B. Lehman

Northeastern University, Boston, Massachusetts, United States

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Publications (130)166.95 Total impact

  • Source
    R.M. Nakagomi, Ye Zhao, B. Lehman
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    ABSTRACT: In the last five years, the desire to improve the control of photovoltaic (PV) arrays working in mismatched conditions has stimulated substantial research and development. Recent research has revived the previously discounted idea of changing the electrical connection between the PV modules according to their operating conditions and load requests. The switching matrix is reliable and allows the additional cost of module-level switching converters to be avoided and also maintains high efficiency under infrequent mismatched conditions. The PV array electrical reconfiguration also enables the best load matching between the PV source and its varying load. Such issues are discussed in this article with an overview of the main aspects introduced in the technical literature.
    IEEE Industrial Electronics Magazine 01/2015; 9(1):62-76. DOI:10.1109/MIE.2014.2360721 · 5.06 Impact Factor
  • Brad Lehman, Arnold J. Wilkins
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    ABSTRACT: How often has this scenario happened? You are driving at night behind a car that has bright light-emitting diode (LED) taillights. When looking directly at the taillights, the light is not blurry, but when glancing at other objects, a trail of lights appears, known as a phantom array. The reason for this trail of lights might not be what you expected: it is not due to glare, degradation of eyesight, or astigmatism. The culprit may be the flickering of the LED lights caused by pulse-width modulating (PWM) drive circuitry. Actually, many LED taillights flicker on and off at frequencies between 200 and 500 Hz, which is too fast to notice when the eye is not in rapid motion. However, during a rapid eye movement (saccade), the images of the LED lights appear in different positions on the retina, causing a trail of images to be perceived (Figure 1). This disturbance of vision may not occur with all LED taillights because some taillights keep a constant current through the LEDs. However, when there is a PWM current through the LEDs, the biological effect of the light flicker may become noticeable during the eye saccade.
    09/2014; 1(3):18-26. DOI:10.1109/MPEL.2014.2330442
  • Dorin O. Neacsu, Brad Lehman
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    ABSTRACT: This paper investigates for the first time the use of a very large SD Card flash memory to the design of a three-phase Pulse Width Modulation (PWM) generator and describes the hardware and software used for implementation. The new digital architecture differs from conventional counter-based implementation and it follows a pre-programmed optimal PWM pattern that is read from memory with magnitude and phase as coordinates. This architecture allows the inclusion of multiple optimization criteria within the PWM pattern. Experimental results are shown with a PICDEM PIC18 Explorer Board and a SD Card Reader PICtail Daughter Board, both from Microchip. The optimal PWM pattern is defined in MATLAB
    IEEE OPTIM 2014 - The 14th International Conference on Optimization of Electrical and Electronic Equipment, Moieciu, Romania,; 05/2014
  • Su Sheng, Brad Lehman
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    ABSTRACT: This paper proposes a method to parallel m digital controlled cascaded H-bridge (CHB) multilevel inverters directly with high bandwidth current sharing capabilities. The proposed method utilizes switch signal synchronization with dead time control to eliminate circulating current and adopts “piecewise” master-slave strategy to achieve equal current distribution in less than one cycle. A prototype is designed, built, and tested to verify the proposed method.
    2014 IEEE Applied Power Electronics Conference and Exposition - APEC 2014; 03/2014
  • Chung-Ti Hsu, Brad Lehman, Ting Qian
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    ABSTRACT: This paper proposes a new approach to efficiently track the desired maximum power point for photovoltaic systems under partial shading conditions. The power stage architecture achieves fast input current change rate by combining a current-adjustable converter with a few converters operating at a constant input current. By taking advantage of fast current change, the control scheme effectively combines perturb-and-observe (P&O) tracking and periodic global maximum power point (MPP) searching, and fulfills fast tracking as well as maintaining ease of implementation.
    2014 IEEE Applied Power Electronics Conference and Exposition - APEC 2014; 03/2014
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    ABSTRACT: This paper presents an equivalent model of an inductive magnetic harvester, taking nonlinear effects of magnetic field into consideration. Conventional models consist of one AC voltage source in series with an inductor and are unable to predict harvester performance precisely; a more accurate approach is introduced by considering the nonlinear effects of the magnetic fields. The new model uses a magnetizing inductance and a leakage inductance rather than a single series inductor. In the proposed equivalent model, magnetizing and leakage inductance can be found by detecting flux variation in the air gap between magnets and coil cores. The finite element analysis (FEA) method is used to estimate the electromagnetic field and flux linkage of the coils. The optimum harvesting system design is only possible based on the more accurate modeling of the inductive magnetic harvester.
    2013 IEEE 14th Workshop on Control and Modeling for Power Electronics (COMPEL); 06/2013
