Phase-shifted PWM Research Articles

Study of start-up decoupling controller for modular multilevel converter

Modular multilevel converters are widely used in power systems because of their significant advantages. In this paper, a dynamic mathematical model is established by analyzing the topology, operating principle and switching function of the modular multilevel converter, so as to construct an inner and outer loop decoupling controller in dq coordinates. The carrier phase-shift pulse-width modulation is selected to control the sub-module operation, and the uncontrolled pre-charging method is adopted to charge the sub-module. Finally, simulation experiments are carried out in MATLAB/Simulink, and the results show that the control method achieves smooth start-up of the modular multilevel converter.

DC Voltage Utilization Improvement to Enlarge Power Balance Constraint Range for Photovoltaic Cascaded Inverter

In the three-phase photovoltaic (PV) cascaded inverter, the output power of PV arrays is not equal due to the difference of solar radiation, temperature and other factors, which leads to the over modulation of PV cascaded inverter. In order to solve the above problems, the carrier phase-shifted PWM (CPS-PWM) control strategy based on third harmonic injection is proposed in this paper, taking a single-stage high-frequency-link isolated cascaded PV inverter topology with clamping circuits as an example. It can not only improve the utilization of DC voltage, but also enlarge the power balance constraint range between the units in the phase of cascaded PV inverter, so as to ensure that all units can avoid over modulation under some serious power imbalance cases, and will not increase the third harmonic component injection of grid current. Firstly, the topology and working principle of the proposed inverter based on CPS-PWM control strategy are introduced. Then, the improved CPS-PWM control strategy which can improve the DC voltage utilization of the PV cascaded inverter is analyzed, and the control strategy of intra-phase power balance is emphatically analyzed in the overall control strategy of PV cascaded inverter system, and it is discussed that the proposed control strategy can also enlarge the power balance constraint range between the units of the cascaded inverter. Finally, the effectiveness of the proposed PV cascaded inverter based on the improved CPS-PWM control strategy is verified by the simulation and experimental results.

Adaptive Bidirectional Inductive Power and Data Transmission System

For the research of wireless power transfer, loosely coupled inductive power transfer (LCIPT) is one of the widely adopted technologies. There are already various researches focusing on development of high-efficiency LCIPT. However, when the transmission and receiver coils are misaligned, the power transfer efficiency will be significantly dropped. This article considers the change of mutual inductance when two induction coils are misaligned. A fuzzy control approach to solve for this problem by adaptively adjusting operational frequency is introduced. Experiments show that it can reduce the power transferring efficiency drop by 11% when two conduction coils are misaligned up to 50 mm. This research also supports adaptive maximum power transfer using tracking mechanism. Before conducting power transmission, the system can choose G2V or V2G according to the vehicle battery capacity via handshaking communication without the need of radio frequency links. In addition, the phase shift pulsewidth modulation technique is proposed for simultaneous data transmission enabling binary quadrature amplitude modulation for digital signal transmission. Fast over current protection is incorporated for power transfer safety when current overload happens. The transmission frequency of this article is controlled to meet the frequency specified by SAE TIR J2954.

A Fast, Decentralized, Self-Aligned Carrier Method for Multicellular Converters

This paper proposes a fast, decentralized method for self-aligning the carriers of a multiphase/multilevel converter operating on the basis of phase-shifted pulse width modulation or level-shifted pulse width modulation. In the proposed method, each cell of the converter synchronizes and updates simultaneously its own carrier angle or carrier level based on the information shared with its neighboring cell, such as its angle/level, its index number, and the total number of activated cells of the converter. Different from the conventional decentralized method (with basic and modified updating rules), which requires some conditions in terms of cell number and initial carrier angles to start up and operate properly, the proposed method can be applied to the system with any number of cells and does not require special conditions of initial carrier angles. Further, while the conventional method needs an iteration process to adjust the inter-carrier phase-shifts and can be applied only to a multiphase converter which uses phase-shifted pulse width modulation, the proposed method offers an accurate and fast alignment of phases (for phase-shifted pulse width modulation) or levels (for level-shifted pulse width modulation) and thus can be applied to both multiphase and multilevel converter types. The simulations and the experimental results are presented in detail to show the validity and the effectiveness of the proposed methods. Further, thorough simulations on multiphase converters with different number of cells also show that the proposed method is much faster than the conventional method in both configuration and reconfiguration processes, especially in case the system has a large number of cells.

