Showing papers on "Pulse-width modulation published in 2012"

A Review of Control and Modulation Methods for Matrix Converters

Abstract: This paper presents a review of the most popular control and modulation strategies studied for matrix converters (MCs) in the last decade. The purpose of most of these methods is to generate sinusoidal current on the input and output sides. These methods are compared considering theoretical complexity and performance. This paper concludes that the control strategy has a significant impact on the resonance of the MC input filter.

A Carrier-Based PWM Strategy With Zero-Sequence Voltage Injection for a Three-Level Neutral-Point-Clamped Converter

Abstract: Performance of a carrier-based pulsewidth modulation (CB-PWM) strategy can be improved by the inclusion of a zero-sequence voltage in the modulation-reference signal. This paper proposes a new CB-PWM strategy for a three-level neutral-point-clamped (NPC) converter, which is based on a zero-sequence voltage injection. By inclusion of the zero-sequence voltage, the sinusoidal-modulation reference is modified to 1) carry out the voltage-balancing task of the dc-link capacitors, with no additional control effort, 2) reduce the switching losses, and 3) reduce the low-frequency voltage oscillations of the neutral point. The proposed strategy is an alternative approach to the nearest three-vector (NTV) space-vector modulation (SVM) strategy and is obtained by the analysis of the NTV-SVM strategy and establishing a correlation between the NTV-SVM and the CB-PWM strategies. The salient features of the proposed scheme, as compared with the NTV-SVM strategy, are: 1) its reduced computational processing time which is attractive for digital implementation and 2) its reduced switching losses. Compared with the existing CB-PWM strategies, the proposed strategy offers 1) capability to balance the capacitor voltages and reduce the NP voltage oscillations and 2) reduced switching losses. Performance of the proposed CB-PWM strategy for a three-level NPC converter based on time-domain simulation studies in the MATLAB/SIMULINK environment is evaluated and also experimentally verified.

Active-Power Control of Individual Converter Cells for a Battery Energy Storage System Based on a Multilevel Cascade PWM Converter

Abstract: The battery energy storage system is an essential enabling device of the smart grid, because it helps grid connection of massive renewable energy resources. This paper has a brief discussion on a battery energy storage system based on a multilevel cascade pulsewidth-modulated (PWM) converter for its practical use. The active-power control of individual converter cells is presented to make it possible to charge and discharge the battery units at different power levels while producing a three-phase balanced line-to-line voltage. This results in the maximum utilization of battery energy even when the power-handling capabilities of the battery units differ. Experimental results obtained from a 200-V, 10-kW, 3.6-kWh battery energy storage system verify the effectiveness of the presented active-power control.

A novel inner current suppressing method for modular multilevel converters

Abstract: Ideally, the inner (the upper or lower arm) current of a modular multilevel converter (MMC) is assumed to be the sum of a dc component and an ac component of the fundamental frequency. However, this current is usually distorted and the peak/RMS value of it is increased compared with the theoretical result. This is because ac current flows through the submodule (SM) capacitors and the capacitor voltages fluctuate with time. The increased currents will increase power losses and may threaten the safe operation of the power devices and capacitors. This paper proposes a novel close-loop method for suppression of the inner current in MMC. This method is very simple and is implemented in stationary frame, and no harmonic extraction algorithm is needed. Hence, it can be applied to single-phase or three-phase MMC. What is more important, this method does not influence the balancing of the SM capacitor voltages. Simulation and experimental results show that the proposed method can suppress the peak and RMS values of the inner currents dramatically. Meanwhile, the harmonic contents in the output current can also be suppressed satisfactorily even when the SM capacitor voltage ripple factor is as large as about ±19%. Therefore, the proposed method can also be adopted to reduce the SM capacitance requirement.

