Power Converter-Based Three-Phase Nonlinear Load Emulator for a ...

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IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 29, NO. 11, NOVEMBER 2014

Power Converter-Based Three-Phase Nonlinear Load Emulator for a Hardware Testbed System Metin Kesler, Member, IEEE, Engin Ozdemir, Senior Member, IEEE, Mithat C. Kisacikoglu, Member, IEEE, and Leon M. Tolbert, Fellow, IEEE

Abstract—A three-phase nonlinear load emulator using a power electronic converter is presented in this study. The proposed nonlinear load emulator is intended to be used in an ultrawide-area grid transmission network emulator, also called hardware testbed (HTB). The emulator converter is controlled in rectifier mode to act as the real nonlinear three-phase diode rectifier load. This paper presents an accurate controller for the nonlinear load emulator based on a three-phase diode rectifier system to be used in the HTB. This study also demonstrates simulation and experimental results for verification of the proposed controller. Index Terms—Three-phase diode bridge rectifier, emulator, hardware testbed (HTB), nonlinear load, power converter.

iabc i∗abc vabc uabc u α , uβ gabc gα , gβ fabc fα , fβ L C Lf Rf Rdc Cdc Vdc

NOMENCLATURE Three-phase source currents. Three-phase nonlinear emulator reference currents. Three-phase source voltages. Three-phase terminal voltages. Terminal voltages in αβ coordinates. Three-phase Heaviside functions. Heaviside functions in αβ coordinates. Three-phase diode voltage functions. Diode voltage functions in αβ coordinates. Three-phase line inductances. DC-bus capacitor. Emulator filter inductance. Emulator filter resistance. Emulator dc link resistance. Emulator dc link capacitor. Emulator dc-link voltage.

Manuscript received August 30, 2013; revised November 24, 2013 and January 5, 2014; accepted January 6, 2014. Date of publication January 21, 2014; date of current version July 8, 2014. This work was supported by the Engineering Research Center Program of the National Science Foundation, the Department of Energy under NSF Award EEC-1041877, the CURENT Industry Partnership Program and TUBITAK (The Scientific and Technological Research Council of Turkey) via its BIDEB-2219 program. This study is also supported by YOK (Turkish Higher Education Council). Recommended for publication by Associate Editor P. Chi-Kwong Luk. M. Kesler is with the Department of Computer Engineering, Bilecik Seyh Edebali University, Bilecik, 11210 Turkey (e-mail: [email protected]). E. Ozdemir is with the Kocaeli University, 41300 Izmit/Kocaeli, Turkey (e-mail: [email protected]). M. C. Kisacikoglu is with the Scientific and Technological Research Council of Turkey (TUBITAK), 06100 Ankara, Turkey (e-mail: [email protected]). L. M. Tolbert is with the University of Tennessee and Oak Ridge National Laboratory, Oak Ridge, TN 37831 USA (e-mail: [email protected]). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TPEL.2014.2301815

dd dq Da Db Dc id , iq i∗d , i∗q vd , vq vα , vβ i∗α , i∗β

Duty ratio in d-axis. Duty ratio in q-axis. Duty ratio in a-axis. Duty ratio in b-axis. Duty ratio in c-axis. Instantaneous source currents in dq coordinates. Instantaneous reference currents in dq coordinates. Instantaneous source voltages in dq coordinates. Instantaneous source voltages in αβ coordinates. Instantaneous reference currents in αβ coordinates. I. INTRODUCTION

