B. Lehman

Northeastern University, Boston, MA, United States

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Publications (116)141.38 Total impact

  • [Show abstract] [Hide abstract]
    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
  • 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: 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 01/2013; 60(9):3784-3795. · 6.50 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: 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
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    ABSTRACT: Methods to estimate the DC wire loss in photovoltaic (PV) systems are proposed. Root-mean-square (RMS) of the irradiance data is first related to wire losses. Then a statistical model is used to characterize the irradiance data for the PV systems. The β-distribution gives reasonable match of the RMS value for the irradiance data. The result is consistent for different geographic locations.
    Telecommunications Energy Conference (INTELEC), 2012 IEEE 34th International; 01/2012
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    ABSTRACT: This paper describes an algorithm for managing a system consisting of “dissimilar” battery packs in parallel without multiple DC/DC converters interfacing each battery pack to the load. The power loss from the circulating current can be avoided so that the operating time of the parallel battery system is prolonged. The proposed algorithm is able to recognize the load demand and determine the number and sequence of the battery packs needed to be connected to the DC bus.
    Telecommunications Energy Conference (INTELEC), 2012 IEEE 34th International; 01/2012
  • 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
  • F. Boico, B. Lehman
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    ABSTRACT: A method to track the maximum power of multiple-input, portable, photovoltaic systems is proposed. The method shares a single current sensor by interleaving Perturb and Observe operations. The system has reduced size and cost, making it attractive for compact portable solar panels and solar battery chargers, such as for cell phones, laptops, and other portable electronics with rechargeable batteries. A flexible total system architecture design is proposed and tested.
    Solar Energy - SOLAR ENERG. 01/2012;
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    ABSTRACT: Because of the non-linear output characteristics of PV arrays, a variety of faults may be difficult to detect by conventional protection devices. To detect and classify these unnoticed faults, a fault detection and classification method has been proposed based on decision trees (DT). Readily available measurements in existing PV systems, such as PV array voltage, current, operating temperature and irradiance, are used as "attributes" in the training and test set. In experimental results, the trained DT models have shown high accuracy of fault detection and fault classification on the test set.
    01/2012;
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    Song Chen, Peng Li, David Brady, Brad Lehman
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    ABSTRACT: This paper proposes a general method of sizing the inverter for a PV system. The method evaluates effects of PV incentive policies, inverter efficiency curves, and inverter protection schemes on optimum inverter sizing through system-level cost analysis. Specifically, different scenarios of PV incentives are discussed and compared to show that the optimal inverter size varies notably by location and context.
    01/2011;
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    ABSTRACT: Fault analysis in solar photovoltaic (PV) arrays is a fundamental task to protect PV modules from damages and to eliminate the risks of safety hazards. This paper examines two types of unique faults found in photovoltaic (PV) array installations that have not been studied in the literature. One is a fault that occurs under low irradiance conditions. In some circumstances, fault current protection devices are unable to detect certain types of faults so that the fault may remain hidden in the PV system, even after irradiance increases. The other type of fault occurs when a string of PV modules is reversely connected, caused by inappropriate installation. This fault type brings new challenges for overcurrent protection devices because of the high rating voltage requirement. In both cases, these unique PV faults may subsequently lead to unexpected safety hazards, reduced system efficiency and reduced reliability.
    Conference Record of the IEEE Photovoltaic Specialists Conference 01/2011;
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    ABSTRACT: This paper discusses the challenges of overcurrent protection devices (OCPD) brought by the maximum power point tracker (MPPT) of the centralized grid-connected inverter in a photovoltaic (PV) array. Since PV arrays have non-linear output characteristics, MPPT algorithms of PV inverters are often used to harvest the maximum output power from PV arrays. Most MPPTs are designed to respond to unexpected environmental variations, such as the change of solar irradiance on PV arrays. For this reason, instead of remaining constant, the fault current in the PV array may be reduced by the MPPT. Sometimes the fault current might evolve to an undetectable level for overcurrent protection devices. As a result, the fault might be hidden in the PV array and become a potential hazard for system efficiency and reliability.