An Improved Pulse Width Modulation Method for Chopper-Cell-Based Modular Multilevel Converters

Abstract: In this paper, an improved pulse width modulation (PWM) method for chopper-cell (or half-bridge)based modular multilevel converters (MMCs) is proposed. This method can generate an output voltage with maximally 2N+1 (where N is the number of submodules in the upper or lower arm of MMC) levels, which is as great as that of the carrier-phase-shifted PWM (CPSPWM) method. However, no phase-shifted carrier is needed. Compared with the existing submodule unified pulse width modulated (SUPWM) method, the level number of the output voltage is almost doubled and the height of the step in the staircase voltage is reduced by 50%. Meanwhile, the equivalent switching frequency in the output voltage is twice that of the conventional SUPWM method. All these features lead to much reduced harmonic content in the output voltage. What is more, the voltages of the submodule capacitors can be well balanced without any close-loop voltage balancing controllers which are mandatory in the CPSPWM schemes. Simulation and experimental results on a MMC-based inverter show validity of the proposed method.

PWM Switching Frequency Signal Injection Sensorless Method in IPMSM

Abstract: The rotor position of an interior permanent-magnet synchronous machine (IPMSM) can be estimated without a position sensor by signal injection sensorless control at standstill and/or in very low speed rotating condition. In the signal injection sensorless control, however, the fundamental control performance is limited by the frequency of the injected signal, and no negligible acoustic noise is generated. If the frequency of the injected voltage signal would increase to pulsewidth modulation (PWM) switching frequency and if the switching frequency is near or above audible range, the dynamics of the sensorless control can be improved, and the acoustic noise can be remarkably reduced or totally eliminated. This paper describes how to extract the rotor position information of IPMSM using the voltage signal injection whose frequency is the same as the PWM switching frequency. Compared to the conventional heterodyning process, the proposed method is simple to implement and appropriate for PWM switching frequency signal injection. The high-frequency voltage signal can be injected in the stationary reference frame or in the estimated rotor reference frame. In this paper, the 5- and 16-kHz signal injections are proposed, implemented, and compared. The experimental results confirm the effectiveness of the proposed method.

Fixed Switching Frequency Sliding Mode Control for Single-Phase Unipolar Inverters

Abstract: Sliding mode control (SMC) is recognized as robust controller with a high stability in a wide range of operating conditions, although it suffers from chattering problem. In addition, it cannot be directly applied to multiswitches power converters. In this paper, a high performance and fixed switching frequency sliding mode controller is proposed for a single-phase unipolar inverter. The chattering problem of SMC is eliminated by smoothing the control law in a narrow boundary layer, and a pulsewidth modulator produces the fixed frequency switching law for the inverter. The smoothing procedure is based on limitation of pulsewidth modulator. Although the smoothed control law limits the performance of SMC, regulation and dynamic response of the inverter output voltage are in an acceptable superior range. The performance of the proposed controller is verified by both simulation and experiments on a prototype 6-kVA inverter. The experimental results show that the total harmonic distortion of the output voltage is less than 1.1% and 1.7% at maximum linear and nonlinear load, respectively. Furthermore, the output dynamic performance of the inverter strictly conforms the standard IEC62040-3. Moreover, the measured efficiency of the inverter in the worst condition is better than 95.5%.

Multilevel inverters: A literature survey on topologies and control strategies

Abstract: Multilevel inverters have been attracting in favor of academia as well as industry in the recent decade for high-power and medium-voltage energy control. In addition, they can synthesize switched waveforms with lower levels of harmonic distortion than an equivalently rated two-level converter. The multilevel concept is used to decrease the harmonic distortion in the output waveform without decreasing the inverter power output. This paper presents the most important topologies like diode-clamped inverter (neutral- point clamped), capacitor-clamped (flying capacitor), and cascaded multilevel with separate dc sources. This paper also presents the most relevant modulation methods developed for this family of converters: multilevel sinusoidal pulse width modulation, multilevel selective harmonic elimination, and space-vector modulation. Authors strongly believe that this survey article will be very much useful to the researchers for finding out the relevant references in the field of topologies and modulation strategies of multilevel inverter.

