Showing papers on "Converters published in 2012"

Minimum Energy and Capacitance Requirements for Single-Phase Inverters and Rectifiers Using a Ripple Port

Abstract: Converters with a dc port and a single-phase ac port must store energy to buffer the inherent double-frequency power flow at the ac port. The minimum energy storage required to isolate the power ripple from the dc port is presented, and leads to the minimum capacitance required for converters that use capacitive energy storage. This paper presents a ripple power port to manage energy storage and decouple capacitor ripple from power ripple. A ripple power port allows the designer to make a choice of capacitor voltage independent of other system voltages. A combination of an ac link converter and a ripple power port leads to a dramatic increase in reliability: it is shown that converters with nominal ratings up to 200 W can be designed with expected mean-time-between-failure ratings on the order of 1.4 × 106 h-sufficient for hundred-year operation in long-life applications such as photovoltaic converters and LED lamps. This large increase in life is achieved with minimal extra cost.

Platform Architecture for Solar, Thermal, and Vibration Energy Combining With MPPT and Single Inductor

Abstract: A platform architecture combining energy from solar, thermal, and vibration sources is presented. A dual-path architecture for energy harvesting is employed that has a peak efficiency improvement of 11%-13% over the traditional two-stage approach. The system implemented consists of a reconfigurable multi-input, multi-output switch matrix that combines energy from three distinct energy-harvesting sources-photovoltaic, thermoelectric, and piezoelectric. The system can handle input voltages from 20 mV to 5 V and is capable of extracting maximum power from individual harvesters all at the same time utilizing a single inductor. A proposed time-based power monitor is used for achieving maximum power point tracking for the photovoltaic harvester. This has a peak tracking efficiency of 96%. The peak efficiencies achieved with inductor sharing are 83%, 58%, and 79% for photovoltaic boost, thermoelectric boost, and piezoelectric buck-boost converters, respectively. The switch matrix and the control circuits are implemented on a 0.35-μm CMOS process.

Control and Circuit Techniques to Mitigate Partial Shading Effects in Photovoltaic Arrays

Abstract: Partial shading in photovoltaic (PV) arrays renders conventional maximum power point tracking (MPPT) techniques ineffective. The reduced efficiency of shaded PV arrays is a significant obstacle in the rapid growth of the solar power systems. Thus, addressing the output power mismatch and partial shading effects is of paramount value. Extracting the maximum power of partially shaded PV arrays has been widely investigated in the literature. The proposed solutions can be categorized into four main groups. The first group includes modified MPPT techniques that properly detect the global MPP. They include power curve slope, load-line MPPT, dividing rectangles techniques, the power increment technique, instantaneous operating power optimization, Fibonacci search, neural networks, and particle swarm optimization. The second category includes different array configurations for interconnecting PV modules, namely series-parallel, total-cross-tie, and bridge-link configurations. The third category includes different PV system architectures, namely centralized architecture, series-connected microconverters, parallel-connected microconverters, and microinverters. The fourth category includes different converter topologies, namely multilevel converters, voltage injection circuits, generation control circuits, module-integrated converters, and multiple-input converters. This paper surveys the proposed approaches in each category and provides a brief discussion of their characteristics.

A Flexible Harmonic Control Approach Through Voltage-Controlled DG–Grid Interfacing Converters

Abstract: The increased penetration of nonlinear loads and power-electronics-based distributed generation (DG) systems may introduce power quality issues to the distribution power system. However, if controlled and regulated properly, the DG-grid interfacing converters are able to improve the distribution system efficiency and power quality. This paper focuses on the distribution system harmonic control through the DG-grid interfacing converters. Two alternative DG systems, namely, current-controlled DG and voltage-controlled DG, are considered. While most of the previous works on harmonic compensation are based on the current-controlled method, a novel harmonic control scheme using a voltage-controlled method is developed in this paper. The voltage-controlled method is more flexible and has similar compensation performance compared to the conventional current-controlled method. In addition, by avoiding the implementation of a harmonic current tracking loop, the proposed voltage-based method can be seamlessly incorporated into a voltage-controlled DG unit, which is important to provide direct voltage and frequency support in a microgrid. Moreover, the possible harmonic circulating current among multiple DG systems is also investigated in this paper. Simulations and experimental results from a three-phase 5-kVA laboratory DG prototype are provided to validate the discussion.

