Showing papers in "IEEE Transactions on Power Electronics in 2005"

Optimization of perturb and observe maximum power point tracking method

Abstract: Maximum power point tracking (MPPT) techniques are used in photovoltaic (PV) systems to maximize the PV array output power by tracking continuously the maximum power point (MPP) which depends on panels temperature and on irradiance conditions. The issue of MPPT has been addressed in different ways in the literature but, especially for low-cost implementations, the perturb and observe (PO moreover, it is well known that the P&O algorithm can be confused during those time intervals characterized by rapidly changing atmospheric conditions. In this paper it is shown that, in order to limit the negative effects associated to the above drawbacks, the P&O MPPT parameters must be customized to the dynamic behavior of the specific converter adopted. A theoretical analysis allowing the optimal choice of such parameters is also carried out. Results of experimental measurements are in agreement with the predictions of theoretical analysis.

Maximum boost control of the Z-source inverter

Abstract: This paper explores control methods for the Z-source inverter and their relationships of voltage boost versus modulation index. A maximum boost control is presented to produce the maximum voltage boost (or voltage gain) under a given modulation index. The control method, relationships of voltage gain versus modulation index, and voltage stress versus voltage gain are analyzed in detail and verified by simulation and experiment.

High step-up converter with coupled-inductor

Abstract: In this study, a high step-up converter with a coupled-inductor is investigated. In the proposed strategy, a coupled inductor with a lower-voltage-rated switch is used for raising the voltage gain (whether the switch is turned on or turned off). Moreover, a passive regenerative snubber is utilized for absorbing the energy of stray inductance so that the switch duty cycle can be operated under a wide range, and the related voltage gain is higher than other coupled-inductor-based converters. In addition, all devices in this scheme also have voltage-clamped properties and their voltage stresses are relatively smaller than the output voltage. Thus, it can select low-voltage low-conduction-loss devices, and there are no reverse-recovery currents within the diodes in this circuit. Furthermore, the closed-loop control methodology is utilized in the proposed scheme to overcome the voltage drift problem of the power source under the load variations. As a result, the proposed converter topology can promote the voltage gain of a conventional boost converter with a single inductor, and deal with the problem of the leakage inductor and demagnetization of transformer for a coupled-inductor-based converter. Some experimental results via examples of a proton exchange membrane fuel cell (PEMFC) power source and a traditional battery are given to demonstrate the effectiveness of the proposed power conversion strategy.

Pulse-width modulation of Z-source inverters

Abstract: Z-Source inverters have recently been proposed as an alternative power conversion concept as they have both voltage buck and boost capabilities. These inverters use a unique impedance network, coupled between the power source and converter circuit, to provide both voltage buck and boost properties, which cannot be achieved with conventional voltage-source and current-source inverters. To facilitate understanding of Z-source inverter modulation, this paper presents a detailed analysis, showing how various conventional pulse-width modulation strategies can be modified to switch a voltage-type Z-source inverter either continuously or discontinuously, while retaining all the unique harmonic performance features of these conventional modulation strategies. This paper starts by analyzing the modulation requirements of a single-phase H-bridge Z-source inverter, and subsequently extends the analysis to cover the more complex three-phase-leg and four-phase-leg Z-source inverters, with carrier-based implementation reference equations derived for all the inverters. The theoretical and modulation concepts presented have been verified both in simulation and experimentally.

Power enhancement of an actively controlled battery/ultracapacitor hybrid

Abstract: An actively controlled battery/ultracapacitor hybrid has broad applications in pulse-operated power systems. A converter is used to actively control the power flow from a battery, to couple the battery to an ultracapacitor for power enhancement, and to deliver the power to a load efficiently. The experimental and simulation results show that the hybrid can achieve much greater specific power while reducing battery current and its internal loss. A specific example of the hybrid built from two size 18650 lithium-ion cells and two 100-F ultracapacitors achieved a peak power of 132 W which is a three-times improvement in peak power compared to the passive hybrid power source (hybrid without a converter), and a seven times improvement as compared to the lithium-ion cells alone. The design presented here can be scaled to larger or smaller power capacities for a variety of applications.