Analysis of PWM Z-Source DC-DC Converter in CCM for Steady State

Abstract: Steady-state analysis of pulse-width modulated (PWM) Z-source dc-dc converter operating in continuous conduction mode (CCM) is presented. Voltage and current waveforms, and their corresponding expressions describing the steady-state operation of the PWM Z-source dc-dc converter have been presented. The input-to-output dc voltage transfer functions, both for ideal and non-ideal PWM Z-source dc-dc converter have been derived. The minimum Z-network inductance required to ensure CCM operation is derived. The voltage ripple due to filter capacitor and its ESR, and their individual effects on the the overall output voltage ripple have been derived and analyzed. Expressions for power loss in each of the components of the PWM Z-source dc-dc converter has been determined. Using the expressions derived to determine the power losses, an expression for the overall efficiency of the PWM Z-source dc-dc has been derived. An example PWM Z-source dc-dc converter is considered. A laboratory prototype is built and the theoretical analysis is in good agreement with the experimental results.

Adaptive Switch Mode LED Driver

Abstract: An adaptive switch mode LED driver provides an intelligent approach to driving multiple strings of LEDs. The LED driver determines an optimal current level for each LED channel from a limited set of allowed currents. The LDO driver then determines a PWM duty cycle for driving the LEDs in each LED channel to provide precise brightness control over the LED channels. Beneficially, the LED driver minimizes the power dissipation in the LDO circuits driving each LED string, while also ensuring that the currents in each LED string are maintained within a limited range. A sample and hold LDO allows PWM control over extreme duty cycles with very fast dynamic response. Furthermore, fault protection circuitry ensures fault-free startup and operation of the LED driver.

Torque Ripple Reduction in BLDC Torque Motor With Nonideal Back EMF

Abstract: In order to improve the speed precision and stabilization of the gimbal servo system of double gimbal magnetically suspended control moment gyro, a comprehensive analysis of the reason of electromagnetic torque ripples of brushless direct current motor with nonideal back electromotive force (EMF) drives in the conduction and commutation regions is presented. A novel automatic control method of torque is proposed. With this method, the current control rule is designed, and the duty cycle of pulse width modulation (PWM) is regulated in real time by measuring the wave function of back EMF. In order to eliminate the diode freewheeling of the inactive phase, the PWM_ON_PWM scheme is used. Simulation and experiment results are given to show that, compared with the conventional current control method, the proposed method can reduce the torque ripple effectively and improve the speed precision and stabilization as well.

Improved Natural Balancing With Modified Phase-Shifted PWM for Single-Leg Five-Level Flying-Capacitor Converters

Abstract: Flying-capacitor converters (FCCs), like most multilevel converter topologies, require a balancing mechanism of the capacitor voltages. FCCs feature natural voltage balancing when a special modulation technique is used. The classic methods, such as phase-shifted pulse width modulation (PS-PWM), result in very slow balancing for some duty-ratio ranges. Previous work has shown that for a single-leg five-level FCC, one time constant is infinite for a zero desired output voltage. In this paper, a modified PS-PWM scheme for a single-leg five-level FCC is presented, which results in faster balancing over the total duty-ratio range. The modified PS-PWM scheme is studied, resulting in an averaged voltage-balancing model. This model is verified using simulations and experiments. The modified PS-PWM scheme solves the slow-balancing problems of the normal PS-PWM method for odd-level FCCs, while maintaining the passive control property, and it provides a self-precharge capability.

PD Modulation Scheme for Three-Phase Parallel Multilevel Inverters

Abstract: Pulsewidth modulation (PWM) strategies and methods for multilevel converters are usually developed for series converters. One of the aims of this paper is to show that they may be applied to parallel converters using interleaving techniques, given that these converters also have multilevel characteristics. PWM methods based on carriers' disposition and on zero sequence injection are studied for parallel multilevel inverters. Analysis show that the best method in terms of load current ripple (phase disposition (PD) centered space vector PWM) has, nevertheless, a bad influence on the current balancing between commutation cells of the same phase. Another objective is to analyze these problems and to propose a solution to cancel current imbalance when using PD strategy. Experimental results validate the analysis presented in this paper as well as the compensation of band transition of the differential mode current observed when PD strategy is used.