Analysis, Modeling, and Implementation of a Multidevice Interleaved DC/DC Converter for Fuel Cell Hybrid Electric Vehicles

Abstract: Multiphase converter topologies for use in high-performance applications have received increasing interest in recent years. This paper proposes a novel multidevice interleaved boost converter (MDIBC) that interfaces the fuel cell with the powertrain of hybrid electric vehicles. In this research, a multidevice structure with interleaved control is proposed to reduce the input current ripples, the output voltage ripples, and the size of passive components with high efficiency compared with the other topologies. In addition, low EMI and low stress in the switches are expected. The proposed dc/dc converter is compared to other converter topologies such as conventional boost converter (BC), multidevice boost converter (MDBC), and two-phase interleaved boost converter (IBC) to verify its dynamic performance. Furthermore, a generalized small-signal model is derived for these dc/dc converters, which has not been previously discussed. A digital dual-loop control is designed to achieve the proper regulator for the converters with fast transient response. The dc/dc converter topologies and their controller are designed and investigated by using MATLAB/Simulink. Finally, the proposed converter (MDIBC) is experimentally validated with results obtained from a 30-kW prototype that has been built and tested in our laboratory based on TMS320F2808 DSP. The simulation and experimental results have demonstrated that the proposed converter is more efficient than other dc/dc converter topologies in achieving high performance and reliability for high-power dc/dc converters.

Enhanced Decoupled Double Synchronous Reference Frame Current Controller for Unbalanced Grid-Voltage Conditions

Abstract: In the last few years, restrictive grid codes have arisen to ensure the performance and stability of electrical networks, which experience a massive integration of renewable energy sources and distributed generation systems that are normally connected to the grid through electronic power converters. In these codes, the injection of positive- and negative-sequence current components becomes necessary for fulfilling, among others, the low-voltage ride-through requirements during balanced and unbalanced grid faults. However, the performance of classical dq current controllers, applied to power converters, under unbalanced grid-voltage conditions is highly deficient, due to the unavoidable appearance of current oscillations. This paper analyzes the performance of the double synchronous reference frame controller and improves its structure by adding a decoupling network for estimating and compensating the undesirable current oscillations. Experimental results will demonstrate the validity of the proposed decoupled DSRF controller.

Digital Control of Power Converters—A Survey

Abstract: Power converters offer a high capability to efficiently manage electrical energy flows. Until a few years ago, their primary use was in supplying motors in industrial applications and in electric traction systems. Nowadays, in addition to those fields they are employed in a very wide range of low, medium, and high power applications including residential applications, renewable energy systems, distributed generation, and automotive. Since digital control represents a key element of modern power converters, this paper presents a review on digital devices [microcontrollers, Field Programmable Gate Arrays (FPGA)], hardware and software design techniques as well as implementation issues useful for designing modern high-performance power converters.

Evaluation and Efficiency Comparison of Front End AC-DC Plug-in Hybrid Charger Topologies

Abstract: As a key component of a plug-in hybrid electric vehicle (PHEV) charger system, the front-end ac-dc converter must achieve high efficiency and power density. This paper presents a topology survey evaluating topologies for use in front end ac-dc converters for PHEV battery chargers. The topology survey is focused on several boost power factor corrected converters, which offer high efficiency, high power factor, high density, and low cost. Experimental results are presented and interpreted for five prototype converters, converting universal ac input voltage to 400 V dc. The results demonstrate that the phase shifted semi-bridgeless PFC boost converter is ideally suited for automotive level I residential charging applications in North America, where the typical supply is limited to 120 V and 1.44 kVA or 1.92 kVA. For automotive level II residential charging applications in North America and Europe the bridgeless interleaved PFC boost converter is an ideal topology candidate for typical supplies of 240 V, with power levels of 3.3 kW, 5 kW, and 6.6 kW.

