B. Lehman

Northeastern University, Boston, Massachusetts, United States

Are you B. Lehman?

Claim your profile

Publications (137)192.24 Total impact

  • D.O. Neacsu · Yue Zheng · B.Lehman
    [Show abstract] [Hide abstract]
    ABSTRACT: This paper demonstrates 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. This digital architecture becomes so valuable since the switching instants can be set in any conceivable manner, away from the rigid constraint of a repetitive sequence of states on each PWM period. Experimental results validate that the proposed new architecture allows simple and low cost implementation of complicated PWM patterns with harmonic reductions that could not otherwise be achieved for low cost.
    IEEE Transactions on Power Electronics 04/2015; DOI:10.1109/TPEL.2015.2424200 · 5.73 Impact Factor
  • [Show abstract] [Hide abstract]
    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 03/2015; 9(1):62-76. DOI:10.1109/MIE.2014.2360721 · 5.06 Impact Factor
  • Brad Lehman · Henry Chung
    [Show abstract] [Hide abstract]
    ABSTRACT: In this paper, a single-phase, single-stage current source inverter-based photovoltaic system for grid connection is proposed. The system utilizes transformer-less single-stage conversion for tracking the maximum power point and interfacing the photovoltaic array to the grid. The maximum power point is maintained with a fuzzy logic controller. A proportional-resonant controller is used to control the current injected into the grid. To improve the power quality and system efficiency, a double-tuned parallel resonant circuit is proposed to attenuate the second- and fourth- order harmonics at the inverter dc side. A modified carrierbased modulation technique for the current source inverter is proposed to magnetize the dc-link inductor by shorting one of the bridge converter legs after every active switching cycle. Simulation and practical results validate and confirm the dynamic performance and power quality of the proposed system.
    IEEE Transactions on Power Electronics 02/2015; 30(2):517-518. DOI:10.1109/TPEL.2012.2229218 · 5.73 Impact Factor
  • Source
    R.M. Nakagomi · Ye Zhao · B. Lehman
  • [Show abstract] [Hide abstract]
    ABSTRACT: Fault detection in solar photovoltaic (PV) arrays is an essential task for increasing reliability and safety in PV systems. Because of PV’s nonlinear characteristics, a variety of faults may be difficult to detect by conventional protection devices, leading to safety issues and fire hazards in PV fields. To fill this protection gap, machine learning techniques have been proposed for fault detection based on measurements, such as PV array voltage, current, irradiance, and temperature. However, existing solutions usually use supervised learning models, which are trained by numerous labeled data (known as fault types) and therefore, have drawbacks: 1) the labeled PV data are difficult or expensive to obtain, 2) the trained model is not easy to update, and 3) the model is difficult to visualize. To solve these issues, this paper proposes a graph-based semi-supervised learning model only using a few labeled training data that are normalized for better visualization. The proposed model not only detects the fault, but also further identifies the possible fault type in order to expedite system recovery. Once the model is built, it can learn PV systems autonomously over time as weather changes. Both simulation and experimental results show the effective fault detection and classification of the proposed method.
    IEEE Transactions on Power Electronics 10/2014; 30(5):2848. DOI:10.1109/TPEL.2014.2364203 · 5.73 Impact Factor
  • Brad Lehman · Arnold J. Wilkins
    [Show abstract] [Hide abstract]
    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
    [Show abstract] [Hide abstract]
    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
    [Show abstract] [Hide abstract]
    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
    [Show abstract] [Hide abstract]
    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
  • Brad Lehman
    IEEE Transactions on Power Electronics 01/2014; 29(1):3-5. DOI:10.1109/TPEL.2013.2271372 · 5.73 Impact Factor
  • Qian Sun · Sumeet Patil · Nian-Xiang Sun · Brad Lehman
    [Show abstract] [Hide abstract]
    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
  • [Show abstract] [Hide abstract]
    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
  • [Show abstract] [Hide abstract]
    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
  • [Show abstract] [Hide abstract]
    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
    [Show abstract] [Hide abstract]
    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
  • Qian Sun · S. Patil · Nian-Xiang Sun · B. Lehman
    [Show abstract] [Hide abstract]
    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
  • Su Sheng · Chung-Ti Hsu · Peng Li · B. Lehman
    [Show abstract] [Hide abstract]
    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
    [Show abstract] [Hide abstract]
    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(1):96–116. DOI:10.1016/j.solener.2012.09.012 · 3.54 Impact Factor
  • Ye Zhao · Brad Lehman · Roy Ball · J.-F. de Palma
    [Show abstract] [Hide abstract]
    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

Publication Stats

1k Citations
192.24 Total Impact Points

Institutions

  • 1996–2015
    • Northeastern University
      • Department of Electrical and Computer Engineering
      Boston, Massachusetts, United States
  • 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
  • 1993–1996
    • 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