PWM for Multilevel Inverter-State of Art and Comparison of its Various Techniques

This paper suggests a comparison across various techniques of pulse width modulation (PWM) such as phase shift PWM (PSPWM), phase disposition PWM (PDPWM) and sine PWM (SPWM) applied across three phase cascaded HBridge inverter to yield seven level output in order to obtain a low distorted output waveform. Simulation has been carried out to analyse the comparison result in the form of waveforms and total harmonic distortion (THD) where it is found that that SPWM delivers lower THD and is reliable for the elimination of harmonics in the system comparatively.

Hybrid DC Converter with Current Sharing and Low Freewheeling Current Loss

A new hybrid high-frequency link pulse-width modulation (PWM) converter using voltage balance capacitor and current balance magnetic coupling is proposed to realize low freewheeling current loss and wide load range of soft switching operation. Series-connected H-bridge converter is adopted for high voltage applications. In addition, a voltage balance capacitor and a current balance magnetic coupling core are employed for achieving voltage and current balance. To extend zero-voltage switching (ZVS) range of switches at lagging-leg of phase-shift PWM converter, soft switching LLC converter is linked to the lagging-leg of phase-shift PWM converter. Therefore, the wide ZVS load operation is realized in the presented hybrid converter. The other high freewheeling current disadvantage in conventional phase-shift PWM converter is improved by a snubber circuit used on low-voltage side. Thus, the primary current during the freewheeling state is decreased and close to zero. In addition, the conduction losses on primary-side components of studied converter are reduced. The secondary-sides of phase-shift PWM converter and LLC resonant converter are series-connected to achieve power transfer between input and output sides. Experimental results using a laboratory prototype are provided to demonstrate the effectiveness of the studied circuit and control algorithm.

Nonlinear control for modular multilevel converters with enhanced stability region and arbitrary closed loop dynamics

This paper presents the nonlinear control design with formal stability proof for a Modular Multilevel Converter (MMC), which is critical for the interaction between AC and DC grids. Lyapunov theory is used to develop the proposed controller, which is based on Backstepping and Feedback Linearization techniques and to perform the stability analysis. The proposed controller allows to asymptotically stabilize all converter’s states, i.e., AC currents, circulating currents, and internal energy. The proposed algorithm allows to manage the converter in a wide range of operation points, by means of arbitrary tuning assignment. Robustness and performances of proposed control are verified by means of Matlab Simscape Electrical simulations, including active and reactive power reference variations and grid imbalance conditions. A detailed MMC switching model, of 450MVA is considered on Matlab validation. A phase-shift PWM is considered, with a sorting algorithm. Also, a comparison with a standard PI controller is performed. In addition, MMC’s internal energy is also controlled, which can contribute to the high-speed frequency control by using this energy for synthetic inertia.

Design of high precision and high power bidirectional adjustable power supply

Abstract ITER (International Thermonuclear Experimental Reactor) sensor steady-state test platform needs high power supply with high steady-state precision output voltage. In the field of high voltage power supply applications, there is often a contradiction between the power supply capability and the switching frequency of high power switching devices. Limited by switching loss, the switching frequency of high power switching devices is low, which will generate a lot of harmonic in the output voltage. In order to meet the requirement of power supply for high-precision output voltage, the design of high power supply adopts the method of combining H-bridge cascaded topology with carrier phase shift PWM control scheme, which can effectively improve output voltage waveform in the design of high power supply. The harmonic analysis and simulation are carried out, and the experimental results verify the validity of the experimental scheme.