Full-Bridge Three-Port Converters With Wide Input Voltage Range for Renewable Power Systems

Abstract: A systematic method for deriving three-port converters (TPCs) from the full-bridge converter (FBC) is proposed in this paper. The proposed method splits the two switching legs of the FBC into two switching cells with different sources and allows a dc bias current in the transformer. By using this systematic method, a novel full-bridge TPC (FB-TPC) is developed for renewable power system applications which features simple topologies and control, a reduced number of devices, and single-stage power conversion between any two of the three ports. The proposed FB-TPC consists of two bidirectional ports and an isolated output port. The primary circuit of the converter functions as a buck-boost converter and provides a power flow path between the ports on the primary side. The FB-TPC can adapt to a wide source voltage range, and tight control over two of the three ports can be achieved while the third port provides the power balance in the system. Furthermore, the energy stored in the leakage inductance of the transformer is utilized to achieve zero-voltage switching for all the primary-side switches. The FB-TPC is analyzed in detail with operational principles, design considerations, and a pulsewidth modulation scheme (PWM), which aims to decrease the dc bias of the transformer. Experimental results verify the feasibility and effectiveness of the developed FB-TPC. The topology generation concept is further extended, and some novel TPCs, dual-input, and multiport converters are presented.

Leakage Current Reduction in a Single-Phase Bidirectional AC–DC Full-Bridge Inverter

Abstract: The leakage current in grid-interface converter systems presents a considerable issue in regard to safety and efficiency. The full-bridge inverter is a well-accepted topology in single-phase power conversion applications. The high-frequency pulsewidth modulation (PWM) modulation schemes are normally applied to the full-bridge topology for smaller ac filter size, which, however, generates a high-frequency dc-side leakage current, resulting in an enormous negative impact on dc components, such as photovoltaic panels and energy storage elements. In this paper, a modified full-bridge inverter topology to reduce the dc-side leakage current as well as to mitigate the ac-side common-mode electromagnetic interference noise is presented. Several considerations are discussed, such as the PWM modulation and filter design. Compared to the other existing methods, the proposed solution provides a reliable performance for bidirectional operation, minimum additional components, low cost, and a simple design process.

Asymmetric PWM Control Scheme During Hold-Up Time for $LLC$ Resonant Converter

Abstract: To narrow down the switching-frequency-variation range of an LLC resonant converter, asymmetric pulsewidth modulation (APWM) of the hold-up time is proposed. During the hold-up time, the modulation method of an LLC resonant converter is changed from frequency modulation (FM) to APWM. Since the LLC resonant converter achieves a higher gain with APWM than it does with FM at the same switching frequency, the switching-frequency-variation range is reduced. Therefore, an optimal design of the magnetic components is possible, resulting in a high efficiency under normal operation and a high power density. The proposed control scheme is applied to an 85-W LLC resonant converter.

A Family of Z-Source Matrix Converters

Abstract: This paper extends the Z-source concept to matrix converters and explores a family of Z-source matrix converters. Their principles and characteristics are analyzed, and a simplified voltage-fed Z-source matrix converter is used as an example to demonstrate its operation, voltage gain versus modulation index, PWM method, boost control, and implementation of the control. Both simulation and experimental results verify operation of the proposed Z-source matrix converter, analytical results, and control method. The Z-source converters can achieve buck and boost operation with reduced number of switches needed, therefore achieving low cost, high efficiency, and reliability, compared to the traditional matrix converters.