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    ABSTRACT: Fault analysis in solar photovoltaic (PV) arrays is a fundamental task to protect PV modules from damage and to eliminate risks of safety hazards. This paper focuses on line-line faults in PV arrays that may be caused by short-circuit faults or double ground faults. The effect on fault current from a maximum-power-point tracking of a PV inverter is discussed and shown to, at times, prevent overcurrent protection devices (OCPDs) to operate properly. Furthermore, fault behavior of PV arrays is highly related to the fault location, fault impedance, irradiance level, and use of blocking diodes. Particularly, this paper examines the challenges to OCPD in a PV array brought by unique faults: One is a fault that occurs under low-irradiance conditions, and the other is a fault that occurs at night and evolves during “night-to-day” transition. In both circumstances, the faults might remain hidden in the PV system, no matter how irradiance changes afterward. These unique faults may subsequently lead to unexpected safety hazards, reduced system efficiency, and reduced reliability. A small-scale experimental PV system has been developed to further validate the conclusions.
    IEEE Transactions on Industrial Electronics 05/2013; 60(9):3784-3795. DOI:10.1109/TIE.2012.2205355 · 6.50 Impact Factor
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    ABSTRACT: This paper proposes a new Non-Superconducting Fault Current Limiter (NSFCL) topology. The bridge-type resistive NSFCL is simple, low-cost, and compact. The proposed topology is optimized to protect against short transients and to work in conjunction with other fuses or circuit breakers. A prototype has been designed and built for a 3-phase 600VRMS,L-L system. It has been tested in a UL certified high power test lab with 5A normal current, and 100kA potential fault current. Both simulation and experimental results are presented for proof of concept.
    2013 IEEE Applied Power Electronics Conference and Exposition - APEC 2013; 03/2013
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    ABSTRACT: Solar photovoltaic (PV) arrays are unique power sources that may have uncleared fault current when utilizing conventional overcurrent protection devices. To monitor the PV operation and detect these unnoticed faults, outlier detection rules have been proposed for fault detection based on instantaneous PV string current. This paper discusses three rules in detail: 3-Sigma rule, Hampel identifier, and Boxplot rule. Unlike other methods, the proposed methods do not require weather measurement or efforts in model training. Our experimental results show that Hampel identifier and Boxplot rule may be recommended for PV fault detection. Furthermore, the proposed models become more reliable as the number of PV measurements increases. The developed methods may be integrated with PV monitoring system for real-time operation.
    2013 IEEE Applied Power Electronics Conference and Exposition - APEC 2013; 03/2013
  • Su Sheng, Peng Li, B. Lehman
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    ABSTRACT: This paper proposes a method to parallel N digital controlled low cost modified sine wave inverters with current sharing capabilities. Existing current sharing methods used to parallel pure sine wave inverters and DC-DC converters cannot be applied directly to modified sine wave inverters because of possible circulating currents. The proposed method utilizes switch signal synchronization with dead time control to eliminate circulating current, implements digital compensator with on/off control to improve the dynamic response of the first stage DC-DC converter, and adopts a “piecewise” master-slave current sharing strategies to achieve equal current distribution. A prototype is designed, built, and tested.
    Energy Conversion Congress and Exposition (ECCE), 2013 IEEE; 01/2013
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    ABSTRACT: Inductive magnetic harvesting technologies have been developing rapidly and have great potential in applications that rotate or vibrate. However, magnetic energy harvesters are often limited by their low voltage outputs, and normally, their placement are in applications that work at low frequency (typically around 100Hz). The traditional rectification with a diode bridge becomes inefficient with the resulting low harvester output voltage due to the diode forward voltage drop. This paper presents a new energy harvesting circuit, which can operate with magnetic harvesters limited by low output voltage. By adding one resonant capacitor and one bi-directional switch, the rectifier input voltage can be increased dramatically with the high voltage generated by resonance between the resonant capacitor and harvester leakage inductance. The simulation and experimental results verify the theoretical predictions.
    Energy Conversion Congress and Exposition (ECCE), 2013 IEEE; 01/2013
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    ABSTRACT: This paper presents a new system architecture for a low cost photovoltaic (PV) battery charging station that can balance: 1) the charging time of each individual battery and 2) the total charging time of all batteries in the system. The control strategy for the new system first charges each individual battery to either the same voltage or same state of charge (SOC) level and then charges multiple batteries in parallel simultaneously. As the paper demonstrates, this method can increase solar energy exploitation so that the total charging time of all batteries is decreased. Experimental results of a prototype of the system validate the effectiveness of the proposed control strategy.
    Control and Modeling for Power Electronics (COMPEL), 2013 IEEE 14th Workshop on; 01/2013
  • Song Chen, Peng Li, David Brady, Brad Lehman
