Showing papers in "Iet Power Electronics in 2013"

Cross-switched multilevel inverter: an innovative topology

Abstract: Multilevel inverters have attractive features which make them one of the most popular fields of investigations. They can operate in high power and high voltage conditions. Multilevel inverters generate output voltage with high quality and low harmonic contents. As the number of output voltage levels increases, the quality of the output voltage is improved leading to reduction of the filtering requirements. However, multilevel inverters have the disadvantage of increased power electronic switches. This leads to higher cost, size and complication. Some advanced topologies have been introduced to reduce the number of power electronic switches, unfortunately, most of these topologies need high voltage switches on their structure making them unsuitable for high voltage conditions. In this study, an innovative topology for multilevel inverter is proposed which reduces the number of switches considerably without using high voltage switches. The proposed topology is a general topology which can be extended for any number of voltage levels. The authors call this topology as cross-switched multilevel inverter. The simulation results obtained in PSCAD/EMTDC as well as the experimental results verify the proposed topology.

Selective harmonic elimination pulse-width modulation of modular multilevel converters

Abstract: The modular multilevel converter (MMC) is the state-of-the-art for multilevel converter topologies. This study presents the operation of the MMC using the multilevel selective harmonic elimination pulse-width modulation (MSHE-PWM) technique. MSHE-PWM offers tight control of the low-order harmonics and the lowest switching frequency for the power semiconductors among all modulation techniques. A comprehensive analysis of the modulation methods for the MMC leads to two different modulation patterns for MSHE-PWM. A method for selecting the number of sub-modules in the phase-legs of the converter is also proposed in this study. Simulation results for both patterns are provided and verified through matching experimental results from a single phase 11-level laboratory prototype.

Study of a non-isolated bidirectional DC–DC converter

Abstract: The study presents a non-isolated bidirectional DC-DC converter, which has simple circuit structure. The control strategy is easily implemented. Also, the synchronous rectifier technique is used to reduce the losses. The voltage gain of the proposed converter is the half and the double of the conventional bidirectional DC-DC buck/boost converter in the step-down and step-up modes, respectively. Therefore the proposed converter can be operated in wide-voltage-conversion range than the conventional bidirectional converter. The voltage stresses on the switches of the proposed converter are a half of the high-voltage side. In addition, the operating principle and steady-state analyses are discussed. Finally, a prototype circuit is implemented to verify the performance of the proposed converter.

Evaluations of current control in weak grid case for grid-connected LCL-filtered inverter

Abstract: For grid-connected inverters, switching harmonics can be effectively attenuated through an LCL-type filter. In order to suppress resonance and guarantee good performance, many strategies (e.g. active damping (AD), harmonic resonant control, repetitive control and grid feedforward) have been proposed. However, the wide variation of grid impedance value challenges system stability in practical applications. The aforementioned methods need to be investigated. This study evaluates the applicability of each part of the overall control in a weak grid case with the use of a stability criterion. It has been demonstrated that the feedback-based AD control can work well in a wide range of grid conditions. However, the resonant and repetitive control methods meet constraints. The grid feedforward method brings in an extra positive feedback path, and consequently results in high harmonics or even instability. Finally, a recommendation for system design has been presented. Simulations and experiments have been provided to verify the analysis.

Two-stage micro-grid inverter with high-voltage gain for photovoltaic applications

Abstract: This study proposes a new two-stage high voltage gain boost grid-connected inverter for AC-module photovoltaic (PV) system. The proposed system consists of a high-voltage gain switched inductor boost inverter cascaded with a current shaping (CS) circuit followed by an H -bridge inverter as a folded circuit and its switches operate at line frequency. The switched inductor boost converter (SIBC) has one switch operates like a continuous conduction mode. The maximum power of the PV module is achieved through the SIBC circuit whereas the grid connection requirements are accomplished using the CS circuit with the H -bridge inverter. The switch of the CS circuit is controlled with a sine wave modulation control scheme. The main advantages of the new proposed system are high boosting gain, lower switching losses and reduces the ground leakage current as all H -bridge switches operate at the line frequency. A 120 W prototype has been built and experimentally tested. It has been found that experimental results have a good matching with the proposed analysis and simulation results.