HE simulation of power electronic converters and electrical drives plays an important role in research and industry. Computer simulations are used in research to analyze the behavior of a new power circuit, which leads to understanding of the circuit characteristics [1]. However, ideal condition-based computer- aided simulations are not efficient to emulate real system behavior and characteristics. There is a demand to emulate the system behavior in real time. Transmission network emulator, which is called the hardware testbed (HTB), is conceptualized to emulate the largescale power system by interconnected converters which emulate power generators and loads. With modular and reconfigurable converters, the HTB can have a flexible network and perform various scenarios. The HTB will allow testing, integration, and demonstration of various key technologies on monitoring, control, actuation, and visualization. With HTB, it is also convenient to test different system architectures, such as emulations of the synchronous generator, induction motor load, PV system, wind turbine, constant impedance, current and power (ZIP) load, high voltage dc (HVdc), and energy storage systems by reconfiguring the system structure [2]–[4]. In this study, a three-phase nonlinear load emulator using a power electronic converter is presented and used in an ultrawide-area grid transmission network emulator, also called HTB. The HTB was established to match the real power system components by changing the configuration or model parameters of the controller. There is an increasing usage of hardware-in-the loop (HIL) technologies that design and test different control systems [5]–[13] in several sectors such as automotive, robotics, aviation, power electronics, and smart grid application. Currently, there are different kinds of real-time simulation platforms and real time digital simulator (RTDS) [14]–[18] proposed in the literature. Real time emulating of electrical loads using power electronic converters is presented in [19] by using an optimal feedback control technique. The concept of real-time analysis

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KESLER et al.: POWER CONVERTER-BASED THREE-PHASE NONLINEAR LOAD EMULATOR FOR A HARDWARE TESTBED SYSTEM

Fig. 2.

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Three-phase diode bridge rectifier.

with the proposed system controller development. Section IV focuses on simulation verification of the proposed system controller for different cases and demonstrates the dynamic behavior of the rectifier in different loading conditions. Section V demonstrates the experimental results of the proposed system controller for the nonlinear load emulator. Fig. 1.

Hardware testbed structure for a reconfigurable grid emulator [4].

II. THREE-PHASE NONLINEAR LOAD MODEL of a smart grid and implementation on a smart grid testbed is experimentally tested in [20]. However, there is not much research in the literature on a three-phase rectifier-based nonlinear load emulation. In this study, a nonlinear load emulation provides a platform where the behavior of a three-phase uncontrolled diode bridge rectifier load can be emulated in hardware at real power levels using controllable power electronics converters. In order to perform a real-load profile in the power system, a robust control method should be designed which would emulate the actual dynamic performance of the system. Power converters in the HTB provide the opportunity to both demand and supply power on the grid emulator side while circulating current on the dc side, which is especially beneficial when working under high-power application. The system operation of the HTB is shown in Fig. 1. A regenerative topology is used, where all the converters are connected to both ac and dc side with a rectifier at the dc side. In the HTB, the produced ac power from the generator emulator is recirculated into the dc bus by the nonlinear loads as they share the same dc and ac bus. The small amount of power consumed in the emulators are due to the conduction and switching losses of the power semiconductors and internal resistance of reactive components. Successful experimental results have contributed to the conceptual design of the HTB, which demonstrates the testing of the electric load and the consequent impact on ac distribution systems. The nonlinear load emulator structure and control algorithm design procedure is discussed in detail, and the performance of the proposed system is presented. The simulation and experimental results clearly show the start-up and steady-state response of the three-phase nonlinear load in different cases. In this study, a 10-kW grid connected power converter-based nonlinear load emulator prototype is set up to verify the effectiveness of the proposed control strategy and circuit configuration, as well as to test the overall system performance. The second section of this paper describes the three-phase diode bridge rectifier system model. Section III is concerned

Three-phase diode bridge rectifiers, basic system configuration given in Fig. 2, are commonly used in industry to produce a dc voltage for motor drives, batteries, and dc—dc converters. The main drawback of these rectifiers is that they inject substantial amount of current harmonics into the power system. These current harmonics can affect the power network by loading capacitors and by affecting the mains voltage at the point of common coupling [21]. Dynamic transient performance of the rectifiers could induce power system voltage and frequency fluctuations, and cause unwanted harmonics on the line voltage and current waveforms. An accurate nonlinear load emulator representing dynamic behavior is crucial in power system analysis. Nonlinear loads such as controlled and uncontrolled singleand three-phase rectifiers form a large part of present electrical loads. The three-phase diode bridge rectifier model [1] associates switching functions (ga , gb , gc ) with each of the bridge phaselegs, representing the conduction state of the diode present in the branch. The switching function is equal to 1 if the diode is conducting current or 0 if it is not. The three-phase voltages ua , ub , uc defined in relation to the neutral point of the threephase ac circuit can be computed by ua = fa Vdc , where

ub = fb Vdc ,

uc = fc Vdc

⎡ ⎤ ⎤ 2ga − gb − gc fa 1 ⎣ fb ⎦ = ⎣ 2gb − gc − ga ⎦. 2 fc 2gc − ga − gb

(1)

⎡

(2)

The dc current is represented by idc = ga ia + gb ib + gc ic .