Z-source inverter for motor drives

Abstract: This paper presents a Z-source inverter system and control for general-purpose motor drives. The Z-source inverter system employs a unique LC network in the dc link and a small capacitor on the ac side of the diode front end. By controlling the shoot-through duty cycle, the Z-source can produce any desired output ac voltage, even greater than the line voltage. As a result, the new Z-source inverter system provides ride-through capability during voltage sags, reduces line harmonics, improves power factor and reliability, and extends output voltage range. Analysis, simulation, and experimental results will be presented to demonstrate these new features.

On the practical design of a sliding mode voltage controlled buck converter

Abstract: This paper presents a simple and systematic approach to the design of a practical sliding mode voltage controller for buck converters operating in continuous conduction mode. Various aspects of the design, including the associated practical problems and the proposed solutions, are detailed. A simple and easy-to-follow design procedure is also described. Experimental results are presented to illustrate the design procedure.

Tapped-inductor buck converter for high-step-down DC-DC conversion

Abstract: The narrow duty cycle in the buck converter limits its application for high-step-down dc-dc conversion. With a simple structure, the tapped-inductor buck converter shows promise for extending the duty cycle. However, the leakage inductance causes a huge turn-off voltage spike across the top switch. Also, the gate drive for the top switch is not simple due to its floating source connection. This paper solves all these problems by modifying the tapped-inductor structure. A simple lossless clamp circuit can effectively clamp the switch turn-off voltage spike and totally recover the leakage energy. Experimental results for 12V-to-1.5V and 48V-to-6V dc-dc conversions show significant improvements in efficiency.

Three-level LLC series resonant DC/DC converter

Abstract: Paper presents a three-level soft switching LLC series resonant dc/dc converter. Zero-voltage switching (ZVS) is achieved for each main switch without any auxiliary circuit. Voltage stress of each main switch is half of input voltage. Zero-current-switching (ZCS) is achieved for rectifier diodes. Wide input/output range can be achieved under low frequency range because of two-stage resonance. Only one magnetic component is required in this converter. Efficiency is higher in high line input, so this converter is a preferable candidate for power products with the requirement of hold up time. For design convenience, relationship between dc gain and switching frequency, load resistance is deduced. Its open load characteristic and short load characteristic are exposed to provide theory basis for no load operation and over current protection. Design consideration of four dead times is presented to assure that voltage stress for main switches is within half of input voltage and ZVS for each main switch is achieved. Finally the principle of operation and the characteristics of the presented converter are verified on a 500V-700V input 54V/10A output experimental prototype, whose efficiency reaches 94.7% under rating condition.

Boost DC-AC inverter: a new control strategy

Abstract: Boost dc-ac inverter naturally generates in a single stage an ac voltage whose peak value can be lower or greater than the dc input voltage. The main drawback of this structure deals with its control. Boost inverter consists of Boost dc-dc converters that have to be controlled in a variable-operation point condition. The sliding mode control has been proposed as an option. However, it does not directly control the inductance averaged-current. This paper proposes a control strategy for the Boost inverter in which each Boost is controlled by means of a double-loop regulation scheme that consists of a new inductor current control inner loop and an also new output voltage control outer loop. These loops include compensations in order to cope with the Boost variable operation point condition and to achieve a high robustness to both input voltage and output current disturbances. As shown by simulation and prototype experimental results, the proposed control strategy achieves a very high reliable performance, even in difficult transient situations such as nonlinear loads, abrupt load changes, short circuits, etc., which sliding mode control cannot cope with.

Torque-ripple reduction in PM synchronous motor drives using repetitive current control

Abstract: The paper deals with the torque-ripple reduction in a permanent magnet synchronous motor (PMSM) drive with distorted back electromotive force. A smooth torque is obtained by tracking a modified current reference which is periodic over one-sixth of the electrical time period in the synchronous reference frame. An accurate tracking involves, however, very high current loop bandwidth, which is usually not achievable with conventional linear controllers. In order to improve current tracking in the presence of periodic reference signals and disturbances, the paper proposes the application of repetitive techniques to the current control in a field-oriented PMSM drive, where the q-axis current reference has been modified to achieve constant torque. The paper investigates the advantages and pitfalls of the method, through a mathematical analysis and an experimental validation obtained on a laboratory prototype. Particular emphasis is placed on the adjustments that have been specifically studied to enhance the overall system performance.