A Fully-Integrated 3-Level DC-DC Converter for Nanosecond-Scale DVFS

Abstract: On-chip DC-DC converters have the potential to offer fine-grain power management in modern chip-multiprocessors. This paper presents a fully integrated 3-level DC-DC converter, a hybrid of buck and switched-capacitor converters, implemented in 130 nm CMOS technology. The 3-level converter enables smaller inductors (1 nH) than a buck, while generating a wide range of output voltages compared to a 1/2 mode switched-capacitor converter. The test-chip prototype delivers up to 0.85 A load current while generating output voltages from 0.4 to 1.4 V from a 2.4 V input supply. It achieves 77% peak efficiency at power density of 0.1 W/mm2 and 63% efficiency at maximum power density of 0.3 W/mm2. The converter scales output voltage from 0.4 V to 1.4 V (or vice-versa) within 20 ns at a constant 450 mA load current. A shunt regulator reduces peak-to-peak voltage noise from 0.27 V to 0.19 V under pseudo-randomly fluctuating load currents. Using simulations across a wide range of design parameters, the paper compares conversion efficiencies of the 3-level, buck and switched-capacitor converters.

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.

Active Capacitor Voltage Balancing in Single-Phase Flying-Capacitor Multilevel Power Converters

Abstract: Two active capacitor voltage balancing schemes are proposed for single-phase (H-bridge) flying-capacitor multilevel converters. They are based on the circuit equations of flying-capacitor converters. Consequently, they can be implemented using straightforward control rules. In particular, the first technique is based on an algorithm which follows the standard multilevel modulation. Then, it utilizes a redundant state selection table for capacitor voltage balancing. In the second method, multiple duty cycles are defined and modulated in direct response to the capacitor voltages. The most important advantage of these two proposed methods is that they can be utilized to converters with any desired number of levels in their output voltage. Moreover, the analysis and implementation of both methods are straightforward. Through simulation and experimental implementation, these methods are shown to be effective on capacitor voltage regulation in flying-capacitor multilevel converters.

Predictive Control of AC–AC Modular Multilevel Converters

Abstract: Multilevel converters can reach medium-voltage operation increasing the efficiency of high-power applications. Among the existing multilevel converter topologies, the modular multilevel converter (MMC) provides the advantages of high modularity, availability, and high power quality. Moreover, the main advantage compared to cascaded multilevel converters is the lack of an input transformer which results in a reduction of cooling requirements, size, and cost. One of the drawbacks of this topology when used as an ac-ac converter is the input and output frequency components in the control loop, resulting in a more complex controller design. In this paper, a single-phase ac-ac MMC predictive control approach is proposed. The controller minimizes the input, output, and circulating current errors and balances the dc voltages. Experimental results show the performance of the proposed predictive control scheme.

Computer-Aided Design and Optimization of High-Efficiency LLC Series Resonant Converter

Abstract: High conversion efficiency is desired in switch mode power supply converters. Computer-aided design optimization is emerging as a promising way to design power converters. In this work a systematic optimization procedure is proposed to optimize LLC series resonant converter full load efficiency. A mode solver technique is proposed to handle LLC converter steady-state solutions. The mode solver utilizes numerical nonlinear programming techniques to solve LLC-state equations and determine operation mode. Loss models are provided to calculate total component losses using the current and voltage information derived from the mode solver. The calculated efficiency serves as the objective function to optimize the converter efficiency. A prototype 300-W 400-V to 12-V LLC converter is built using the optimization results. Details of design variables, boundaries, equality/inequality constraints, and loss distributions are given. An experimental full-load efficiency of 97.07% is achieved compared to a calculated 97.4% efficiency. The proposed optimization procedure is an effective way to design high-efficiency LLC converters.