Inverse Watkins–Johnson Topology-Based Inverter

Abstract: A Z-source inverter (ZSI) uses an L-C impedance network between the source and the voltage source inverter (VSI). It has the property of stepping down or stepping up the input voltage, as a result, the output can be either higher or lower than the input voltage as per requirement. This topology also possesses robust electromagnetic interference noise immunity, which is achieved by allowing shoot through of the inverter leg switches. This letter proposes an inverter circuit based on the inverse Watkins-Johnson (IWJ) topology that can achieve similar advantages as that of a ZSI. The proposed circuit requires two switches and one pair of an LC filter apart from the VSI. The systematic development of this inverter topology is described starting from the basic IWJ circuit. Steady-state analysis and implementation of the proposed topology are also described. The pulse width modulation control strategy of the inverter is explained. An experimental prototype is used to validate the proposed circuit.

Novel Switching Sequences for a Space-Vector-Modulated Three-Level Inverter

Abstract: A three-level inverter produces six active vectors, each of normalized magnitudes 1, 0.866, and 0.5, besides a zero vector. The vectors of relative length 0.5 are termed pivot vectors. The three nearest voltage vectors are usually used to synthesize the reference vector. In most continuous pulsewidth-modulation (PWM) schemes, the switching sequence begins from a pivot vector and ends with the same pivot vector. Thus, the pivot vector is applied twice in a subcycle or half-carrier cycle. This paper proposes and investigates alternative switching sequences, which use the pivot vector only once but employ one of the other two vectors twice within the subcycle. The total harmonic distortion (THD) in the fundamental line current pertaining to these novel sequences is studied theoretically as well as experimentally over the whole range of modulation. Compared with centered space vector PWM, two of the proposed sequences lead to reduced THD at high modulation indices at a given average switching frequency.

PWM Plus Phase Angle Shift (PPAS) Control Scheme for Combined Multiport DC/DC Converters

Abstract: Multiport dc/dc converters are widely employed in hybrid energy generation systems to provide stable power to key loads with high power density. In this paper, the switch duty cycle and the phase angle of the interleaved converters are employed as two control freedoms to achieve decoupled voltage regulation within a certain operating range among different ports, which is referred to as pulsewidth modulation plus phase angle shift (PPAS) control scheme. An interleaved bidirectional buck-boost converter and a full-bridge converter are integrated together to derive a combined three-port dc/dc converter for photovoltaic (PV)-battery hybrid energy systems, which is adopted as a typical example to explore the clear performance of the proposed PPAS control strategy. The bidirectional buck-boost converter and the full-bridge converter share the same power MOSFETs in the primary side, which simplifies the circuit structure and improves the power density. The duty cycle of the interleaved bidirectional buck-boost converter is adopted to realize the maximum power point tracking and the voltage balance between the battery and the PV cell in the primary side. Furthermore, the phase angle of the interleaved buck-boost converter is employed as another control freedom to achieve accurate secondary output voltage regulation. Finally, a 100-W PV-Battery energy system is designed and tested to verify the effectiveness of the proposed scheme.

Evaluation of different carrier-based PWM methods for modular multilevel converters for HVDC application

Abstract: The outstanding features of modular multilevel converters (M2C) make it attractive for high voltage direct current (HVDC) systems. In order to achieve high efficiency in HVDC converter stations, the switching frequency and the capacitor voltage ripple of the converter should be minimized. A suitable modulation algorithm should achieve an optimal tradeoff between these two requirements. This paper evaluates different carrier-based PWM algorithms and discusses the most challenging technical aspects of an efficient M2C. It is observed that decoupling the waveform synthesis from the selection of which cell to switch at each instant has beneficial impact on operation performance. The evaluation is done by time-domain simulation considering a grid connected, three-phase M2C converter and an advanced control system. Results of this study can be used for implementing more economical HVDC converters.

Fuzzy logic controller based SEPIC converter of maximum power point tracking

Abstract: This paper presents a fuzzy logic controller (FLC) based single-ended primary-inductor (SEPIC) converter of maximum power point tracking (MPPT). The proposed scheme ensures optimal use of PV array and proves its efficacy in variable load conditions, unity and lagging power factor at the inverter output (load) side as it is used also there. The real-time implementation of the MPPT SEPIC converter is done by digital signal processor (DSP); TMS320F28335. The performance of the converter is tested in both simulation and experiment at different operating conditions. The results are compared to PI based ones due to its severe use in industries. The results show that the proposed FLC based SEPIC-MPPT scheme can accurately track the reference signal and transfer power around 4.8% more than conventional PI based system.