Suppressing Leakage Current for Cascaded H-Bridge Inverters in Renewable Energy and Storage Systems

Leakage current in a transformerless cascaded H-bridge (CHB) inverter is a problem that deteriorates the system performance and causes safety concerns. In this article, a common-mode equivalent circuit is established for analyzing the occurrence of leakage current in an m-level CHB inverter with either asymmetrical or symmetrical inductance output filter configurations. The analysis provides a comparison between traditional phase shifted pulsewidth modulation, phase disposition pulsewidth modulation, and the proposed leakage current reduction pulsewidth modulation. It is reported that grid leakage current cannot be suppressed in an asymmetrical inductance filter configuration solely based on modulation methods. The proposed leakage current reduction pulsewidth modulation pulsewidth modulation can effectively reduce the grid leakage current in a symmetrical filter configuration nine-level CHB inverter. Simulation and experimental studies for aforementioned methods are provided and their performances are evaluated.

Imbalance phenomenon caused by CPS-PWM strategy for MMC systems

The carrier phase-shift PWM strategy is widely used in modular multilevel converters to satisfy the driver demands of the numerous switches in submodules. However, the traditional carrier phase-shift PWM strategy can cause an imbalance phenomenon in the submodule capacitor voltages, which deteriorates the system working performance. This study develops a mathematical model of an MMC system, which is the basis of a quantitative analysis of the relations between the PWM strategy and the imbalance phenomenon. A novel method is then proposed to provide an accurate description of the cause of the imbalance phenomenon in relation to the PWM strategy. Based on the above analysis, a quantitative analysis of the imbalance phenomenon in the capacitor voltages is used to further measure the magnitudes of the imbalance phenomenon. The effect of the PWM strategy on the capacitor voltages is finally demonstrated via experiments.

Phase-Shifted Model Predictive Control to Achieve Power Balance of CHB Converters for Large-Scale Photovoltaic Integration

Cascaded H-bridge (CHB) converters are attractive candidates for next generation photovoltaic (PV) inverters. CHB converters present a reduced voltage stress per power switch and a high modularity. Therefore, the plant can be divided in several PV strings that can be connected to each H-bridge cell. However, due to variability on solar irradiance conditions, each PV string may present different maximum available power levels, which difficult the overall converter operation. To address this issue, this article presents a model predictive control (MPC) strategy, which works along with a phase-shifted pulsewidth modulation (PS-PWM) stage; hence, its name phase-shifted MPC (PS-MPC). The novelty of this proposal is the way both interbridge and interphase power imbalance are directly considered into the optimal control problem by a suitable system reference design. Thus, the interphase imbalance power is tackled by enforcing the converter to operate with a proper zero-sequence voltage component. Then, by exploiting the PS-PWM working principle, PS-MPC is able to handle each H-bridge cell independently. This allows the predictive controller to also deal with an interbridge power imbalance using the same control structure. Experimental results on a 3-kW prototype are provided to verify the effectiveness of the proposed PS-MPC strategy.

A High-Frequency Isolated Multilevel Cascaded-Type Bipolar Direct PWM AC–AC Converter for Utility Voltage Compensation

A high-frequency isolated multilevel cascaded-type bipolar direct PWM ac-ac converter is proposed. By cascading N-unit cells, the converter can produce symmetric bipolar voltage gain of ±ND. It does not require a separate ac source for each unit cell, and can provide higher ac output voltages with multiple levels using low-voltage rating devices, decreasing output voltage harmonics and filtering requirements. Moreover, phase-shift PWM switching is employed, increasing the effective frequency of the output filter, further reducing its requirements. The converter can be utilized as a dynamic voltage restorer (DVR) for grid voltage regulation, with the following features: No need for an external dc source, separate ac sources, and a line-frequency voltage injection transformer. Injection of higher voltage magnitudes than the line voltage, mitigating deep voltage sags. Injection of a negative series voltage to provide compensation for voltage swells. The detailed circuit operation is explained together with component design guidelines and application as DVR. Finally, experimental results are provided using an 800-W scale-down laboratory prototype.