Multiple-input DC/DC converter topology for hybrid energy system

Abstract: In this study, a multiple-input non-isolated DC/DC converter topology is presented. The proposed multiple-input DC/DC converter is proficient for energy diversification from renewable and storage energy sources individually or simultaneously. It can be operated in buck, boost and buck–boost modes of operation with the capability of bidirectional power flow to achieve desired voltage level on either side. The development of small-signal modelling based on state-space averaging has been discussed. In addition, a power management control scheme for the proposed converter has also been presented. The proposed concept has been investigated through simulation using the MATLAB/Simulink environment and validated experimentally on a laboratory prototype using dSPACE 1103 real time digital controller.

Efficiency analysis of a sliding-mode controlled quadratic boost converter

Abstract: A quadratic boost converter with high DC gain to step up the voltage of a standard photovoltaic panel up to 400 V is analysed. First of all, a comparative analysis among boost, quadratic boost and cubic boost reveals that the quadratic boost exhibits the best trade-off between duty ratio range and converter efficiency. A hysteretic comparator is employed as a modulator to ensure the converter operation with high values of the duty ratio without risk of modulation saturation. A two- loop sliding-mode control is used to regulate the output voltage. An internal loop controls cycle by cycle the input current whose reference is established by an outer loop that processes the output voltage error by means of a proportional-integral compensating network. The measured results in a 100 W prototype operating in continuous conduction mode of average values, ripples, frequency and efficiency for different equilibrium points are in good agreement with the theoretical predictions.

Modified space vector modulation for fault-tolerant operation of multilevel cascaded H-bridge inverters

Abstract: In this study, fault-tolerant operation of multilevel cascaded H-bridge (CHB) inverters is presented and a novel switching strategy based on space vector modulation is proposed. A faulty power switch in high power converters can lead to expensive downtime, loss of productivity and increased costs. Various power switch faults and their influence on space vector diagram of CHB inverters are investigated. In the event of a fault, the output voltage of inverter is reduced and redundant switching states are used to generate balanced line-to-line voltages. By adding some devices to the basic structure, CHB inverter will operate with maximum achievable output voltage. Simulation and experimental results are shown for a five-level CHB inverter to validate the proposed modulation technique.

Novel three-phase asymmetrical cascaded multilevel voltage source inverter

Abstract: Series connection of power cells in asymmetrical cascaded configurations helps to cancel redundant output levels and maximise the number of different levels generated by the inverter. A new configuration of three-phase multilevel asymmetrical cascaded voltage source inverter is presented. This structure consists of series-connected sub-multilevel inverters blocks. The number of utilised switches, insulated gate driver circuits, voltage standing on switches, installation area and cost are considerably reduced. Cascaded-cell DC voltages in each inverter leg form an arithmetic sequence with common difference of E. With the selected inverter DC sources, high-frequency pulse-width modulation (PWM) control methods can be effectively applied without loss of modularity. Low-frequency and sinusoidal PWM techniques were successfully applied. Hence, high flexibility in the modulation of the proposed inverter is demonstrated. The prototype of the suggested inverter was manufactured and the obtained simulation and hardware results ensured the feasibility of the configuration, and the compatibility of both modulation techniques was accurately noted. Lastly, the semiconductor losses in the converter were calculated using simulation models. Based on the analysis of the total power losses, the proposed inverter provided high efficiency at different operating conditions.

Cascaded cross-switched multilevel inverter in symmetric and asymmetric conditions

Abstract: This study proposes and analyses a new cascaded multilevel inverter in both symmetric and asymmetric conditions. Firstly, the topology is presented in general form and then it is optimised. For any given number of voltage levels, the proposed topology reduces the number of switches. In the symmetric condition, the proposed topology offers capability of charge balance control method while reducing the number of switches. Also, unlike the other reduced switch topologies, the proposed topology does not increase the total standing voltage of the switches in comparison with the conventional cascaded H-bridge (CHB) multilevel inverter. In the asymmetric condition, the proposed topology has highest output voltage resolution while keeping the total standing voltage equal to the CHB topology. In other words, in both symmetric and asymmetric conditions the number of switches in the proposed topology is lower than that of the CHB topology. However, both the proposed topology and also the CHB topology have the same total standing voltage on the switches. Other asymmetric topologies use higher number of switches and have higher total standing voltage in comparison with the proposed asymmetric topology. A modified phase-shifted-pulse width modulation is also presented for the proposed symmetric topology. The proposed topology is verified with simulation and experimental results.