Modulation Strategies to Reduce Common-Mode Voltage for Indirect Matrix Converters

Abstract: Common-mode voltage (CMV) is responsible for overvoltage stress to the winding insulation and bearing damage of an ac motor. High dv/dt of CMV raises leakage currents, which can cause serious problems such as motor damage and electromagnetic noise to the equipment installed near the converter. This paper proposes two modulation strategies for indirect matrix converters that substantially eliminate CMV. The first method is developed by using the suitable couple of nonzero space vectors instead of zero space vectors in the inverter stage. The maximum voltage transfer ratio of this method is unaffected and remains 0.866. The second one, which is useful for low-voltage operation, is approached by choosing the medium and the lowest positive line-to-line input voltages in the rectifier stage to generate the dc-link voltage and by placing suitable zero space vectors in the inverter stage. In the second method, the maximum voltage transfer ratio is 0.5. Both methods can mitigate the peak value to 42%. In addition, while the first method provides the same performance to the conventional method, the second method can reduce significantly the harmonic components of line-to-line output voltage. Simulation and experimental results are shown to verify the effectiveness of the proposed methods.

A Novel Single-Phase Five-Level Inverter With Coupled Inductors

Abstract: In this paper, a novel single-phase five-level inverter is proposed using coupled inductors This inverter can output five-level voltage with only one dc source No split of the dc voltage capacitor is needed, totally avoiding the voltage balancing problem in conventional multilevel inverters The level of the output voltage is only half of the dc-link voltage in all conditions, leading to much reduced dv/dt This inverter is based on the widely used three-arm power module and the voltage stresses on all the power switches are the same, making it very easy to construct Operation mechanism of this inverter is analyzed and the possible switching patterns are investigated Based on these analyses, a novel optimized modulation scheme is presented With this modulation method, no dc components exist in the inductor currents, which is very helpful for minimization of the inductors Simulation and experimental results show the validity of the proposed inverter together with the optimized modulation scheme

Hysteresis current control of hybrid active power filters

Abstract: This article presents a hysteresis pulse width modulation study for a inductor-capacitor (LC)-coupling hybrid active power filter (HAPF). As the coupling LC impedance yields a non-linear inverter current slope, this can affect the controllability of using the conventional hysteresis control method and generate unexpected trigger signals to the switching devices. This results in deteriorating the system operating performances. On the basis of the proposed modelling, the linearisation of the hysteresis control for the HAPF is firstly studied, investigated and compared with the linear active power filter. Two limits are proposed in this study that divides the HAPF into non-linear, quasi-linear and linear operation regions. The design criteria of hysteresis band and sampling time can then be derived. Single-phase simulation and experimental results are given to verify the hysteresis control study of HAPF compared with active power filter. Finally, representative simulation and experimental results of a three-phase four-wire centre-split HAPF for power quality compensation are presented to demonstrate the validity of the hysteresis linearisation study.

Modelling and optimization of cut quality during pulsed Nd:YAG laser cutting of thin Al-alloy sheet for straight profile

Abstract: Thin sheets of aluminium alloys are widely used in aerospace and automotive industries for specific applications. Nd:YAG laser beam cutting is one of the most promising sheetmetal cutting process for cutting sheets for any profile. Al-alloy sheets are difficult to cut by laser beam because of its highly reflective nature. This paper presents modelling and optimization of cut quality during pulsed Nd:YAG laser cutting of thin Al-alloy sheet for straight profile. In the present study, four input process parameters such as oxygen pressure, pulse width, pulse frequency, and cutting speed and two output parameters such as average kerf taper (Ta) and average surface roughness (Ra) are considered. The hybrid approach comprising of Taguchi methodology (TM) and response surface methodology (RSM) is used for modelling whereas multi-objective optimization is performed using hybrid approach of TM and grey relational analysis (GRA) coupled with entropy measurement methodology. The entropy measurement methodology is employed for the calculation of weight corresponding to each quality characteristic. The results indicate that the hybrid approaches applied for modelling and optimization of the LBC process are reasonable.