Novel Level-Shifted PWM Technique for Equal Power Sharing Among Quasi-Z-Source Modules in Cascaded Multilevel Inverter

Phase-shifted pulsewidth modulation (PS-PWM) is a well-known switching technique for quasi-Z-source (qZS)-based cascaded multilevel inverters (qZS-CMI). PS-PWM ensures equal power distribution (among operating modules) and equal switchings in all the semiconductors of the every given module; however, this technique suffers from higher number of switchings. On the other hand, level-shifted pulsewidth modulation (LS-PWM) ensures optimal switching but suffers from uneven distribution of power among the all the operating modules and also all the semiconductors of the same module of qZSI. This article proposes a novel switching technique which combines the advantages of both PS-PWM and LS-PWM, such as equal power distribution (among operating modules) and optimal switching sequence along with equal switch utilization. The proposed method is phase opposed disposed PS-PWM. This article details the switching technique along with switching loss analysis and compares it with the PS-PWM technique to show its superiority. Simulation results are carried out to study the cascaded multilevel inverter (CMLI) performance. Experimental prototype is developed to validate the performance of the proposed modulation.

Discontinuous-PWM Method for Multilevel $N$-Cell Cascaded H-Bridge Converters

The cascaded H-bridge converter (CHB) is a very suitable solution with inherent modularity for many applications such as flexible ac transmission systems and motor drives. In this article, the discontinuous pulsewidth modulation (DPWM) by clamping some power cells is used in the CHB because it permits to reduce the switching losses in the power devices. However, this provokes a high harmonic distortion (basebands and sidebands harmonics) in the CHB output voltage compared with the quality obtained avoiding to clamp cells, just applying the conventional phase-shifted PWM (PS-PWM) technique. This has been studied for the three-cell CHB topology but the extension for an N-cell CHB with several clamped cells is a challenge that is addressed in this article. A generalized CHB harmonic voltage analysis applying the DPWM method is presented considering multiple clamped cells. The harmonic distortion mitigation target can be decoupled into two objectives. The proposed method cleverly groups the cells (one clamped + one or two nonclamped cells) to mitigate the basebands harmonic distortion. Simultaneously, a variable-angle PS-PWM technique is applied to the CHB in order to mitigate the sidebands harmonics content. Experimental results demonstrate the good performance of the proposed method.

Modeling and analysis of hybrid multilevel converter for constant DC and fuel cell sources

AbstractThis article proposes a symmetrical hybrid multilevel converter topology with a constant DC source and a fuel cell source (FCS) for enhancing the voltage levels and minimization of total harmonic distortions (THD). The proposed topology is employed using a modified phase shifted pulse width modulation (PS‐PWM) in order to control the power electronic switches. In this hybrid topology, the first converter operated through low voltage with high frequency and the second converter through high voltage with fundamental frequency for improving the voltage levels. The THD for the hybrid multilevel converter with respect to current and voltage is 1.97% and 15.10%, respectively. The results show that the proposed topology performs better with a constant DC source compared to the fuel cell source. Furthermore, the less component count of the proposed topology is compared with the existing topologies. Comparisons are made to validate the results of the proposed topology with different sources and the existing topologies.

Using Gated Recurrent Units for Selective Harmonic Current Mitigation-PWM in Grid-Tied Cascaded H-Bridge Converters

Grid-tied converters become more popular for electric vehicle, renewable energy, and energy storage systems applications. Different modulation techniques such as low-frequency (e.g., selective harmonic elimination-PWM, selective harmonic mitigation-PWM, and selective harmonic current mitigation-PWM (SHCM-PWM)) and high-frequency modulation techniques (e.g., phase shift-PWM and space vector modulation) are used in the literature for grid-tied converters. Low-frequency modulation techniques have a high-efficiency due to the low-switching power losses. One of the main challenges of low-frequency modulation techniques such as SHCM-PWM is how to implement the proposed technique in real-time, due to the transcendental Fourier equations of SHCM-PWM. In this paper, a deep learning technique (i.e., gated recurrent units) is adopted to implement the SHCM-PWM technique in real-time. To evaluate the effectiveness of the proposed learning-based technique, both simulation and experiments are performed for a single-phase 3-cell 7-level CHB converter.