Multilevel inverter topology based on series connected switched sources

Abstract: A new topology for multilevel DC–AC conversion is presented in this study. It consists of isolated symmetric input DC sources alternately connected in opposite polarities through power switches. The structure allows synthesis of multilevel waveform using reduced number of power switches as compared to the classical topologies. The working principle of the proposed topology is explained with the help of a single-phase five-level inverter. Simulation studies are carried out in MATLAB/Simulink environment and experimental validations are obtained on a laboratory prototype. An exhaustive comparison of the proposed topology against the classical cascaded H-bridge topology indicates reduction in number of power switches, losses, installation area and converter cost.

Z-source-based isolated high step-up converter

Abstract: Continuous input currents and high step-up ratios are desired from the dc-dc converters that work with renewable energy sources such as photovoltaic panels and fuel cells. Most dc-dc converters use transformers to have high step-up ratios. There is another type of converters that use coupled inductors, but they need to operate at high duty cycles to increase the gain. These converters suffer from low efficiency values because of the leakage inductance values and long reverse recovery times of the diodes. A new dc-dc converter based on Z-source topology is proposed in this study. It uses coupled inductors to increase the step-up ratio. Its main features are high step-up ratios, continuous input currents, high efficiency and galvanic isolation between input and output. Simulation and experimental results are given for a prototype converter that converts 25-400 V dc at 300 W with over 91% efficiency.

Asymmetrical multilevel converter topology with reduced number of components

Abstract: In this study, firstly a new basic unit is proposed for multilevel converters. The proposed basic units are used as building blocks to form a cascaded multilevel converter. In other words, the proposed topology consists of cascaded basic units. The proposed basic unit and the proposed multilevel converter use lower number of switching devices and gate driver circuits. In the proposed topology, two design parameters are available: the number of cascaded basic units and the number of dc voltage sources in each basic unit. These two parameters can be used to design the desired multilevel converter based on the operational conditions. Therefore the proposed topology offers good flexibility in designing. An algorithm for determining the values of the dc voltage sources is given in order to generate maximum number of voltage levels. The comparison results with some recently introduced topologies show that the proposed topology effectively reduces the components count. The simulation results obtained in PSCAD/EMTDC as well as the experimental results of a 51-level inverter based on the proposed topology are presented to verify its performance.

Improved power quality bridgeless cuk converter fed brushless DC motor drive for air conditioning system

Abstract: This study presents a bridgeless Cuk converter-fed brushless DC (BLDC) motor drive for an air conditioning system. A new approach of speed control of BLDC motor is proposed by controlling the voltage at DC bus using a single voltage sensor. The proposed drive utilises a bridgeless Cuk converter operating in discontinuous inductor current mode (DICM) for the power factor correction (PFC) and improved power quality (PQ) at the AC mains for a wide range of speed control. The bridgeless Cuk converter operating in a DICM gives an inherent PFC and requires a simple voltage follower approach for the voltage control. The bridgeless converter topology is designed for obtaining the low conduction losses and requirement of low size of heat sink for the switches. The proposed system is designed and its performance is simulated for an air conditioning system to operate over a wide range of speed control with near unity power factor at AC mains. Finally, the performance of the proposed drive obtained in simulation is validated with test results on a developed drive for its operation over a wide range of speed control with improved PQ at AC mains.

Fault diagnosis scheme for open-circuit faults in field-oriented control induction motor drives

Abstract: In this study, a new model-based fault detection and isolation (FDI) strategy is proposed for field-oriented control (FOC) induction motor (IM) drives. Actuator faults are addressed, and specifically, single open-circuit faults are considered in this study. The residual signals are synthesised by taking the resulting closed-loop dynamics when a FOC strategy is applied, that is, the residuals are referenced to the synchronous reference frame (dqe -coordinates), which are generated by using a bank of variable structure observers to obtain a robust FDI scheme. Thus, subsystems sensitive to a specific fault, but decoupled from other faults are obtained in a natural way, where only two stator currents and the mechanical position are required for fault isolation purposes. Residual evaluation is carried out in the stator reference frame (dq-coordinates) for the IM model, where the residual direction (angle) is employed to isolate a fault in each one of the six power switches in a voltage source inverter. In addition, the observer FDI scheme can be combined with a fault re-configuration strategy in order to improve the reliability of the motor drive. Experimental results are illustrated for a three-phase 3/4 HP IM drive at different reference frequencies and load torques with single open-circuit faults that verify the ideas presented in this work.