Performance of dimming control scheme in visible light communication system

Abstract: We investigate the performance of visible light communication (VLC) system with a pulse width modulation (PWM) dimming control scheme. Under this scheme, the communication quality in terms of number of transmitted bits and bit error rate (BER) of less than 10(-3) should be guaranteed. However, for on-off-keying (OOK) signal, the required data rate becomes 10 times as high as the original data rate when the duty cycle of dimming control signal is 0.1. To make the dimming control scheme easy to be implemented in VLC system, we propose the variable M-QAM OFDM VLC system, where M is adjusted according to the brightness of LED light in terms of duty cycle. The results show that with different duty cycles the required data rates are not higher than the original value and less LED lamp power is required to guarantee the communication quality, which makes the dimming control system that satisfies both communication and illumination requirements easy to be implemented and power-saving.

Hybrid Selective Harmonic Elimination PWM for Common-Mode Voltage Reduction in Three-Level Neutral-Point-Clamped Inverters for Variable Speed Induction Drives

Abstract: This paper proposes a hybrid selective harmonic elimination pulsewidth modulation (SHEPWM) scheme for common-mode voltage reduction in three-level neutral-point-clamped inverter-based induction motor drives. The scheme uses the conventional SHEPWM (C-SHEPWM) to control the inverter at high frequency (≥ 0.9 motor rated frequency) and uses the modified SHEPWM (M-SHEPWM) to control the inverter at low frequency. It also uses a scheme to ensure the smooth transition between the two SHEPWM schemes. As a result, at high frequency, the C-SHEPWM provides the required high modulation index for the motor, while at low frequency, when a passive filter is less effective for common-mode voltage reduction, the M-SHEPWM is used to suppress the common-mode voltage. Experimental results show that the proposed hybrid SHEPWM scheme could meet the modulation index need of the motor and reduce the common-mode voltage in the drive, and the two SHEPWM schemes could transition smoothly.

Pulse-Width Modulation

Abstract: Pulse-width modulation (PWM) is the foundation of control in power electronics. Its design affects almost all aspects of converter operation, from steady-state and dynamic control performance to electromagnetic interference (EMI) and thermal design. The basic principle of commonly used PWM methods has been well established and easy to understand. Yet achieving optimal performance in terms of control, power quality and EMI requires in-depth understanding of the modulation process as well as its spectral and dynamic characteristics, which are mathematically much more involved due to the highly nonlinear nature of the PWM process.

Digital Control of Actual Grid-Connected Converters for Ground Leakage Current Reduction in PV Transformerless Systems

Abstract: The design of a photovoltaic (PV) grid-connected converter usually comprehends a galvanic isolation between the grid and the photovoltaic panels. Recently, in low power systems, the galvanic isolation has been removed with the aim to increase efficiency and reduce the cost of the converter. Due to the presence of a parasitic capacitance between the photovoltaic cells and the metal frame of the PV panel, usually connected to earth, a high value of common mode current (i.e., ground leakage current) can arise. In order to limit the ground leakage current (which deteriorates the power quality and generates EMI), new converter topologies have been proposed. Their effectiveness is based on the symmetrical (ideal) commutations of the power switches and some of them adopt a further voltage level derived from a capacitive divider of the DC bus voltage. Unfortunately, in actual implementations, asymmetrical power switches transients and variations of this added voltage lead to higher ground leakage current with respect to the ideal case. After a review of the state of the art this paper investigates these two issues and presents a particular solution (based on digital control and PWM strategy) that, in conjunction with a compensation strategy of power switches actual commutations, guarantees low ground leakage current regardless the parameters tolerance of the power circuit. Simulation and experimental results confirm the effectiveness of the proposed solution.