(3)

A dc voltage is present on the dc side of the three-phase diode bridge rectifier, such as dc capacitor voltage or a voltage source on the dc side. Rf and Lf represent the ac side resistance and inductance of the mains, respectively. Three-phase voltages

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Fig. 3.


Proposed nonlinear load emulator control block diagram for the HTB.

(va , vb , vc ) are defined as follows: ⎡ ⎤ ⎡ ⎤ ⎡ ⎤ ⎡ ⎤ ia ia ua va d ⎣ vb ⎦ = Rf ⎣ ib ⎦ + Lf ⎣ ib ⎦ + ⎣ ub ⎦. dt vc ic ic uc

(4)

The produced voltage and current signals are converted to αβ coordinates. With the transformation, the computed dc current can be obtained by idc = gα i∗α + gβ i∗β where Vdc can be calculated by

Vdc = Rdc idc + Cdc then,

uα uβ

= Vdc

(5)

idc dt

fα . fβ

Voltages in αβ coordinates are defined as given below ∗ d i∗α iα u vα = Rf ∗ + Lf + α . vβ iβ uβ dt i∗β

(6)

(7)

(8)

III. DESIGN OF A NONLINEAR LOAD CONTROLLER The three-phase diode bridge rectifier model results in reference current signals (i∗a , i∗b , i∗c ) as output parameters by using grid voltages (va , vb , vc ), and input parameters (Rf , Lf , Cdc , Rdc ) in the model. By modifying parameters of the model, the rectifier load behavior can easily be changed. The modeling of the diode bridge rectifier is done utilizing three heaviside functions (g) to determine when the diode is conducting or in blocking stage as given in (1), (2), and (3) [1]. Then, terminal node voltages (uα , uβ ) are calculated using f functions by multiplying the dc voltage (Vdc ) as given in (1). The dc-link (idc ) current of the diode rectifier model is calculated in

(5) by using gα and gβ functions. Three-phase current references (i∗abc ) are produced by voltages in αβ coordinates (vα β ) using variable load resistance and the capacitor connected to the dc bus as given in (8). Equations (1) to (8) can be represented by the proposed nonlinear load emulator control block diagram for the HTB shown in Fig. 3. The calculations of duty ratios in dq and abc coordinates are summarized in (9) and (10), respectively 1 ed + vd + 3ωLiq dd = (9) dq Vdc eq + vq − 3ωLid ⎤ ⎡ ⎤ ⎡ sin(ωt) cos(ωt) Da 2 ⎣sin(ωt − 2π/3) cos(ωt − 2π/3)⎦ dd . (10) ⎣ Db ⎦ = dq 3 sin(ωt + 2π/3) cos(ωt + 2π/3) Dc The three-phase reference currents (i∗abc ) are converted to dq coordinates and compared with actual grid currents (idq ) in dq coordinates. Then using two PI controllers and the dc bus voltage, finally switching signals in dq coordinates are generated accordingly and fed to the PWM converter. The phase angle of the three-phase voltage is obtained through the phase-locked loop (PLL) algorithm. The PLL takes three-phase voltages as inputs, and reduces a sinusoidal signal of the same frequency as the frequency of the supply voltages. It also calculates the phase angle (θ) for synchronization with the grid voltages as described in [22]. IV. SIMULATION VERIFICATION OF THE PROPOSED CONTROLLER The three-phase nonlinear load emulator consists of a power electronic converter used in an ultrawide-area grid transmission network emulator. The proposed three-phase diode bridge rectifier nonlinear load emulator used in the HTB can demonstrate the dynamic behavior of the rectifier under different loading


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TABLE I SYSTEM PARAMETERS USED IN THIS STUDY

Fig. 5. Simulated waveforms of converter-side grid currents (Case 2) (L f = 4 mH, R f = 1 mΩ, R d c = 10 Ω, C d c = 500 μF).

