonics performance improvement in paralleled voltage-source inverters fed permanent magnet synchronous motor (PMSM) drive. By using the proposed scheme, the current harmonics around the switching frequency and some of its multiples in PMSM are eliminated. Besides, the peaky harmonics around all multiples of switching frequency are spread out such that the peaky harmonics are further suppressed with the irregular switching frequencies. The experiments are given to verify the performance of the proposed scheme in this paper.

Purpose. The main objective of this work is to demonstrate the advantages brought by the use of space vector modulation technique in the direct torque control of the induction motor. To achieve this purpose, two different direct torque control approaches (with space vector modulation) are proposed and studied from a comparative aspect with each other and with the conventional direct torque control. The novelty of this work consists in the employment of an Integral-Proportional (IP) speed controller in the two proposed direct torque control approaches and a more in-depth evaluation for their performance mainly the switching frequency of inverter semiconductor components and motor torque ripples. Methods. Two different direct torque control approaches that use the space vector modulation strategy and/or fuzzy-logic control, are described in detail and simulated with IP speed controller. The simulation experiments are carried out using Matlab/Simulink software and/or fuzzy-logic tools. Results. Practical value. Comparison results show that the two proposed direct torque control structures (with space vector modulation) exhibit a large reduction in torque ripples and can also avoid random variation problem of switching frequency (over a wide range of speed or torque control). On the other hand, the use of IP speed regulator ensured good dynamic performance for the drive system as well as considerably minimized peak overshoot in the speed response. Practically all of these benefits are achieved while retaining the simplicity and the best dynamic characteristics of the classical direct torque control, especially with the modified direct torque control approach in which the design or implementation requires minimal computational effort.