A digital controlled PV-inverter with grid impedance estimation for ENS detection

Abstract: The steady increase in photovoltaic (PV) installations calls for new and better control methods in respect to the utility grid connection Limiting the harmonic distortion is essential to the power quality, but other requirements also contribute to a more safe grid-operation, especially in dispersed power generation networks For instance, the knowledge of the utility impedance at the fundamental frequency can be used to detect a utility failure A PV-inverter with this feature can anticipate a possible network problem and decouple it in time This paper describes the digital implementation of a PV-inverter with different advanced, robust control strategies and an embedded online technique to determine the utility grid impedance By injecting an interharmonic current and measuring the voltage response it is possible to estimate the grid impedance at the fundamental frequency The presented technique, which is implemented with the existing sensors and the CPU of the PV-inverter, provides a fast and low cost approach for online impedance measurement, which may be used for detection of islanding operation Practical tests on an existing PV-inverter validate the control methods, the impedance measurement, and the islanding detection

System identification of power converters with digital control through cross-correlation methods

Abstract: For digitally controlled switching power converters, on-line system identification can be used to assess the system dynamic responses and stability margins. This paper presents a modified correlation method for system identification of power converters with digital control. By injecting a multiperiod pseudo random binary signal (PRBS) to the control input of a power converter, the system frequency response can be derived by cross-correlation of the input signal and the sensed output signal. Compared to the conventional cross-correlation method, averaging the cross-correlation over multiple periods of the injected PRBS can significantly improve the identification results in the presence of PRBS-induced artifacts, switching and quantization noises. An experimental digitally controlled forward converter with an FPGA-based controller is used to demonstrate accurate and effective identification of the converter control-to-output response.

Power converter line synchronization using a discrete Fourier transform (DFT) based on a variable sample rate

Abstract: Line synchronization of grid connected power converters is a well recognized problem when the grid is weak, or derives from a remote area power supply with poor frequency regulation. Such systems can suffer significant line voltage distortion due to notches caused by power device switching and/or low frequency harmonic content, which can easily corrupt the output of a conventional zero crossing detector. This paper presents a method of filtering the incoming grid voltage using a recursive discrete Fourier transform (DFT). The filter provides a high degree of noise immunity but does produce a phase shift between the incoming grid voltage and the filtered output voltage when the DFT time window does not match the grid period. Two methods of compensating this phase shift are presented, based on tracking the drift in the phase predicted by the recursive DFT. The first method makes a deadbeat adjustment to the time window (thereby changing the sampling rate) while the second approach calculates the phase error based on the linear phase response of the DFT. These compensation algorithms can correct for discrepancies of at least 25% between the DFT time window and the system period, and can track grid frequencies with slew rates as high as 40 Hz/s with negligible phase shift (<2/spl deg/) between the grid voltage input and the filtered output waveforms.

Analysis of multiphase space vector pulse-width modulation based on multiple d-q spaces concept

Abstract: Multiphase motors are usually designed to have the concentrated winding and nonsinusoidal airgap flux density distribution in order to maximize the torque per ampere. This means that the phase voltage of a multiphase motor has the nonsinusoidal waveform. Accordingly, the conventional analysis on a multiphase space vector pulse-width modulation (SVPWM), which is confined to a sinusoidal phase voltage, should be extended to a nonsinusoidal phase voltage. In this paper, based on a multiple d-q spaces concept a novel analysis on a multiphase SVPWM to synthesize an arbitrary nonsinusoidal phase voltage is proposed. Throughout this paper, a five-phase inverter is used as a practical example. The basic concepts can be easily extended to an n-phase inverter.

Optimal pulse-width modulation for three-level inverters

Abstract: The three-level neutral-point-clamped voltage source inverter is widely used in high power, medium voltage applications. This paper studies continuous and discontinuous pulse-width modulation for this inverter. Detailed analysis of discontinuous modulation shows that the average switching frequency is not directly proportional to the carrier or sampling frequency, since additional switching transitions occur between different regions of discontinuity. At typical switching frequencies for high power applications (up to 2 kHz) these additional transitions contribute significantly to the inverter's total losses, so that a proper comparison of the harmonic performance can only be carried out under constant loss conditions with varying carrier frequency. This comparison is performed for a typical industrial medium voltage inverter. The paper then considers the major issues of neutral-point voltage balancing and loss distribution within the inverter, for the identified optimal modulation schemes.