Fig. 6. Simulated waveforms of converter-side currents (Case 3) (L f = 0.7 mH, R f = 1 mΩ, R d c = 12 Ω, C d c = 200 μF).

Fig. 4. Simulation results for start-up and steady-state performance of the HTB (Case 1). (a) converter grid currents and (b) reference currents (L f = 1 mH, R f = 1 mΩ, R d c = 12 Ω, C d c = 1100 μF).

conditions. The system parameters used in the simulation and in the experimental study are listed in Table I. The stability of the nonlinear load was analyzed based on the Powersim (PSIM) [23] simulator model shown in Fig. 3. The simulation results are shown in Fig. 4 for start-up and steady-state performance of the HTB in nonlinear load analysis. Fig. 4 shows the change in converter grid currents and reference currents when the proposed controller receives a start command at 0.015 s. The simulation results verify the performance of the proposed control strategy for start up and steady state using parameters Lf = 1 mH, Rf = 1 mΩ, Rdc = 12 mΩ, Cdc = 1100 μF. The simulated waveforms of reference line currents and converter-side grid currents are depicted in Figs. 5–7 for different load parameters (Rf , Lf , Cdc , Rdc ). These figures show the line current harmonics under different loading conditions. Therefore, they verify the controller performance at different loading levels. Although there is some increased ripple observed in the negative cycle of the current waveforms in the simulation results (Figs. 4– 7), these have been eliminated in the final experimental results that are explained in the next section. V. EXPERIMENTAL STUDIES The nonlinear load experimental prototype is designed using a Vacon X series power converter unit which was donated to the University of Tennessee by Vacon, an ac drive manufacturer. The converter utilizes three legs of Semikron IGBT modules each having two IGBTs and two antiparallel diodes.

Fig. 7. Simulated waveforms of converter-side currents and together with reference line currents (Case 4) (a) converter grid currents and (b) reference currents (L f = 1 mH, R f = 1 mΩ, R d c = 25 Ω, C d c = 1100 μF).

The converter controller board has been replaced by a floatingpoint Texas Instruments F28335 digital signal processor (DSP) board, a custom DSP interface-board, and an ac grid voltage sampling board. The nonlinear load HTB emulator controller code is developed in C language and embedded into the system. Prior to the implementation stage, the controller code is first tested and developed using PSIM simulation software [23]. As depicted in Fig. 3, calculation begins from sampling of three-phase voltages (vabc ). The nonlinear load model is described in dq coordinates with relations among voltage, current, load resistance, capacitance, and inductance. After each iteration cycle, in the switching angle calculation period in the DSP, three-phase current references are produced and fed to the gate drive signals of the IGBTs in the HTB emulator converter. The proposed nonlinear load emulator has been implemented and tested to verify the principle of operation of the proposed nonlinear load in the HTB. The grid phase voltage and frequency

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Fig. 8.


Nonlinear load HTB emulator structure. Fig. 10. Phase voltage and the three-phase grid current from nonlinear load emulator. (L f = 4 mH, R f = 1 mΩ, R d c = 10 Ω, C d c = 500 μF).

Fig. 11. Phase voltage and three-phase grid current waveforms for different parameter condition. (L f = 2 mH, R f = 1 mΩ, R d c = 12 Ω, C d c = 1100 μF).