Multi-input voltage-summation converter based on switched-capacitor

Abstract: This study is to propose a multi-input converter which is designed based on switched-capacitor instead of magnetic components. The number of its inputs can be any figure. The output voltage level of the converter is the summation of its multiple input levels which share with a common neutral. There is no need to have any level-shift or boost-strap for adjusting the inputs. The number of active switches and capacitors required both are equal to the number of input sources. Only a pair of complementary pulse signals is needed to control all of these switches. The converter is operated under high frequency. Non-resonant version does not require resonant inductors and therefore they are of small size and simple structure. Small resonant inductor is also added in the resonant version to realise soft-switching and improve the conversion efficiency. The circuit analysis, design method and experimental verification are shown.

Experimental evaluation of four-phase floating interleaved boost converter design and control for fuel cell applications

Abstract: Power electronic converters are essential part of hybrid fuel cell automotive application systems. The converter needs to provide high-voltage ratio for a wide range of input voltage. In addition, the converter should have high efficiency for a wide range of duty cycle control range. A four-phase floating interleaved boost converter (FIBC) is analysed and a small-signal AC model using an averaged pulse-width modulation (PWM) switch technique is used for supporting the feedback controller and aiding a frequency response design. The small-signal AC model as well as the current controller are validated by simulation and evaluated by experimental results. The proposed converter has competitive device ratings, the total inductance volume and weight is decreased, current ripple is minimised and converter efficiency and reliability are improved. Proof of concept of the proposed topology is demonstrated through an experimental prototype.

Single-stage multi-port DC-DC converter topology

Abstract: A single-stage multi-port DC–DC converter topology is proposed. It can accommodate arbitrary number of input sources and output loads, and employs a single inductor. The voltage of each output port can be less than the minimum input voltage or exceed the maximum input voltage. Power regulation can be provided for the input ports in addition to providing voltage regulation for the output ports. The steady-state and dynamic characteristics of the converter are analysed and a control scheme is proposed. Hardware measurements and numerical switch-level simulations verify the analytical results.

Single-phase 9-level hybridised cascaded multilevel inverter

Abstract: This study proposes a single-phase, 9-level, cascade multilevel inverter topology. The multicarrier, phase disposition pulse width modulation scheme is employed to generate the gating signals for the power switches. Operational principles with switching functions are given. By controlling the modulation index, the desired number of levels: 3, 5, 7 and 9, of the inverter's output voltage can be achieved. For modulation indices of: 0.24, 0.4, 0.6 and 0.8, the proposed inverter configuration was subjected to an R-L load and the respective numbers of output voltage level were synthesized. Fast Fourier transform analyses of the output voltage waveforms were carried out and the corresponding THD values were obtained. For a modulation index of 0.8, a THD value of 10.12% has been achieved in the output voltage waveform of the proposed inverter configuration. Comparison of the proposed inverter configuration and the classical single-phase topologies is given based on the power circuit component count. Moreover, analysis of the conduction power losses in the power semiconductor switches of the proposed inverter topology is given. To verify the performance of the proposed inverter architecture, simulations and experiments are carried out on a 2.12 kW rated prototype of the proposed inverter for an R-L load; and adequate results are presented.

Indirect output voltage regulation of DC-DC buck/boost converter operating in continuous and discontinuous conduction modes using adaptive backstepping approach

Abstract: In this study, an adaptive non-linear controller is designed for DC-DC buck/boost converter which is robust and stable against converter load changes, input voltage variations and parameter uncertainties. The proposed controller is developed based on input-output linearisation using an adaptive backstepping approach. The controller can be applied in both continuous and discontinuous conduction modes (CCM and DCM). Owing to non-minimum phase nature of buck/boost converter, the output voltage of this converter is indirectly controlled by tracking the inductor reference current. The inductor reference current is generated by a conventional PI controller. Using a MATLAB/Simulink toolbox and a stand-alone TMS320F2810 digital signal processor from Texas Instruments, some simulations and practical results are presented to verify the capability and effectiveness of the proposed control approach.