    Conference Record of the IEEE Photovoltaic Specialists Conference 01/2011;
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    ABSTRACT: Solar photovoltaic (PV) arrays behave distinctively from conventional power sources so that they need special consideration in fault analysis and protection. The faults inside PV arrays usually cause overcurrent that may damage PV components. This paper focuses on the challenges to overcurrent protection devices (OCPDs) in a PV array under two types of unique fault scenarios. One is a line-line fault that occurs under low irradiance conditions. In this circumstance, the fault current may not be large enough to trip the OCPDs in the PV array, even when high irradiance occurs later in the day. The other fault scenario is that when PV blocking diodes are used in the PV array, the reverse current may be greatly limited. However, OCPDs might not detect the reverse current properly. In both fault scenarios, the fault may not be cleared successfully by conventional OCPDs. Therefore, faults may remain undetected, which could lead to reduced system efficiency, reduced system reliability, and even unexpected safety hazards.
    01/2011;
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    T. Qian, B. Lehman
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    ABSTRACT: This study presents a concept of connecting two-stage DC-DC converters in an input-series connection. An application example is discussed in detail where the first stage utilises two series connected buck converters that have reduced voltage stress. A single second stage is a half-bridge converter and is able to regulate the charge balance of the first stage. The benefits of the topology include: reduced primary switch voltage stress, simple self-driven synchronous rectification for wide input voltage range, self-voltage balancing on intermediate bus capacitors and simple housekeeping power supply. Further, the topology exhibits an unusual ripple match concept that can be utilised to suppress the current ripple of the second stage. Based on the detailed analysis, prototypes with 500-700-V input and 5-V/30-A output are built. Experimental results verify the principle and performance of the new topology.
    IET Power Electronics 12/2010; · 1.52 Impact Factor
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    Song Chen, Peng Li, D. Brady, B. Lehman
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    ABSTRACT: This paper investigates the time behavior of over-irradiance events in which the photovoltaic (PV) array outputs more power than the rated power of the inverter. A new dynamic interpretation of such events is proposed and is compared to the conventional static viewpoint. Facts revealed under such dynamic view may lead to new guidelines for system integrators and inverter designers in both sizing and designing inverters. A way to extend overload operation time of inverters is also proposed.
    Control and Modeling for Power Electronics (COMPEL), 2010 IEEE 12th Workshop on; 07/2010
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    ABSTRACT: This paper focuses on fault evolution in a photovoltaic array during night-to-day transition. The effect of a maximum power point tracker on fault current is first discussed. When a PV fault occurs in daylight, overcurrent protection devices work properly. However, when the same fault occurs at night, our results demonstrate that the fault current is difficult to detect. As a result, the fault might be hidden in the PV array and become a potential hazard for system efficiency and reliability.
    Control and Modeling for Power Electronics (COMPEL), 2010 IEEE 12th Workshop on; 07/2010
  • Source
    R.M. Nakagomi, Ye Zhao, B. Lehman
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    ABSTRACT: This paper presents an approach to generate three-phase multi-level AC voltage output using a switching matrix device and photovoltaic (PV) panels. The approach is based on the dynamic reconfiguration of PV panels distributed in a matrix. The principle is similar to methods used in multi-level inverters related to the technique of matrix reconfiguration. The concept is to switch in and out solar panels in series to create a higher three-phase voltage level. The focus of the project is optimizing the PV array usage and improving the output current while generating three-phase multi-level waveforms.
    Control and Modeling for Power Electronics (COMPEL), 2010 IEEE 12th Workshop on; 07/2010

Publication Stats

812 Citations
141.38 Total Impact Points

Institutions

  • 1996–2012
    • Northeastern University
      • Department of Electrical and Computer Engineering
      Boston, MA, United States
  • 2009
    • Diversified Technologies, Inc
      Bedford, Massachusetts, United States
  • 1996–2007
    • Boston University
      • Department of Electrical and Computer Engineering
      Boston, MA, United States
  • 2000
    • Rockwell Collins
      Cedar Rapids, Iowa, 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
  • 1992–1994
    • Georgia Institute of Technology
      • School of Electrical & Computer Engineering
      Atlanta, GA, United States
  • 1989
    • University of Illinois, Urbana-Champaign
      Urbana, Illinois, United States