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.

A Direct AC–AC Converter for Inductive Power-Transfer Systems

Abstract: This paper proposes a direct ac-ac converter to generate a high-frequency current for inductive power-transfer (IPT) systems. Unlike traditional dc-ac converters for IPT systems, the proposed converter can achieve a high-frequency current generation directly from an ac power source without a dc link. The power converter can maintain high-frequency resonance by natural circuit oscillation and discrete energy injection control. A variable-frequency control strategy is developed to vary the switching frequency to keep the high-frequency current operating with zero-current switching. Thus, the switching stress, power losses, and electromagnetic interference of the converter are reduced. Theoretical analysis, simulation, and experimental results show that the output track current is fully controllable with good waveforms for contactless power-transfer applications.

A Unified Approach to Reliability Assessment of Multiphase DC–DC Converters in Photovoltaic Energy Conversion Systems

Abstract: A systematic framework for reliability assessment and fault-tolerant design of multiphase dc-dc converters deployed in photovoltaic applications is presented. System-level steady-state models allow a detailed specification of component failure rates, and in turn establish the effects of ambient conditions and converter design on reliability. Markov reliability models are derived to estimate the mean time to system failure. Case studies applied to two- and three-phase, 250-W converters demonstrate that topological redundancy does not necessarily translate to improved reliability for all choices of switching frequency and capacitance. Capacitor voltage rating is found to be the dominant factor that affects system reliability.

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.

Interleaved Buck Converter Having Low Switching Losses and Improved Step-Down Conversion Ratio

Abstract: This paper proposes a new interleaved buck converter (IBC) having low switching losses and improved step-down conversion ratio, which is suitable for the applications where the input voltage is high and the operating duty is below 50%. It is similar to the conventional IBC, but two active switches are connected in series and a coupling capacitor is employed in the power path, such as Cuk, Sepic, and Zeta converters. The proposed IBC shows that since the voltage stress across all the active switches is half of the input voltage before turn-on or after turn-off when the operating duty is below 50%, the capacitive discharging and switching losses can be reduced considerably. This allows the proposed IBC to have higher efficiency and operate with higher switching frequency. In addition, the proposed IBC has a higher step-down conversion ratio and a smaller output current ripple compared with a conventional IBC. The features, operation principles, and relevant analysis results of the proposed IBC are presented in this paper. The validity of this study is confirmed by the experimental results of prototype converters with 150-200 V input, 24 V/10 A output.

Closed-loop control of DC-DC dual active bridge converters driving single-phase inverters

Abstract: A solid-state transformer (SST) is a high-frequency power electronic converter that is used as a distribution power transformer. A common three-stage configuration of an SST consists of ac-dc rectifier, isolated dc-dc dual-active-bridge (DAB) converter, and dc-ac inverter. This study addresses the controller design issue for a dc-dc DAB converter when driving a regulated single-phase dc-ac inverter. Since the switching frequency of the inverter stage is much higher than that of the DAB stage, the single-phase inverter is modeled as a double-line-frequency (e.g., 120 Hz) current sink. The effect of 120-Hz current by the single-phase inverter is studied. The limitation of a PI-controller, low gain at 120 Hz, is investigated. Two methods are proposed to improve the regulation of the output voltage of DAB converters. The first one uses a bandstop filter and feedforward, while the second method uses an additional proportional-resonant controller in the feedback loop. Theoretical analysis, simulation, and experiment results are provided.