Design of a zero-voltage-switching large-air-gap wireless charger with low electric stress for electric vehicles

Abstract: This study proposes a design and development of a wireless power transfer system to charge the battery in electric vehicles. A parallel-parallel topology is adopted to realise 10-15 cm-distance power transfer using the resonance theory. Finite-element method is used to extract the coil parameters. The advantages of the proposed design compared with the previous similar research are (i) low operational frequency (42 kHz) which avoids the electromagnetic interference to the on-board automotive electronics equipment and (ii) low electric stress to the semi-conductor switches through using zero-voltage-switching technique. A 2 kW prototype to charge 200 V battery was built to experimentally verify the theoretical analysis. The overall system efficiency is ~86%.

Space vector pulse-width modulation algorithm and DC-side voltage control strategy of three-phase four-switch active power filters

Abstract: A space vector pulse-width modulation (SVPWM) algorithm and a DC-side voltage control strategy for three-phase four-switch shunt active power filters are proposed. The basic principle of three-phase four-switch inverters to output three-phase balanced voltages is investigated in detail. Based on an advanced coordinate transformation, the proposed algorithm ensures basic voltage vectors to be right on the coordinate axes of αβ-coordinate system, which has the advantages of less computation and higher accuracy than the traditional. It synthesises zero vectors by equalising the duration time of two basic voltage vectors, which are equivalent in amplitude and opposite in phase, to simplify control logic and decrease harmonics of output voltages. To stabilise the DC bus voltage and balance the DC-side capacitor voltages, a DC-side voltage control strategy is presented. The former objective is obtained by controlling the active partition of the output currents, and the latter one by adapting the direct partition of phase C output current. An overall control method combining the proposed SVPWM algorithm with the DC-side voltage control strategy is presented. Experimental results have shown the validity and feasibility of the proposed algorithm and strategy.

Types-1 and -2 fuzzy logic controllers-based shunt active filter I d -I q control strategy with different fuzzy membership functions for power quality improvement using RTDS hardware

Abstract: This research paper proposes the shunt active filter (SHAF), which is used to improve the power quality of the electrical network by mitigating the harmonics with the help of Types-1 and -2 fuzzy logic controllers (Types-1 and -2 FLC) using different fuzzy membership functions (MFs). To carry out this analysis, active and reactive current (I d -I q ) control strategy is chosen. Threephase reference current waveforms generated by proposed scheme are tracked by the three-phase voltage source converter in a hysteresis band control scheme. The performance of the proposed control strategy has been evaluated in terms of harmonic mitigation and DC-link voltage regulation under various source conditions. To maintain DC-link voltage constant and to generate the compensating reference currents, the authors have developed Types-1 and -2 FLC with different fuzzy MFs (trapezoidal, triangular and Gaussian). The I d -I q control strategy with proposed Type-2 FLC is able to eliminate the uncertainty in the system and SHAF gains outstanding compensation abilities. The detailed real-time results using real-time digital simulator are presented to support the feasibility of proposed control strategy.

Dual-output dc-dc buck converters with bidirectional and unidirectional characteristics

Abstract: This study presents single-input double-output unidirectional and bidirectional dc-dc buck converters, for applications that demand two dc-controlled output voltages. The proposed converters present a component count reduction and it is considered as a monolithic device with two output voltages generated by the same power converter. The main advantages of the proposed converters are reduction of costs and better power losses distribution among the semiconductor devices. Besides the proposal of suitable power converters, this study presents their models, control strategies, modulation approaches and design specification. The selected simulated and experimental results are presented as well.

Design of backstepping control for high-performance inverter with stand-alone and grid-connected power-supply modes

Abstract: In this study, the backstepping control (BSC) design for a high-performance inverter with the functions of stand-alone and grid-connected power supply is developed so that distributed generation units can operate individually or in a micro-grid mode. In the stand-alone power-supply mode, the output ac voltage can supply to ac loads. In the grid-connected power-supply mode, the goal of power management can be achieved by controlling the amplitude and direction of the output current in the inverter. As a result, the proposed high-performance inverter with the BSC scheme has the output voltage with a low total harmonic distortion in the stand-alone power-supply mode and the output current with a high-power factor in the grid-connected power-supply mode to provide an ac output with high-performance power quality. The effectiveness of the proposed high-performance inverter with the BSC is verified by experimental results of a 3 kW prototype, and the merit of the proposed BSC scheme is indicated in comparison with previous proportional-integral control, proportional-resonant control and adaptive total sliding-mode control strategies.