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    ABSTRACT: This paper discusses the practical factors that may influence the inverter sizing strategy. Effects of various factors are studied separately by isolating parameters in the simulations. These factors include irradiance and temperature conditions of the photovoltaic (PV) installation location, PV incentives, electricity rates, and inverter intrinsic parameters such as overload protection schemes and efficiency curves. Specifically, examples of nine different geographic locations in the US are simulated and discussed with realistic parameters to show that the optimum inverter size varies notably by location and context.
    Solar Energy 01/2013; 87:96–116. DOI:10.1016/j.solener.2012.09.012 · 3.54 Impact Factor
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    ABSTRACT: Fault detection in solar photovoltaic (PV) arrays is an essential task for increasing reliability and safety in PV systems. Fault classification allows identification of the possible fault type so that to expedite PV system recovery. However, because of the non-linear output characteristics of PV arrays, a variety of faults may be difficult to detect using conventional protection devices. Supervised learning methods have been previously proposed to detect and classify solar PV arrays. These methods rely on numerous labeled data for training models and, therefore, have drawbacks: 1) The labeled data on solar PV arrays is difficult or expensive to obtain; 2) The model requires updates as environmental conditions change. To solve these issues, this paper proposes a fault detection and classification method using graph-based semi-supervised learning (SSL). The proposed method only uses a few labeled data points, but relies instead on a large amount of inexpensive unlabeled data points. The method demonstrates self-learning ability in real-time operation. Simulation and experimental results verify the proposed method.
    Energy Conversion Congress and Exposition (ECCE), 2013 IEEE; 01/2013
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    ABSTRACT: This paper proposes a method to merge PV cells or PV panels within the internal components DC-DC converters. The purpose of this merged structure is to reconfigure the PV modules between series and parallel connections using high switching frequencies. This leads to multi-levels of voltages and currents that become applied to the output filter of the converter. As an example, reconfigurable PV-buck converter topology is presented. Its analysis, simulation and experimental verification are presented. Benefits and drawbacks of the new approach are discussed.
    Control and Modeling for Power Electronics (COMPEL), 2013 IEEE 14th Workshop on; 01/2013
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    ABSTRACT: For most linear loads, finding the maximum power point by using the traditional perturb-and-observe algorithm to track the maximum output current of the DC-DC converter in a PV system is a proven technique. However, when the load is nonlinear and exhibits negative impedance, traditional methods might no longer be applicable. This paper presents a method to overcome this challenge. The proposed maximum power point tracking algorithm still only requires typically measured signals, yet is suitable for both linear and periodic nonlinear loads.
    Energy Conversion Congress and Exposition (ECCE), 2013 IEEE; 01/2013
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    ABSTRACT: The design of a new-type of inductive magnetic energy harvesting system with high permeability magnetic material combined with a dual polarity boost converter is presented. By using the strong magnetostatic interaction between the coils and permanent magnets, the output power and power density of the energy harvester is enhanced compared to conventional air cored inductive harvesters. In the proposed dual polarity boost converter, the coil leakage inductor of the harvester is utilized to form the input inductor of the boost converter to minimize the converter size. Therefore, the magnetic energy harvester and the power converter share the same magnetic core. Optimum system design becomes a balance of core size, required leakage inductance and required magnetizing inductance in order to extract maximum energy.
    2012 IEEE Energy Conversion Congress and Exposition (ECCE); 09/2012
  • Ye Zhao, Ling Yang, B. Lehman
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    ABSTRACT: This paper presents a reconfigurable solar photovoltaic (PV) battery charger. A traditional PV charger either utilizes a DC/DC converter between the PV array and load, or it connects the battery directly to the PV array without any power conversion. Different from these approaches, our proposed PV charger avoids conventional DC/DC converters by using a reconfigurable switch matrix. This switch matrix can configure the PV array automatically into optimal series-parallel configuration to charge batteries with different voltage ratings.
    Telecommunications Energy Conference (INTELEC), 2012 IEEE 34th International; 01/2012

Publication Stats

1k Citations
166.95 Total Impact Points

Institutions

  • 1996–2015
    • Northeastern University
      • Department of Electrical and Computer Engineering
      Boston, Massachusetts, United States
  • 2010
    • University of Essex
      • Department of Psychology
      Colchester, ENG, United Kingdom
  • 1996–2007
    • Boston University
      • Department of Electrical and Computer Engineering
      Boston, MA, United States
  • 1997
    • Clark Atlanta University
      • Department of Engineering
      Atlanta, GA, United States
  • 1994–1996
    • Mississippi State University
      • Department of Electrical and Computer Engineering
      Starkville, MS, United States
  • 1990–1994
    • Georgia Institute of Technology
      • School of Electrical & Computer Engineering
      Atlanta, Georgia, United States
  • 1989
    • University of Illinois, Urbana-Champaign
      • Department of Electrical and Computer Engineering
      Urbana, Illinois, United States