Fig. 9. (a) Nonlinear load emulator experimental setup photograph and (b) HTB system photograph.

are selected as 208 V and 60 Hz. A 100 kVA rated converter was used at 10 kVA connected to the grid. The prototype system specifications are selected as given in the simulation section in Table I. The experimental setup was implemented using the converter configuration shown in Fig. 8 to demonstrate the dynamic

experimental grid current of the converter emulating a diode bridge rectifier in the HTB. The nonlinear load model is calculated during each PWM period of converter switching signals inside the DSP controller. Line current outputs from the measurement and current references calculated in the nonlinear load model are compared in the HTB controller in the DSP. Then, the current references are obtained from the nonlinear load model in the DSP code. The proposed nonlinear load emulator successfully replicates the behavior of the calculated values in the nonlinear load model and thus could be used in the hardware test bed as a dynamic system model. Fig. 9 shows the nonlinear load emulator experimental setup and HTB system photograph. The experimental waveforms for nonlinear load emulation for phase voltage (va ) and the three-phase grid current waveforms are shown in Fig. 10. Figs. 11 and 12 show the experimental results for phase voltage and three-phase grid currents with different parameters in the nonlinear load emulator. When the filter inductance (Lf ) is decreased, the three-phase currents are highly distorted as shown in the experimental results. The experimental results show that the proposed controller can achieve successful nonlinear load emulation. The system is capable of emulating a three-phase diode bridge rectifier load in real time using power electronic converters. The system is controlled in the DSP controller in the HTB emulator system with proper design of the control parameters.


Fig. 12. Phase voltage and the three-phase grid current waveforms for different parameters. (L f = 0.7 mH, R f = 1 mΩ, R d c = 12 Ω, C d c = 200 μF).

VI. CONCLUSION This paper discusses nonlinear load dynamic modeling of a three-phase diode bridge rectifier, controller design, and finally simulation and experimental results of the proposed HTB emulator system. The power converter is controlled in threephase rectifier mode to behave like a real nonlinear load whose model is programmed in the DSP controller in the HTB emulator. The experiments successfully demonstrate that the power converter-based nonlinear load emulator prototype could represent a three-phase diode bridge rectifier with proper design of the control parameters. The main advantages of the proposed emulator are reduced expenses and set-up time due to energy savings, low air conditioning requirements, space savings, and low peak power demands. Finally, simulation and experimental results from nonlinear load prototype verify the validity of the proposed system and effectiveness of the designed controller. This paper discusses nonlinear load emulation which is capable of emulating an electrical load in real time using power electronic converters in an HTB. The proposed testing approach eliminates the need of using real active loads in the design process and, therefore, greatly reduces the overall design cost. The proposed controller has been designed and implemented in a DSP platform and has been shown to perform satisfactorily under steady-state and transient conditions. The system can be operated to emulate various types of nonlinear load models such as R, RL and RLC loaded three-phase diode bridge rectifier without the need to actually construct a rectifier with all of the different parameters. REFERENCES [1] G. D. Marques, “A simple and accurate system simulation of three-phase diode rectifiers,” in Proc. 24th Annu. Conf. IEEE Ind. Electron. Soc., 1998, pp. 416–421. [2] J. Wang, L. Yang, Y. Ma, X. Shi, X. Zhang, L. Hang, K. Lin, L. M. Tolbert, F. Wang, and K. Tomsovic, “Regenerative power converters representation of grid control and actuation emulator,” in Proc. IEEE Energy Convers. Congr. Expo., Sep. 2012, pp. 2460–2465. [3] L. Yang, X. Zhang, Y. Ma, J. Wang, L. Hang, K. Lin, M. Tolbert, Leon, F. Wang, and K. Tomsovic, “Hardware implementation and control design of generator emulator in multi-converter system,” in Proc. IEEE Appl. Power Electron. Conf. Expo., 2013, pp. 2316–2323. [4] J. Wang, Y. Ma, L. Yang, L. M. Tolbert, and F. Wang, “Power converterbased three-phase induction motor load emulator,” in Proc. IEEE Appl. Power Electron. Conf. Expo., 2013, pp. 3270–3274.