Three-phase four-leg flying-capacitor multi-level inverter-based active power filter for unbalanced current operation

Abstract: This study describes a new application of flying capacitor multi-level inverters whereby a three-phase, four-leg three-level inverter is used for harmonic current cancellation and unbalanced current compensation in a four-wire system. A direct three-dimensional-pulse-width modulation (3D-PWM) scheme is developed to deal with the increased state redundancy in the four-leg flying capacitor multi-level inverter. Test results show much superior performance in the four-leg case, which may allow the use of smaller DC supply capacitors. A new modified dead-beat current control algorithm is proposed and combined successfully with the 3D-PWM modulation technique in a hardware demonstrator of a complete active filter.

Low-voltage ride-through concept in flyback inverter-based alternating current–photovoltaic modules

Abstract: The flyback inverter-based alternating current-photovoltaic modules' behaviour under voltage rise/drop conditions is investigated. Specifically, the aim is to calculate the module steady-state operational characteristics of distributed generators, such us generation loss and root mean square current rise, based on the low-voltage ride-through capability requirements. The proposed analysis stands for the widely known discontinuous conduction mode of operation and it can also account for high power inverter topologies. Furthermore, the derived expressions are corroborated by means of PSPICE simulation along with the selected experimental results and insightful discussions.

Research on fast transient and 6n ± 1 harmonics suppressing repetitive control scheme for three-phase grid-connected inverters

Abstract: The repetitive control technique is widely adopted in ac systems, because of its excellent steady-state tracking performance and low total harmonic distortion. The repetitive control method with one-sixth of the system fundamental period T 0 as the delay time has been proposed to achieve fast transient response. In this study, an improved control scheme based on the T 0/6 repetitive control is proposed for three-phase grid-connected inverters. The proposed scheme adopts T 0/6 as the delay time in the positive-rotating and negative-rotating synchronous reference frames to suppress the 6n ± 1 harmonics. Meanwhile, the proportional-integral regulator and the plug-in repetitive controller are combined to reinforce the system performance. A new auxiliary function based on the linear interpolation is proposed to maintain the ideal repetitive control performance when one-sixth of the ratio of the sampling frequency to the grid fundamental frequency is non-integer. The effectiveness of the proposed scheme on improving the T 0/6 repetitive control is confirmed by simulation and experimental results finally.

Wireless battery chargers for portable applications: design and test of a high-efficiency power receiver

Abstract: In this study, the authors present a 5 W wireless battery charger for handheld devices. Efficiency-related issues are addressed. A power receiver architecture which improves power conversion efficiency is proposed. Design hints are provided for further applications. Comparison with a conventional architecture is provided as well. Laboratory prototypes of both the proposed and the conventional architectures have been realised. Both prototypes are tested by using the same power transmitter to perform a valuable comparison. As shown by the experimental results, in the receiver side power conversion efficiency is increased up to 99% thanks to the proposed solution.

Analysis and design of split-capacitor resistive-inductive passive damping for LCL filters in grid-connected inverters

Abstract: Grid-connected inverters require a third-order LCL filter to meet standards such as the IEEE Std. 519-1992 while being compact and cost-effective. LCL filter introduces resonance, which needs to be damped through active or passive methods. Passive damping schemes have less control complexity and are more reliable. This study explores the split-capacitor resistive-inductive (SC-RL) passive damping scheme. The SC-RL damped LCL filter is modelled using state space approach. Using this model, the power loss and damping are analysed. Based on the analysis, the SC-RL scheme is shown to have lower losses than other simpler passive damping methods. This makes the SC-RL scheme suitable for high power applications. A method for component selection that minimises the power loss in the damping resistors while keeping the system well damped is proposed. The design selection takes into account the influence of switching frequency, resonance frequency and the choice of inductance and capacitance values of the filter on the damping component selection. The use of normalised parameters makes it suitable for a wide range of design applications. Analytical results show the losses and quality factor to be in the range of 0.05-0.1% and 2.0-2.5, respectively, which are validated experimentally.