Sliding-Mode Control for DFIG Rotor- and Grid-Side Converters Under Unbalanced and Harmonically Distorted Grid Voltage

Abstract: Regarding doubly fed induction generator (DFIG) operation, unbalanced and harmonically distorted grid voltage conditions have been treated as two separate control problems. This paper reports a solution for the rotor- and grid-side power converters, which allows one to keep the DFIG successfully in operation under both grid voltage conditions. The proposed solution is based on sliding-mode control (SMC). The rotor-side converter is commanded so that the electromagnetic torque and the stator reactive power remain free of fluctuations that arise during grid voltage disturbances. Meanwhile, the grid-side converter ensures both constant DC-link voltage and steady active power output from the overall system. The developed algorithms turn out being robust against parameter variations and of fast dynamic response. In addition, none of the converters need either voltage or current positive and negative sequences extraction. The simulation results presented demonstrate the appropriateness of SMC to face such disturbed scenarios. Finally, the stability proof of both converters' control algorithms is provided in the appendices.

High Power Current Sensorless Bidirectional 16-Phase Interleaved DC-DC Converter for Hybrid Vehicle Application

Abstract: A new 16-phase interleaved bidirectional dc/dc converter is developed featuring smaller input/output filters, faster dynamic response and lower device stress than conventional designs, for hybrid vehicle applications. The converter is connected between the ultracapacitor (UC) pack and the battery pack in a multisource energy storage system of a hybrid vehicle. Typically, multiphase interleaved converters require a current control loop in each phase to avoid imbalanced current between phases. This increases system cost and control complexity. In this paper, in order to minimize imbalance currents and remove the current control loop in each phase, the converter is designed to operate in discontinuous conduction mode (DCM). The high current ripple associated with DCM operation is then alleviated by interleaving. The design, construction, and testing of an experimental hardware prototype is presented, with the test results included. Finally, a novel soft switch topology for DCM operation is proposed for future research, to achieve zero-voltage switching (ZVS), or zero-current switching (ZCS) in all transitions.

A Space-Vector Modulation Scheme for Multilevel Open-End Winding Five-Phase Drives

Abstract: Open-end winding three-phase variable speed drives with dual-inverter supply have been extensively investigated for various applications, including series hybrid powertrains and propulsion motors. The topology is simple to realize while offering a higher number of switching states without the need for capacitor voltage balancing algorithms, when compared to the standard multilevel converters. This paper extends the open-end winding concept to a five-phase drive. A relatively simple space-vector modulation (SVM) algorithm, based on the already well-understood five-phase two-level drive SVM method, is developed. The proposed modulation technique has the advantage of being straightforward to implement and, like its two-level counterpart, is able to generate output voltages with minimum low-order harmonic content. The method generates up to 17-level output phase voltage and, therefore, offers superior harmonic performance when compared to the two-level five-phase modulation. The developed scheme is verified via detailed simulations and experiments, using a five-phase induction machine operating under open-loop V/f control.

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.

General Derivation Law of Nonisolated High-Step-Up Interleaved Converters With Built-In Transformer

Abstract: First, the limitations of the conventional interleaved boost converters in high-step-up and high-output-voltage applications are addressed in this paper. Then, a general derivation law of the nonisolated converters from their isolated counterparts is proposed and studied to give a universal solution for high-performance topology deduction. By employing the direct energy transfer concept, a family of nonisolated high-step-up interleaved boost converters is originated to make the turns ratio of a built-in transformer as another design freedom for the voltage gain extension. The derived converters have the advantages of large voltage conversion ratio, low power switch voltage stress, small input current ripple, and zero-voltage-switching soft-switching performance. The steady-state operation of the derived converter is analyzed, and the circuit performance is summarized to explore its advantages in the high-step-up, high-output-voltage, and large-current conversion systems. Finally, a 1-kW prototype with 40-V input and 380-V output voltages is implemented and tested to show the effectiveness of the derived converters. One of the main contributions of this paper is that a clear picture is made on the universal derivation law to generate high-step-up and high-performance dc/dc converters.