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[5] W. C. Lee and D. Drury, “Development of a hardware-in-the-loop simulation system for testing cell balancing circuits,” IEEE Trans. Power Electron., vol. 28, no. 12, pp. 45 949–5959, Dec. 2013. [6] B. Lu, X. Wu, H. Figueroa, and A. Monti, “A low-cost real-time hardwarein-the-loop testing approach of power electronics controls,” IEEE Trans. Ind. Electron., vol. 54, no. 2, pp. 919–931, Apr. 2007. [7] H. Li, M. Steurer, K. L. Shi, S. Woodruff, and D. Zhang, “Development of a unifed design, test, and research platform for wind energy systems based on hardware-in-the-loop real-time simulation,” IEEE Trans. Ind. Electron., vol. 53, no. 4, pp. 1144–1151, Jun. 2006. [8] W. Ren, M. Steurer, and L. Qi, “Evaluating dynamic performance of modern electric drives via power-hardware-in-the-loop simulation,” in Proc. IEEE Int. Symp. Ind. Electron., 2008. [9] J. Jin-Hong, K. Jong-Yul, K. Hak-Man, K. Seul-Ki, C. Changhee, K. JangMok, A. Jong-Bo, and N. Keo-Yamg, “Development of hardware in-theloop simulation system for testing operation and control functions of microgrid,” IEEE Trans. Power Electron., vol. 25, no. 12, pp. 2919–2929, Dec. 2010. [10] M. Faruque and V. Dinavahi, “Hardware-in-the-loop simulation of power electronic systems using adaptive discretization,” IEEE Trans. Ind. Electron., vol. 57, no. 4, pp. 1146–1158, Apr. 2010. [11] Z. R. Ivanovic, E. M. Adzic, M. S. Vekic, S. U. Grabic, N. L. Celanovic, and V. A. Katic, “HIL evaluation of power flow control strategies for energy storage connected to smart grid under unbalanced conditions,” IEEE Trans. Power Electron., vol. 27, no. 11, pp. 4699–4710, Nov. 2012. [12] M. S. Vekic, S. U. Grabic, D. P. Majstorovic, I. L. Celanovic, N. L. Celanovic, and V. A. Katic, “Ultralow latency HIL platform for rapid development of complex power electronics systems,” IEEE Trans. Power Electron., vol. 27, no. 11, pp. 4436–4444, Nov. 2012. [13] A. Sarikhani and O. A. Mohammed, “HIL-based finite-element design optimization process for the computational prototyping of electric motor drives,” IEEE Trans. Energy Convers., vol. 27, no. 3, pp. 737–746, Sep. 2012. [14] V. R. Dinavahi, M. R. Iravani, and R. Bonert, “Real-time digital simulation of power electronic apparatus interfaced with digital controllers,” IEEE Trans. Power Del., vol. 16, no. 4, pp. 775–781, Oct. 2001. [15] S. Abourida, C. Dufour, M. G. Belanger, J., N. Lechevin, and Y. Biao, “Real-time PC-based simulator of electric systems and driver,” in Proc. IEEE Appl. Power Electron. Conf., 2002, pp. 433–438. [16] R. Champagne, L. A. Dessaint, H. Fortin-Blanchette, and G. Sybille, “Analysis and validation of a real-time AC drive simulator,” IEEE Trans. Power Electron., vol. 19, no. 2, pp. 336–345, Mar. 2004. [17] G. G. Parma and V. Dinavahi, “Real-time digital hardware simulation of power electronics and drives,” IEEE Trans. Power Del., vol. 22, no. 2, pp. 1235–1246, Apr. 2007. [18] M. Aung and V. Dinavahi, “FPGA-based real-time emulation of power electronic systems with detailed representation of device characteristics,” IEEE Trans. Ind. Electron., vol. 58, no. 1, pp. 358–368, Jan. 2011. [19] R. Y. Srinivasa and C. C. Mukul, “Real-time electrical load emulator using optimal feedback control technique,” IEEE Trans. Ind. Electron., vol. 57, no. 4, pp. 1217–1225, Apr. 2010. [20] V. Salehi, A. Mohamed, A. Mazloomzadeh, and O. Mohammed, “Laboratory-based smart power system—Part I: Design and system development,” IEEE Trans. Smart Grid, vol. 3, no. 3, pp. 1394–1404, Sep. 2012. [21] K. L. Lian, K. P. Brian, and P. W. Lehn, “Harmonic analysis of a threephase diode bridge rectifer based on sampled-data model,” IEEE Trans. Power Del., vol. 23, no. 2, pp. 1088–1096, Apr. 2008. [22] M. Kesler and E. Ozdemir, “Synchronous reference frame SRF based control method for UPQC under unbalanced and distorted load conditions,” IEEE Trans. Ind. Electron., vol. 58, no. 9, pp. 3967–3975, Sep. 2011. [23] (2013, May). Powersim. [Online]. Available: http://www.powersimtech. com Metin Kesler (M’12) received the M.Sc. and Ph.D. degrees in electrical education from Kocaeli University, Kocaeli, Turkey, in 2005 and 2010, respectively. In 2013, he was a Visiting Professor at the University of Tennessee, Knoxville, TN, USA, where he was also working on a project at the CURENT Laboratory, Knoxville, TN, and also worked on a project at the Oak Ridge National Laboratory, Oak Ridge, TN. He is an Associate Professor with the Computer Engineering Department, Faculty of Engineering, Bilecik Seyh Edebali University, Bilecik, Turkey. His research interests include power quality, plug-in electric vehicles and renewable energy systems, and distributed energy resources.