Evaluation of current control methods for wind turbines using doubly-fed induction machines

Abstract: In this paper, different rotor current control methods are investigated with the objective of eliminating the influence of the back electromotive force (EMF), which is that of, in control terminology, a load disturbance, on the rotor current. It is found that the method that utilizes both feed-forward of the back EMF and so-called "active resistance" manages best to suppress the influence of the back EMF on the rotor current, particularly when voltage sags occur, of the investigated methods. This method also has the best stability properties. In addition it is found that this method also has the best robustness to parameter deviations.

Implementation and experimental investigation of sensorless speed control with initial rotor position estimation for interior permanent magnet synchronous motor drive

Abstract: In this paper, a new approach to sensorless speed control and initial rotor position estimation for interior permanent magnet synchronous motor (IPMSM) drive is presented. In rotating condition, speed and rotor position estimation of IPMSM drive are obtained through an extended Kalman filter (EKF) algorithm simply by measurement of the stator line voltages and currents. The main difficulty in developing an EKF for IPMSM is the complexity of the dynamic model expressed in the stationary coordinate system. This model is more complex than that of the surface PMSM, because of the asymmetry of the magnetic circuit. The starting procedure is a problem under sensorless drives, because no information is available before starting. The initial rotor position is estimated by a suitable sequence of voltage pulses intermittently applied to the stator windings at standstill and the measurement of the peak current values of the current leads to the rotor position. Magnetic saturation effect on the saliency is used to distinguish the north magnetic pole from the south. To illustrate our work, we present experimental results for an IPMSM obtained on a floating point digital signal processor (DSP) TMS320C31/40 MHz based control system.

A fixed-frequency pulsewidth modulation based quasi-sliding-mode controller for buck converters

Abstract: This paper presents the design and analysis of a fixed-frequency pulsewidth modulation (PWM)-based quasi-sliding-mode voltage controller for buck converters from a circuit design perspective. A practical design approach that aims at systematizing the procedure for the selection of the control parameters is presented. In addition, a simple analog form of the controller for practical realization is provided. The resulting controller exhibits the same structure as a PWM proportional derivative (PD) linear controller, but with an additional component consisting of the instantaneous input voltage and the instantaneous output voltage. Simulation and experimental results show that the performance of the converter agrees with the theoretical design.

Selective harmonic control: a general problem formulation and selected solutions

Abstract: Selective harmonic elimination/control has been a widely researched alternative to traditional pulse-width modulation techniques. Previous and current work has made fundamental assumptions that enforce output waveform quarter-wave symmetry, presumably in order to reduce the complexity of the resulting equations. However, the quarter-wave symmetric assumption is not strictly necessary. It restricts the solution space, which can result in sub-optimal solutions with regards to the uncontrolled harmonic distribution. A more general formulation is proposed, removing the quarter-wave symmetry constraint for two classes of the m-level, n-harmonic harmonic control problem. The special cases of two- and three-level harmonic elimination are presented in detail along with representative solutions for each harmonic control problem. New solutions previously unattainable based on quarter-wave symmetric techniques are identified.

A new active clamping zero-voltage switching PWM current-fed half-bridge converter

Abstract: A new active clamping zero-voltage switching (ZVS) pulse-width modulation (PWM) current-fed half-bridge converter (CFHB) is proposed in this paper. Its active clamping snubber (ACS) can not only absorb the voltage surge across the turned-off switch, but also achieve the ZVS of all power switches. Moreover, it can be applied to all current-fed power conversion topologies and its operation as well as structure is very simple. Since auxiliary switches in the snubber circuit are switched in a complementary way to main switches, an additional PWM IC is not necessary. In addition, it does not need any clamp winding and auxiliary circuit besides additional two power switches and one capacitor while the conventional current-fed half bridge converter has to be equipped with two clamp windings, two ZVS circuits, and two snubbers. Therefore, it can ensure the higher operating frequency, smaller-sized reactive components, lower cost of production, easier implementation, and higher efficiency. The operational principle, theoretical analysis, and design considerations are presented. To confirm the operation, validity, and features of the proposed circuit, experimental results from a 200-W, 24-200Vdc prototype are presented.