Variable-Angle PS-PWM Technique for Multilevel Cascaded H-Bridge Converters With Large Number of Power Cells

Modular converters such as the multilevel cascaded H-bridge (CHB) are an attractive option for multiple applications mainly because of inherent modularity and fault-tolerant operation. This article is focused on the CHB converter operating with unbalanced conditions (different dc voltages and/or modulation indexes). Under these circumstances, applying the conventional control and modulation strategies, the output voltage harmonic spectrum is degraded. In this article, a generalized variable-angle phase-shifted pulsewidth modulation (PS-PWM) technique for CHB converters with a large number of power modules <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$(&gt;\!\!3)$</tex-math></inline-formula> is presented. The method considers all possible cells’ combinations to form groups and assigns the role of each cell in the group. This cell role defines the identifier of the cell in the variable-angle PS-PWM technique. In the steady state, in each group of cells, the harmonic distortion of the CHB output voltage located at twice the carrier frequency <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$f_c$</tex-math></inline-formula> is eliminated, while the distortion at <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$4f_c$</tex-math></inline-formula> is also diminished. Experimental results show how the proposed technique achieves superior harmonic performance without introducing any significant disadvantage.

Resonant LLC DC–DC Converter Employing Fixed Switching Frequency Based on Dual-Transformer With Wide Input-Voltage Range

In this article, a dual-transformer-based LLC resonant converter is proposed, which is a hybrid combination of a full-bridge and half-bridge LLC circuits. The output voltage is regulated through fixed-frequency phase-shift pulsewidth modulation control scheme, whereas the switching frequency is equal to the resonant frequency, which helps to design the magnetic components. The turns-ratio design of the two transformers is analyzed by the current stress and power distribution of both transformers with two different cases. The proposed converter is capable of realizing soft switching over wide input-voltage and whole load range. Finally, a prototype converter of 1 kW is built and the experiments are conducted under wide input-voltage and full load range. The experimental results show that the proposed converter can achieve high efficiency throughout the full load range.

Minimum voltage control for reliability improvement in modular multilevel cascade converters-based STATCOM

This work proposes an adaptive dc-link voltage strategy applied to a double-star chopper cell-based static synchronous compensator (DSCC-STATCOM). The proposed technique aims to improve the converter reliability by controlling the cell reference voltage according to the minimum voltage required in each STATCOM operation condition. It is important to remark that the proposed strategy does not require any hardware modification. The technique is evaluated in terms of dynamics and reliability. For this purpose, two modulation strategies were analyzed: phase-shifted pulse-width modulation (PS-PWM) and nearest-level control with cell tolerance band algorithm (NLC-CTB). The results show that the proposed technique can reduce the switching losses and increase the system reliability. For the evaluated case study, the DSCC-STATCOM B1 lifetime increases 16% and 23% for PS-PWM and NLC-CTB, respectively, when the minimum voltage control strategy is employed.

Self-Balanced 13-Level Inverter Based on Switched Capacitor and Hybrid PWM Algorithm

A 13-level inverter based on switched-capaci-tor technique is proposed in this article. It consists of ten transistors, four diodes, and four capacitors with self-balanced voltages. The ten transistors form two H-bridges and one half-bridge resulting in simple structure and easy design of gate drivers. With a hybrid of level-shifted and phase-shifted pulsewidth modulation algorithm, voltage ripples of capacitors and low harmonic components of output voltage are suppressed simultaneously. Compared with the existing solutions, the proposed 13-level inverter has a simpler structure and the lower cost per level. Experimental results demonstrate that the proposed inverter has the advantages of high boosting factor, self-balanced capacitor voltages, low harmonics, and high efficiency, and the maximum efficiency is up to 97.2%.