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Engin Ozdemir (S’98–M’00–SM’07) was born in Izmit, Turkey. He received the M.Sc. degree in electrical engineering from Yildiz Technical University in Istanbul, Turkey, in 1994, and the Ph.D. degree in electrical engineering in Kocaeli University, Turkey, in 1999. In 2007, he was a visiting professor at the University of Tennessee, Knoxville, TN, where he also worked on a project at the Oak Ridge National Laboratory (ORNL), in Oak Ridge, TN. Since 2013, he has been a Professor in Department of Energy Systems Engineering in the Faculty of Technology in Kocaeli University, Turkey. His research interests are renewable energy system and power electronics interface, multilevel converters, energy storage systems, active power filters and power quality.

Mithat C. Kisacikoglu (S’04–M’14) received the B.S. degree from Istanbul Technical University, Istanbul, Turkey in 2005, the M.S. degree from University of South Alabama, Mobile, AL, USA, in 2007, and the Ph.D. degree from the University of Tennessee, Knoxville, TN, USA, in 2013, all in electrical engineering. He is currently an R&D Specialist at The Scientific and Technological Research Council of Turkey, operating under the Ministry of Science, Industry, and Technology, Ankara, Turkey. He was a Researcher at the University of Tennessee and at the Oak Ridge National Laboratory (parttime) between 2007 and 2013. His research interests include plug-in electric vehicles, renewable energy sources, and power electronic converters.

Leon M. Tolbert (S’88–M’91–SM’98–F’13) received the B.E.E., M.S., and Ph.D. degrees in electrical engineering from the Georgia Institute of Technology, Atlanta, GA, USA, in 1989, 1991, and 1999, respectively. He joined Oak Ridge National Laboratory (ORNL) in 1991 and worked on several electrical distribution projects at the three U.S. Department of Energy plants in Oak Ridge, TN, USA. He joined the University of Tennessee in 1999, and he is presently the Min H. Kao Professor and Head of the Department of Electrical Engineering and Computer Science. He is also a part time Senior Research Engineer at ORNL. His research interests include the areas of electric power conversion for distributed energy sources, motor drives, multilevel converters, hybrid electric vehicles, and the application of SiC power electronics. Dr. Tolbert is a Registered Professional Engineer in the state of Tennessee. He is a member of the following societies: Industry Applications, Industrial Electronics, Power and Energy, and Power Electronics. He was elected as a member-at-large to the IEEE Power Electronics Society Advisory Committee for 2010–2012, and he served as the Chair of the PELS Membership Committee from 2011–2012. He was an Associate Editor of the IEEE TRANSACTIONS ON POWER ELECTRONICS from 2007 to 2012 and an associate editor of the IEEE POWER ELECTRONICS LETTERS from 2003 to 2006. He was the Chair of the Education Activities Committee of the IEEE Power Electronics Society from 2003 to 2007. He received the 2001 IEEE Industry Applications Society Outstanding Young Member Award, and he has four prize paper awards from the IEEE.