Power budgeting of a multiple-input buck-boost converter

Abstract: The use of a multiple-input buck-boost converter for budgeting power between different energy sources is discussed. It is shown mathematically that the idealized converter can accommodate arbitrary power commands for each input source while maintaining a prescribed output voltage. Power budgeting is demonstrated experimentally for a real converter under various circumstances, including a two-input (solar and line-powered) system. A closed-loop control example involving simultaneous tracking of output voltage and set-point tracking of the solar array shows that an autonomous system is realizable.

Power loss-oriented evaluation of high voltage IGBTs and multilevel converters in transformerless traction applications

Abstract: The low-frequency line transformer in todays ac rail vehicles suffers from poor efficiency and a substantial weight. Future traction drives may operate directly from the mains without this transformer. A feasible concept for a transformerless drive system consists of series connected medium voltage converters applying modern high-voltage insulated gate bipolar transistors (HV-IGBTs). In a first design step, the switching characteristics and losses of 6.5-kV IGBTs are compared to 3.3-kV and 4.5-kV IGBTs which are already commercially used in traction applications. Based on the considered HV-IGBTs, the properties of multilevel converters are analyzed and their applicability to the transformerless system is evaluated. The paper focusses on a loss analysis of the converters. Reliability aspects and harmonic spectra are briefly discussed. Taking these design aspects into account, the three-level neutral point clamped converter turns out to be a reasonable solution to realize line and motor converter modules in a transformerless traction system.

A 6.6-kV transformerless shunt hybrid active filter for installation on a power distribution system

Abstract: This paper presents a fully-digital-controlled shunt hybrid filter for damping of harmonic propagation in power distribution systems. The harmonic propagation is caused by resonance between line inductances and power capacitors installed for power factor correction. A possible solution to damping out harmonic propagation is based on installation of a shunt pure active filter at the end of a feeder. This paper proposes a shunt hybrid active filter characterized by series connection of a seventh-tuned LC filter per phase and a small-rated three-phase active filter. Like the pure filter, the hybrid filter is connected to the end bus of a feeder. The capacitor of the LC filter imposes a high impedance to the fundamental frequency, so that the fundamental voltage appears across the capacitor. This unique feature allows us to directly connect the hybrid filter to the 6.6-kV power line without step-down transformers. Furthermore, the capacitor used in this hybrid filter is lighter, cheaper and smaller than the transformer used in the pure filter. Theoretical analysis, along with experimental results obtained from a 200-V, 20-kW laboratory system, verifies the viability and effectiveness of the proposed hybrid filter.

Implementation and control of distributed PWM cascaded multilevel inverters with minimal harmonic distortion and common-mode voltage

Abstract: Cascaded multilevel inverters can be implemented through the series connection of single-phase modular power bridges. This work presents details on how these bridges should be implemented and operated to synchronize their pulse-width-modulation (PWM) carriers, fundamental references and sampling instances to implement a network-controlled cascaded inverter with distributed PWM computation and overall optimal system performance. The paper begins by detailing the development and control of an integrated power bridge, designed with its own digital signal processor and associated control circuitry. Details describing the networked control algorithm and signal protocol needed for synchronizing the multiple power bridges through a dynamically fast data communication network, are then presented to achieve optimum harmonic cancellation and reduced common-mode voltage. The practicality and performance of the presented modular implementation concepts have been confirmed through the close match between simulation and experimental results obtained using a modular cascaded five-level inverter prototype.

An integrated inverter with maximum power tracking for grid-connected PV systems

Abstract: An inverter for grid-connected photovoltaic systems is presented in this paper. It can globally locate the maximum power point of the panel over wide insolation and feed the solar energy to the grid. Its structure mainly integrates a previously developed maximum point tracking method and output current shaping function into a buck-boost-derived converter and then inverts the shaped current through a grid frequency bridge to the grid. Instead of having a storage capacitor connecting in parallel with the converter output, series connection is used, so that the required capacitor voltage rating is lower than that in classical inverters. Most importantly, the inverter output current harmonics are less sensitive to the capacitor value. A 30-W laboratory prototype has been built. The tracking capability, inversion efficiency, and large-signal responses at different insolations have been investigated. Detailed analysis on the inverter performance has been performed. The theoretical predictions are verified with the experimental results.