Novel Nonisolated High-Voltage Gain DC–DC Converters Based on 3SSC and VMC

Abstract: This paper introduces a new family of dc-dc converters based on the three-state switching cell and voltage multiplier cells A brief literature review is presented to demonstrate some advantages and inherent limitations of several topologies that are typically used in voltage step-up applications In order to verify the operation principle of this family, the boost converter is chosen and investigated in detail The behavior of the converter is analyzed through an extensive theoretical analysis, while its performance is investigated by experimental results obtained from a 1-kW laboratory prototype and relevant issues are discussed The analyzed converter can be applied in uninterruptible power supplies, fuel cell systems, and is also adequate to operate as a high-gain boost stage with cascaded inverters in renewable energy systems Furthermore, it is suitable in cases where dc voltage step-up is demanded, such as electrical fork-lift, audio amplifiers, and many other applications

AC circulating currents suppression in modular multilevel converter

Abstract: Modular multilevel converter is a next generation multilevel converters for medium to high voltage conversion applications, such as medium voltage motor drive and high voltage direct current transmission. One potential issue of this type of converter is the AC circulating current, which increases the current stress and brings additional conduction loss to the system. This paper proposes modified control architecture for modular multilevel converters, aiming at suppressing the AC components in the circulating current. Specifically, a proportional-resonant type minor loop is incorporated to regulate the most AC components of the circulating current to zero in addition to the DC regulation loop. The proposed minor loop can also be applied to single phase MMC, which is not available in previous methods. Simulation results for a three-phase MMC operating as an inverter are provided to demonstrate the feasibility of the proposed method.

Analysis of EMI Terminal Modeling of Switched Power Converters

Abstract: A generalized terminal modeling technique was proposed earlier to predict conducted electromagnetic interference from a dc-dc boost converter. The predictions of these conducted emissions showed that there was a good agreement up to 50 MHz. This paper extends the generalized terminal modeling approach to converters with the buck-type input. Both dc and ac applications are discussed. The technique is developed for the electromagnetic interference modeling of switched power converters in aerospace applications where the requirements on electromagnetic pollution are very strict. The model is shown to successfully predict conducted emissions for a buck converter and a three-phase voltage source inverter up to 100 MHz with an error of 6 dB or less at most frequencies.

Coupling Effect Reduction of a Voltage-Balancing Controller in Single-Phase Cascaded Multilevel Converters

Abstract: This paper presents a new voltage-balancing controller for cascaded multilevel converters, especially for single-phase cascaded multilevel converters. It proposes a control algorithm that devotes itself not only to balancing the floating dc capacitors but also to eliminating the coupling effect between the voltage-balancing controller and the original system controller (controller without additional voltage-balancing controllers). Specifically, the average model in the d-q coordinate frame is derived and the control law is established. Then, the coupling effect between the voltage-balancing controller and the original system controller is identified and a new expression for duty cycle modification is proposed thus to eliminate the effect. Furthermore, this paper gives the design considerations of the pro- posed method, including the derivation of key transfer functions and effective voltage-balancing area, for the completeness of the discussion. Moreover, the reference generation techniques of the voltage-balancing controller are also discussed. This paper investigates the voltage imbalance in the soft-start process caused by an unsuitable reference, and presents a simple modified reference generation solution. Finally, both simulation and experimental results verify the performance of the proposed control system.

Classification and comparative evaluation of PV panel integrated DC-DC converter concepts

Abstract: Strings of photovoltaic panels have a significantly reduced power output when mismatch between the panels, such as partial shading, occurs since integrated diodes are then partly bypassing the shaded panels. With the implementation of DC-DC converters on panel level, the maximum available power can be extracted from each panel regardless of any shading. In this paper, different concepts of PV panel integrated DC-DC converters are presented, comparative evaluation is given and the converter design process is shown for the buck-boost converter which is identified as the best suited concept. Furthermore, the results of high precision efficiency measurements of an experimental prototype are presented and compared to a commercial MIC.