An adaptive dead-time compensation strategy for voltage source inverter fed motor drives

Abstract: This paper presents an adaptive dead-time compensation strategy to obtain fundamental phase voltage for inverter fed vector controlled permanent magnet synchronous motor drives. The amplitude of phase dead-time compensation voltage (DTCV) to compensate disturbance voltage due to undesirable characteristics of inverter, such as dead-time, turn-on/off time of switching devices, and on-voltages of switching devices and diodes is adaptively determined according to a dead-time compensation time (DTCT). DTCT is identified on-line with using a /spl delta/-axis disturbance voltage in the current reference frame that is synchronized with current vector. The /spl delta/-axis disturbance voltage is estimated by a disturbance observer. The accuracy of identified DTCT is experimentally confirmed by calculating the mean absolute percentage error (MAPE) between a calculated active power and a measured one. MAPE for adaptive DTCT is almost within 5% at any operating point.

Suppression of line voltage related distortion in current controlled grid connected inverters

Abstract: The influence of selected control strategies on the level of low-order current harmonic distortion generated by an inverter connected to a distorted grid is investigated through a combination of theoretical and experimental studies. A detailed theoretical analysis, based on the concept of harmonic impedance, establishes the suitability of inductor current feedback versus output current feedback with respect to inverter power quality. Experimental results, obtained from a purpose-built 500-W, three-level, half-bridge inverter with an L-C-L output filter, verify the efficacy of inductor current as the feedback variable, yielding an output current total harmonic distortion (THD) some 29% lower than that achieved using output current feedback. A feed-forward grid voltage disturbance rejection scheme is proposed as a means to further reduce the level of low-order current harmonic distortion. Results obtained from an inverter with inductor current feedback and optimized feed-forward disturbance rejection show a THD of just 3% at full-load, representing an improvement of some 53% on the same inverter with output current feedback and no feed-forward compensation. Significant improvements in THD were also achieved across the entire load range. It is concluded that the use of inductor current feedback and feed-forward voltage disturbance rejection represent cost-effect mechanisms for achieving improved output current quality.

A novel winding-coupled buck converter for high-frequency, high-step-down DC-DC conversion

Abstract: This paper analyzes the fundamental limitations of the buck converter for high-frequency, high-step-down dc-dc conversion. Further modification with additional coupled windings in the buck converter yields a novel topology, which significantly improves the efficiency without compromising the transient response. An integrated magnetic structure is proposed for these windings so that the same magnetic cores used in the buck converter can be used here as well. Furthermore, it is easy to implement a lossless clamp circuit to limit the device voltage stress and to recover inductor leakage energy. This new topology is applied for a 12V-to-1.5V/25A voltage regulator module (VRM) design. At a switching frequency of 2MHz, over 80% full-load efficiency is achieved, which is 8% higher than that of the conventional buck converter.

Magnetics on silicon: an enabling technology for power supply on chip

Abstract: Data from the ITRS2003 roadmap for 2010 predicts voltages for microprocessors in hand-held electronics will decrease to 0.8V with current and power increasing to 4A and 3W, respectively. Consequently, low power converters will move to multimegahertz frequencies with a resulting reduction in capacitor and inductor values by factors of 5 and 20, respectively. Values required at 10 MHz, for a low power buck converter, are estimated at 130 nH and 0.6 uF, compatible with the integration of magnetics onto silicon and the concept of power supply-on-chip (PSOC). A review of magnetics-on-silicon shows that inductance values of 20 to 40nH/mm/sup 2/ can be achieved for winding resistances less than 1/spl Omega/. A 1-/spl mu/H inductance can be achieved at 5 MHz with dc resistance of 1/spl Omega/ and a Q of four. Thin film magnetic materials, compatible with semiconductor processing, offer power loss density that is lower than ferrite by a factor of 5 at 10 MHz. Other data reported includes, lowest dc resistance values of 120 m/spl Omega/ for an inductance of 120 nH; highest Q of 15 for an inductance of 350 nH and a current of 1 A for a 1- /spl mu/H inductor. Future technology challenges include reducing losses using high resistivity, laminated magnetic materials, and increasing current carrying capability using high aspect-ratio, electroplated copper conductors. Compatible technologies are available in the power switch, control, and packaging space. Integrated capacitor technology is still a long-term challenge with maximum reported values of 400 nF/cm/sup 2/.